1
How to Avoid Conductive Anodic Filaments (CAF)
Ling Zou & Chris Hunt
22 January 2013
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NPL Management Ltd - Commercial
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NPL EI Free Technology WebinarsBenefits of Using Nitrogen During Soldering and determining PPM LevelsThursday 14 February
Attraction of Sn Tin WhiskersWednesday 6 March
BGA Reliability - The Effects of Solder Joint Voiding and Uneven Stand-Off HeightTuesday 30 April
Reuse of Electronic ProductsWednesday 15 May
Through Hole Reliability for High Aspect via HolesTuesday 11 June
Functionalisiation and Applications of Nanomaterials for Electronic ApplicationsMonday 18 July
Environmental Robustness: Performance Coatings & High Temperature InterconnectsTuesday 3 September
www.npl.co.uk/ei
30ft
20ft
SIR Testing Microscope
Contamination Flux Test Kits
Aqueous Cleaner
Ionic Tester
AOI for Flux & Coatings
Ion Chromatography Ionic Tester
“IPC/NPL Cleaning & Contamination Testing Center Live” stand 1549
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AV & Seminars
http://www.ipcapexexpo.org/html/main/cleaning.htm
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How to Avoide Conductive Anodic Filaments (CAF)
Ling Zou & Chris Hunt
22 January 2013
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NPL Management Ltd - Commercial
Three main drive for CAF concern in past ten years
CAF failure mechanism was first reported in the 1970;s by Bell laboratory.
Three main drives: The drive to increase circuit density with smaller
printed wiring board geometries.
The rapid increase of the use of electronics in harsh environments and for high reliability and safety critical application.
Lead-free soldering process affect laminate stability and increase CAF material choices.
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Conductive Anode Filament (CAF)
CAF formation inside the PCB is an important failure mode for electronic circuit. It is a electrochemical process, and initially caused by anodic Cu corrosion.
CAF is Cu corrosion products grow along the glass/resin interface from anode to cathode
R
+-
NPL Management Ltd - Commercial
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Mechanisms
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Ionic species
Moisture
Bias
Susceptibility
Low pHAnode
corrosion
reflow pathway
• Electrochemical cell• Mobile corrosion
products• Migration down
pathway
• pH increase• Deposition of saltCAF
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Electrochemical process for CAF
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-Cu
+Cu
H+
M+n
M+nH+
C6H9O-HO-
MH2
V
HO-
Corrosion
of metal
Cu Cu+n + ne
Anode
2H2O O2 + H+ + 4e
Cathode
Cu+n + ne Cu
O2 + H2O + 4e 4OH -H+ + 2e H2
Cu+2 + 2OH - Cu (OH)2 Cu (OH)2 Cu O + H2O
CuO: Black CuCl2: Yellow brown CuSO4 5H2O : Blue
v
+-
The Cu salt or Cu (OH)2 can be deposited at high pH
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Conditions for CAF formation
Electrical charge carriers must be present to form Electrochemical cell Ionic species inside PCB, H+ and OH- from water
Water must be present to dissolve the ionic material and sustain them in their mobile ionic state Moisture, humidity
Acid environment around conductors is needed to initiate Cu corrosion at anode. Acid contaminations from glass fibre surface, fluxes from assembly
process or/and acid residues from plating process
Pathway is needed for Ions to move Delamination between glass fibre and resin due to reflow
Bias acts as driving force for ion transport Circuits need to be powered up in service
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Traditional CAF measurement
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Test PCBs at high temperature and humidity (85°C/85% RH) condition with bias (50V) for 1000 hours.
Monitor Insulation Resistance (IR) continually.
Take long time for the evaluation.
