ROLLER-CAST POLY-DIMETHYLSILOXANE AS A NON-HERMETIC

ENCAPSULANT FOR MEMS PACKAGING

by

SHEM BENJAMIN LACHHMAN

Submitted in partial fulfillment of the requirements

For the degree of Master of Science

Thesis Advisor: Professor Christian A. Zorman

Thesis Co-Advisor: Professor Wen H. Ko

Department of Electrical Engineering and Computer Science

CASE WESTERN RESERVE UNIVERSITY

January, 2012

1

CASE WESTERN RESERVE UNIVERSITY

SCHOOL OF GRADUATE STUDIES

We hereby approve the thesis/dissertation of

Shem Benjamin Lachhman

candidate for the Master of Science degree *.

(signed) Christian A. Zorman .

(chair of the committee)

Wen H. Ko .

Philip X.L. Feng .

(date) 12/08/2011 .

*We also certify that written approval has been obtained for any proprietary material contained therein.

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Table of Contents

List of Tables……………………………………………………………………………...5

List of Figures………………………………………………………………………....…..6

Acknowledgements………………………………….………………………….………....9

Abstract.…………………………...………………………………..……………………10

1. Introduction………………………………………………………………….…….…11

1.1 Implantable Microelectromechanical Systems………….…………...…….…...11

1.1.1 Applications…………………………………………………………..….12

1.1.2 Challenges of Packaging…………..……………………………………..12

1.1.3 Hermetic vs. Non-hermetic Packaging Techniques……………………...13

1.1.4 Packaging Materials……………………………….……………………..15

1.1.5 PDMS…………...……………………………………………………….17

1.2 Dow Corning MDX4-4210 Biomedical Grade Elastomer………...……………18

1.2.1 Material Properties……………………………………………….………18

1.2.2 Conventional Methods of Application………………………………...…20

1.2.3 Advantages of Dow Corning MDX4-4210 Biomedical Grade

Elastomer……………………………………………………………...…21

1.3 Thesis Investigation…………………………………………………………….21

2. Experimental Methods……………………………………………………………….22

2.1 Accelerated Lifetime Testing…………………………………………………...22

2.1.1 Principles…………………………………………………………………22

2.1.2 Accelerated Lifetime Testing Techniques…………………………….....22

3

2.1.3 Accelerated Lifetime Transforms and Reliability Functions………….…24

2.1.4 Surface Insulation Resistance……………………………………...…….28

2.1.5 Sample Preparation………………………………………………………29

2.1.6 Design of Experiments…………………………………………...………41

2.2 Bond Strength Testing……………………………………………………...…..56

2.2.1 Method…………………………………………………………..……….56

2.2.2 Pull Testing Apparatus…………………………………………………..57

2.2.3 Sample Preparation………………………………………………………58

2.2.4 Experimental Design………………………………………………..……59

3. Data Analysis………………………………………………………………...………60

3.1 Accelerated Lifetime Study………...…………………………………………..60

3.1.1 Preliminary Multilayer Accelerated Lifetime Studies………...…...…….60

3.1.2 Primary Accelerated Lifetime Study…………………………….………61

3.2 Adhesion Strength Data Analysis……………………………………….……78

3.2.1 Mounting Adhesive……..………………………………………...……..78

3.2.2 Pull Off Testing………………………..…………………………..…….79

4. Conclusions and Future Work……………………………………..……………….81

4.1 Conclusions………………...…………………………………………………...81

4.2 Future Work………………………………………………………………….....82

Appendix A: Interdigitated Electrode Array Mask………………………..………...…..83

Appendix B: Interdigitated Electrode Array Fabrication…………………………….….84

Appendix C: Cleaning Method for Samples…………………………………………….87

Appendix D: Schematic for the Lid of the Temperature Controlled Baths……………..88

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Appendix E: Pinning Information and Truth Table for the Intersil DG406 16:1

Multiplexer………………….……………………………………………90

Appendix F: Pinning Information and Truth Table for the NXP

74HC40518:1Multiplexer……………….………………………..……....92

Appendix G: Circuit Diagram for the Data Acquisition System……………………..…93

Appendix H: Arduino Uno Software……………………………………………………94

Appendix I: Dimensional Schematic of the Pull Tester…………………………………98

References………………………………………………………………………....……102

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List of Tables

Table 1-1: Properties of Dow Corning Silastic MDX4-4210 Biomedical Grade Elastomer

[25, 29]……………………………….…………………………………...….19

Table 2-1: Groups of each set of devices with the type of surface treatment administered

………………………………………...……………….………………….….40

Table 2-2: Coefficients for the K-type thermocouple voltage-to-temperature

relationship………………………………………………………………..….52

Table 2-3: List of fixed resistor values utilized for testing the multiplexing

circuit……………………………………………………………………..….53

Table 3-1: List of samples containing the amount of layers and lifetimes for each sample

in the preliminary accelerated lifetime study……….……………………….60

Table 3-2: Number of devices that failed within each 10 day period of testing for the

primary accelerated lifetime study at 85oC…………………..………….…...62

Table 3-3: Lifetime in hours and days and thickness for the samples with no surface

treatment for the primary accelerated lifetime study at 85oC…………..…....63

Table 3-4: Lifetime in hours and days and thickness for the samples with oxygen plasma

surface activation treatment for the primary accelerated lifetime study at

85oC………………………………………………………………………….64

Table 3-5: Calculated reliability functions based on the lifetime data obtained in the

primary accelerated lifetime study at 85oC………….………………………65

Table 3-6: Pull off test results taken on samples before soak testing and after soak testing

at 85oC………………………….…………………………………….………79

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List of Figures

Figure 2-1: Routes for leakage current between two conduction electrodes 1: Air. 2:

Surface contamination. 3: Along the surface of the substrate. 4: Through the

substrate ………………………………………………………………….... 28

Figure 2-2: Schematic of the narrow 0.5 mm (right) and wide 1.0 mm (left) gap IDEs for

one soak testing sample……………………………………….…………….30

Figure 2-3: Process flow using lithography and etching for negative and positive photo-

resists………………………………………………………………………..31

Figure 2-4: Sircle Lam SL-3500-6R 14” Pouch Laminator……………………………...32

Figure 2-5: Kinsten KVB-30d Exposure Unit Vacuum Tray Table……………………..33

Figure 2-6: Branson 5510 Ultrasonic Cleaner…………………………………………...34

Figure 2-7: Schematic of how multiple layers can overlap defects on individual layers to

decrease the probability of failure…………………………………………..38

Figure 2-8: Schematic of the four imaginary columns used for reference while the coating

procedure is performed ……………..…………..…………………………..39

Figure 2-9: Schematic of the data acquisition system designed for the accelerated lifetime

study of Dow Corning MDX4-4210 Biomedical Grade Elastomer………...42

Figure 2-10: Wheatstone Bridge Circuit where R1, R3, R4 = 100 Mega-Ohms and Vs = 5

V. R2 is the unknown surface insulation resistance of the sample….….…..46

Figure 2-11: Software program developed in National Instruments Labview version 7.0 to

read and record the temperature of the 85oC and 40oC temperature baths,

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output voltage from the Wheatstone bridge, and control the 16:1 multiplexer

switching………………………………………………………………..…..48

Figure 2-12: Flow Chart depicting the order of operation for one iteration of the VI

developed in Labview 7.0 to control multiplexer switching and read the

output voltage from the Wheatstone bridge.…………………...……...……50

Figure 2-13: Calibration data acquired from the K-type thermocouples used to monitor

the room temperature, 40oC, and 85oC temperature controlled baths. An

Omega Digital Thermometer was used as a reference.………...………...…55

Figure 2-14: A photograph of the pull tester used to measure the adhesion of 85oC

accelerated lifetime study samples before and after soak testing .……..…...57

Figure 3-1: Enhanced photograph of an interdigitated electrode coated with Dow Corning

MDX4-4210 Biomedical Grade Elastomer…………………………….…...66

Figure 3-2: Photograph of a sample after soak testing depicting the inflated pockets

formed……………………………………………………………………...66

Figure 3-3: Photograph of a sample after soak testing depicting the deflated pockets…67

Figure 3-4: Photograph of the pockets formed by electrolysis…………………………..68

Figure 3-5: Average surface insulation resistance (ohms) vs. Time (days) plot for a

sample failure within a 10 day testing period for the 85oC accelerated lifetime

study………………………………………………………………………...70

Figure 3-6: Average surface insulation resistance (ohms) vs. Time (days) plot for a

sample failure within a 20 day testing period for the 85oC accelerated lifetime

study………………………………………………………………………...71

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Figure 3-7: Average surface insulation resistance (ohms) vs. Time (days) plot for a

sample failure within a 30 day testing period for the 85oC accelerated lifetime

study………………………………………………………………………...72

Figure 3-8: Average surface insulation resistance (ohms) vs. Time (days) plot for a

sample failure within a 40 day testing period for the 85oC accelerated lifetime

study………………………………………………………………………...73

Figure 3-9: Average surface insulation resistance (ohms) vs. Time (days) plot for a

sample failure within a 50 day testing period for the 85oC accelerated lifetime

study………………………………………………………………………...74

Figure 3-10: Average surface insulation resistance (ohms) vs. Time (days) plot for the

first days of testing where the strong decay in surface insulation resistance

occurs for the 85oC accelerated lifetime study……………….…………….75

Figure 3-11: Average surface insulation resistance (ohms) vs. Time (days) plot for the

40oC accelerated lifetime study…………………………………………….77

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Acknowledgements

I would like to thank:

…my advisor, Dr. Zorman, for his technical guidance, enthusiasm,

inspiration, and abundant advice

…to Dr. Ko, for his passion for research, ecstatic and inspiring attitude,

and massive amount of technical wisdom

…to Prof. Feng, for serving on my committee

…to Chris Roberts for his technical guidance, enthusiasm, encouragement

to keep going and good times

…to Jeremy Dunning for his expertise, and kindness

…to Andrew Barnes for his expertise, friendliness, and for being a great

laboratory mate

…to Rui Zhang for training me on the lab equipment and for being a great

laboratory mate

…to Lilliana for her continuous love, support, and patience throughout my

thesis

…and to my family and parents for their constant encouragement,

patience, and inspiration to achieve a masters and beyond

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ROLLER-CAST POLY-DIMETHYLSILOXANE AS A NON-HERMETIC

ENCAPSULANT FOR MEMS PACKAGING

Abstract

by

SHEM LACHHMAN

This thesis describes the development of a roller-casting process for multi-layered

poly-dimethylsiloxane films to be used as an encapsulant for implantable MEMS.

Accelerated lifetime studies were conducted at 40oC and 85oC in saline solution on

sixteen samples each containing eight interdigitated electrode arrays and encapsulated

with Dow Corning MDX4-4210 Biomedical Grade Elastomer. A custom environmental

testing system designed for long-term, unmonitored testing was built to acquire resistance

measurements. Select samples received surface treatments prior to PDMS deposition to

enhance adhesion and were evaluated by pull-off tests before and after soak testing. Test

results show lifetimes up to 40 days for samples tested at 85oC. Samples in 40oC saline

solution are still under test with no device failures after 68 days. The estimated maximum

lifetime of packages made using the roller casting method based on previously published

data is 5.5 years at physiological temperature (37ºC) and 22.2 years at 25ºC.

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1. INTRODUCTION

1.1 Implantable Microelectromechanical Systems

Microelectromechanical Systems (MEMS) technology leverages microfabrication

techniques to create functional microscale structures for sensing and control of their local

environment. The sensing and actuation functions inherent in MEMS necessitate direct

interaction with the local environment while the control electronics require complete

isolation, thus making the packaging of MEMS much more complex than that for

integrated circuits. MEMS technology is particularly attractive for medical implant

applications due to the additional functionality that potentially can be realized by such

systems. Current approaches to package implantable MEMS have several limitations in

terms of material selection, size, robustness, biocompatibility, and lifetime. At present,

packaging options for implantable MEMS are significantly limited. Packages based on

metallic or ceramic enclosures guarantee long device lifetime, but limit miniaturization

and mechanical flexibility. Polymeric packages offer mechanical flexibility, but are not

suited for long-term implantation. Increasing the lifetime and robustness of a polymer-

based packaging technology provides many potential benefits: an increased protection

against the environment, increased usable lifetime of the device, and increased implant

lifetime. Decreasing the size of the packaging can reduce negative side effects such as

irritation to the host of the implantable device. Utilizing polymer films, such as

polydimethysiloxane (PDMS), are of particular interest for packaging technology because

of their ease of use, low cost, and excellent material properties which provide protection

against harsh environments such as the human body.

