I have been doing quality control analysis on immersion gold electroless
nickel through-hole platings (gold on nickel on copper) on a PCB
recieved from a supplier. After solderability test with wave soldering
and reflow soldering, we analyized a cross section of the SMT pads and
PTH and discovered a row of holes or bubbles in the surface barrier
between the nickel and the solder. We didn't find any holes or bubbles
in the mass of the solder, just at the barrier. We send part of PCB to
Auger Electronic Spectroscopy (AES) and found the contamination of
Nickel Oxide (NiO2) in the gold surface along with copper nickel and
aluminum. We measured the percentage of phosphorous in the nickel
coating, and the result was 11.1 atomic percentage.

Our question is where the aluminum contamination may be coming from,
what is the impact on reliability of the phosphorous percentage, how to
prevent oxidation and bubbles in the solder barrier? Finally, is it
recommended/reliable to use immersion gold process with BGA? Any ponters
to papers on the subject will be appreciated.

Avshalom Rozenthal
IAI
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