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Time to failure
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5
6
7
8
9
10
11
12
0 50 100 150 200 250 300Time (hour)
Log
SIR
(ohm
217 hours217 hours217 hours
Log
IR (
)
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Traditional CAF measurements
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300µm
In-line
Vertical
400µm
In-line
Vertical
550µm
In-line
Vertical
800µm
In-line
Vertical
400µm
In-line
Horizontal
400µm
Staggered
800µm
In-line
Horizontal
400µm geometry
No via
400µm
Anti-Pad
Inner Pads
Ground Anode
400µm
Anti-Pad
Inner Pads
Via Anode
400µm
Anti-Pad
No Pads
Ground Anode
400µm
Anti-Pad
No Pads
Via Anode
A
F
E
DB
C
H
G
L
K
J
I
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In-Line /staggered combs
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Via to via
In-Line Staggered
m 300 400 550 800
+ _
+
- Fibre Weave
+
-
+
-
Fibre Weave
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Anti-pad combs
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+-Comb I +-Comb I + -Comb J + -Comb J
+-Comb K +-Comb K +Comb L -+Comb L -
Cathode Via Anode Via In
ner
Lay
er P
ads
Inn
er L
ayer
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Parameters tested
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Factors affect CAF formation: Via wall gap
Bias
Pattern orientation
Pattern arrangement
Via polarity
Presence of Inner-layer pads
Surface finish
Drill speed
Reflow conditions
PCB materials
Board house
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Effect of tested variables on CAF formation-failure time
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Conclusions- how to avoide CAF
• Via to via gap• Fail faster with smaller gap
• Bias• High bias significantly decrease the time to fail
• Laminate• CAF resistance material reduce risk of CAF formation• Identically specified laminates from different suppliers have different CAF performance
• Manufacture• The in house process of laminate at board house resulting in different CAF performance
• Via and ground planes• Anodic via fail faster than cathodic via
• Alignment to reinforcement• Warp and weft direction• Via staggered at 45° has greater CAF resistance
• Reflow condition• High peak temperature in reflow is harmful to CAF performance of the PCB
• Board finish, drill speed, inner pads, Tg of laminate have a less significant effect on CAF performance
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Images courtesy of IsolaWicking, & inter yarn voiding
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Publication
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Lead-Free and Other Process Effects on Conductive Anodic Filamentation Resistance of Glass-Reinforced Epoxy Laminate” Ling Zou, Alan Brewin & Christopher Hunt, The ELFENT Book on Failure Mechanisms, Testing Method, and Quality Issues of Lead-Free Solder Interconnects, 2011 p255-271
Susceptibility of Glass- Reinforced Epoxy Laminates to Conductive Anodic Filamentation, Alan Brewin, Ling Zou & Christopher Hunt, MATC(A)155, January 2004
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Identifying CAF
Typically CAF occurs in buried layers inside PCBs
It is difficult to find and inspect, and hence it is difficult to study
Work at NPL has investigated a method for controlling the growth of CAF, and then subsequently studying the growth.
NPL has developed a sample structure where glass fibres are introduced, encapsulated, and then provide the correct electrochemical conditions to grow the CAF in relatively shot time.
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NPL Management Ltd - Commercial
Simulated Test Vehicle (STV)
Simulated Test Vehicle (STV) was developed, which provides a controlled way to grow CAF.
This enables us to investigate different variables separately.
