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Ni/Au Immersion

We are currently working on a set of problems regarding premature failures of electroless Ni/gold immersion pads and BGAs. We have seen a high O, C, with P and Ni that is black in color at the fractured sites. The bulk P content is approximately 5-6 atomic percent on bare boards but may slightly higher on the fracture surface, maybe 7 or 8. We have also seen, in a few instances, what appears to be (and for a lack of better terminology) a dual or two onion layer electroless Ni plating under the gold. The gold tends to be about 4 microinches and the Ni is > 200 microinches. I do know that ITRI and many others in the electronics industry are wrestling with this problem as we speak. What is the current understanding and information as to the cause of

The degraded solder joints that are made on some pads/BGAs after the Ni/Au platings? Second question is, what are the best techniques to quantitatively determine the porosity or void distribution/content of immersion gold coatings that are 3-6 microinches thick? Thanks for your consideration of my questions.

Harry Wachob
Exponent Failure Analysis Associates - Menlo Park, California
1998

I am not familiar with the BGA design. So my first question is, what is the substrate material for the Ni coating?

Mandar Sunthankar
- Fort Collins, Colorado
1998