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1.1.1 Applications

Implantable MEMS have begun to impact several application areas in the medical

field, including: cardiac pacemakers [1], blood pressure monitors for heart disease

patients [2], retinal prosthesis for the blind and the visually impaired [2-4], renal

replacement systems for renal disease victims [2], MEMS accelerometer microphones for

cochlear prosthesis [5], accelerometers for the detection of heart parameters during the

postoperative period following heart surgery [6], implantable drug delivery systems for

treatment in ambulatory emergency care [7], continuous monitoring of glucose levels in

diabetics [8], pressure sensors that serve as monitors for endovascularly-repaired

abdominal aortic aneurysms [9], optical sensors for continuous monitoring of arterial

blood oxygen saturation [10], and electrode arrays for neural prosthesis to control

artificial or paralyzed limbs [11]. Successful translation of these technologies to clinical

use requires a robust, biocompatible, noninvasive packaging technology.

1.1.2 Challenges of Packaging

For implantable MEMS devices, principal function of the package is to protect the

environmentally sensitive device components from the body environment while

simultaneously protecting the body environment from adverse affects caused by the

presence of the device [12]. To protect a MEMS device from the body environment, a

packaging design must provide isolation of environmentally-sensitive device structures

from the penetration and ingress of moisture and ions from bodily fluids by way of

package-penetrating electrical leads and feedthroughs. Moisture is a common source of

failure for implantable MEMS, causing current leakage and chemical corrosion, which if

left unchecked, could eventually lead to catastrophic failure. Structural integrity of the

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packaging must also be taken into consideration as outside mechanical forces such as

friction against body tissue may cause damage to system components. Size and weight of

the packaged device is also of great importance. Large scale packaged devices may

apply pressure to surrounding tissue which may affect blood circulation and also cause

irritation to the host [12].

Selection of biocompatible materials to prevent negative side effects by the

package to the body environment is imperative for devices designed for long-term

implantation [12]. The surface of the packaged device should be mechanically soft,

flexible, and absent of sharp edges and corners to reduce irritation to the body.

Secondary requirements of the package include low manufacturing cost and durability in

high temperature environments (e.g., such as during sterilization). Packages used with

implantable MEMS should allow for signal distribution (including wireless transmission)

and cannot limit device operation in any manner [13, 14], in particular the sensors and

actuators. All of these requirements limit the selection of materials and packaging design

available.

1.1.3 Hermetic vs. Non-hermetic Packaging Techniques

Unlike the integrated circuit technology for which packaging is highly

standardized, there is no general packaging method applicable for all implantable MEMS

devices. Each micro-scale implantable system requires a specific design to suit the needs

for the device operation. However, packaging techniques can be split into two main

categories: hermetic and non-hermetic encapsulation.

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Hermetic sealing involves the use of a metal or ceramic capsule that is bonded to

a substrate containing the microsystem or device by four main sealing techniques:

soldering, brazing, glass sealing, and welding [13, 14]. All of these techniques involve

high temperatures to form a reaction between the capsule and substrate interface in order

to achieve bonding. Non-hermetic encapsulation utilizes polymer thin film materials to

cover and seal the implanted device. Polymer films can be deposited by numerous

methods including spin coating, dip coating, or by specialized deposition equipment.

Hermetic packaging is known for offering a much higher level of protection against the

ingress of water vapor and moisture from device electronics than its non-hermetic

counterparts [12-14].

Moisture is the main cause of device failure for implantable MEMS [12]. As

stated previously, hermetic packaging utilizes ceramics, glass, or metals that are

impermeable to moisture. However, hermetic techniques often require expensive

equipment and can be difficult to implement in a manufacturing environment. Another

concern of hermetic packaging for implantable devices is the effect of the package on its

surrounding environment. Materials such as ceramics and metals, which are rigid and

have high Young’s moduli, are commonly used in hermetic packaging designs. Sharp

edges and hard surfaces such as those found in the hermetic packaging designs are not

desirable for implantable MEMs packaging as they can cause irritation to the host.

Non-hermetic packaging utilizing thin film polymers presents itself as an

excellent packaging technology for implantable devices. Thin film polymers are

attractive due to their ability to occupy smaller volumes, contain less weight, and softer

surfaces than ceramic and metal-based hermetic packaging. Polymers can be easily

15

deposited with minimal cost and provide excellent protection against ions and moisture.

Techniques utilizing multiple layers of the same material or alternating layers of different

materials can be used to increase the lifetime and the reliability of non-hermetic

packaging. Polymer films are generally compatible with wireless communications due to

their non-conducting characteristics. In fact, multiple polymer thin film packages have

previously been utilized [16-17], to encapsulate an implantable telemetry unit and an RF

coil integrated with an RF identification chip.

1.1.4 Packaging Materials

The selection of appropriate materials for the encapsulation of implantable micro-

systems is one of the most important aspects for the encapsulation design. It is

imperative that the packaging engineer knows the characteristics and behavior of the

materials in advanced to select the correct material, and adapt the application method to

the particular geometric characteristics of the device to be packaged. Since there is no

single polymeric material that can achieve all of the requirements for successful long-

term encapsulation of implantable devices (i.e., unlike ceramics and metals, polymers are

not completely impervious to moisture penetration) several materials and/or multiple

layers of the same material must be used to provide robust, impermeable, and

biocompatible encapsulation. Recent research efforts have focused mainly on the use of

thin films in the packaging design. Thin films offer a wide range of advantages: they

occupy a small volume (i.e., they have an inherently small form factor), they can be

deposited with a variety of techniques, and they can take any shape or form. Thin film

materials for packaging implantable micro-systems can be split into two main categories:

inorganic and organic [13].

16

Inorganic materials for packaging purposes include thin films of ceramics such as

silicon carbide, silicon nitride, polycrystalline diamond, and metal thin films such as

gold, aluminum or silver. Ceramic thin films offer excellent protection against water

permeation and allow RF signals to be transmitted throughout the package. Ceramics

also bring forth relatively small amounts of tissue response in the human body [18].

Unfortunately their mechanical properties are not favorable as they can fracture with

minimal deformation under stress. Another drawback is the high temperatures needed for

a good conformal, pinhole free deposition.

Metal thin films are also attractive due to the excellent barrier against moisture

they provide. An additional insulating layer is necessary when using metal films for

encapsulation to provide electrical isolation from the encapsulated devices [12, 13].

Careful selection of the insulating layer is necessary to achieve a successful encapsulation

with very little parasitic capacitance.

Organic packaging materials which are primarily used in non-hermetic

encapsulations are composed of thin film polymeric materials. Their low cost,

nonconductive nature, low-temperature, and simple deposition processes make them

extremely attractive as packaging materials. Polymers are deposited by special

deposition processes such as spin coating, dam and fill procedures, or dip coating. The

most popular polymers primarily used for encapsulation of implantable devices include

polyurethanes, epoxies, parylenes, and poly-dimethylsiloxane or PDMS [13].

17

1.1.5 PDMS

Poly-dimethylsiloxane, also known as PDMS or silicone, was originally

developed in the early 1940s by General Electric and Corning. Since then, the chemistry

of poly-dimethylsiloxane has been modified to meet various applications. PDMS possess

a silicon-to-carbon chemical backbone instead of a carbon-to-carbon backbone (which

most organic polymers have) which enables it to have high thermal resistance and

excellent electrical properties in high temperature and humid environments. PDMS is

particularly well suited for use in the human body [19].

Silicone is a preferred material for implantable micro-device encapsulation

because of its short and relatively simple processing procedures. Processing of silicones

require the introduction of a catalyst to a silicone resin base to support curing, which

occurs by one of two mechanisms: condensation curing and addition curing [19]. RTV or

room-temperature-vulcanizing silicones have been widely used for medical device

encapsulation [12, 13, 16, 20, 21]. Most RTV silicones have two part kits which require a

1:10 mixing ratio of catalyst to base. RTV silicones polymerize and cure at room

temperature within a 24 hour period. The cure time can be reduced by raising the curing

temperature [19].

While PDMS is known to have good bonding to most substrates, techniques have

been developed to increase the adhesion properties of PDMS to glass substrates. Oxygen

plasma treatment of PDMS has been shown to form good adhesion to several substrates

including glass and PDMS [22, 23]. PDMS was also shown to have an increased

adhesion to silicon, glass, and aluminum with the application of a chemical adhesion

promoter to the substrate before PDMS deposition [24].

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Other properties of silicones, such as alpha particle protection, high purity, and

high moisture resistance, make them an attractive material for implantable micro-

electromechanical systems [19]. PDMS has previously been utilized and evaluated for its

use as a packaging material by several researchers [16, 26, 27]. In the case of Refs. 16

and 26, test boards coated with Dow Corning MDX4-4210 Biomedical Grade Elastomer

using a roller cast deposition method were immerged into 40oC saline solution and

exhibited device lifetimes of 38 and 21 days, respectively. In Ref. 27, a dam and fill

deposition technique was used to encapsulate an implantable microstimulator with Nusil

1137 medical grade silicone rubber. The package was soak tested in room temperature

saline solution, exhibiting a lifetime of 30 days. Accelerated lifetime tests performed on

PDMS packages at 85oC with 85% relative humidity (also known as 85/85 tests and will

be discussed in the later sections of the thesis) exhibited a maximum lifetime of 1000

hours , which is equivalent to roughly 42 days [28].

1.2 Dow Corning MDX4-4210 Biomedical Grade Elastomer

1.2.1 Material Properties

Dow Corning MDX4-4210 Medical Grade Elastomer is a biocompatible, room-

temperature-vulcanized silicone designed for medical device encapsulating and mold-

making applications. Curing can be performed at room temperature for several hours or

at elevated temperatures for shorter cure times. A full cure at room temperature can be

achieved in three days [25]. The material properties for Dow Corning MDX4-4210

Biomedical Grade Elastomer are presented in Table 1-1.

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Table 1-1: Properties of Dow Corning Biomedical Grade Elastomer [25, 29].

Property Dow Corning MDX4‐4210

Tensile Strength 5.0 Mpa

Young's Modulus < 2Mpa

Elongation 470%

Dielectric Constant at 100Hz 3.01

Dielectric Constant at 100kHz 3.00

Surface Resistivity >7x1016 Ohms

Dow Corning MDX4-4210 possesses desirable characteristics which make it an

excellent material for implantable micro-system encapsulation. A low Young’s modulus

indicates a soft surface that will minimize negative side effects such as irritation to

surrounding tissue. A low dielectric constant and high surface resistivity contributes to

its attractiveness as an encapsulating material that will allow the transmission of RF

signals with minimal interference.

Although literature does not report a value for the water absorption and

permeability of Dow Corning MDX4-4210, it has been widely accepted that silicones

provide good protection against the intrusion of moisture [12, 14]. Dow Corning MDX4-

4210 has been previously evaluated as an encapsulation material for implantable micro-

systems by Leping Bu at Case Western Reserve University. Samples are seen to have a

38 day lifetime in saline solution at 40oC [16].

Biocompatibility tests show that MDX4-4210 meets or exceeds the cytotoxicity,

sensitization, irritation/intracutaneous reactivity, systemic toxicity and subchronic

toxicity requirements of ISO 10993-1 [25].

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1.2.2 Conventional Methods of Application

Dow Corning MDX4-4210 follows the conventional methods of deposition for

PDMS: spin coating, dip coating, pouring, and molding. Spin coating uses specialized

equipment to apply uniform PDMS coatings by the use of centrifugal forces onto flat

surfaces. Dip coating requires manually immersing the implant device or test structure

into uncured and degassed PDMS. Molding, which is primarily used in microfluidic

device fabrication, is a method that uses a mold or predefined cast to define the shape and

size of the package. The uncured PDMS is poured or spun onto the mold which, for

microfluidic applications, is defined by SU-8 photoresist [30]. Pouring requires the user

to dispense the PDMS while in the uncured state onto the device structure until the device

is completely encapsulated. After deposition curing is performed either in room

temperature for 24 hours or at elevated temperatures for a shorter cure time.

