How to Avoid Conductive Anodic Filaments (CAF) · PDF fileHow to Avoid Conductive Anodic...

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How to Avoid Conductive Anodic Filaments (CAF)

Ling Zou & Chris Hunt

22 January 2013

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SIR Testing Microscope

Contamination Flux Test Kits

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Ion Chromatography Ionic Tester

“IPC/NPL Cleaning & Contamination Testing Center Live” stand 1549

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How to Avoide Conductive Anodic Filaments (CAF)


22 January 2013

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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

+-


8

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


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.


Time to failure

12

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 (

)

7


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


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

15

+-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


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


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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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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+-

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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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Test sample preparation


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


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


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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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.


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


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


CAF formation

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+-

+-

+-

- +

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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.


CAF formation – desmear process

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DesmearedNo-desmear

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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


56789

10111213

0 30 60 90 120 150 180Time (hour)

Log

R (

ohm

) 1213141516


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More CAF formed with contamination increase


Desmear process increase CAF formation, but not significant.

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+-

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56789

10111213

0 30 60 90 120 150 180Time (hour)

Log

R (

ohm

) 12345


56789

10111213

0 30 60 90 120 150 180Time (hour)

Log

R (

ohm

) 67891011


56789

10111213

0 30 60 90 120 150 180Time (hour)

Log

R (

ohm

) 1213141516


CAF formation with desmear-Phenolic cured resin & 7628 finished glass fibre

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CAF formation on drilling sample

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+-

++

+-

-

-+

-


CAF formation

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+-

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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

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How to Avoide Conductive Anodic Filaments (CAF)


22 January 2013

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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

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