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Conductive Anodic Filament (CAF) Formation in Printed Circuit Boards (PCBs)…And What Sherlock Can Do For You

October 11, 2012

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What is Conductive Anodic Filament (CAF) Formation?

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This is CAF…

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…and this…

Z

Z Z:Z Cross-SectionThe photo in A shows a cross-sectional view of conductive anodic filaments between two plated through holes (PTHs). An oblique slice through the copper filament is shown in B.

A B

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…and this.

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o IPC-9691 Users Guide for the IPC-TM-650, Method 2.6.25 o A growth consisting of a conductive copper-containing salt. It is

created electrochemically and grows from the anode toward the cathode subsurface along the epoxy/glass interface

o IPC-TM-650, Method 2.6.25 Conductive Anodic Filament (CAF) Resistance Test: X-Y Axiso A form of electrochemical migration within a printed wiring

board (PWB)

o What is electrochemical migration?

Definitions of Conductive Anodic Filament (CAF)

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o IPC-TR-476A, Electrochemical Migration: Electrically Induced Failures in Printed Wiring Assemblieso The growth of conductive metal filaments or dendrites on or

through a printed board under the influence of a DC voltage bias

o DfR Solutionso Movement of metal through an electrolytic solution under an

applied electric field between insulated conductors

Definition of Electrochemical Migration

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Ano

de

Cat

hode

Cations

Electrolyte

DC voltage source

_+

Electrolytic Cell

Anode: the positive electrode of an electrochemical cell at which oxidation occurs

e-

Schematic of CAF

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Why do I care about CAF?

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CAF causes failures…

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…and more failures…

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…and even more failures (typically with burning)

The copper seen here along the outside of these fibers caused a short in this PCB

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How Does CAF Occur?

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o Traditional electrochemical migration involves four stepso Path Formationo Electrodissolutiono Ion Migrationo Electrodeposition

The Four Steps of ECM

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o Hollow Fibers

o Drilling Damage

o Triple Points or Poor Wet Out

o Interfacial Separation

Path Formation

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After Path Formation

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CAF and Hollow Fibers

o Translation: It’s the glass manufacturer’s fault

100 m

Hollow fibers form from decomposed impurities in the glass melt

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Hollow Fibers

o Generally, CAF is a two-step processo Dependent on debonding between the glass fibers and epoxy resin

matrix to provide a path for copper migration

o With appearance of hollow fibers inside the laminates, CAF can happen as a one step process. o Concentration of hollow fibers in laminate becomes critical to reliability

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Images of Hollow Fibers

Hollow fibers can be seen as white lines traveling along the fiber bundle weave in both of these pictures.

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Images of Hollow Fibers (cont.

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o Control your supply chaino Laminate manufacturer can not change glass

supplier without prior approval

How to Prevent Hollow Fibers

Who selects the glass fibers?

Laminate manufacturer?

Glass weave supplier?

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o Qualify your supply chaino Request the PCB manufacturer to periodically

perform hollow fiber assessment

How to Prevent Hollow Fibers (part 2)

o Hollow Fiber Assessmento Burn-off the epoxyo Cut the weave along the diagonal

(avoid double counting)o Seal the edges with waxo Immerse in an index-matching oil

o Goal is zero hollow fibers per 100 cm2

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o Drilling damage can accelerate CAF througho Fiber/resin

delamination, o Creation of

paths for moisture to accumulate

o Wicking due to cracking of the board material

Drilling and CAF

Drilling Damage

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Wicking

Wicking can be serious if it extends sufficiently to deter the dielectric strength or internal resistance breakdown between PTHs. It also provides a convenient starting point for CAF as it effectively decreases the conductor spacing.

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o Drill bit manufacturers provide PCB manufacturers guidance on key process parameterso Speeds and feedso Entry and exit materialo Number of drilling operations before repointingo Stackup guidelines (number of PCBs of a given thickness that can

be stacked during drilling)o Number of repoints / sharpening

o There is no ‘right’ answer for process parameterso PCB manufacturer may buy a more expensive drill bit, but repoint

more often

Avoiding Drilling Damage

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o Kyocerao FR-4, Multilayer,

High Tg

o Chiploado Feed / (Speed x

# of Flutes)

o SFM (surface feet per minute)o Feed x Dia. x

Feeds / Speeds (example)

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o The key to success is verification/control and compliance/reward

o Did the PCB manufacturer perform their own DoE to understand and verify guidelines from the drill bit manufacturer?o Capture influence of high glass content, heavy copper, fill particles, etc.

o Are incoming and resharpened bits subjected to automatic inspection?

o Are vacuum gauges alarmed and monitored?o Is SPC of drill runout checked before production?

o Is the PCB manufacturer confirming employee compliance with defined drilling parameters?o Are bonuses inline with process parameters (compensation cannot be

increased by exceeding recommendations)

Drilling (cont.)

