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Trouble in Your Tank: Optimizing the Soldermask Process, Part 2
July 25, 2013 |Estimated reading time: 1 minute
Introduction
Last month, Trouble in Your Tank presented the soldermask process in general terms. In this installment, critical process parameters are presented in more detail.
Soldermask Coating Thickness
The mask overall thickness is primarily determined by the actual wet weight of the ink as well as the solids content. As stated previously, excessively thick wet weights will present their own set of issues relating to proper solder paste printing. And low wet weights lead to potentially dangerous situations including ink skips over traces, flaking of the mask and the potential for oxygen to penetrate to the bare copper leading to more mask peeling and/or lifting.
While wet weights are important, there is another, perhaps less well understood principle known as thixotropy. Thixotropy is the property exhibited by certain gels of becoming fluid when stirred or shaken and returning to the semisolid state upon standing [1]. Essentially, soldermask ink tends to become more fluid when being screened or sprayed onto the surface of a circuit board, reacting under sheer conditions where the bonds are broken yielding a lower viscosity state. However, once the sheer is removed, the ink reverses to its prior state as the molecules reorient themselves and the viscosity of the ink increases. This prevents ink from running off the traces.
Editor's Note: This article originally appeared in the June 2013 issue of The PCB Magazine.