Over the past years, electroless nickel/immersion gold (ENIG) plating continues to be applied significantly in the electronics industry for its excellent planarity, solderability and wire bondability during multiple solder reflowing, wave soldering and/or aluminum wirebonding operations in the assembly process. However, inherently associated with the use of this particular surface finish of choice is the potential risk popularly known as the “black pad” phenomenon, which manifests itself as a dark gray to black trademark on the underlying nickel surface after a failed or open solder joint ensues. Through numerous studies and investigations of the “black pad” failures over the years in the industry [1-7], a clearer picture of this issue has come to light. It’s generally accepted that the phosphorus content in the EN deposits plays a critical role on its corrosion resistance and solderability properties. While a high phosphorus EN coating has good corrosion resistance, it can induce a solderjoint embrittlement with the growth of intermetallics through phosphorus enrichment during the ensuing soldering processes and other thermal excursions. On the other hand, too low phosphorus co-deposited in the EN coating has poor acid corrosion resistance, with more aggressive attack and greater amount of nickel dissolved in the subsequent acidic immersion gold bath, resulting in a phosphorus rich EN surface under the thin gold coating. During the assembly process, this thin layer of immersion gold dissolves quickly into the solder, exposing the poorly solderable EN surface, which results in dewetting and gray to black appearance on the pads, the typical symptoms of the “black pad” phenomenon.

There are a few test methods available to assess the corrosion resistance properties of the EN deposits. It is not an uncommon practice to routinely strip off the immersion gold layer (with the use of cyanide solution) on the pad features to discern the extent of nickel corrosion underneath by visual inspection or under SEM for more distinct classification, or through highly polished microsections under high magnification microscope/SEM to look for the signatory “tooth” marks propagating from the EN grain boundaries [1, 2, 5, 7]. The main drawback of these methods is that it is time-consuming and not all PCB shops have the luxury of owning a sophisticated SEM analytical instrument, or even a high-power stereomicroscope. Moreover, the samples are prepared after the full ENIG process, and we know from experiences and studies that the key process parameters affecting the corrosion properties are from the EN bath; immersion gold bath is typically relatively stable and easy to control, although we can not neglect its influence once out of control. Hence the focus of this article is to investigate a quick and simple method that can be implemented practically to assess the acid corrosion resistance property of the as-plated EN deposits, as an additional in-process monitoring tool for an early detection of possible “black pad” phenomenon. The common oxidizing agents (to simulate the immersion gold displacement reaction, which is essentially a corrosive attack on EN layer) are the strong, concentrated acids such as sulfuric acid (H₂SO₄), hydrochloric acid (HCl) and nitric acid (HNO₃). Among these, nitric acid corrosion resistance test has been reported in at least two sources, one is by quantification through the percentage weight loss of EN deposits [8], while the other by assessing whether the EN surface can survive the nitric acid (i.e., not turning “black”) through visual inspection within a stipulated dip time [9]. The test methodologies utilizing SO₂ (Kesternich Test) and HCl acid have also been outlined in Reference 9, but these are more involved and are not as appealing as the quick and simple nitric acid test by visualization of the “black nickel” appearance.

Experimental Set-Up

Figure 1. Experimental set-up.

In this experiment, a fresh half ounce copper clad (~0.2mm thick), 18”x24” in dimension, was processed through a freshly prepared EN bath (at ~zero nickel metal turnover, MTO) under production conditions along with production panels going through the ENIG process. After EN plating, the test panel was rinsed and dried, and strips of 20mm x 60mm nickel samples were cut and prepared out of it. 18 such strips were randomly selected and subjected to the nitric acid corrosion test in a lab-scale beaker environment (see the detailed set-up in Figure 1).

The time taken (in sec.) for the nickel strip to turn “black” spontaneously by visual observation was recorded using a stopwatch. The concentration of the nitric acid used was maintained at 40%v/v at ambient temperature (through prior rounds of trial and error to derive a reasonable and practical timeframe for the endpoint to be detected). Take note that the nitric acid solution (~60-ml) is only meant for a single use per nickel strip, and the strip should be submerged and remain stationary (no agitation) in the middle of the solution without contacting any of the sidewalls of the 100-ml glass beaker. The experiment is repeated for the same EN bath at 2.5 MTO (mid-life) and 5 MTO (end of bath life). Prior to the acid corrosion test, the nickel surface topography and % phosphorus content at each as-is condition were determined through SEM/EDX respectively, and repeated for every 5 sec. up to 30 sec. acid dip time. At subsequent different EN bath MTOs, the same measurements were taken.