Spin coating, dip coating, pouring, and molding are promising and simple

deposition methods for packaging purposes but with minor flaws that cannot be

overlooked. Spin coating can cause the formation of bubbles in the coating during the

spinning process. Bubbles have been shown to cause failure in samples coated with Dow

Corning MDX4-4210. Longer spin times are also necessary to achieve thinner coatings.

Dip coated interdigitated electrode arrays using Dow Corning MDX4-4210 were

previously evaluated by researchers at CWRU [16]. They report the dip coating method

as producing uneven coating layers, and air bubbles. Producing a bubble free mold with

a uniform defect free finish can be difficult due to the long vacuuming time necessary to

allow entrapped air to be removed. Pouring the PDMS into the mold can cause several

folds and inclusions to form [12]

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1.2.3 Advantages of Dow Corning MDX4-4210 Biomedical Grade

Elastomer

Dow Corning MDX4-4210 Biomedical Grade Elastomer is an easily processed

material with desirable characteristics for device encapsulation if a suitable deposition

process can be developed. Previous studies of MDX4-4210 have shown it has excellent

moisture permeability and the potential to serve as a packaging material [16]. Its low

Young’s modulus and biocompatibility allow it to be implanted into the body with

minimal to no negative side effects to surrounding tissue. A low dielectric constant and

high surface resistivity contributes to its attractiveness as an encapsulating material that

will allow the transmission of RF signals with minimal interference. But as stated

previously, the majority of the traditional deposition processes for PDMS give rise to

defects that can cause catastrophic failure to implanted devices by the intrusion of

moisture through those failures. Therefore development of a processing method for

PDMS that yields defect-free coatings is essential if a PDMS-based encapsulation

technology for long term implantation of MEMS is to be realized.

1.3 Thesis Investigation

To overcome issues related to the processing of PDMS thin films for device

encapsulation, this work investigates a novel multi-layer roller casting technique for

PDMS films (Dow Corning MDX4-4210). Preliminary accelerated lifetime studies were

conducted in saline solution at 85oC to characterize the roller-cast PDMS films. Primary

accelerated lifetime studies at 40oC and 85oC in saline solution and the characterized

coating method were conducted on interdigitated electrode array test structures coated

with Dow Corning MDX4-4210. Oxygen plasma surface activation and Dow Corning

22

P5200 adhesion promoter were treated on two separate groups of samples to evaluate

adhesion enhancement strategies. Pull tests were performed on samples before soak

testing, and after device failure in soak testing.

2. Experimental Methods

2.1 Accelerated Lifetime Testing

2.1.1 Principles

Accelerated lifetime testing (ALT) is an experimental method used to decrease

the time for a failure to occur on a component, analyze the mechanism of failure, and

formulate a solution to prevent or decrease the probability of that failure occurring. ALT

is used to identify what failure mechanisms will occur at a certain time under accelerated

lifetime conditions, calculate the time that those failures will occur under normal

conditions, and compare the performance of materials or processes used. Components

are subjected to exaggerated mechanical, environmental and/or electrical conditions to

bring forth these failures. Several techniques have been developed for accelerated

testing, all of which can be grouped into three main categories: mechanical tests,

electrical tests, and environmental tests [14].

2.1.2 Accelerated Lifetime Testing Techniques

Mechanical tests use temperature cycle changes to aggravate mismatches in the

coefficients of thermal expansion (CTE) for components under mechanical load. There

are also several accelerated mechanical tests that test for vibration resistance, g-force, and

drop shock [14]. Electrical testing utilizes either a constant high voltage or pulsed

electrostatic discharge (ESD). The most commonly used standard of ESD testing is IEC-

61000-4-2 Human Body Model. This test simulates electrostatic discharge from a person

23

to a device [14]. The method was first developed to understand the precautions needed

for packaging and handling ICs. For the purpose of this study, mechanical tests are not

suitable since the packaged devices are to be deployed where mechanical load is to be

kept to a minimum. The main concern for this study is to develop and test packaging

materials that will provide excellent protection against ion carrying bodily fluids and

moisture. As such, environment testing is more appropriate for this thesis.

Environmental testing subjects samples to simultaneously high temperature and

high humidity to induce failures. Accelerated environmental testing first appeared in the

1960s when researchers placed device components into boiling water (100oC). Western

Electric later developed an 85oC and 85% relative humidity (RH) accelerated test [14].

The test became known as the 85/85 test. At that time, 1000 hours (6 weeks) was the

standard threshold for a component to pass the 85/85 test. The 85/85 lifetime test

provided a standard for decreasing contamination in the fabrication and packaging of

materials. By the early 1980s device components were routinely exceeding 1000 hours

due to the reduction of contamination levels in the fabrication and packaging of

components [14]. For further testing, the highly accelerated stress test (HAST) was

developed. HAST utilizes environmental testing chambers to increase the pressure,

temperature, and RH conditions relative to standard 85/85 test.

24

2.1.3 Accelerated Lifetime Transforms, and Reliability Functions

2.1.3.1 Accelerated Lifetime Transforms

Accelerated lifetime transforms were developed to transform the device lifetime

under accelerated conditions to an operational lifetime under normal operating

conditions. Normal conditions are considered to be an environment where the device is

under no intentional forces that would induce a failure. The accelerated lifetime

transform for 85/85 lifetime studies was first developed by Peck for epoxy packages [31].

Peck suggested the acceleration transform for environmental lifetime studies take the

form:

exp∆

Equation 2-1

where RH is the relative humidity (%), n is an empirically found constant, ∆Ea is an

empirically found activation energy for failure to occur (eV), k is the Boltzmann’s

constant (eV/K), and T is the absolute temperature in Kelvin [14]. The transform is

adopted from the standard Arrhenius life-stress relationship and has been widely used

when the stress variable is temperature. Swedish physical chemist Svandte Arrhenius

developed and proposed the Arhennius reaction rate equation in 1887 [32]. For the

epoxy samples, Peck found values for n and ∆Ea to be n = 2.66 and ∆Ea = 0.79 eV,

respectively by fitting data of 61 accelerated studies as a function of T and RH [31].

Another 26 studies were subsequently performed determining n to be 3.00 and ∆Ea = 0.9

eV for epoxy [14].

25

To calculate the time to failure of an accelerated test to normal conditions or vice

versa, the ratio of Equation 2-1 is taken for two relative humidity and temperature values.

The equation can be seen below:

∆ Equation 2-2

This transform is extremely useful for durable packages since lifetimes under

normal conditions may be extremely long (ie, 20 years). With this type of transform, the

experimenter can calculate when the component is due to fail under normal conditions

based on observed failures under accelerated conditions and knowledge of the activation

energy. The activation energy is a material dependent property and therefore will be

specific to PDMS.

To the best of our knowledge, no one has yet to explicitly report an activation

energy for PDMS. However, Zhang conducted a set of experiments from which an

activation energy can be estimated [26]. Zhang performed accelerated lifetime

experiments on roller coated PDMS at 40oC and 85oC in saline solution [26]. From his

testing results, he calculated a conversion factor by taking the ratio of the time to failure

for his PDMS coated samples at the two temperatures. Zhang simply used the left side of

Equation 2-2 to arrive at the conversion factor and did not calculate the corresponding

activation energy. For his samples, Zhang found the conversion factor to be 38.8.

Analyzing Zhang’s data using Equation 2-2 yields an activation energy of 0.79 eV for

PDMS.

26

The accuracy of this activation energy could easily be called to question since the

data used in making the calculation comes from a single test specimen. However, the

value is comparable to epoxy (also a polymer) and the experiment itself does not appear

to be flawed. In light of these issues and restricted by the lack of published data

concerning the activation energy of PDMS, it deemed appropriate to use the activation

energy calculated from Zhang’s data to determine a rough estimate of package lifetime at

various temperatures using the 85ºC accelerated lifetime data collected in this thesis as

the baseline.

2.1.3.2 Reliability Functions

Reliability functions are used to predict failure as a function of time, allowing an

experimenter to utilize a finite amount of data to predict the behavior of devices under

test. There are four main reliability functions: reliability function, cumulative failure

function, failure density function and the hazard rate [14].

The reliability function, R(t), represents the fraction of the original set of devices

that are still operating at time t. .

#

Equation 2-3

The cumulative failure function, F(t), is the percentage of original devices that

have failed by time t.

#

Equation 2-4

27

Both of these functions have a value between zero and unity and the sum of these

two functions is always equal to 1. With these functions a reliability engineer can

estimate the number of devices that will fail by a specific time t, and the probability of a

device being operational by a time t [14].

The failure density function is the rate of failure of the original number of devices

at a given time t. This is the time derivative of the cumulative failure function and takes

the form:

∆

∆ Equation 2-5

The failure rate will decrease for a population of devices under test because of the

increasing amount of devices that fail. Therefore, there are fewer devices to fail because

of the smaller population of devices under test. As a result of the continuous decline in

the population of viable devices, the failure density function will eventually reach zero

[14].

Since the failure density function represents a failure at a specific time based on

the original population of devices, it does not give much information about the rate of

failure for devices that are still under test. However, the hazard rate provides the failure

rate for devices that are still working. The hazard rate is defined as the rate of failure for

the number of devices still in operation and takes the form:

Equation 2-6

28

2.1.4 Surface Insulation Resistance

The electrical performance of packaging materials can be evaluated through

surface insulation resistance (SIR) measurements. Surface insulation resistance is the

resistivity measured across the surface of a substrate between adjacent conductors [33].

In a package prone to failure, leakage current is induced by the accumulation of ionic

contamination on the surface of the substrate. There are four possible routes for the

passage of leakage current between adjacent conductors as can be seen in Figure 2-1,

namely; air, contamination along the surface of the encapsulant, along the

encapsulant/substrate interface, and through the bulk of the substrate [33].

Figure 2-1: Routes for leakage current between two conduction electrodes 1: Air. 2:

Surface contamination. 3: Along the surface of the substrate. 4: Through the substrate.

Interdigitated electrodes are commonly utilized as testing structures to monitor

surface insulation resistance during accelerated lifetime testing of packaging since they

closely simulate the adjacent metal features in real-world devices and can be easily used

to measure leakage current [28, 33]. Polymer packaging materials are deposited over

Substrate

Conductor #1 Conductor #2

1

4

2

3

Surface Contamination

29

these electrodes and placed into 85/85 conditions [28], or soaked in saline solution [3].

The surface insulation resistance is monitored over time and failure mechanisms and

trends from the data are observed.

2.1.5 Sample Preparation

2.1.5.1 Interdigitated Electrode Design and Fabrication

There are two types of components that can go under test when conducting

accelerated testing: the actual component or a test structure specifically designed to fail

by a certain prescribed mechanism [14]. For accelerated tests conducted on packaging

materials, it is crucial that the failure is due to only the packaging material as it will be

difficult to pinpoint the mechanism of failure if it is due to an active device component.

Interdigitated electrodes (IDEs) on printed circuit boards are utilized for the

lifetime studies performed in this thesis. IDEs consist of only one metal layer and are

easy to fabricate using a simple one mask process. IDEs will exhibit only one failure

mode and can be easily tested prior to packaging by monitoring electrical continuity

before packaging material is deposited.

Single sided 1/16 inch thick printed circuit boards with 1 oz. of deposited copper

were used for the fabrication of the IDEs. Each 9 x 6 inch board is cut into six individual

3 x 3 inch test samples after lithographic patterning and etching. Each test sample

contains eight devices of different finger gap spacing but connected in parallel and all

sharing a common ground. The different finger gaps are used to evaluate the penetration

of contamination and water through the packaging material. The narrow gap is 0.5 mm,

and the wider gap is 1.0 mm. Figure 2-2 contains the schematic of an individual test

sample. The mask for the entire 9 x 6 board can be found in Appendix A.

30

Figure 2-2: Schematic of the narrow 0.5 mm (right) and wide 1.0 mm (left) gap IDEs

for one soak testing sample.

