hkpca_ipc 2003_bhlee

Implementing a Simple Implementing a Simple Corrosion Test Method for Corrosion Test Method for

Early Detection of “Black Pad” Early Detection of “Black Pad” Phenomenon In ENIG PlatingPhenomenon In ENIG Plating

BabHui Lee11th Dec 2003

HKPCA-IPC Conference

2BabHui Lee HKPCA-IPC 2003

IntroductionIntroduction

• Forms during ENIG plating, but manifests itself at the board assembly stage• Brittle, interfacial solder joint failure• Selective and occurs at low ppm level• Can not be predicted or detected before assembly, causing catastrophic failures

What’s “Black Pad”?



What’s “Black Pad”? On bare board level…

Electroless nickel pad surface appears “black” after stripping off immersion gold layer under visual inspection



What’s “Black Pad”? On bare board level…

Electroless nickel pad surface exhibiting “mud-crack” signature after stripping off immersion gold layer under SEM, top scan (6,000X)



What’s “Black Pad”? On assembly level…

Solder jointfailure

Solder jointfailure

Brittle solder joint showing separation Pin lifted from SMT pad


What’s “Black Pad”? On assembly level…


Pin side (SEM 6,000X) Pad side (SEM 6,000X)

Nickel-like nodular interfaces with “mud cracks” are exposed



What’s “Black Pad”? Cross-Section/SEM

Large regions of severe black pad with spikes protruding into nickel layer



Upon solder reflow, interconnects are wetted and solder joints appear in normal form




Failure occurs after reflow, resulting in open joints




After assembly, esp. on BGA…



The root cause is not completely understood, but the following conclusions can be made:

• The Black Pad defect is the result of an interaction between process control and other factors, such as circuit board design (ITRI)

• It results in corrosion of the Ni layer during accelerated Au deposition in the immersion process (N. Biunno, ITRI)

• Ni corrosion occurs under galvanic cell conditions (N. Biunno, P. Snugovsky, K. Johal)

What causes “Black Pad”?



The Ni layer is more susceptible to corrosion if • It is thin, < 120 µ”• P < 6 wt %• High level of micro- structure defects such as nodule layer grain boundaries

What causes “Black Pad”?


BackgroundBackground

Not all PCB shops have the luxury of SEM/EDX available

for process control & troubleshooting

• Often, this is an after-the-fact, ie. when “black pad” occurs,

it’s already too late to take preventive actions

• Cross-section preparation is both time-consuming & costly

• Cyanide-stripping of Au layer can post health hazard if not

performed under controlled environment

Constraints on current common “Black Pad” detecting methods:


ObjectiveObjective

To devise & implement an optimum acid corrosion

test methodology for early detection of “black pad”

phenomenon prevalent in the PCB industry pertaining

to the use of ENIG. The preferred method should

have the following characteristics:

Simple & yet sensitive, a quick “Acid-Test”

Repeatable (least variation & reliance on operator)

Predictable & with strong correlation to failure mode

Serves as an in-process control checkpoint


Test MethodologyTest MethodologyNickel Strips

20mm x 60mm, 0.2mm thick

Cut from 18”x24” EN-plated dummy

panel per Ni bath MTO

Ni Bath MTO: 0~5

Ni thickness: 3~4 µm

Nitric Acid

Conc: ~40% (v/v)

Volume: ~60 ml in 100-ml

glass beaker

Temperature: 24~25oC

Single use per Ni strip

Test Procedure

18 strips per MTO were used

Each strip was immersed in the beaker of nitric acid as illustrated

Time (sec) for the strip to turn “black” was recorded using a stopwatch

Note: The hold time for freshly prepared Ni strips should not exceed 12-hour before testing

20 ml

40 ml

60 ml

80 ml

100 ml

2.0 cm

6.0 cm

2.0 cm


5 M

TO

2.5

MT

O

0 M

TO

45

40

35

30

25

20

Ni MTO

Tim

e (s

ec)

Boxplots of Time (sec) by Ni MTO

(means are indicated by solid circles)

0 MTO 2.5 MTO 5.0 MTO

39 25 27

38 27 24

31 33 23

44 30 27

35 34 28

36 28 24

43 25 23

37 32 25

32 33 27

37 36 24

28 33 23

33 33 25

44 41 22

26 32 24

36 35 22

34 35 24

33 32 25

33 34 25

Test Results – MTO vs. HNOTest Results – MTO vs. HNO33 Time Time

From the box plots, there is a clear influence of Ni MTO on the time to fail (ie. for the nickel strip sample to turn “black”), esp. at 5 MTO – this is affirmed with one-way ANOVA analysis with Tukey’s pairwise comparisons.


