Conductive Anodic Filament (CAF)

Conductive Anodic Filament
(CAF) Formation in Printed
Circuit Boards (PCBs)
…And What Sherlock Can Do For You
October 11, 2012
1
What is Conductive Anodic Filament (CAF)
Formation?
2
This is CAF…
3
…and this…
Z
B
A
Z
Z:Z Cross-Section
The 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.
4
…and this.
5
Definitions of Conductive Anodic Filament (CAF)
o
IPC-9691 Users Guide for the IPC-TM-650, Method 2.6.25
o
o
IPC-TM-650, Method 2.6.25 Conductive Anodic Filament
(CAF) Resistance Test: X-Y Axis
o
o
6
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
A form of electrochemical migration within a printed wiring
board (PWB)
What is electrochemical migration?
Definition of Electrochemical Migration
o
IPC-TR-476A, Electrochemical Migration: Electrically
Induced Failures in Printed Wiring Assemblies
o
o
DfR Solutions
o
7
The growth of conductive metal filaments or dendrites on or
through a printed board under the influence of a DC voltage
bias
Movement of metal through an electrolytic solution under an
applied electric field between insulated conductors
Schematic of CAF
Electrolytic Cell
DC voltage source
e-
Anode
+
Cations
Cathode
_
Electrolyte
Anode: the positive electrode of an electrochemical cell at which oxidation occurs
8
Why do I care about CAF?
9
CAF causes failures…
10
…and more failures…
11
…and even more failures (typically with burning)
The copper seen here along the outside of these fibers
caused a short in this PCB
12
How Does CAF Occur?
13
The Four Steps of ECM
o
Traditional electrochemical
migration involves four steps
o
o
o
o
14
Path Formation
Electrodissolution
Ion Migration
Electrodeposition
Path Formation
o
Hollow Fibers
o
Drilling Damage
o
Triple Points or Poor Wet Out
o
Interfacial Separation
15
After Path Formation
16
CAF and Hollow Fibers
Hollow fibers form from decomposed impurities in the
glass melt
100 m
o
17
Translation: It’s the glass
manufacturer’s fault
Hollow Fibers
o
o
18
Generally, CAF is a two-step process
o
Dependent on debonding between the glass fibers and epoxy resin
matrix to provide a path for copper migration
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
Images of Hollow Fibers
Hollow fibers can be seen as white lines traveling along the
fiber bundle weave in both of these pictures.
19
Images of Hollow Fibers (cont.
20
How to Prevent Hollow Fibers
o
Control your supply chain
o Laminate manufacturer can not change glass
supplier without prior approval
Who selects the glass fibers?
Laminate manufacturer?
Glass weave supplier?
21
How to Prevent Hollow Fibers (part 2)
o
o
Qualify your supply chain
o Request the PCB manufacturer to periodically
perform hollow fiber assessment
Hollow Fiber Assessment
o
o
o
o
o
22
Burn-off the epoxy
Cut the weave along the diagonal
(avoid double counting)
Seal the edges with wax
Immerse in an index-matching oil
Goal is zero hollow fibers per 100 cm2
Drilling and CAF
o
Drilling damage
can accelerate
CAF through
o
o
o
23
Fiber/resin
delamination,
Creation of
paths for
moisture to
accumulate
Wicking due to
cracking of the
board material
Drilling Damage
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.
24
Avoiding Drilling Damage
o
Drill bit manufacturers provide
PCB manufacturers guidance on
key process parameters
o
o
o
o
o
o
There is no ‘right’ answer for process parameters
o
25
Speeds and feeds
Entry and exit material
Number of drilling operations before repointing
Stackup guidelines (number of PCBs of a given thickness that can
be stacked during drilling)
Number of repoints / sharpening
PCB manufacturer may buy a more expensive drill bit, but repoint
more often
Feeds / Speeds (example)
o
Kyocera
o
o
Chipload
o
o
Feed / (Speed x
# of Flutes)
SFM (surface feet
per minute)
o
26
FR-4, Multilayer,
High Tg
Feed x Dia. x 
Drilling (cont.)
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
o
o
o
o
Are incoming and resharpened bits subjected to automatic
inspection?
Are vacuum gauges alarmed and monitored?
Is SPC of drill runout checked before production?
Is the PCB manufacturer confirming employee compliance with
defined drilling parameters?
o
27
Capture influence of high glass content, heavy copper, fill particles, etc.
Are bonuses inline with process parameters (compensation cannot be
increased by exceeding recommendations)
How Much Drilling Damage is Too Much?
o
Current IPC specification is inadequate
o
o
IPC-A-600G does provide an accept / reject condition for
wicking
o
o
Ranging from 80 to 125 microns, depending on class designation
Wicking is the preferred defect assessment because
standard construction analysis is in a vertical orientation
o
28
Acceptability of Printed Boards, IPC-A-600G, provides a target
and reject condition for haloing of unsupported holes only (not
plated through holes)
Haloing is easier to identify through non-standard horizontal
sectioning
Industry Specifications (cont.)