The fabrication procedure for the IDEs utilizes conventional microfabrication

techniques such as photolithography and etching. Photolithography is a method utilized

in microfabrication to transform a complex image into three-dimensional replicas by use

of light and chemical reactions. Photoresist, a light sensitive material, is first deposited

31

onto the surface of a substrate. A mask containing the pattern to be printed is then placed

over the surface of the substrate and exposed to light. The polar properties of the

exposed areas of the photoresist undergo a chemical change due to light exposure. A

chemical developing process is then used to harden or remove the exposed areas

depending on if the resist is negative or positive, respectively [3]. The resist is then used

as a mask to selectively etch underlying material. Etching is a process to selectively

remove the exposed areas of a film to create a desired pattern [3]. Wet etching with

chemical solution is utilized to fabricate the IDEs. A process flow using lithography and

etching with both negative and positive resists is shown in Figure 2-3.

Figure 2-3: Process flow using lithography and etching for negative and positive

photo-resists.

For this thesis, negative dry film photoresist from MG Chemicals was applied to

the copper surface of the PC boards. The resist was applied with a Sircle Lam SL-3500-

32

6R 14” Pouch Laminator. The temperature and speed of the laminator was set to 110oC

and speed setting No. 6 respectively. The boards were placed under the rollers of the

laminating tool and passed through twice to increase the adhesion of the resist to the

copper surface and to remove bubbles that can affect the trace lines of the IDEs. A

photograph of the laminating tool is shown in Figure 2-4.

Figure 2-4: Sircle Lam SL-3500-6R 14” Pouch Laminator

Prior to exposure, the mask was aligned to the PC board and placed under vacuum

to ensure contact between the mask and the resist. An exposure time of 60 seconds with

a Kinsten KVB-30d Exposure Unit was used. The Kinsten KVB-30d can be used for

single sided or double sided printed circuit boards. The wavelength of light provided by

the exposure tool is 350 to 400 µm. A photograph of the exposure unit is shown in Figure

2-5.

33

Figure 2-5: Kinsten KVB-30d Exposure Unit Vacuum Tray Table.

To complete the lithographic patterning procedure, the boards were developed in

a 1:10 potassium carbonate and water solution. A soft bristle paint brush was gently

swiped over the surface of the exposed area to help accelerate the development process.

The boards are rinsed in deionized (DI) water to allow excess resist particles to be

removed. Finally, the boards were etched in a sodium persulfate solution in a plastic

etchant tank from MG Chemicals. Bubbles provided from an air pump through drilled

rubber tubing are supplied to the etch tank to accelerate the etching process. To ensure

that the devices had no broken trace lines, a continuity test was conducted for each line

using a Keithley Model 2000 digital multimeter. A detailed process for the fabrication of

the IDEs is presented in Appendix B.

2.1.5.2 Cleaning Method

One of the most critical processing steps in device packaging is the pre-

encapsulation cleaning process [36]. Leakage current and device failure can be induced

by the accumulation of contaminants directly on the surface of a substrate [33].

34

Contamination can be in the form of mobile ions, salts, dust, or particulates. As part of

this thesis, a cleaning method utilizing isopropanol, acetone, and DI water was developed

to remove surface contamination on the IDEs. To reduce the probability of failure from

particulates, all cleaning steps were performed in the Microfabrication Laboratory, a class

100 clean room facility at Case Western Reserve University. Details regarding this

procedure can be found in Appendix C.

Most of the steps in the cleaning method are solution based and performed in a

heated Branson 5510 Ultrasonic Cleaner. Ultrasonic cleaning is a method in which high

frequency vibrations create cavitation bubbles to aid in the removal of contaminates from

a surface. The process can thoroughly remove embedded particles from nearly every

surface, recess and opening [37]. A photograph of the Branson Ultrasonic Cleaner can

be seen in Figure 2-6.

Figure 2-6: Branson 5510 Ultrasonic Cleaner.

35

Acetone is a commonly used solvent for stripping photoresist. Isopropanol or

IPA, which is widely known for its effectiveness in cleaning processes, has been

employed by several research groups for the removal of surface contamination [32, 36-

39]. For this thesis, the IDEs were sequentially cycled through separate dishes containing

IPA, acetone, and DI water. Each dish was placed in the ultrasonic cleaner. All boards

were cleaned with these chemicals in the ultrasonic cleaner for 3 minutes. The

combination of the clean room environment, and chemical cleaning in acetone, IPA, and

high purity DI-Water under ultrasonic agitation proved to be adequate for cleaning the

surface of the PC boards.

2.1.5.3 Deposition of PDMS by Roller Casting

The roller casting method for PDMS packages was previously investigated by two

student-led efforts at CWRU. Leping Bu et. al., were the first to explore the use of roller

casting for PDMS [16]. This roller casting method differs from conventional methods

such as dip coating and spin coating in that a force is exerted by the roller that presses the

PDMS film directly onto the surface of the substrate during the deposition process. The

force eliminates the presence of bubbles in the deposited material and also allows a

conformal deposition process over patterned surfaces (e.g., IDEs). For this effort, the

PDMS was MDX4-4210 and the substrates were conventional PC boards with electronic

components. Before the samples are coated with MDX4-4210, epoxy was spread over the

solder joints to provide structural integrity for the wires and also reduce the probability of

failure at the solder joints themselves. Devices coated using this method exhibited a

lifetime of 38 days at 40oC in saline solution. Rui Zhang continued the work of Bu et al.,

and reported a 21 day lifetime in 40oC saline solution [26]. Zhang also conducted

36

experiments at 85oC in saline with two testing samples, achieving a 13 hour lifetime for

one sample while the other sample failed immediately upon testing. The thicknesses of

these samples were in the range of 100 to 400 microns which is substantially thicker than

the 40 to 70 micron-thick samples produced for this thesis. Thickness measurements for

the roller cast coatings produced for this thesis can be found in Table 3-6.

This thesis builds upon the work by Zhang and Bu by implementing several

changes to the roller casting process that are designed to improve the lifetime of PDMS

coatings for encapsulating films that are substantially thinner than those previously used.

The first proposed improvement is in the area of substrate cleaning. Previous work

utilized a simple cleaning process which involved soaking the PC boards in alcohol and

baking them at 80oC for 1 hour in vacuum. Unfortunately, there is no mention of how

long the boards were soaked in the alcohol. Also, the use of specialized cleaning

equipment such as an ultrasonic cleaner to help loosen particulates from the surface of

their samples is not mentioned. A principal hypothesis of this thesis is that a rigorous,

multi-step substrate cleaning procedure that includes ultrasonic agitation will yield

PDMS coatings with substantially longer lifetimes in saline solution than what has

previously been observed.

The second proposed improvement to the process involves the reduction of

particulate contamination during the roller coating procedure. All processing steps used

by Bu and Zhang were performed in a regular open air laboratory which contains

numerous particulate sources that may have contributed to eventual coating failure. To

address this issue, the packaging procedures used in this thesis were performed in a Class

100 cleanroom to reduce the probability of failure by airborne particulates.

37

The third proposed improvement involves the method used to roller cast the

PDMS films. Perhaps because their work was at the proof-of-concept stage, little

quantitative information was reported regarding the roller casting process. Parameters

such as the motion used to roll the PDMS, the rate at which the PDMS is applied to the

substrate, the pressure between the roller and substrate during roller casting, and the

period over which a single roller cast layer was applied were not quantized.

2.1.5.3.1 Multi-Layer Thin Film Technique

To improve upon the roller-cast method of Ref. 16, we proposed to create the

PDMS encapsulant not as a single layer, but rather as a multilayered stack of thin PDMS

films. For other material systems, the multilayer technique decreases the probability of a

defect causing failure by utilizing multiple thin film layers to overlap defects on

previously deposited layers [40]. If a defect such as an air bubble is present on the first

layer, a second layer will overlap and terminate that defect, thus decreasing the

probability of the bubble causing a failure. A third layer further decreases the probability

of a failure resulting from a large defect in the first layer and terminates most defects in

the second layer. Since defects originating in the second layer do not penetrate the first

layer, the overall probability of defect-related failure decreases substantially. Figure 2-7

depicts the multilayer theory for decreasing the probability of failure caused by defects.

38

Figure 2-7: Schematic of how multiple layers can overlap defects on individual layers

to decrease the probability of failure.

For the PDMS films used in this thesis, the roller casting method was performed

in the Microfabrication Laboratory at CWRU. To prepare the space in which the films

were prepared, all surfaces that the test boards came in contact with, as well as the

surrounding areas, were wiped with IPA before sample preparation. The standard

method of processing MDX4-4210 as provided by Dow Corning was religiously

followed. The curing agent and elastomer were mixed into a 1:10 mixture, respectively

and allowed to degas for 30 minutes under vacuum in a desiccator. Degassing was used

to remove air bubbles from the catalyst and elastomer mix. Following the degassing

procedure, the mixture was left in air for 10 minutes.

A systematic approach was developed to apply the PDMS films uniformly to the

IDE substrates. The sample area is much larger than the roller length, therefore the

samples were split into four imaginary column-like regions for PDMS application as

shown in Figure 2-8.

39

Figure 2-8: Schematic of the four imaginary columns used for reference while the

coating procedure is performed.

Select samples were designated to receive a surface treatment prior to PDMS

deposition to evaluate the efficacy of such treatments on the adhesion of roller cast

PDMS. For such samples, the surface treatment is applied just after cleaning the substrate

and just prior to PDMS deposition. Table 2-1 describes these samples.

40

Table 2-1: Groups of each set of devices with the type of surface treatment

administered.

Group Number of

IDEs Surface Treatment Temperature

A 32 None 40oC

B 32 None 85oC

C 32 Dow Corning P5200 Adhesion

Promoter 85oC

D 32 Oxygen/Plasma Surface Treatment 85oC

Dow Corning P5200 Adhesion Promoter is a versatile primer composed of silane

coupling agents. The P5200 adhesion promoter requires moisture in the air for a

complete cure at 1 to 2 hours [41]. Oxygen plasma cleaning is often used in MEMS

fabrication to enhance and enable adhesion of incompatible materials [30]. The standard

recipe developed by our group for PDMS bonding to glass substrates utilizes a rf power

of 25 W, a time of 25 s at a pressure of 900 millitorr in O2.

Following the application of a surface treatment (if performed), the samples were

ready for PDMS deposition by roller casting. The roller was first coated with PDMS by

dipping it into the newly prepared PDMS and thinning the coating by moving the roller

across a Teflon substrate. After several strokes, the roller was placed on the sample at the

center of Column #1. Using gradually increasing strokes, the PDMS was spread over

Column 1 in a longitudinal motion (up and down in Fig. 2.8). This process was repeated

for each column until all of the columns (2, 3, and 4) have received a coating of PDMS.

Beginning with Column #1, the roller was placed at the bottom edge of the board. The

roller was moved from the bottom edge to the top edge of the sample and back to the

bottom edge, completing one stroke. Each column received 100 strokes at a rate of 1.5

41

strokes per second. A force equivalent to 250 to 300 grams was applied by the roller

during the coating procedure. To measure and monitor the applied force; samples were

placed on a Kilotech KLB 400-R Digital Balance and weight measurements were

recorded upon the application of the coating. Once all columns received 100 strokes the

samples were inspected visually for large defects. To create a conformal coating using

the roller, the samples received an upward motion from the bottom edge to the top edge

until no humps or other visual defects were seen. The coatings were cured at 70oC under

vacuum for 1 hour. After curing, the aforementioned roller casting process was repeated

a second and third time with a curing step performed between each process. The third

coating received a cure of 16 hours.

2.1.6 Design of Experiments

Accelerated lifetime studies of the roller-cast films were conducted at 40oC and 85oC.

Samples were separated into three groups, each containing 32 IDEs with two different

surface treatments. Leakage current for each coated device was monitored over time.

Packaging materials were considered to have failed when the resistance drops below 100

MΩ. Accelerated lifetime studies are typically long term experiments which are best

performed using a robust, automated testing system designed to control the testing

environment and acquire measurement data from large sample sets. Significant effort was

put forth on this project to design and construct such a system.

2.1.6.1 System Design

In order to maximize the information received, a data acquisition system was

designed and fabricated to monitor the leakage current of 144 samples over a time period

ranging from weeks to months. The purpose of designing this system was not only to

42

monitor the lifetime of the coated samples but to record, plot, and analyze the surface

insulation resistance data. A schematic of the system is provided in Figure 2-9.

Figure 2-9: Schematic of the data acquisition system designed for the accelerated

lifetime study of Dow Corning MDX4-4210 Biomedical Grade Elastomer.