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How Much Drilling Damage is Too Much?

o Current IPC specification is inadequateo Acceptability of Printed Boards, IPC-A-600G, provides a target

and reject condition for haloing of unsupported holes only (not plated through holes)

o IPC-A-600G does provide an accept / reject condition for wickingo Ranging from 80 to 125 microns, depending on class designation

o Wicking is the preferred defect assessment because standard construction analysis is in a vertical orientationo Haloing is easier to identify through non-standard horizontal

sectioning

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Industry Specifications (cont.)

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Debonding and Wetout

o Isola Group’s statement on laminate / prepreg CAF performanceo “… wet out and interfacial chemistry

override other factors.”

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Poor Wet Out

Insufficient infiltration of epoxy into glass weave can result in ‘triple points’

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o Glass spread

o Cleanliness of the glass fiber before silane treatment

o Silane finish (coupling agent)

o Impregnation process parameters (temperature, flow rates, etc.)

How to Prevent Wet-Out

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o PCB laminates (and prepregs) are fabricated with a variety of glass styles

o The closed structure of low resin content glass styles (e.g., 7628) can prevent adequate resin flow during impregnationo Can also trap ash after heat clean

(removal of starch-oil coating/sizing)

Glass Style Glass Style

Resin Volume Content

Fiber Volume Content

1027 0.86 0.14

1037 0.86 0.14

106 0.84 0.16

1067 0.84 0.16

1035 0.83 0.17

1078 0.82 0.18

1080 0.79 0.21

1086 0.78 0.22

2313 0.74 0.26

2113 0.72 0.28

2116 0.71 0.29

3313 0.71 0.29

3070 0.68 0.32

1647 0.66 0.34

1651 0.66 0.34

2165 0.66 0.34

2157 0.66 0.34

7628 0.64 0.36

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o Trend is processes that improve wetting through spreading of yarn bundles or opening capillaries between filaments.

o Top of the line PCB shops will require suppliers spec degree of spreadness and provide a lot certification

Glass Spread

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Interfacial Separation

o Classic CAF is along the fiber/ epoxy interface

o Exposure to elevated temperature-humidity conditions weakens glass /polymer bonds based on silanes

o Hydrolysis reactiono Si2O + H2O ↔ 2SiOH

o Attempts at improving the bonding at this interface can result in improved CAF performance

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o Classic Engineering Problemo Properties good for one thing are not good for anothero The best choice is the most expensive

o Interface between the fiber and resin varies from tightly bound siloxanes at the fiber wall to unbound siloxanes blending into the epoxy matrix. o Unbound siloxanes permit penetration of the epoxy resin into the

interface region and strengthens the epoxy-glass bondo Tightly bound siloxanes restrict moisture absorption.

o The proper ratio of bound to unbound siloxanes results in the optimum interface

Silane Conundrum

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o A method of increasing adhesion is to improve the reactivity of surface treating agents

o Improved reactivity with resins can result in a rigid and thin layer on the interfaces that can elevate residual stress

o The use of surface treating agents together with long chain polysiloxanes will reduce the residual stress, but will tend to decrease intrinsic interfacial adhesion

Silane Conundrum (cont.)

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o Dow Corning’s Z-6032 tends to dominate the marketo Vinylbenzylaminoethylaminopropyltrimethoxysilane

[C6H4-CH2-NHC2H4NHC3H6-Si(OCH3)3)] o High water resistance and universal coupling agent

(pretty much works on all epoxy formulations)

o Problemso Universal is not really universal (not all supply chains re-validate

compatibility with changes in resin)o Z-6032 is expensive and instable (requires cooling); strong

motivation to select lower cost options

Silane Finale

This is where your 5% price reduction comes from!

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Evidence of Non-Optimized Silane

Optical micrograph of copper filaments in the area of fiber/resin delamination

X-ray Map of Cu

Electron micrograph of area of fiber/resin delamination. EDS shows evidence of copper filaments.

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Lot Qualification for Glass/Epoxy Bonding

o The process for ensuring good bonding at the glass/epoxy interface, outside of CAF testing, is the use of the test method IPC TM-650 2.6.16 Pressure Vessel Method for Glass Epoxy Laminate. o Requires exposing laminate coupons to pressure cooker conditions

(121C, 100%RH, 15 psi) for 30 minutes and then immersing the coupons in a solder pot heated to either 260C or 288C.

o Called out in IPC-4101B Specifications for Base Materials for Rigid and Multilayer Printed Boardso Optional testo If incorporated, IPC-4101B recommends that it be used for both

conformance and qualification testing, with testing performed on every lot.

o The test method provides a grading system of 1 to 5, with laminates graded 4 to 5 often rejected by the PCB supply chain

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What is Everyone Else Doing About CAF?