CAF formation: Different resin systems and glass fibres: STV & STV with drilled
hole Reflow process: STV Desmear process: STV with drilling hole Glass bundle size: STV
CAF can be easily seen using microscope with backlight
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STV1 sample
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Polyimide substrate with 2 oz Cu
Resin powder dissolved in acetone
Resin cured at 150°C for 60 minute
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- +
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Test sample preparation
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Different variables
0.3mm
5mm
0.025ul
Acid condition at anode (plating solution CuSO4+H2SO4 )
Ionic contamination inside PCB (NaCl coated fibres) Pathway between two conductors (Reflow process not
necessary)
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Test different resins and glass fibres - STV
Anode contaminated with or without plating solution
Resin Glass fibres Suppler
DICY Cured*
Phenolic cured
1080 finished
7628 finished
A
7628 finished
7628 heat cleaned no finish
7628 loom state no finish
B
*DICY absent DICY cured resin 20m
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Phenolic cured resin - 7628 heat clean fibre
Anode: no contamination Fibres: clean CAF formed on reflowed sample
56789
10111213
0 30 60 90 120 150 180Time (hour)
Log
R (
ohm
)
12345678
56789
10111213
0 30 60 90 120 150 180Time (hour)
Log
R (
ohm
)
12345678
+- +-
Not reflowed Reflowed
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Phenolic cured resin - 1080 finished fibre
56789
10111213
0 30 60 90 120 150 180Time (hour)
Log
R (
ohm
)
12345678
56789
10111213
0 30 60 90 120 150 180Time (hour)
Log
R (
ohm
)
12345678
Anode: 100% plating solution
Fibres: clean
No CAF formed on both reflowed and no reflowed samples
NPL Management Ltd - Commercial
CAF formation with different resins and glass fibres
7628 fin ished (A ) xx
1080 fin ished (A )100% p la ting so lu tion
7628 loom s ta te
x
P heno l ic cu red
7628 hea t c leaned
7628 loom s ta te
x7628 hea t c leaned
N one
7628 loom s ta te
7628 hea t c leaned
7628 fin ished (B )
7628 fin ished (A )
7628 fin ished (B )
xx
D IC Y absen t D IC Y cu red
1080 fin ished (A )
100% p la ting so lu tion
re flow edN o t re flo w ed
C A F fo rm atio nR es inG lass fib resA n o d e co n tam in a tio n
7628 fin ished (A ) xx
1080 fin ished (A )100% p la ting so lu tion
7628 loom s ta te
x
P heno l ic cu red
7628 hea t c leaned
7628 loom s ta te
x7628 hea t c leaned
N one
7628 loom s ta te
7628 hea t c leaned
7628 fin ished (B )
7628 fin ished (A )
7628 fin ished (B )
xx
D IC Y absen t D IC Y cu red
1080 fin ished (A )
100% p la ting so lu tion
re flow edN o t re flo w ed
C A F fo rm atio nR es inG lass fib resA n o d e co n tam in a tio n
+- +-
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NPL Management Ltd - Commercial
Different resins and glass fibres
With anode contamination only: There is CAF formation for heat cleaned and loom
state fibres with both resins (DICY cured and phenolic cured).
There is no CAF formation for all finished fibres with both resins.
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Finished glass fibre - DICY resin
Anode contamination
Fibre coated in NaCl
Reflow & no reflow
Anode Fibre Reflow 100% plating solution Yes 100% plating solution No 20% plating solution Yes 20% plating solution
3% NaCl No
100% plating solution Yes 100% plating solution No 20% plating solution Yes 20% plating solution
1% NaCl No
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Conclusion for finished fibres
Three factors must be met for CAF formation Low pH at anode (Plating solution contamination)
Ionic contamination inside PCB (NaCl coated fibres)
Pathway between two conductors (Reflow process)
Anode Fibre Reflow CAF formation 100% plating solution Yes Yes 100% plating solution No No 20% plating solution Yes Yes 20% plating solution
3% NaCl No No
100% plating solution Yes No 100% plating solution No No 20% plating solution Yes No 20% plating solution
1% NaCl No No
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CAF formation - different glass fibre bundle size
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Phenolic cured resin
Anode: no contamination
Fibres: Clean heat and loom state glass fibres
Bundle size: small (~10) & large (30~50)
Reflowed
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Phenolic cured resin -Heat cleaned fibre loom state fibre
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56789
10111213
0 30 60 90 120 150 180Time (hour)
Log
R (
oh
m)
12345678
Small bundle
56789
10111213
0 30 60 90 120 150 180Time (hour)
Log
R (
oh
m)
12345678
Large bundle
56789
10111213
0 30 60 90 120 150 180Time (hour)
Log
R (
ohm
) 12345678
Small bundle
56789
10111213
0 30 60 90 120 150 180Time (hour)
Log
R (
ohm
) 910111213141516
Large bundle
NPL Management Ltd - Commercial
CAF formation
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+-
+-
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- +
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NPL Management Ltd - Commercial
STV with drilled hole
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100% 30% 10%
+-+
Tap
Polyimide
Cu
-
STV is sitting on adhesive tape.