The system incorporates two Precision 182 temperature controlled water baths

filled with DI water. The Precision 182 can supply a maximum temperature of 99.9oC

which is adequate for the needs of this experiment. The dimensions of the chambers are

11.5 x 6 x 6 inches, providing enough volume to accommodate all packaged devices

under test. The temperature of each bath is set to a constant temperature of 40oC or 85oC.

K-type thermocouples are used to monitor the temperature of both the baths as well as the

43

laboratory room. The voltage output from each of the thermocouples recorded by a

National Instruments PXI 6030E Data Acquisition board which interfaces with a custom-

designed Labview program which is described in detail in a later section of this thesis.

Steel pans filled with saline solution are placed within the DI water baths. A lid

was designed and fabricated out of 1/8” acrylic to hold the devices vertically in the baths

and to prevent the evaporation of the saline solution. A schematic of the lid is provided

in Appendix D. To further reduce the rate of evaporation, two layers of plastic seal wrap

were placed over both lids and sealed with adhesive tape. A Hudson Valve 3” diameter

float valve was installed in the 85oC bath to maintain a constant water level over the

course of the accelerated lifetime experiment. The valve is supplied with DI water from a

5 gallon tank. Teflon tape is wrapped around all threads of the connecting tubes to the

valve to prevent DI water from leaking.

A National Instruments PXI-8186 Controller Module with a Windows XP

operating system running Labview 7.0 Express with a NI PXI-6527 Digital I/O and NI

PXI-6030E Multifunction DAQ are used for system automation and data acquisition.

Multiplexer switching, temperature monitoring, and voltage measurement and recording

are all controlled with Labview 7.0. A Back-UPS 350 backup power supply is installed

to prevent a failure to the system due to power outages. The monitor and National

Instruments PXI-8186’s power cords are plugged into the rear outlets of the backup

power supply. The supply is rated for up to 300 Joules of surge protection, and sustains a

200 W load for 9 minutes. The test circuit is placed inside a faraday cage to reduce noise

on all signals, and also for protection of the circuit from outside mechanical forces.

44

2.1.6.2 Circuit Design

Nine Intersil DG406 16-to-1 (16:1) multiplexers (MUXs) are utilized to monitor

the surface insulation resistance of 128 PDMS-coated IDEs and 16 uncoated IDEs. The

low cost 16:1 DG406 has sixteen analog switches (S1-S16), a TTL and CMOS

compatible digital decode circuit for channel selection, a voltage reference for logic

thresholds, an ENABLE input for device selection when several multiplexers are used in

a system, low power consumption (<1.2mW), a fast switching time (300 ns), and low

charge injection which make them a suitable circuit component for the purposes of this

experiment. The 8:1 74HC4051 features an 8 channel analog multiplexer/demultiplexer

(S1-S8), three digital select inputs (A0-A2), an active-LOW enable input, and low ON

resistance. A schematic and truth table for both the DG406 and 74HC4051 are provided

in Appendix E and F respectively.

A parallel circuit configuration is used to be able to provide an operating voltage

of 5 V to all nine 16:1 MUXs and to give the user the option of selecting which MUX is

ON or OFF. The ENABLE lines of all nine 16:1 MUXs and the select lines of the 8:1

MUX are controlled by an Arduino UNO. When the ENABLE line is sent a HIGH signal

(4 –5 V), it is in the ON state which will allow the selection of the input channels.

Alternatively, when it is sent a LOW signal (0 to 0.3 V) it is in the OFF state and all

switches are off and inactive. A HIGH signal corresponds to a ‘1’ and a LOW signal to a

‘0’ in binary logic. The multiplexers use binary logic and a decoder to select which

switch is ON or OFF.

The Arduino UNO is a micro-controller board based on the ATMEGA 328

programmable microcontroller and uses a programming language that is very similar to

45

the popular and widely used line by line code ‘C’. It is used to control the switching of

the 8:1 MUX input channels and select which 16:1 MUX is in the ON state. Address

lines for the 8:1 and ENABLE lines for the 16:1’s are connected in parallel to the I/O

pins of the Arduino UNO. Address line of the 16:1 MUX are connected to the SCB-100

break out board for the NI PXI-6527 Digital I/O.

IDEs are connected in parallel with one terminal connected to ground and the

other to the input channel of the 16:1 MUXs. Eight of the 16:1 MUXs’ outputs are

connected in parallel to the input channels of a 74HC4051 8-to-1 (8:1) MUX. The output

of the 8:1 MUX and the output of the ninth 16:1 MUX are connected to a Wheatstone

bridge. A Wheatstone bridge circuit is used for several reasons, namely it uses a constant

voltage supply, it is relatively insensitive to temperature variation, and a simple

calculation can be used to find the unknown surface insulation resistance of the test

samples. The Wheatstone bridge circuit is shown in Figure 2-10.

46

Figure 2-10: Wheatstone Bridge Circuit where R1, R3, R4 = 100 Mega-Ohms and Vs = 5

V. R2 is the unknown surface insulation resistance of the samples.

.

For the Wheatstone bridge circuit used in this thesis, R1, R2, R3 are one percent

100 MΩ resistors, Vs is a 5 V supply, Vout is the resulting output voltage, and R4 is the

unknown surface insulation resistance of the test samples. When the bridge is balanced

the resulting output is zero indicating a device failure. The output voltage of the

Wheatstone bridge circuit is given by:

Equation 2-3

The output of the bridge is buffered by a National Semiconductor LMC6482 dual

rail-to-rail input and output operational amplifier. The OP AMP is used to prevent

47

loading down of the circuit on the National Instruments SCB-68 breakout board for the

NI PXI-6030E Multifunction DAQ by the Wheatstone bridge since the bridge contains a

high input impedance. The voltage is read with the PXI-6030E.

K-type thermocouples are used to measure the temperature of the water baths and

the temperature of the laboratory. Three AD595 monolithic thermocouple amplifiers

with cold junction compensation are used to calibrate the thermocouples. The AD595

has a temperature uncertainty of +/- 2.2oC, cold junction compensation, amplification,

and rejects common-mode noise voltage. The voltage output by all three thermocouples

is read with the NI PXI-6030E. A simple calculation is performed by the Labview

program used to automate this system to provide the resulting temperature from the

voltage output by the thermocouples. A circuit diagram/schematic is provided in

Appendix G.

2.1.6.3 Software Design

Labview (Laboratory Virtual Instrumentation Engineering Workbench) is a visual

programming language developed by National Instruments that is used for automating

measuring equipment in a laboratory or industrial setting. Labview programs are referred

to as VI’s. Each VI contains several functional blocks called subVIs that automatically

build code to perform a certain action. A schematic of the VI developed in Labview

version 7.0 can be seen in Figure 2-11.

48

Figure 2-11: Software program developed in National Instruments Labview version

7.0 to read and record the temperature of the 85oC and 40oC temperature baths, output

voltage from the wheatstone bridge, and control the 16:1 multiplexer switching.

The VI was developed to control the switching of the multiplexers, monitor the

temperature of both temperature baths, and read the voltage output for the Wheatstone

bridge. Communication with the Arduino is the first step. A separate program is first

uploaded to the Arduino (See Appendix H for details). Labview communicates with the

Arduino UNO through serial port COM 3 with VISA: a common I/O language for

instrumentation control and communication. VISA first opens and configures the serial

port for communication with the “VISA CONFIGURE SERIAL” subVI. The baud rate,

data bits, parity, stop bits, and flow control are 9600, 8, 0:none, 1.0, 0:none respectively.

49

These parameters are configured to match the configuration of the serial port in Windows

XP. A mismatch in configuration will lead to an error in the VI.

Nested for loops are used for automatic switching of the MUXs. A for loop is a

programming structure that allows code to be repeated for a specific amount of

repetitions which is set by the user. The outer for loop in the VI is used to send a number

sequentially ranging from 0 to 8 to the Arduino through the serial port with the VISA

Write subVI while the inner for loop switches through all 16 input channels on the 16:1

MUX, records the voltage, and saves it to a .txt file. The inner for loop iterates 16 times

for each iteration of the outer for loop. Sixteen is chosen since there are 16 input

channels on the 16:1 MUXs. Nine iterations were used for the outer for loop since there

are nine 16:1 multiplexers. A flow chart for one iteration of the program is presented in

Figure 2-12.

50

Figure 2-12: Flow Chart depicting the order of operation for one iteration of the VI

developed in Labview 7.0 to control multiplexer switching and read the output voltage

from the Wheatstone bridge.

51

When the number ‘0’ is sent to the Arduino UNO, the first input channel on the

8:1 MUX is selected, and the first 16:1 MUX’s ENABLE line is sent a HIGH signal to

turn it to the ON state. The number is sent in 8 bit data in a string through the serial port.

Once the corresponding 8:1 input channel and 16:1 MUX are selected, the inner for loop

initiates. The order of operation of the inner for loop is as follows: select the input

channel on the 16:1 MUX, initiate a predetermined time delay, read the voltage across

Wheatstone bridge and thermocouples, save the voltage readings to a .txt file. Both for

loops are placed within a while loop with a 1 hour delay. The for loops run through all

iterations before the delay is initiated. The outer while loop runs continuously unless

stopped manually by the user which also stops the entire program.

K-type thermocouple conversion equations obtained from the National Institute of

Standards and Technology (NIST) and transfer functions provided by the AD595 product

data sheet were used to convert the output voltage of all three thermocouples to

temperature in Celsius. Transfer functions were necessary since thermocouple output

voltage is nonlinear with respect to temperature and the AD595 amplifies the signal

linearly. The transfer function takes the form:

595 16 193.4 Equation 2-4

The voltage to temperature relationship obtained from NIST takes the form:

Equation 2-5

52

where T is the temperature in oC, V is the output voltage from the thermocouples, and C0

to C9 are constants. The coefficients are listed in Table 2-2:

Table 2-2: Coefficients for the K-type thermocouple voltage to temperature

relationship

Coefficient Value

C0 0

C1 2.508355 x 101

C2 7.860106 x 10-2

C3 -2.503131 x 10-1

C4 8.315270 x 10-2

C5 -1.228034 x 10-2

C6 9.804036 x 10-4

C7 -4.413030 x 10-5

C8 1.057734 x 10-6

C9 -1.052775 x 10-8

The equations are combined and entered into the Formula subVI. The input to the

Formula subVI is taken from the measured output voltage from the AD595. The DAQ

Assistant subVI was used to automatically configure the physical channels of the SCB-68

breakout board for the PXI-6030E DAQ to read in an analog signal from the Wheatstone

bridge and the thermocouple amplifiers. DAQ Assistant helps the user save on time and

complex programming to complete a task by automatically generating the programming

needed.

2.1.6.4 Calibration

To ensure the robustness and accuracy of the system several calibration

experiments were performed on the circuits of the system. The logic on the multiplexers

53

was verified by probing the address line pins with a Keithley 2000 DMM. The switching

on the multiplexers were manually changed in Labview instead of automatically by the

for loops. This allows time for the user to probe the address line pins accurately. The

ENABLE lines for each 16:1 multiplexer are also probed to ensure that the selected

multiplexer in the software corresponds to the actual multiplexer in the hardware. A set

of 16 fixed resistors were used to verify the accuracy and switch of every switch for all

nine 16:1 multiplexers. Every other fixed resistor was 100 MΩ in value. The resistance

for the fixed resistors was verified with a Keithley 2000 DMM. Table 2-3 contains the

list of fixed resistors used.

Table 2-3: List of fixed resistor values utilized for testing the multiplexing circuit.

Resistor Number Value (MΩ) 1 18.18 2 98.6 3 5.62 4 102.2 5 2.37 6 99.4 7 7.2 8 99.4 9 1.7 10 99.3 11 5.642 12 93.5 13 18.4 14 98.6 15 2.705 16 103.5

A voltage divider configuration instead of a Wheatstone bridge was first used

because of its simplicity, ease of design, and simple calculation. The fixed resistors are

54

connected to the input channels of one 16:1 MUX. The user first initiates the Labview

program, selects which MUX is ON, selects the input channel of the MUX that is ON,

verifies that the correct channel is selected by probing the address lines with the Keithley

DMM, and verifies that the voltage reading corresponds to the appropriate resistor on that

particular channel. The process is repeated until all 16:1 multiplexers were tested.

