All

Now that everybody has had their say on ENIG vs. electrolytic Ni-Pd-Au, I have a few comments. Having worked with both systems for several years, in the downhole industry, I know that either will work if some care and thought is used on assembly and fabrication techniques.

With the electroplated Ni-Pd-Au finish there is no problem maintaining thin gold electroplate 3-8 microinches, but due to the galvanic difference between gold and copper the 2mil line/spacing fabrication is not at all easy; however, the 6mil line/spacing is a normal process.  The Ni-Pd-Au electroplate is most often used as the etch resist, and special techniques must be used for very fine lines to decrease undercutting.

The electroplated Ni-Pd-Au board has an improved retention of solderability when high humidities and long term bakes are encountered during assembly.  The more porous ENIG gold is not much of a barrier to nickel oxidation when subjected to high temperatures and high humidities, and solderability drops off rapidly under these conditions.  The downhole industry is often based in humid areas and may keep the boards, mainly made from polyimide laminate, for several days in an oven at 125-150C while doing assemblies.  ENIG or just Cu with thin Au will not maintain solderability if subjected to this less than friendly environment.  The ENIG system requires protection before and during assembly especially if sequential solder operations are required, dry boxes instead of ovens. The pd (10-20) microinches thick  appears to act as barrier layer and maintains solderability of the gold surface.  Fresh boards from either system solder similarly. 

The electrolytic nickel plate in the holes strengthens them so barrel cracking is reduced, not as much so for ENIG.  Small holes (6-10 mils) are a problem getting thickness (100 microinches min.) of the nickel plate in electrolytic systems, but solderability is not often required in the very small holes and they are often filled for heat transfer.

Several reliable shops make the Ni-Pd-Au board on polyimide or other high-temp. substrates.  These shops also make boards with other finishes and often do ENIG and electrolytic in the same area for different customers.  Their identity is no secret. 

For this area of the industry I have tested the electroless gold over electroless nickel and over copper, but solderability maintenance was not adequate for the environment and cost was considerably greater.     

Black pad can be seen with ENIG, but a similar condition can occur with thicker Pd, and after aging at higher temperatures, the components fall.  Keep the Pd thckness under 25 microinches.

Phil Hinton
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