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o Most electronic OEMs do nothing in regards to CAFo Blissful ignoranceo Not susceptible (design is too coarse to be an issue)

o Some electronic OEMs use ‘gut feel’o Very conservative on voltage/spacing design rules

(graybeards)o Require CAF-resistant laminate in PCB drawings

(could mean anything)o Moved to more robust laminate with change to Pb-free

(higher Tg/Td/T288, lower moisture absorption)

Blissful Ignorance

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Who is Concerned with CAF?

o Enterprise / Telecom / High-End Computingo E.g., Cisco Systems, Sun (Oracle), IBM, etc.o Aggressive Designs

o High I/O (>1000) BGAs with High Layer Count (18+) creates many potential initiation sites

o Constant bias, but controlled environmento Need for high availability

o Military / Avionics / Automotive / Industrialo E.g., TRW, Rockwell Collins, etc.o Modestly dense designso Uncontrolled environmento Long life requirements (10+ years)

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Preventing CAF

Design Rules

Supplier Qualification

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CAF – Critical Paths

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Design Rules – Critical Paths

o Some debate over ‘critical’ paths

o PTH-to-PTHo Potential for greatest internal

damage (2X drilling)o Larger exposed surface areao 20 to 30 mil spacings (12 mil drill

diameter minus 32 to 40 mil pitch)

o PTH-to-Planeo Less potential for damage (1X drilling)o 7 to 10 mil clearance

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Proposed Trends in Conductor Pitch in PCBs

1985 1990 1995 1999 2002 2004 20060

20

40

60

80

100

Reduction in Pitch

m i

l s

Drilled Hole Size

ViaEdge

toVia

Edge

When CAF?

Susceptibility of Glass-Reinforced Epoxy Laminates to Conductive Anodic Filamentation by Chris Hunt Circuitree; March 1, 2007

Determining the Limits of TelCordia Compliance for Printed Wiring Boards by Karl SauterCircuitree, June 2000

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Current Industry Trends on Wall-to-Wall Spacing

o Conservative Designso Based on ball grid array (BGA) with 0.8mm (32mil) pitch

and a 0.3mm (12 mil) drill hole o Wall-to-wall spacing of 0.5mm (20mil)

o Aggressive Designso Down to 0.25mm (10 mil) wall-to-wall spacing

o IPC Class 2 allows 100 microns of wicking

o Try to avoid spacings less than knuckle-to-knuckle distance on the glass weave

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o A revolutionary automated design analysis tool that brings insight and prediction earlier than ever into the product development process

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o Since time-to-failure cannot realistically be determined for CAF, Sherlock uses scoring to identify at risk designs

o 10 is in accordance with industry best practices

o 7 to 10 is designated green; indicates a preferred design

o 5 is in accordance with minimum acceptable practice

o 4 to 6 is designated yellow; indicates a marginal design

o 0 suggests a high likelihood of failure during lifetime

o 0 to 3 is designated red; indicates a high risk design

Sherlock and CAF Avoidance

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o Scoring is based on combination of wall-to-wall spacing, degree of overlap (orthogonality), and qualification protocolso Simple premise: The more aggressive the design, the more robust

the qualification method

o Exampleso Industry best practice (10) allows for 20mil spacing if each lot is

qualifiedo Industry minimum practice (5) allows for 20 mil spacing if no

qualification is performedo 10 mil spacing is a marginal, but not high risk, design (4) if each

lot is qualifiedo Product qualification (design/material combination) is not

sufficient

CAF Scoring

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Identifying At-Risk Sites for CAF

X1 (in) Y1 (in) Diam1 (mil) X2 (in) Y2 (in) Diam2 (mil) Distance (mil) Overlap (%)

13.02 4.365 12 13.02 4.39 12 13.0 100.0

11.745 3.565 12 11.745 3.59 12 13.0 100.0

14.61 4.5 12 14.635 4.5 12 13.0 100.0

14.61 4.53 12 14.635 4.53 12 13.0 100.0

9.65 4.62 12 9.675 4.62 12 13.0 100.0

10.245 2.58 12 10.27 2.58 12 13.0 100.0

13.025 4.25 12 13.05 4.25 12 13.0 100.0

13.11 5.025 12 13.135 5.025 12 13.0 100.0

8.97 2.735 12 8.98 2.76 12 14.9 16.7

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CAF Scoring

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Future Improvements

o Electric fieldo Leveraged through existing use of net list

o Use CAF-resistant grade laminateo Poorly defined at this time

o Comparison of laminate properties (Tg, Td, T288, moisture absorption) to assembly temperatures

o Board thickness and stackupo Presence of non-plated through-holeso Parameters of CAF qualification testo Supplier capability (‘sweet spot’)