A Fischer Scientific Digital Thermometer with a K-type thermocouple is used as a

reference for calibrating the thermocouples installed on the system. A Pyrex dish is filled

with water and allowed to reach room temperature. The dish is heated with a Thermo

Scientific Hot Plate containing a feedback system to ensure a stable temperature. All

three K-type thermocouples are placed into the dish along with the reference. The

temperature is raised by 10oC and the temperature readings are recorded from the

reference and the three thermocouples until 100oC is reached. Figure 2-13 contains the

graph of the temperatures for all three thermocouples and the reference.

55

Figure 2-13: Calibration data acquired from the K-type thermocouples used to

monitor the room temperature, 40oC, and 85oC temperature controlled baths. An

Omega Digital Thermometer was used as a reference.

When the baths are sealed, the temperature increases due to an increase in

pressure. For this reason the reference thermocouple is also used to set the temperature

controlled water baths to a constant 40 and 85oC.

Reference

Forty 0

20

40

60

80

100

120

1 2 3 4 5 6 7 8 9

Reference

Room

Forty

Eighty‐Five

56

2.2 Bond Strength Testing

Adhesion tests are performed on samples to observe a correlation between the

adhesion of PDMS coated samples and their lifetime. Adhesion tests were performed

before and after soak testing to observe changes in adhesion after devices have failed in

the lifetime study.

2.2.1 Method

There are several common methods employed to measure the adhesion of PDMS

to a surface: blister tests [30], t-peel tests [42], die shear tests [43], and pull off tests [44].

Most bonding test techniques require some alteration of the substrate. In this thesis, the

pull-off test was selected as the means to determine adhesion due to the low cost of

constructing the apparatus, and simple preparation of samples to perform the pull off test.

Pull off tests were performed on samples before and after accelerated lifetime soak

testing in saline A dolly of certain surface area was glued onto the surface of the PDMS

sample with a strong adhesive and left to cure. Once the adhesive had fully cured, the

edges of the dolly were cut to ensure that the adhesion during pull off is directly affected

by vertical forces directly under the dolly-PDMS substrate. The sample was clamped

down and the dolly was lifted off with a pull off test stand. The dolly was lifted off until

the PDMS separated from the substrate.

57

2.2.2 Pull Testing Apparatus

A photograph of the pull test apparatus can be seen in Figure 2-14.

Figure 2-14: A photograph of the pull tester used to measure the adhesion of 85oC

accelerated lifetime study samples before and after soak testing.

The apparatus was designed and fabricated out of stacked ¼” clear acrylic sheets.

Acrylic sheets were used due to their availability and low cost. Four unthreaded ½” steel

poles with ½”-13 threaded ends were bolted to the base to ensure stability. Steel hex

bolts were used to fasten the poles to the base. A series of 19/32” diameter holes are cut

into the horizontal manifold. The unthreaded support rods are passed through the holes

cut into the center manifold and greased with WD-40 to reduce friction. Sleeve bearings

58

are fitted on the top and bottom of the holes to ensure the center manifold does not

wobble in any direction when moved up and down. Hex-nuts are inserted and glued to a

middle hole with the center threaded rod screwed threw both hex nuts. The center pole is

comprised of a ½” steel threaded rod. The middle rod is bolted to a 1-1/8” ball bearing

that was pressed into the base of the apparatus to allow the rod to spin and the center

manifold to move up and down. A glass filled nylon crank handle is assembled on the

other end of the middle threaded rod to allow the user to spin the middle rod. The crank

handle features a revolving handle to ensure a smooth motion is applied when spinning

the middle rod.

A faceplate used to attach an Imada 110 push-pull force gauge is fastened to the

center manifold with 90o, 1” long steel brackets. The brackets are held into place with a

¼” hex bolt and nut. Samples are clamped down tightly to the base of the pull test

apparatus with a faceplate. The faceplate contains ½” holes to allow the studs to pass

through and connect to the force gauge with a threaded insert standoff. The standoff was

used to ensure alignment when pulling off the studs from the test samples. The

dimensions for the apparatus for each part are found in Appendix I.

2.2.3 Sample Preparation

Samples for the pull off test are prepared using the method described previously.

Two samples for each surface treatment were fabricated for adhesion tests before soak

testing. Four steel flush headed studs with ¼” threaded ends are mounted onto the

surface of PDMS coated samples with Dow Corning RTV silicone sealant/adhesive. The

adhesive was left to cure for 3.5 days before pull off tests were administered. The same

mounting procedure was used to prepare for pull testing samples failed during accelerated

59

lifetime testing. After the dollies were mounted, the samples were soaked again in saline

at 85oC. The edges around the studs were cut to ensure the force measured is solely in

the vertical plane.

2.2.4 Experimental Design

Preliminary tests were conducted with several adhesives to ensure a clean pull off

was obtained. A clean pull off would occur when only the PDMS separated from the

PCB substrate with no delamination between the PDMS and adhesive or the adhesive and

stud. Super glue, Dow Corning MDX4-4210 Medical Grade Elastomer, Epoxy, and Dow

Corning RTV 732 silicone adhesive were evaluated as adhesives for mounting the

samples. All adhesives with the studs for the preliminary study are cured for 1, 2, and 3

days. The test samples were clamped down to the base of the pull test apparatus to ensure

that no movement occurs when the studs are pulled off. The studs are screwed onto the

standoff first before clamping the samples down. This ensured correct alignment and a

lift off only in the vertical direction. In order to accurately observe the force output by the

Imada 110, pull off tests were recorded with a video camera and observed repeatedly. A

total of four samples before accelerated lifetime tests and four samples after device

failure during the accelerated tests were subjected to pull off testing.

60

3. Data Analysis

3.1 Accelerated Lifetime Study

3.1.1 Preliminary Multilayer Accelerated Lifetime Studies

Preliminary tests performed on five, single layer coated samples were performed

using the same coating method as described in Ref. 16. Double and triple layer coated

IDEs were evaluated for Samples 6 and 7, and 8 through10 respectively. Samples 6 and

7 were prepared using the same PDMS application method as Samples 1 through 5. Table

3-1 shows a list of samples with their respective number of layers and lifetime for each

sample.

Table 3-1: List of samples containing the amount of layers and lifetimes for each

sample in the preliminary accelerated lifetime study.

Sample Number

Number of Layers

Lifetime (Hours)

1 1 0

2 1 0

3 1 0

4 1 0

5 1 0

6 2 0

7 2 0

8 3 0

9 3 480

10 3 1248

The first seven samples utilizing the previous coating method with a single layer

for Samples 1 through 5 and double layers for Samples 6 and 7 failed immediately. An

immediate failure is an event in which the surface insulation resistance of the sample

61

dropped to or below 100 MΩ within 5 hours of the commencement of the accelerated

study. Sources of failure observed under a microscope included dust, traces of human

hair, air bubbles, and blemishes to the coating in the wet state before curing. It was

concluded that careful handling of the samples, and a more extensive cleaning method

was necessary to achieve a longer lifetime. The cleaning method is one of the

determining factors for a good lifetime of an encapsulation [36].

Samples 8 through 10 utilized the roller casting method described in previously,

in the Experimental Methods section, which incorporates more strokes and a more

extensive cleaning method. The immediate failure of Sample 8 was attributed to

contamination at the solder humps caused by soldering flux.

Sample 9 was soak-tested in saline solution for 20 days with no failure. The

experiment was stopped before the sample exhibited failure in order to inspect the surface

of the substrate under a microscope and compare the surface with the previous samples.

The surface for Sample 9 contained no traces of human hair, particulates, air bubbles, or

physical damage to the surface. Sample 10 did not exhibit failure until day 52 of testing.

The positive results achieved with Samples 9 and 10 motivated the design of a more

extensive accelerated lifetime study of Dow Corning MDX4-4210 using the newly

developed roller casting method.

3.1.2 Primary Accelerated Lifetime Study

Primary accelerated lifetime testing was conducted as described previously in the

Experimental Methods section of this thesis. All 32 devices that were treated with Dow

Corning P5200 adhesion promoter failed immediately. About 20% of the plasma surface

activated samples also exhibited immediate failure. Table 3-2 lists the number of device

62

failures for each 10 day period of testing. Tables 3-3 and 3-4 show the lifetimes for the

samples containing no surface treatment and plasma surface activation treatment,

respectively.

Table 3-2: Number of devices that failed within each 10 day period of testing for the

primary accelerated lifetime study at 85oC.

Days of field usage

Number of failed plasma treated devices

Number of failed untreated devices

10 8 11 20 13 9 30 0 4 40 1 3 50 1 5

63

Table 3-3: Lifetime in hours and days and thickness for the samples with no surface

treatment for the primary accelerated lifetime study at 85oC.

Board # Device

# Lifetime (Hours) Days

Thickness (m)

No Surface Treatment #1 1 276 11.5 40

2 277 11.54

3 277 11.54

4 118 4.92

5 155 6.46

6 197 8.21

7 197 8.21

8 373 15.54

No Surface Treatment #2 1 731 30.46 40

2 388 16.17

3 415 17.29

4 382 15.92

5 376 15.67

6 514 21.42

7 522 21.75

8 525 21.88

No Surface Treatment #3 1 970 40.42 43

2 786 32.75

3 745 31.04

4 563 23.46

5 975 40.63

6 976 40.67

7 978 40.75

8 973 40.54

No Surface Treatment #4 1 272 11.33 60

2 214 8.92

3 26 1.08

4 27 1.13

5 113 4.71

6 98 4.08

7 112 4.67

8 100 4.17

64

Table 3-4: Lifetime in hours and days and thickness for the samples with oxygen

plasma surface activation treatment for the primary accelerated lifetime study at 85oC.

Board # Device # Lifetime (Hours) Days Thickness (m)

Plasma #1 1 3 0.13 70

2 1 0.04

3 1 0.04

4 Immediate failure 0.00

5 Immediate failure 0.00

6 Immediate failure 0.00

7 Immediate failure 0.00

8 Immediate failure 0.00

Plasma #2 1 36 1.50 60

2 358 14.92

3 83 3.46

4 100 4.17

5 198 8.25

6 Immediate failure 0.00

7 370 15.42

8 380 15.83

Plasma #3 1 459 19.13 42

2 459 19.13

3 74 3.08

4 75 3.13

5 74 3.08

6 342 14.25

7 326 13.58

8 967 40.29Plasma #4 1 272 11.33 70

2 293 12.21

3 249 10.38

4 247 10.29

5 256 10.67

6 247 10.29

7 240 10.00

8 826 34.42

65

The data presented in Table 3-3 and 3-4 were used to calculate the reliability,

cumulative failure, failure density, and hazard rate functions for each 10 day period of

device failure. Table 3-5 contains the calculations for the reliability functions.

Table 3-5: Calculated reliability functions based on the lifetime data obtained in the

primary accelerated lifetime study at 85oC.

Time (days) R(t) F(t) f(t) x 10-3 h(t) x 10-3

10 0.654545455 0.345454545 34.5 52.8 20 0.6 0.4 40 66.7 30 0.927272727 0.072727273 7.27 7.83 40 0.927272727 0.072727273 7.27 7.84 50 0.890909091 0.109090909 10.9 12.2

The probability of a packaged device failing after 10, 20, 30, and 40, and 50 days

is 34.5%, 40%, 7.3%, 7.3%, and 10.9%, respectively. Samples were seen to fail in

groups within 10 day periods. The majority of the sample failures occurred within the 10

and 20 day periods. IDEs that were in the same area of the PC board were seen to fail in

groups with a lifetime ranging from hours to 1 or 2 days between failures.

Large voids with irregular geometric form were present on all samples coated

with MDX4-4210 after failure in the accelerated lifetime tests. All samples were

thoroughly inspected under microscope before each soak test to ensure no bubbles,

pinholes, or particulates were present in the PDMS coating, so these defects formed

during the test. Figure 3-1, Figure 3-2, and Figure 3-3 show photographs of a

representative sample before soak testing, the voids that were observed when samples

66

were pulled from the data acquisition system upon device failure, and a zoom in

photograph of a void, respectively.

Figure 3-1: Enhanced photograph of an interdigitated electrode coated with Dow

Corning MDX4-4210 Biomedical Grade Elastomer.

Figure 3-2: Photograph of a sample after soak testing depicting the inflated pockets formed.

