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A circuit board is made of copper. Usually, final finishes are applied after the solder mask process. In some cases, for special applications, the final finish may be applied before solder mask. In this case, we have solder mask on ENIG or galvanic nickel-gold. It is also possible to have tin or tin-lead under solder mask; this was an old technology that no longer plays a role today.
The methods for surface preparation of PCBs with copper are very well known. All these methods of pretreatment, chemically or mechanically, increase the surface roughness and ensure good solder mask adhesion. Many technical papers provide details on this topic. Yet, there is almost no literature available on the topic of surface preparation of ENIG or galvanic nickel-gold.
The main criterion for copper preparation is to achieve the desired level of surface roughness. But the gold surface is even, glossy, and very thin. I don’t recommend increasing the surface roughness of the gold, as the gold surface itself will be destroyed. As a consequence, mechanical methods—such as brushing, pumicing, or jet scrubbing—are not applicable to gold.
In addition, chemical microetches, such as mixtures of acids with oxidants, are not recommended. The gold surface generally resists these blends, but due to the fact that the gold thickness is just 0.03–0.07 µm after ENIG, the result is that the gold surface is porous. Acids and oxidants can attack the less noble metals beneath the gold with a negative impact on solderability and reliability. Even after galvanic gold plating with a higher thickness, the surface is porous. In addition, the edges of the tracks are generally not plated, allowing another point of entry for the oxidants.
Nickel-gold Surface (ENIG, ENIIPIG, ENIPIG, Galvanic Ni-Au)
PCBs with a nickel-gold finish can be processed directly over solder mask coating. But to do so, the rinse process following the nickel-gold plating step is good. The rinse water should not exceed 10 µS conductivity. The hold time between ENIG plating and solder mask coating must be held to less than one hour, as well.
To read this entire article, which appeared in the February 2020 issue of PCB007 Magazine, click here.