29
Debonding and Wetout
o
Isola Group’s statement on laminate /
prepreg CAF performance
o
30
“… wet out and interfacial chemistry
override other factors.”
Poor Wet Out
Insufficient infiltration of epoxy
into glass weave can result in
‘triple points’
31
How to Prevent Wet-Out
o
o
o
o
32
Glass spread
Cleanliness of the glass fiber
before silane treatment
Silane finish (coupling agent)
Impregnation process parameters
(temperature, flow rates, etc.)
Glass Style
o
o
PCB laminates (and prepregs) are
fabricated with a variety of glass styles
The closed structure of low resin content
glass styles (e.g., 7628) can prevent
adequate resin flow during impregnation
o
33
Can also trap ash after heat clean
(removal of starch-oil coating/sizing)
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
Glass Spread
o
o
34
Trend is processes that improve wetting through spreading of
yarn bundles or opening capillaries between filaments.
Top of the line PCB shops will require suppliers spec degree of
spreadness and provide a lot certification
Interfacial Separation
o
o
o
Classic CAF is along the fiber/
epoxy interface
Exposure to elevated temperaturehumidity conditions weakens glass
/polymer bonds based on silanes
Hydrolysis reaction
o
o
35
Si2O + H2O ↔ 2SiOH
Attempts at improving the bonding
at this interface can result in
improved CAF performance
36
Silane Conundrum
o
Classic Engineering Problem
o
o
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
o
o
37
Properties good for one thing are not good for another
The best choice is the most expensive
Unbound siloxanes permit penetration of the epoxy resin into the
interface region and strengthens the epoxy-glass bond
Tightly bound siloxanes restrict moisture absorption.
The proper ratio of bound to unbound siloxanes results in the
optimum interface
Silane Conundrum (cont.)
o
o
o
38
A method of increasing adhesion is to improve the
reactivity of surface treating agents
Improved reactivity with resins can result in a rigid and
thin layer on the interfaces that can elevate residual stress
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 Finale
o
Dow Corning’s Z-6032 tends to dominate the market
o
o
o
Vinylbenzylaminoethylaminopropyltrimethoxysilane
[C6H4-CH2-NHC2H4NHC3H6-Si(OCH3)3)]
High water resistance and universal coupling agent
(pretty much works on all epoxy formulations)
Problems
o
o
Universal is not really universal (not all supply chains re-validate
compatibility with changes in resin)
Z-6032 is expensive and instable (requires cooling); strong
motivation to select
lower cost options
This is where your 5% price
reduction comes from!
39
Evidence of Non-Optimized Silane
Optical micrograph of copper filaments
in the area of fiber/resin delamination
Electron micrograph of area
of fiber/resin delamination.
EDS shows evidence of
copper filaments.
X-ray Map of Cu
40
Lot Qualification for Glass/Epoxy Bonding
o
o
o
41
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.
Called out in IPC-4101B Specifications for Base Materials for Rigid and
Multilayer Printed Boards
o
Optional test
o
If incorporated, IPC-4101B recommends that it be used for both
conformance and qualification testing, with testing performed on every
lot.
The test method provides a grading system of 1 to 5, with laminates
graded 4 to 5 often rejected by the PCB supply chain
What is Everyone Else
Doing About CAF?
42
Blissful Ignorance
o
Most electronic OEMs do nothing in regards to CAF
o
o
o
Some electronic OEMs use ‘gut feel’
o
o
o
43
Blissful ignorance
Not susceptible (design is too coarse to be an issue)
Very conservative on voltage/spacing design rules
(graybeards)
Require CAF-resistant laminate in PCB drawings
(could mean anything)
Moved to more robust laminate with change to Pb-free
(higher Tg/Td/T288, lower moisture absorption)
Who is Concerned with CAF?
o
Enterprise / Telecom / High-End Computing
o
o
E.g., Cisco Systems, Sun (Oracle), IBM, etc.
Aggressive Designs
o
o
o
o
Constant bias, but controlled environment
Need for high availability
Military / Avionics / Automotive / Industrial
o
o
o
o
44
High I/O (>1000) BGAs with High Layer Count (18+) creates many
potential initiation sites
E.g., TRW, Rockwell Collins, etc.
Modestly dense designs
Uncontrolled environment
Long life requirements (10+ years)
Preventing CAF
Design
Rules
45
Supplier
Qualification
CAF – Critical Paths
46
Design Rules – Critical Paths
o
Some debate over ‘critical’ paths
o
PTH-to-PTH
o
o
o
o
Potential for greatest internal
damage (2X drilling)
Larger exposed surface area
20 to 30 mil spacings (12 mil drill
diameter minus 32 to 40 mil pitch)
PTH-to-Plane
o
o
47
Less potential for damage (1X drilling)
7 to 10 mil clearance
Proposed Trends in Conductor Pitch in PCBs
Determining the Limits of TelCordia Compliance for
Printed Wiring Boards by Karl Sauter
Circuitree, June 2000
Reduction in Pitch
100
Drilled Hole Size
Susceptibility of Glass-Reinforced Epoxy Laminates to
Conductive Anodic Filamentation by Chris Hunt
Circuitree; March 1, 2007
mils
80
Via
Edge
to
Via
Edge
48
60
40
When CAF?
