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90 PCB007 MAGAZINE I FEBRUARY 2020 Article by Nikolaus Schubkegel A circuit board is made of copper. Usually, final finishes are applied after the solder mask process. In some cases, for special applica- tions, 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 pos- sible 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 me- chanically, increase the surface roughness and ensure good solder mask adhesion. Many tech- nical papers provide details on this topic. Yet, there is almost no literature available on the topic of surface preparation of ENIG or galvan- ic 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, mechani- cal 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 rec- ommended. 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. Ac- ids and oxidants can attack the less noble met- als beneath the gold with a negative impact on solderability and reliability. Even after gal- vanic gold plating with a higher thickness, the surface is porous. In addition, the edges of the tracks are generally not plated, allowing anoth- er point of entry for the oxidants. Nickel-gold Surface (ENIG, ENIIPIG, ENIPIG, Galvanic Ni-Au) PCBs with a nickel-gold finish can be pro- cessed directly over solder mask coating. But to do so, the rinse process following the nick- el-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. In this case, there is no need for pretreat- ment. Nevertheless, it is always recommended to perform a pretreatment prior to solder mask coating. In this case, developing as a pretreat- PCB Surface Preparation Before Solder Mask on Non-copper Finishes

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