Statistically, 5 MTO samples have lower mean time to failure than 0 & 2.5 MTOs (around 10 sec less). The general trend is that the acid corrosion resistance of the as-plated EN surface reduces as Ni MTO rises. Repeatability is also better at 5 MTO from the obvious lower standard deviation attained. This may be attributed to a more steady state reached towards the end of the useful electroless nickel bath life (just an attempted theorized explanation).

One-way ANOVA: Time (sec) versus Ni MTO

Analysis of Variance for Time (sec)Source DF SS MS F PNi MTO 2 1130.1 565.1 38.59 0.000Error 51 746.7 14.6Total 53 1876.8 Individual 95% CIs For Mean Based on Pooled StDevLevel N Mean StDev ----+---------+---------+--------0 MTO 18 35.500 4.997 (----*---) 2.5 MTO 18 32.111 3.984 (---*----) 5 MTO 18 24.556 1.756 (---*----) ----+---------+---------+---------+---------+Pooled StDev = 3.826 24.0 28.0 32.0 36.0

Test Results – MTO vs. HNOTest Results – MTO vs. HNO33 Time Time


Test Results – Reliability PlotsTest Results – Reliability Plots

Shape

7.896

8.897

14.614

Scale

37.638

33.809

25.379

AD*

1.040

1.171

1.393

F/C

18/0

18/0

18/0

0 MTO2.5 MTO

5 MTO

5 MTO

2.5 MTO0 MTO

20 30 40

1

5

10

20

304050607080909599

Weibull Probability

Pe

rce

nt

20 30 40

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Survival Function

Pro

ba

bil

ity

20 30 40

0

1

2

Hazard FunctionR

ate

20 30 40

0.0

0.1

0.2

Probability Density Function

Overview Plot for 0 MTO-5 MTOML Estimates - Complete Data

As expected, 5 MTO has the most hazardous function, and all 5 MTO samples are expected to fail within 30 sec.


0 MTO

2.5 MTO

5 MTO

20 30 40 50

1

2

3

5

10

20

30

40 50 60 70 80

90 95

99

Time to Failure

Per

cent

Probability Plot for 0 MTO-5 MTOWeibull Distribution - ML Estimates - 95.0% CI

Complete Data

Shape Scale AD* F/C

7.8959 37.638 1.04 18/0

8.8969 33.809 1.17 18/0

14.614 25.379 1.39 18/0

More than 90% of the samples (across the full Ni MTO range) should be able to survive the 40% nitric acid test for at least 20 sec.

Test Results – Reliability PlotsTest Results – Reliability Plots


SEM Scan/EDX Analysis (%P)SEM Scan/EDX Analysis (%P)

For each Ni MTO (0, 2.5 & 5) and at an interval of 5 sec (up to 30 sec) of 40% nitric acid dip test, the following were obtained:

These shall be used (in parallel with the predicted reliability data) as a basis for the acceptance level of the degree of nickel corrosion subjected to nitric acid test, and as a quick indication of the corrosion resistance of the as-plated electroless nickel surface under normal production conditions.

SEM Scan at 2,500X – Surface topography depicting Ni grain boundary structure and any sign of nickel attack / corrosion.

EDX for surfcae %P content to show any correlation with Ni bath MTO and resulting attack from nitric acid dip test.


