Implementing a Simple Corrosion Test Method for Early Detection of “Black Pad” Phenomenon In ENIG Plating

1. Implementing a Simple Corrosion Test Method for Early Detection of “Black Pad” Phenomenon In ENIG Plating BabHui Lee 11th Dec 2003 HKPCA-IPC Conference

3. Introduction What’s “Black Pad”? On bare board level… Electroless nickel pad surface appears “black” after stripping off immersion gold layer under visual inspection

4. Introduction 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)

5. Introduction What’s “Black Pad”? On assembly level… Brittle solder joint showing separation Pin lifted from SMT pad Solder joint failure Solder joint failure

6. Introduction 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

7. Introduction What’s “Black Pad”? Cross-Section/SEM Large regions of severe black pad with spikes protruding into nickel layer

8. Introduction Upon solder reflow, interconnects are wetted and solder joints appear in normal form What’s “Black Pad”? Cross-Section/SEM

9. Introduction Failure occurs after reflow, resulting in open joints What’s “Black Pad”? Cross-Section/SEM

10. Introduction After assembly, esp. on BGA…

12. Introduction 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”?

16. Test Results – MTO vs. HNO 3 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. 25 34 33 25 32 33 24 35 34 22 35 36 24 32 26 22 41 44 25 33 33 23 33 28 24 36 37 27 33 32 25 32 37 23 25 43 24 28 36 28 34 35 27 30 44 23 33 31 24 27 38 27 25 39 5.0 MTO 2.5 MTO 0 MTO

17. 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 P Ni MTO 2 1130.1 565.1 38.59 0.000 Error 51 746.7 14.6 Total 53 1876.8 Individual 95% CIs For Mean Based on Pooled StDev Level 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. HNO 3 Time

18. Test Results – Reliability Plots As expected, 5 MTO has the most hazardous function, and all 5 MTO samples are expected to fail within 30 sec.

19. 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 Plots

21. 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

22. SEM/EDX – 5 sec HNO 3 Dip 0 MTO, 8.79%P 2.5 MTO, 9.16%P 5 MTO, 8.37%P Clean nickel surfaces

23. SEM/EDX – 10 sec HNO 3 Dip 0 MTO, 7.82%P 2.5 MTO, 9.66%P 5 MTO, 8.05%P Signs of minor attack at 5 MTO

24. SEM/EDX – 15 sec HNO 3 Dip 0 MTO, 8.46%P 2.5 MTO, 9.81%P 5 MTO, 10.42%P Signs of slight attack at 5 MTO

25. SEM/EDX – 20 sec HNO 3 Dip 0 MTO, 7.87%P 2.5 MTO, 9.36%P 5 MTO, 10.36%P Signs of some attack at 5 MTO

26. SEM/EDX – 25 sec HNO 3 Dip 0 MTO, 8.28%P 2.5 MTO, 10.64%P 5 MTO, 8.59%P Signs of heavier attack at 5 MTO

27. SEM/EDX – 30 sec HNO 3 Dip 0 MTO, 10.38%P 2.5 MTO, 11.94%P 5 MTO, 10.20%P Signs of heavier attack at 5 MTO

28. Test Results - %P vs. MTO, HNO 3 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, HNO 3 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

29. 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. Test Results - %P vs. MTO, HNO 3

30. Test Results - %P vs. MTO, HNO 3

31. Test Results - %P vs. MTO, HNO 3

33. Recommendation 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.

34. Power and Sample Size One-way ANOVA Sigma = 3.826 Alpha = 0.05 Number of Levels = 3 Sample Target Actual Maximum SS 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 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).

35. Frequency 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.

36. Acceptance 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.