67

Figure 3-3: Photograph of a sample after soak testing depicting the deflated pockets.

The blue background seen in Figure 3-1 is generated from the blue stand used to

hold the samples while taking a photograph. The ripples seen on the surface of the

sample in Figure 3-1 are produced by the roller casting method as the roller passes over

the copper features on the samples.

Liquid was found inside of the voids when they were probed with a pin,

indicating that in these regions, significant moisture either permeated through the PDMS

film, a failure occurred at the edge of the test specimens, or moisture penetrated along the

interface between the PDMS and the electrical feed-throughs. The large size of the voids

can be attributed to the electrolysis of water. Electrolysis is a method used to decompose

water into oxygen and hydrogen gas. Electrolysis applies a voltage potential across two

metal electrodes submerged into salt or pure water. Salt water requires less applied

voltage than fresh water since ions are more prevalent in salt water. The ions are forced

68

to undergo oxidation at the anode and reduction at the cathode to produce oxygen or

hydrogen, respectively.

Figure 3-3 depicts the voids spanning across several copper trace lines associated

with multiple devices explaining why sample failures occurred in groups and not one at a

time. Voids appear to initially form at or near the electrode structures as shown in Figure

3-4.

Figure 3-4: Photograph of the pockets formed by electrolysis.

The void-related failure mechanism is a fairly simple process. First, moisture

reaches the PDMS/substrate interface in regions of high defect density. The 5 V bias used

to monitor leakage current causes electrolysis to occur in regions of high moisture

concentration. As a result of electrolysis, hydrogen and oxygen gas are produced,

causing the flexible PDMS to expand thus forming a large void. Over time the pockets

expand and cause the polymer to delaminate from the substrate, causing a multiple device

69

failures within a few hours to one or two days. The expansion of the pockets is a result of

the constant generation of gas by electrolysis.

None of the primary study samples contained devices that were able to achieve a

52 day lifetime under 85ºC accelerated lifetime testing like Sample 10 in the preliminary

study. Several subtle differences in the processing of Samples 9 and 10 in the

preliminary study and the samples for the primary accelerated lifetime study may be

responsible for the shorter lifetimes in the primary experiment. A longer time was used

to coat the samples prepared for the preliminary study as compared with those for the

primary study. The samples prepared for the preliminary study utilized 1/64” PC board

while those for the primary study used 1/16” boards. In hindsight, the thicker boards may

have required a longer cure time than the thinner boards. A new roller was used for the

primary experiment and the process may be sensitive to roller conditions in ways that

could not be determined in this project.

Figure 3-5, Figure 3-6, Figure 3-7, Figure 3-8, and Figure 3-9 are the average

surface insulation resistance vs. time semi-log plots for samples that failed within the 10,

20, 30, 40, and 50 day periods, respectively. Due to the large amount of data acquired by

the automatic control system, the average resistance for every 24 hour test period is

calculated and plotted instead of each data point collected.

70

Figure 3-5: Average surface insulation resistance (ohms) vs. Time (days) plot for a

sample failure within a 10 day testing period for the 85oC accelerated lifetime study.

71

Figure 3-6: Average surface insulation resistance (ohms) vs. Time (days) plot for a

sample failure within a 20 day testing period for the 85oC accelerated lifetime study.

72

Figure 3-7: Average surface insulation resistance (ohms) vs. Time (days) plot for a

sample failure within a 30 day testing period for the 85oC accelerated lifetime study.

73

Figure 3-8: Average surface insulation resistance (ohms) vs. Time (days) plot for a

sample failure within a 40 day testing period for the 85oC accelerated lifetime study.

74

Figure 3-9: Average surface insulation resistance (ohms) vs. Time (days) plot for a

sample failure within a 50 day testing period for the 85oC accelerated lifetime study.

The graphs all seem to exhibit three distinct periods in the data. The first period

is characterized by a strong drop in resistance within the first 2 to 4 days of testing which

may be attributed to bulk moisture absorption by the PDMS. The second period tends to

be a long period of fairly constant resistance that lies significantly above the threshold

resistance. In some samples, the resistance exhibits a slow decrease over this period, but

always remains above the threshold. The third period is only seen in devices that fail and

is described as a steep drop in resistance to a value below the threshold. Once below the

threshold, resistance values remain below this value. Mechanisms responsible for the

behavior observed in the second and third period are beyond the scope of this thesis. .

Figure 3-7 best illustrates the three distinct regions. Red dots on the plots indicate the

75

beginning of each period. Figure 3-10 plots the measured surface insulation resistance

for 5 samples for the first 6 days of testing for samples that failed within a 10, 20, 30, 40,

and 50 day testing period. The 6 day span fully encompasses the first period for each of

the samples. It is interesting to note that devices with a shorter lifetime exhibit a shorter

first period than devices with long lifetimes.

Figure 3-10: Average surface insulation resistance (ohms) vs. Time (days) plot for the

first days of testing where the strong decay in surface insulation resistance occurs for

the 85oC accelerated lifetime study.

As shown in Tables 3-3 and 3-4 which summarize the results of the 85oC test, all

of the samples that received no surface pre-treatment and most of the oxygen plasma

treated samples exhibit lifetimes that greatly exceed the maximum lifetime reported by

Zhang for his roller-cast samples tested under the same conditions (16 hrs) [26]. Refs. 16

and 27 did not report testing of specimens at 85ºC. It is particularly noteworthy that the

0.00E+00

5.00E+08

1.00E+09

1.50E+09

2.00E+09

2.50E+09

3.00E+09

3.50E+09

4.00E+09

4.50E+09

5.00E+09

1 2 3 4 5 6

Average

Resistan

ce (Ohms)

Time (Days)

10 Day Device Failure Period

20 Day Device Failure Period

30 Day Device Failure Period

40 Day Device Failure Period

50 Day Device Failure Period

76

films in this thesis range in thickness from 40 to 70 microns and yet exhibit much longer

lifetimes at 85ºC than the 100 to 400 micron thickness range in previous work. This may

be attributed to the improved processing techniques combined with the multi-layer

approach developed in this thesis.

How the lifetime data in Tables 3-3 and 3-4 translate from accelerated testing

conditions at 85ºC to physiological conditions (37oC) can, at best, only be estimated

using the calculated activation energy from Zhang’s data, an accurate determination of

activation energy (and hence translation of lifetime data at 85ºC to other temperatures)

requires actual lifetime data from the experiments at 40oC. Samples under the 40oC test

are ongoing and have not exhibited any device failures after 68 days of testing (1,632

hours). A representative graph of the surface insulation vs. time plots for a 40oC sample

that is still under test can be seen in Figure 3-11. To date, these are the longest running

accelerated lifetime experiments using the roll casting deposition method for PDMS; and,

to the best of our knowledge, and the longest running accelerated lifetime tests for

PDMS-based packages. By comparison, Refs 16 and 27 maximum lifetimes of 38 and 30

days respectively for PDMS coatings tested at 40ºC in saline. It should be noted that this

thesis is the first to evaluate a large population of PDMS-based packaged samples under

77

accelerated lifetime conditions.

Figure 3-11: Average surface insulation resistance (ohms) vs. Time (days) plot for the

40oC accelerated lifetime study

.

By visual inspection of Fig 3-11, the average surface insulation resistance is

constant for devices under test at 40ºC after roughly Day 6, making it extremely difficult

to predict a point at which the resistance value will drop below the failure threshold. As

such, an estimate of the activation energy for PDMS cannot be estimated by extrapolating

the lifetime from these data. Using the estimated activation energy for PDMS from

Zhang’s data (0.79 eV) and the results from the 85ºC accelerated lifetime tests in this

thesis from the longest running sample that survived for 40 days, analysis using Equation

2-2 yields an estimated lifetime for the roller-cast PDMS encapsulant of 4.32 years at

78

40oC, is 5.5 years at 37oC, and 22.2 years at 23ºC. As stated previously, these lifetimes

can only be considered rough maximum estimates given the potential issues related to the

activation energy used in the calculation. That being said accelerated lifetime

experiments at 40oC on 32 devices are currently ongoing in the automated testing

apparatus built for this thesis. To date, no devices have failed testing. Once the lifetime

tests at 40oC are complete, a statistically significant activation energy can calculated, and

from this, an accurate estimate of device lifetime at other relevant temperatures can be

determined.

3.2 Adhesion Strength Data Analysis

Experiments as described previously in Experimental Methods were performed to

select a mounting adhesive with the ability to successfully test the adhesion of the

MDX4-4210 Medical Grade Elastomer on a PC board substrate. Once an appropriate

mounting adhesive was identified, pull off tests were performed on PDMS-coated

samples before and after soak testing in saline. The resulting pull off force for each

sample was recorded and analyzed.

3.2.1 Mounting Adhesive

Super glue, Dow Corning MDX4-4210 Medical Grade Elastomer, Epoxy, and

Dow Corning RTV 732 silicone adhesive were evaluated as mounting adhesives for the

pull-off testing. At issue was the ability of the mounting adhesive to adhere to the PDMS

with sufficient strength to enable testing of the adhesion of the PDMS to the PC board.

Dow Corning RTV 32 silicone after a cure time of 3.5 days was the only mounting

adhesive capable of providing a clean pull-off of the PDMS from the PC-board substrate.

79

The other adhesives were seen to either delaminate from the stud surface or cause half of

the area underneath the stud to be pulled off.

3.2.2 Pull-Off Testing

Pull off tests were conducted on untreated, plasma treated, and adhesion before

and after soak testing the samples. Tests were conducted following the procedures

detailed in the Experimental Methods Section of this thesis. Pull off tests were

performed on soaked samples when all of the interdigitated electrode devices on a

particular PC board failed. This practice was selected to avoid unintentional damage to

interdigitated electrodes that were still undergoing electrical testing. Samples undergoing

pull tests before soaking were not tested electrically due to irreversible damage to the

PDMS/PC board interface caused by the pull test. Table 3-6 contains the adhesion

strength results for all samples before and after soak testing.

Table 3-6: Pull off test results taken on samples before soak testing and after soak

testing at 85oC.

Before Plasma (N) Untreated (N) Adhesion Promoter (N)

1 73.4 83.8 150.1

2 74.8 95.3 140.4

3 73.2 75.8 145.7

4 40.5 85.7 144.6

After

1 12.5 38.5 55.4

2 25.2 14 57.9

3 5.5 38.9 63.7

4 18.8 37.3 55.1

80

As shown in Table 3-6, the samples treated with the adhesion promoter possessed

the highest bonding strength prior to the soak test. Oxygen plasma surface treated

samples were expected to have a higher bonding strength than the untreated samples;

however this was not the case. It should be noted that the oxygen plasma treatment

recipe was that developed for bonding of PDMS to glass. No effort was made to develop

a plasma treatment process specifically for PC boards. Samples treated with Dow

Corning’s P5200 adhesion promoter were left for 10 more days in saline solution after the

immediate failures were observed.

As mentioned previously, the adhesion promoter samples will be excluded from

the bonding strength analysis due to their short lifetimes which do not provide much

information. The bonding strength of oxygen plasma surface activated treated samples

and untreated samples decreased after the soak testing was completed. The decrease in

bonding strength suggests that the bonding between the packaging material-substrate

interface must be strong to achieve a longer lifetime and must also maintain a high

adhesion throughout implantation. Furthermore, this suggests that not only the material

properties such as the rate of moisture permeation of the polymeric packaging material

are of great importance but also the surface of the substrate to which the polymer is

encapsulating. Untreated samples were seen to have longer lifetimes and higher bonding

than oxygen plasma treated samples suggesting that higher bonding strength of packaging

materials will provide a longer lifetime and better reliability.

81

4. Conclusions and Future Work

4.1 Conclusions

The multi-layer rolling technique along with Dow Corning MDX4-4210 Biomedical

Grade Elastomer has shown to be an excellent encapsulating technique for packaging

implantable micro-electromechanical systems. The rolling technique has proven itself as

a simple deposition process that can be reproduced with minimal cost, and produce thin

film PDMS layers with no pinholes or bubbles.

Samples under the accelerated lifetime study at 85oC were seen to reach a

maximum 40 day lifetime. The average lifetime of all 32 of the untreated samples is 17.5

days with 13 exhibiting lifetimes over 20 days, 4 over 30 days, and 3 exhibiting lifetimes

slightly above 40 days. Coating the samples in a clean room environment has shown to

reduce failure by particulates as none were observed on the failed samples.