SEM/EDX – As Is (Before Test)SEM/EDX – As Is (Before Test)

0 MTO, 9.41%P

2.5 MTO, 7.81%P

5 MTO, 9.46%P

Clean nickel surfaces


SEM/EDX – 5 sec HNOSEM/EDX – 5 sec HNO33 Dip Dip

0 MTO, 8.79%P

2.5 MTO, 9.16%P

5 MTO, 8.37%P

Clean nickel surfaces



0 MTO, 7.82%P

2.5 MTO, 9.66%P

5 MTO, 8.05%P

Signs of minor attack at 5 MTO



0 MTO, 8.46%P

2.5 MTO, 9.81%P

5 MTO, 10.42%P

Signs of slight attack at 5 MTO



0 MTO, 7.87%P

2.5 MTO, 9.36%P

5 MTO, 10.36%P

Signs of some attack at 5 MTO



0 MTO, 8.28%P

2.5 MTO, 10.64%P

5 MTO, 8.59%P

Signs of heavier attack at 5 MTO



0 MTO, 10.38%P 2.5 MTO, 11.94%P

5 MTO, 10.20%P

Signs of heavier attack at 5 MTO


Test Results - %P vs. MTO, HNOTest Results - %P vs. MTO, HNO33

Statistically, the Ni MTOs & nitric acid dip time do not seem to affect %P content significantly in the Ni deposits within the ranges under test. However, caution should be exercised as residual analysis only indicates some reasonable fit.

General Linear Model: %P versus Ni MTO, HNO3 Time

Factor Type Levels Values

Ni MTO fixed 3 0.0 2.5 5.0

HNO3 Tim fixed 7 0 5 10 15 20 25 30

Analysis of Variance for %P, using Adjusted SS for Tests

Source DF Seq SS Adj SS Adj MS F P

Ni MTO 2 3.9341 3.9341 1.9670 2.26 0.146

HNO3 Time 6 10.5961 10.5961 1.7660 2.03 0.139

Error 12 10.4230 10.4230 0.8686

Total 20 24.9531


HNO3 TimeNi MTO

10.5

10.0

9.5

9.0

8.5

%P

Main Effects Plot - Data Means for %P

Nonetheless, from the main effects & interaction plots, it’s discernible that prolonged nitric acid at 30 sec tends to induce phosphorus enrichment at the attacked nickel surface (which is not difficult to predict by intuition) – this is especially pronounced at 2.5 Ni MTO.




12

10

8

12

10

8

Ni MTO

HNO3 Time30

25

20

15

10

5

0

5.0

2.5

0.0

30

25

20

15

10

5

0

5.0

2.5

0.0

Interaction Plot - Data Means for %P



12111098

95% Confidence Interval for Mu

10.09.59.08.5

95% Confidence Interval for Median

Variable: %P

8.4306

0.8546

8.7696

Maximum3rd QuartileMedian1st QuartileMinimum

NKurtosisSkewnessVarianceStDevMean

P-Value:A-Squared:

9.9373

1.6130

9.7865

11.940010.2800 9.3600 8.3250 7.8100

21-1.8E-01

0.5090381.247661.116999.27810

0.4530.344

95% Confidence Interval for Median

95% Confidence Interval for Sigma

95% Confidence Interval for Mu

Anderson-Darling Normality Test

Descriptive Statistics


Conclusion & SummaryConclusion & Summary

At different nickel bath MTOs (Metal Turnover), the extent of nickel corrosion varies as

the exposure time to nitric acid increases – this is especially pronounced towards the end of

bath life at 5 MTO, where the attack is more severe and the time to failure (complete attack,

manifested by “blackened” surface almost spontaneously) is significantly lower than fresher

EN baths.

As contrary to the general belief, the %P variation of the nickel deposits as the nickel bath

ages is not significant, and %P alone cannot explain why the nickel surface is more

susceptible to attack as the MTO rises. There could be some interaction effects from the

bath contaminants build-up and the balance of other organic/inorganic additives, stabilizers

&/or complexors, etc. which may all contribute to the observed phenomenon. This could be

subject for future study/research.

A quick & easy nitric acid test methodology for assessing the corrosion resistance nature

of as-plated electroless nickel surface can be practically implemented as an additional

monitoring & control item for early detection & prevention of “black pad” that has serious

deteriorating impact and consequences on solderability & reliability performances.