20
0
1985
1990
1995
1999
2002
2004
2006
Current Industry Trends on Wall-to-Wall Spacing
o
Conservative Designs
o
o
o
Based on ball grid array (BGA) with 0.8mm (32mil) pitch
and a 0.3mm (12 mil) drill hole
Wall-to-wall spacing of 0.5mm (20mil)
Aggressive Designs
o
Down to 0.25mm (10 mil) wall-to-wall spacing
o
o
49
IPC Class 2 allows 100 microns of wicking
Try to avoid spacings less than knuckle-to-knuckle distance
on the glass weave
o
50
A revolutionary automated design analysis tool that
brings insight and prediction earlier than ever into the
product development process
Sherlock and CAF Avoidance
o
51
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
CAF Scoring
o
Scoring is based on combination of wall-to-wall spacing,
degree of overlap (orthogonality), and qualification protocols
o
o
Examples
o
o
o
52
Simple premise: The more aggressive the design, the more robust
the qualification method
Industry best practice (10) allows for 20mil spacing if each lot is
qualified
Industry minimum practice (5) allows for 20 mil spacing if no
qualification is performed
10 mil spacing is a marginal, but not high risk, design (4) if each
lot is qualified
o Product qualification (design/material combination) is not
sufficient
Identifying At-Risk Sites for CAF
53
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
CAF Scoring
54
Future Improvements
o
Electric field
o
o
Use CAF-resistant grade laminate
o
o
o
o
o
o
55
Leveraged through existing use of net list
Poorly defined at this time
Comparison of laminate properties (Tg, Td, T288, moisture
absorption) to assembly temperatures
Board thickness and stackup
Presence of non-plated through-holes
Parameters of CAF qualification test
Supplier capability (‘sweet spot’)
Qualifying Suppliers: Select a Test Board
o
IPC-9253 / IPC-9254 / PCQR2
o
o
Wall-to-wall ranges from 10 to 25mil
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
56
Test Boards (cont.)
o
o
57
Most company specs
require between 500 to
2000 initiation sites
IPC recommends 4200
initiation sites across
25 coupons
CAF Test Coupon (Margining)
58
Qualifying Suppliers: Select a Test Condition
o
Temperature / Humidity (sometimes with preconditioning)
o
o
o
o
o
65C / 88%RH
75C / 85%RH
50C / 80%RH
60C / 90%RH, 85C / 85%RH, etc.
Voltage
o
o
o
o
59
IPC TM-650, 2.6.25:
IPC-9151D (PCQR2):
IBM:
Others:
Not standardized
Debate about high voltage (50V / 100V) vs. low voltage
(5V / 10V / 15V) and if bias voltage should equal test voltage
IPC allows up to 100V (meets E-field limitation of 10V/mil)
DfR Recommendation: Highest voltage (at appropriate spacing)
and smallest spacing (at appropriate voltage)
Qualifying Suppliers: Define Failure
o
Driven by measurement approach
o
o
o
Failure definition
o
o
o
Resistance (100 megaohms)
Change in resistance (10X)
Define test time
o
o
60
Continuous monitoring (rare)
Periodic (24 to 72 hours)
Enterprise: 300 to 600 hours
Automotive: 500 to 2000 hours
Defining Time to Failure (IPC-9691)
61
CAF is an Infant Mortality Defect
Test Condition (85C / 85%RH)
62
CAF in the Future
o
Will the risk of CAF increase in future designs?
o
o
The rate of feature size reduction is very limited with PCBs
o
o
Industry roadmaps have barely moved (except for
substrates, which tend to rely on laser drilling and resincoated copper without glass fibers)
Pb-free transition is almost complete
o
63
Not really
Some movement to lower temperature alloys (e.g., SnBi)
PCB Industry Plated Through Hole Capability
95% of Industry
95% of Industry
5% of Industry
5% of Industry
1.2 – 1.6mm PCB Thickness
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
64
2.3 – 3.2mm PCB Thickness
IBM PCB-OS Symposium 2007, Roadmap Technology Verification,
Conductor Analysis Technologies (CAT)
o
Minimal technological
progress over past 10
years
Summary / Conclusion
o
Conductive Anodic Filament (CAF) formation does happen
o
o
CAF behavior is relatively stable
o
o
Limited change in key PCB technology (pitch, materials,
assembly)
CAF mitigation is well known (execute it!)
o
o
65
When it happens, it can cause a lot of pain
Evaluate your designs
Qualify your suppliers
Disclaimer & Confidentiality
o
o
o
ANALYSIS INFORMATION
This 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.
DISCLAIMER
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and implied, concerning the content of this report, including, but not limited to the existence of
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arising out of, connected with, or resulting from, the information presented within this report.
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the prior written authorization of DfR Solutions, is strictly prohibited.
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Best Regards,
Dr. Craig Hillman, CEO
66