Reliability of Conformal Coated Surface Mount Parts

Accelerated Stress Testing and Reliability Conference

Reliability of Conformal Coated Surface Mount Parts

Deng Y. Chen and Dr. Michael Osterman

Center for Advanced Life Cycle Engineering (CALCE)

University of Maryland, College Park, MD 20742

ASTR 2016, Sep 9 - 11, St. Cambridge, MA January-4-17 1


Introduction

• Conformal coating is a protective polymer layer that is uniformly applied on the surfaces of PCB assemblies.

• Coatings are used to protect the electronics from hazardous environments

• Some conformal coatings have been used for tin whisker mitigation.

www.ieee-astr.org September 28- 30 2016, Pensacola Beach, Florida 2

[1] “How To Guide for Atomised Spray conformal Coating of Printed Circuit Boards”, http://www.conformalcoatingconsultancy.com/cms/How-Spray-Coat, Accessed June 14, 2016 [2] “PCB Coatings,” http://precisiongraphics.us/conformal-coating.html, Accessed June 14, 2016

http://www.conformalcoatingconsultancy.com/cms/How-Spray-Coat





http://precisiongraphics.us/conformal-coating.html




Conformal Coated Interconnect Reliability (1/2)

• Parylene conformal coating was found to be beneficial for solder joint reliability under temperature shock between -196oC and room temperature [3].

• Pippola studied the thermoshock life of solder joint and found that polyurethane and epoxy coatings reduce the reliability of solder joints [4].


[3] H. M. Tong, L. S. Mok, K. R. Grebe, H. L. Yeh, K. K. Srivastava, and J. T. Coffin “Effects of Parylene Coating on the Thermal Fatigue Life of Solder Joints in Ceramic Packages”, IEEE Transactions on Components, Hybrids and Manufacturing Technology, Vol. 16, No. 5, pp 571-576, 1993. [4] Pippola, J., T. Marttila, and Laura Frisk. "Effect of protective casting materials on product level reliability under accelerated test conditions." In Microelectronics Packaging Conference (EMPC), 2013 European, pp. 1-6. IEEE, 2013.


Conformal Coated Interconnect Reliability (2/2) • Dunn et al examined the effect that various conformal coatings have on the

reliability of solder joints through 1000 cycles of -40oC to 100oC temperature cycling and found that conformal coatings have negative impact [5]. – Polyurethane, epoxy, and silicone coatings had the worst impacts.

• Lall also found that conformal coating had negative impact on solder joint reliability under temperature cycling, although the type of coating was not specified [6].

• Hunt concluded in his study that conformal coating improves the thermomechanical reliability of solder joints under -55oC to 125oC temperature cycling [7]. – Hunt hypothesized that pliable coatings are beneficial to solder joint reliability.


[5] B. D. Dunn, P. Desplat, and W. R. Burke, Evaluation of conformal coatings for future spacecraft applications, European Space Agency, 1994. [6] P. Lall, M. N. Islam, J. Evans, J. C. Suhling, T. Shete , “Damage Mechanics of Electronics on Metal-Backed Substrates in Harsh Environments”, IEEE Transactions on Components Packaging and Technology, Vol. 29, No. 1, pp 204-212 2006. [7] C. Hunt and M. Dusek, “Lead-free Solders and PCB Finish Effects on Solder Joint Reliability”, Proceedings of the 1st Electronics Systemintegration Technology Conference, pp169-180, 2006.


Motivation

• Limited studies on the effects of conformal coating on the interconnect reliability are presented in the literature.

• The studies in the literature present disagreements on the effects of conformal coating on the interconnect reliability under thermomechanical loadings.

• No study has been reported on the impact drop and vibrational reliability of conformal coated PCB assemblies.