RecommendationRecommendation

From the standpoint of reliability performance, a minimum

nitric acid (40% v/v) withstanding timing of 20 sec can be

stipulated based on the highest Ni MTO at 5 (the normal

useful life of the electroless nickel bath), and the acceptable

nickel surface topography (degree of corrosion) & %P content

after 20 sec exposure to the nitric acid attack. This will depict

an average failure rate probability of at most 3% (from the

reliability probability plot for the worst case scenario, ie. at 5

Ni MTO).

Sample size determination, frequency of test and the

acceptance judgement are proposed on the following slides.


Power and Sample Size

One-way ANOVA

Sigma = 3.826 Alpha = 0.05 Number of Levels = 3

Sample Target Actual MaximumSS Means Size Power Power Difference 12.5 13 0.8000 0.8235 5 12.5 14 0.8500 0.8545 5 12.5 16 0.9000 0.9027 5 18.0 9 0.8000 0.8046 6 18.0 10 0.8500 0.8519 6 18.0 12 0.9000 0.9175 6 24.5 7 0.8000 0.8101 7 24.5 8 0.8500 0.8704 7 24.5 9 0.9000 0.9133 7 32.0 6 0.8000 0.8407 8 32.0 7 0.8500 0.9051 8 32.0 7 0.9000 0.9051 8 40.5 5 0.8000 0.8403 9 40.5 6 0.8500 0.9178 9 40.5 6 0.9000 0.9178 9 50.0 5 0.8000 0.9103 10 50.0 5 0.8500 0.9103 10 50.0 5 0.9000 0.9103 10

Sample Size DeterminationSample Size Determination

A test of power was conducted based on the original test results, and it’s gathered that a sample size of 10 should suffice for a power target of 85% with detectability of maximum difference of 6 sec (which makes practical sense too, considering the variation within the samples). Hence it’s recommended that the sample size to be consisted of 10 freshly prepared Ni strips per test (in accordance to the test methodology as outlined in the beginning).


Frequency of TestFrequency of Test

A test frequency is proposed at once/week for a start – this also serves to supplement the current SEM/EDX analyses that have been put in place on a monthly basis. Based on the past historical data collected over a prolonged period of time, our current tight process control on ENIG bath does not warrant a more stringent & frequent check of this additional control item to be implemented soon. As the sample is easy to prepare, and the test itself is quick and simple, once/week test frequency will not incur too much burden on the current workload. It’s also suggested that Chem Lab shall schedule & conduct this test weekly. Of course, for troubleshooting purpose, whenever there is doubt cast on poor Ni bath performance or suspected “black pad” issue, this test should be carried out immediately.


Acceptance CriteriaAcceptance Criteria

As mentioned earlier, a minimum of 20 sec nitric acid resistance

time is proposed based on the test results and the associated

reliability probability plots. The nitric acid resistance time is defined

as the time taken for the nickel strip sample exposed to the 40%

nitric acid to turn “black” completely under the stipulated test

conditions. For a sample size of 10, the acceptance judgement is

based on the fact that the min. average acid resistance of the 10

samples shall exceed 20 sec, with at most one strip below 20 sec,

correlating to a failure rate of 10% over the full range of the nickel

bath conditions over its useful bath life – we can also infer from the

probability plots that more than 90% of the samples (across the full

Ni MTO range) should be able to survive the 40% nitric acid test for

at least 20 sec.


AcknowledgementAcknowledgement

Special thanks & acknowledgement are credited to the following persons:

1. Choice Lee (B5 PE) for preparing the samples & conducting the tests, after numerous excruciating trials & errors.

2. John Ke & Bill Slough (AMD Lab) for churning out all the SEM images & EDX analyses under extreme time constraints.

3. Derris Chew (B3 QA) for some advice in statistical analysis.

4. Jim Poon (Corporate VP,QA) for propounding & supporting this initiative.

5. All others who help in one way or another.

hkpca_ipc 2003_bhlee

Business

introduction whats black

black pad defect

ni bath mto ni bath

ni mto range

current common black

ni layer

open joints whats black

mto samples