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o IPC-9253 / IPC-9254 / PCQR2

o Wall-to-wall ranges from 10 to 25mil

o Alternative (should be based on your design rules!)o Probably more common than IPC designs

o Some test boards qualify a specific design (zero failures)

o Some test boards assess margin (trying to cause failure)

o Material and stackup should be the same as actual product

Qualifying Suppliers: Select a Test Board

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o IPC recommends 4200 initiation sites across 25 coupons

Test Boards (cont.)

o Most company specs require between 500 to 2000 initiation sites

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CAF Test Coupon (Margining)

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Qualifying Suppliers: Select a Test Condition

o Temperature / Humidity (sometimes with preconditioning)o IPC TM-650, 2.6.25: 65C / 88%RHo IPC-9151D (PCQR2): 75C / 85%RHo IBM: 50C / 80%RHo Others: 60C / 90%RH, 85C / 85%RH, etc.

o Voltageo Not standardizedo Debate about high voltage (50V / 100V) vs. low voltage

(5V / 10V / 15V) and if bias voltage should equal test voltageo IPC allows up to 100V (meets E-field limitation of 10V/mil)o DfR Recommendation: Highest voltage (at appropriate spacing)

and smallest spacing (at appropriate voltage)

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o Driven by measurement approacho Continuous monitoring (rare)o Periodic (24 to 72 hours)

o Failure definitiono Resistance (100 megaohms)o Change in resistance (10X)

o Define test timeo Enterprise: 300 to 600 hourso Automotive: 500 to 2000 hours

Qualifying Suppliers: Define Failure

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Defining Time to Failure (IPC-9691)

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CAF is an Infant Mortality Defect

Test Condition (85C / 85%RH)

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o Will the risk of CAF increase in future designs?o Not really

o The rate of feature size reduction is very limited with PCBso Industry roadmaps have barely moved (except for

substrates, which tend to rely on laser drilling and resin-coated copper without glass fibers)

o Pb-free transition is almost completeo Some movement to lower temperature alloys (e.g., SnBi)

CAF in the Future

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o Minimal technological progress over past 10 years

PCB Industry Plated Through Hole Capability

1.2 – 1.6mm PCB Thickness 2.3 – 3.2mm PCB Thickness

95% of Industry

5% of Industry

95% of Industry

5% of Industry

DDI Tech Roadmap 2011 PCB Thickness (mil / mm)

Via Diameter(mil / um) Standard Advanced Engineering

6 / 150 N/A 39 / 1.0 60 / 1.5

8 / 200 64 / 1.6 80 / 2 96 / 2.4

10 / 250 100 / 2.5 120 / 3 160 / 4

IBM PCB-OS Symposium 2007, Roadmap Technology Verification, Conductor Analysis Technologies (CAT)

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o Conductive Anodic Filament (CAF) formation does happeno When it happens, it can cause a lot of pain

o CAF behavior is relatively stableo Limited change in key PCB technology (pitch, materials,

assembly)

o CAF mitigation is well known (execute it!)o Evaluate your designso Qualify your suppliers

Summary / Conclusion

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Disclaimer & Confidentiality

o ANALYSIS INFORMATIONThis report may include results obtained through analysis performed by DfR Solutions’ Sherlock software. This comprehensive tool is capable of identifying design flaws and predicting product performance. For more information, please contact sales@dfrsolutions.com.

o DISCLAIMERDfR represents that a reasonable effort has been made to ensure the accuracy and reliability of the information within this report. However, DfR Solutions makes no warranty, both express and implied, concerning the content of this report, including, but not limited to the existence of any latent or patent defects, merchantability, and/or fitness for a particular use. DfR will not be liable for loss of use, revenue, profit, or any special, incidental, or consequential damages arising out of, connected with, or resulting from, the information presented within this report.

o CONFIDENTIALITYThe information contained in this document is considered to be proprietary to DfR Solutions and the appropriate recipient. Dissemination of this information, in whole or in part, without the prior written authorization of DfR Solutions, is strictly prohibited.

From all of us at DfR Solutions, we would like to thank you for choosing us as your partner in quality and reliability assurance. We encourage you to visit our website for information on a wide variety of topics. To help us continually improve, please send any feedback or comments to iso@dfrsolutions.com.

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