Research Objective

To investigate the reliability of effect of conformal coating on surface mount interconnect reliability under temperature cycling, harmonic vibration, and drop tests



Approach


Design of Test Vehicle

Conformal Coating Application

Temperature Cycling -55 to 125oC, 15 mins dwell

Harmonic Vibration 3G and 4G

Drop Testing 3000G

Failure Data Comparison


Test Vehicle Specifications (1/2)


Board Details Coating Components Count

− 4 Layers

− 9"x4.5"x0.063"

− Surface Finish: ImAg

− Solder: SAC305

− CTEx :13.8 ppm/°C

− CTEy 15.1ppm/°C

− Acrylic (50µm)

− Parylene like (50µm)

− None

BGA 4

QFP 4

QFN 4

Resistor (50% Pad

Width)

4

Resistor (50% Pad

Length)

4


Test Vehicle Specifications (2/2)


BGA 192IO’s 14mm × 14mm CTExy 7.6(ppm/oC) Solder mask defined

QFP 44 IO’s 10mm × 10mm CTExy 15.1 (ppm/oC)

QFP 52IO’s 8mm × 8mm CTExy 16.2(ppm/oC)

2512 Chip Resistor 25mil × 12mil CTExy 4.3 (ppm/oC)


Temperature Cycling Test


• About 6000 cycles were conducted.

• Resistance monitoring is carried out for all components on the board to determine failure. Failure is defined based on the IPC-9701 standard, which defines failure as a 20% increase in initial resistance value for five consecutive

scans.

(-55 to 125oC, 15 min dwell)

Components

Coating Type

None Acrylic Parylene

Number of Failures

BGA 8/8 8/8 8/8

QFP 0/8 0/8 0/8

QFN 8/8 8/8 8/8

2512

Resistors

(50% Nominal Length) 8/8 8/8 8/8

(50% Nominal Width) 8/8 8/8 8/8


Failure Distribution of BGAs


• Acrylic coated BGAs yield a characteristic life that is very close to the one of noncoated BGAs.

• Characteristic life of parylene coated BGAs are about twice the characteristic life of noncoated BGAs.

• Both coating improved the shape parameter of the BGA failure distribution.

.

Temperature Cycling Test (-55 to 125oC, 15 min dwell)


Failure Distribution of QFNs


• Both coatings improved the characteristic life of QFNs under temperature cycling.

• Acrylic coated QFNs have higher characteristic life than parylene coated QFNs

Temperature Cycling Test (-55 to 125oC, 15 min dwell)


Harmonic Vibration Test Setup


• 3G and 4G base excitation vibration were exerted on the fixture.

• The resonant frequency of the boards is about 190Hz.

• At the resonant frequency, a transmissibility about 28 was observed.

Schematic of test setup

Direction of vibration

Clamped Ends

6.0''


Failure Distributions of BGA


• Both acrylic coating and parylene coating improved the mean cycles to failure of BGAs under 3G vibration.

• However, arylic coated BGAs have the lowest first time to failure.

• Parylene coated BGAs have larger improvements.

Harmonic Unidirectional Vibration (3G at fixture)


Failure Distributions of BGA


• At 4G vibration, acrylic coating has little to no impact on the reliability of BGAs under harmonic vibration.

• Parylene coated BGAs have higher characteristic life than both noncoated BGAs and acrylic coated BGAs.

Harmonic Unidirectional Vibration (4G at fixture)


Drop Test


3"

• Drop tests were performed on a Lansmont M23 drop tower.

• Boards were constrained on aluminum plates with 3 inches of unconstraind span.

• 3000G impact acceleration with 0.35 millisecond pulse width on the fixture.


Failure Distributions (BGA) (1/2)


Shock Impact Testing (3000G at fixture)

• Noncoated BGAs have the highest characteristic life out of all the tested boards under drop test.

• The failure distributions are very close to each other.


Failure Distributions (BGAs) (2/2)


The difference in the failure distribution between the BGAs seems insignificant when plotted with 90% confidence interval.

Shock Impact Testing (3000G at fixture)


Summary

• Both acrylic and parylene like conformal coatings improved the reliability of all of the types of packages under -55oC to 125oC temperature cycling.

• Conformal coating also improved the reliability of surface mount components under 3G and 4G harmonic vibrational loadings.

• The impact of conformal coating on the surface mount packages seems to be insignificant under drop test.


Documents

Reliability of Conformal Coated Surface Mount Parts