Hole filled with 1µl different concentration plating solutions.
Filling solution dried for 2 hours at room temperature before CAF testing.
NPL Management Ltd - Commercial
CAF formation – desmear process
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DesmearedNo-desmear
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NPL Management Ltd - Commercial
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CAF formation without desmear- Phenolic cured resin & 7628 finished glass fibre (B)
56789
10111213
0 30 60 90 120 150 180Time (hour)
Log
R (
ohm
) 12345
100% plating solution
56789
10111213
0 30 60 90 120 150 180Time (hour)
Log
R (
ohm
) 67891011
30% plating solution
56789
10111213
0 30 60 90 120 150 180Time (hour)
Log
R (
ohm
) 1213141516
10% plating solution
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More CAF formed with contamination increase
NPL Management Ltd - Commercial
Desmear process increase CAF formation, but not significant.
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56789
10111213
0 30 60 90 120 150 180Time (hour)
Log
R (
ohm
) 12345
100% plating solution
56789
10111213
0 30 60 90 120 150 180Time (hour)
Log
R (
ohm
) 67891011
30% plating solution
56789
10111213
0 30 60 90 120 150 180Time (hour)
Log
R (
ohm
) 1213141516
10% plating solution
CAF formation with desmear-Phenolic cured resin & 7628 finished glass fibre
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NPL Management Ltd - Commercial
CAF formation on drilling sample
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+-
++
+-
-
-+
-
NPL Management Ltd - Commercial
CAF formation
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+-
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NPL Management Ltd - Commercial
STV1 and STV1 with drilled hole
A STV has been successfully used to evaluate the effect of different resin systems, different glass fibres, desmearprocess, reflow process and glass fibre bundle size on CAF failure. Heat cleaned and loom state fibres form CAF more easily than
finished fibres. Loom state fibre has the highest propensity to form CAF compared with others.
Phenolic cured resin promote more CAF than DICY cured resin, this is probably because the DICY was removed when the resin was dissolved in acetone in our sample preparation.
Desmear process can increase CAF formation, but not significant.
Reflow process increase CAF formation significantly. Large bundle size increase CAF formation, but not significant.
Further developments of a STV are underway
41
How to Avoide Conductive Anodic Filaments (CAF)
Ling Zou & Chris Hunt
22 January 2013
42
22
Stand 1549 PresentationsTuesday 12.00noon “Cleaning and Contamination Failures – Causes and Cures”
2.00pm “Cleaning Under Low Stand Off Components”3.00pm “Testing Printed Board Assemblies For Cleanliness”
Wednesday 12.00noon “Evaluating a Cleaning Process Performance”2.00pm “Reliability of Flux Residues Under Low Stand Off Devices”3.00pm “Cleaning Package On Package Assemblies”
Thursday 11.00am “Conductive Anodic Filament CAF Failures and Defect Analysis”12.00 noon “Testing Adhesion of Conformal Coatings and Failures”1.00pm “Testing Components for Cleaning Process Compatibility”
Each presentation will be 15-20mins
A copy of the presentation material can be obtained by visiting the stand
http://www.ipcapexexpo.org/html/main/cleaning.htm
NPL EI Free Technology WebinarsBenefits of Using Nitrogen During Soldering and determining PPM LevelsThursday 14 February
Attraction of Sn Tin WhiskersWednesday 6 March
BGA Reliability - The Effects of Solder Joint Voiding and Uneven Stand-Off HeightTuesday 30 April
Reuse of Electronic ProductsWednesday 15 May
Through Hole Reliability for High Aspect via HolesTuesday 11 June
Functionalisiation and Applications of Nanomaterials for Electronic ApplicationsMonday 18 July
Environmental Robustness: Performance Coatings & High Temperature InterconnectsTuesday 3 September
www.npl.co.uk/ei