Failures were seen to occur in 10 day testing periods and on multiple devices in

the same region on the coated samples. Electrolysis was found to occur which formed

pockets by outgassing and cause the PDMS to delaminate from the PC board substrate.

Testing at 40oC is ongoing with no device failures to date, making it difficult to calculate

an activation energy. However, an activation energy of 0.79 was estimated from

previously published data for PDMS and yielded estimated lifetimes of 5.5 years at

physiological temperature (37ºC) and 22.2 years at 25ºC.

Pull off performed on samples before and after soak testing confirms that the

surface of the sample should also be considered when designing a package. The force

necessary to pull the stud off of the samples was seen to dramatically decrease on

samples after soak testing was completed.

82

4.2 Future Work

Further investigation of the roller casting technique is required to further develop its

use for encapsulating implantable micro-electromechanical systems. Parameters such as

cure time, the number of strokes, and the amount of force exerted by the roller onto the

substrate should be optimized. Mechanisms responsible for the behavior of the surface

insulation resistance versus time plots in each of the three periods should be thoroughly

investigated. Optimal parameters for the oxygen plasma treatment should also be

investigated for increasing the bonding of Dow Corning MDX4-4210 to various

substrates.

Future work should entail the use of the optimized rolling technique on actual

implantable devices for in vivo testing into animal subjects with other materials such as

Parylene-C and epoxy. Optimal bonding strength between MDX4-4210 and other

biocompatible materials must be researched in order to develop a long term robust

packaging technology.

83

Appendix A: Interdigitated Electrode Array Mask

84

Appendix B: Interdigitated Electrode Fabrication

1. Power ON the Sircle Lam SL-3500-6R 14” Pouch Laminator and select

temperature and speed setting 110oC and speed setting No. 6. Allow the

laminator to warm up to 110oC for approximately 4 – 8 minutes.

2. Cut the MG Chemicals Negative Dry film resist with scissors to cover the

entire PC board surface area with approximately a half inch extra of resist film

hanging off all edges of the board.

3. Peel of the protective clear thin film layer by hand carefully. Be sure not to

tear resist.

4. Carefully place the side with no protective layer on the PCB. Inspect the

surface for bubbles and wrinkles.

5. Pass the boards under the rollers of the laminating tool two to three times to

increase the adhesion of the resist to the copper surface.

6. Place the PCB with resist in the tray of the Kinsten KVB-30d Exposure tool

and align the mask to the left edge of the board.

7. Turn on the vacuum until all of the bubbles from the tray have been eradicated

85

8. Set the exposure timer for 60 seconds and turn on the UV-lights of the

exposure tool.

9. Mix a 1: 10 solution of MG Chemicals potassium carbonate and water.

10. Brush a soft bristle paint brush over the surface of the exposed area to help

accelerate the developing process. This step will take about 15 minutes.

11. Rinse the board in DI Water for 30 seconds when the develop step is complete

to wash away resist particles.

12. Slowly dilute 1 kg of Sodium Persulfate crystals from MG Chemicals into 4

liters of H2O in the MC Chemicals plastic etchant tank.

13. Place boards inside the etchant tank and turn on pumps to allow bubbles to

flow. Etch will take 30 – 40 minutes.

14. Rinse boards in DI Water for 30 seconds to remove particulates from the

surface after the etch is complete.

15. Cut the boards and leave about .75 inches between the most right trace line of

the device and the edge of the board.

86

16. Place a small amount of soldering flux on the top copper pads of the samples.

17. Solder Red 30 American Wire Gauge (AWG) to each pad with soldering gun.

Use careful soldering technique is to ensure there are no sharp, or pointed

ends present at the solder interface as these defects can cause a failure to the

packaging.

18. Separate connection strip into 8 sockets. Solder the other end of the wire to

the female end of a 0.1” connection socket strip.

87

Appendix C: Cleaning Method for Samples

1. 3 minute DI water rinse in heated Branson 5510 sound bath.

2. 2 minute DI water rinse under DI water tap. The DI water will flow from top to bottom to wash away loose particles.

3. 3 minute Acetone rinse in heated Branson 5510 sound bath.

4. 3 minute DI water rinse in heated Branson 5510 sound bath.

5. 2 minute DI water rinse under DI water tap. The DI water will flow from top to bottom to wash away loose particles.

6. 3 minute 2-Propanol (98% alcohol) rinse in heated Branson 5510 sound bath.

7. 3 minute DI water rinse in heated Branson 5510 sound bath.

8. 2 minute DI water rinse under DI water tap. The DI water will flow from top to bottom to wash away loose particles.

9. Bake PCB in a vacuum oven @ 70oC for 30 minutes or until completely cured.

88

Appendix D: Schematic for the Lid of the Temperature Controlled Baths.

Acrylic lid for 40oC temperature controlled bath:

89

Acrylic lid for 85oC temperature controlled bath:

90

Appendix E: Pinning Information and Truth Table for the Intersil DG406 16:1 Multiplexer

Pinning Information for the Intersil DG406 16:1 Multiplexer:

91

DG 406 Truth Table

A3 A2 A1 A0 EN ON

SWITCH

X X X X 0 None

0 0 0 0 1 1

0 0 0 1 1 2

0 0 1 0 1 3

0 0 1 1 1 4

0 1 0 0 1 5

0 1 0 1 1 6

0 1 1 0 1 7

0 1 1 1 1 8

1 0 0 0 1 9

1 0 0 1 1 10

1 0 1 0 1 11

1 0 1 1 1 12

1 1 0 0 1 13

1 1 0 1 1 14

1 1 1 0 1 15

1 1 1 1 1 16

92

Appendix F: Pinning Information and Truth Table for the NXP 74HC4051 8:1 Multiplexer

Pinning Information:

74HC4051 Truth Table

S2 S1 S0 Ē

ON SWITCH

X X X 1 None

0 0 0 0 1

0 0 1 0 2

0 1 0 0 3

0 1 1 0 4

1 0 0 0 5

1 0 1 0 6

1 1 0 0 7

1 1 1 0 8

93

Appendix G: Circuit Diagram for the Data Acquisition System

94

Appendix H: Arduino Uno Software

int incomingByte; // a variable to read incoming serial data into #include <EEPROM.h> void setup() for (int i = 0; i < 512; i++) EEPROM.write(i, 0); // initialize serial communication: Serial.begin(9600); pinMode(2,OUTPUT); // 8:1 S2 ADDRESS LINE pinMode(3,OUTPUT); // 8:1 S1 ADDRESS LINE pinMode(4,OUTPUT); // 8:1 S0 ADDRESS LINE pinMode(5,OUTPUT); // Enable 16:1 #1 pinMode(6,OUTPUT); // Enable 16:1 #2 pinMode(7,OUTPUT); // Enable 16:1 #3 pinMode(8,OUTPUT); // Enable 16:1 #4 pinMode(9,OUTPUT); // Enable 16:1 #5 pinMode(10,OUTPUT); // Enable 16:1 #6 pinMode(11,OUTPUT); // Enable 16:1 #7 pinMode(12,OUTPUT); // Enable 16:1 #8 pinMode(13,OUTPUT); // Enable 16:1 #9 digitalWrite(2,LOW); // Set all outputs LOW digitalWrite(3,LOW); digitalWrite(4,LOW); digitalWrite(5,LOW); digitalWrite(6,LOW); digitalWrite(7,LOW); digitalWrite(8,LOW); digitalWrite(9,LOW); digitalWrite(10,LOW); digitalWrite(11,LOW); digitalWrite(12,LOW); digitalWrite(13,LOW); void loop() if (Serial.available() > 0) // see if there's incoming serial data. incomingByte = Serial.read(); // read the oldest byte in the serial buffer. if (incomingByte == '0') digitalWrite(2,LOW); // S0 ON 8:1 MUX digitalWrite(3,LOW); digitalWrite(4,LOW); digitalWrite(5,HIGH); // 16:1 MUX #1 ENABLED digitalWrite(6,LOW); digitalWrite(7,LOW); digitalWrite(8,LOW); digitalWrite(9,LOW); digitalWrite(10,LOW); digitalWrite(11,LOW);

95

digitalWrite(12,LOW); digitalWrite(13,LOW); if (incomingByte == '1') digitalWrite(2,LOW); // S1 ON 8:1 MUX digitalWrite(3,LOW); digitalWrite(4,HIGH); digitalWrite(5,LOW); digitalWrite(6,HIGH); // 16:1 MUX #2 ENABLED digitalWrite(7,LOW); digitalWrite(8,LOW); digitalWrite(9,LOW); digitalWrite(10,LOW); digitalWrite(11,LOW); digitalWrite(12,LOW); digitalWrite(13,LOW); if (incomingByte == '2') digitalWrite(2,LOW); // S2 digitalWrite(3,HIGH); digitalWrite(4,LOW); digitalWrite(5,LOW); digitalWrite(6,LOW); digitalWrite(7,HIGH); // 16:1 MUX #3 ENABLED digitalWrite(8,LOW); digitalWrite(9,LOW); digitalWrite(10,LOW); digitalWrite(11,LOW); digitalWrite(12,LOW); digitalWrite(13,LOW); if (incomingByte == '3') digitalWrite(2,LOW); // S3 digitalWrite(3,HIGH); digitalWrite(4,HIGH); digitalWrite(5,LOW); digitalWrite(6,LOW); digitalWrite(7,LOW); digitalWrite(8,HIGH); // 16:1 MUX # 4 ENABLED digitalWrite(9,LOW); digitalWrite(10,LOW); digitalWrite(11,LOW); digitalWrite(12,LOW); digitalWrite(13,LOW); if (incomingByte == '4')

96

digitalWrite(2,HIGH); // S4 digitalWrite(3,LOW); digitalWrite(4,LOW); digitalWrite(5,LOW); digitalWrite(6,LOW); digitalWrite(7,LOW); digitalWrite(8,LOW); digitalWrite(9,HIGH); // 16:1 MUX #5 ENABLED digitalWrite(10,LOW); digitalWrite(11,LOW); digitalWrite(12,LOW); digitalWrite(13,LOW); if (incomingByte == '5') digitalWrite(2,HIGH); // S5 digitalWrite(3,LOW); digitalWrite(4,HIGH); digitalWrite(5,LOW); digitalWrite(6,LOW); digitalWrite(7,LOW); digitalWrite(8,LOW); digitalWrite(9,LOW); digitalWrite(10,HIGH); // 16:1 MUX #6 ENABLED digitalWrite(11,LOW); digitalWrite(12,LOW); digitalWrite(13,LOW); if (incomingByte == '6') digitalWrite(2,HIGH); // S6 digitalWrite(3,HIGH); digitalWrite(4,LOW); digitalWrite(5,LOW); digitalWrite(6,LOW); digitalWrite(7,LOW); digitalWrite(8,LOW); digitalWrite(9,LOW); digitalWrite(10,LOW); digitalWrite(11,HIGH); // 16:1 MUX #7 ENABLED digitalWrite(12,LOW); digitalWrite(13,LOW); if (incomingByte == '7') digitalWrite(2,HIGH); // S7 digitalWrite(3,HIGH); digitalWrite(4,HIGH); digitalWrite(5,LOW); digitalWrite(6,LOW);

97

digitalWrite(7,LOW); digitalWrite(8,LOW); digitalWrite(9,LOW); digitalWrite(10,LOW); digitalWrite(11,LOW); digitalWrite(12,HIGH); // 16:1 MUX #8 ENABLED digitalWrite(13,LOW); if (incomingByte == '8') digitalWrite(2,LOW); digitalWrite(3,LOW); digitalWrite(4,LOW); digitalWrite(5,LOW); digitalWrite(6,LOW); digitalWrite(7,LOW); digitalWrite(8,LOW); digitalWrite(9,LOW); digitalWrite(10,LOW); digitalWrite(11,LOW); digitalWrite(12,LOW); digitalWrite(13,HIGH); // 16:1 MUX #9 ENABLED

98

Appendix I: Dimensional Schematic of the Pull Tester Base:

99

Face plate to hold force gauge:

100

Center Manifold Middle Sheet:

101

Center Manifold Outer Sheet:

102

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