Results

Figure 2. Boxplots of resistance time vs. Ni MTOs.

The times taken for the nickel strips to turn “black” at different Ni MTOs are summarized in the following boxplots (see Figure 2):

From the plots, there is a clear influence of Ni MTO on the time to failure (i.e., for the nickel strip sample to turn “black”), especially 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 and 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 EN bath life (just an attempted theorized explanation).

Figure 3. Weibull probability plots for 0~5 MTO.

On the other hand, the acid corrosion resistance behavioral characteristics of the EN bath at different MTOs can also be described from a reliability standpoint. A Weibull distribution and its associated probability functions are deployed to analyze and predict such reliability performance (see Figure 3).

Figure 4. Probability plot for 0 MTO~5 MTO.

As expected, 5 MTO has the most hazardous function, and all 5 MTO samples are expected to fail within 30 sec. This can be discerned easily from the characteristic shape of the associated Weibull probability distribution as depicted in Figure 4—the steeper the curve, the more hazardous is the probability function.

Figure 5. Descriptive statistics for %P content.

Based on the above probability plots, 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.

The %P contents at different Ni MTOs and nitric acid dip times are summarized in the descriptive statistics below (Figure 5):

From the descriptive statistics summary, the %P content averages around 9%, a typical value for a mid-range phosphorous EN bath used in the experiment.

Fig. 6a. As-plated, 0 MTO. Fig. 6b. As-plated, 2.5 MTO. Fig. 6c. As-plated, 5 MTO. Fig. 6d. 5 MTO, 5s HNO3. Fig. 6e. 0 MTO, 10 s HN03. Fig. 6f. 5 MTO, 10 s HN03. Fig. 6g. 5 MTO, 20 s HN03. Fig. 6h. 5 MTO, 30 s HN03.

Some of the SEM micrographs (2,500X) of the nickel topography at selective MTOs and HNO₃ dip times are illustrated in Figure 6.

From the SEM micrographs, it became conceivable that the nitric acid attack initiated at the nickel intergranular boundaries, followed by the dome-shaped nickel grain surfaces, which appeared darkened and more pronounced as the acid dip time progressively extended, as well as when the Ni MTO rose. This can also be correlated to the visual appearance and nitric acid resistance time observed from the nickel strips in the original experiment. Eventually this will lead to the “mud-crack,” fully flattened and corroded nickel topography as reported in the references [1, 2, 5, 7], the signatory “black pad” phenomenon.

A general linear model using Minitab statistical software was employed for the statistical analysis of the effects of Ni MTO and the nitric acid dip time on the variation of %P content, with one replicate stipulated.

Figure 7.

Statistically, the Ni MTOs and nitric acid dip time do not seem to affect %P content significantly in the EN deposits within the ranges under test (P-value > 0.05, a = 5%). However, caution should be exercised as residual analysis only indicated some reasonable fit.

Figure 8.

Nonetheless, from the main effects and interaction plots (Figures 7 and 8), 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 fathom by intuition as more severe nickel corrosion will leave behind more phosphorus within EN deposits)—this is especially pronounced at 2.5 Ni MTO.

Conclusion

At different nickel bath MTOs, 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 EN 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 and/or complexors, etc., which may all contribute to the observed phenomenon. This could be a subject for future study/research.

A quick and easy nitric acid test methodology for assessing the corrosion resistance nature of as-plated EN surface can be practically implemented as an additional monitoring and control item for early detection and prevention of “black pad” that has serious deteriorating impact and consequences on solderability and reliability performances.

From the standpoint of reliability performance, a minimum nitric acid resistance time of 20 sec. can be stipulated based on the highest Ni MTO at 5 (the normal useful life of the EN bath), and the acceptable nickel surface topography (degree of corrosion) and %P content. This will depict an average failure rate probability of at most 3% (from the reliability probability plot for the worst case scenario, i.e., at 5 Ni MTO).

A test frequency is recommended at once/week (towards the end of EN bath life) with a sample size of 10 nickel strips per test to ensure the EN bath is not behaving abnormally. For trouble-shooting purpose, whenever there is doubt cast on poor EN bath performance or suspected “black pad” issue, this test should be carried out immediately.

For a sample size of 10, the acceptance judgment is based on the fact that the minimum 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.

Acknowledgements

Advertisement

The author would like to extend special thanks to fellow colleagues: Choice Lee (for test sample preparation), John Ke and Bill Slough (for SEM micrographs and EDX analyses), Derris Chew and Jim Poon (for advice in statistics) and all others who helped in one way or another for making this test report possible.