by Rudy Sedlak
In all plating baths there are at least two competing reactions to the principal plating reaction. The two major (desired) reactions that occur are:
Anode reaction: M - e- ---> M++ (Metal Anode) (electron) (Metal ion in solution) Cathode reaction: M++ + e- -----> M (Metal ion) (electron) (Plated deposit)
The water itself competes with the metal being plated in both reactions:
Anode reaction: H2O - e- ----> O2(gas) + H+ Cathode reaction: H2O + e- ----> H2(gas) + OH-
Thus, Hydrogen gas (H2) can form on the cathode (work being plated) if the conditions are right. If Hydrogen gas forms on the work being plated, under the photoresist, it will cause the photoresist to lift. The conditions required to get Hydrogen gas to form are typically: some combination of high current density and/or low metal content. The "low metal" can be either actual low metal content, or apparent low metal caused by the pH being too high.
In a cyanide bath, the cyanide complexes ("chelates") the metal in solution, thus rendering it less available, effectively lowering the metal concentration. The lower the pH, the more of the cyanide that is present in the Hydrogen Cyanide forms:
CN- + H+ ---> HCN cyanide ion Hydrogen Cyanide
The more of the cyanide that is in the HCN form, the less available to complex the metal, and thus the more "available" the metal is for plating, or effectively the higher the metal concentration. Thus the lower the pH, the greater the current density that can be used without forming Hydrogen gas under the resist. And conversely, the higher the pH (in a cyanide bath), the greater the tendency for Hydrogen gas formation.
None of the above is meant to advise going out of the suppliers recommendations on pH, or any other parameter. Rather, it is meant to stress the interdependence of all the variables in the plating bath/process, and the need to monitor every variable, especially in plating printed circuit boards.
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