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72 The PCB Magazine • March 2015 by Michael Carano OMG ELECTrOnIC ChEMICaLS trouble iN your taNk More Pesky Soldermask Problems: Plugged Via Leading to Skip Plating ColuMn Introduction Many factors are in play when it comes to preventing ink from remaining in vias. Two fac- tors are presented here, and include phototool quality, the concept of D-min and D-max and optimal set-up of the developing step. I am sure you have seen it, but may not have been sure of the root cause: solder not flowing in the vias and the lack of a plated solderable finish in the vias. Some of the vias have a final finish in them and others do not, and all of this happening on the same boards! A closer examination yields solder mask ink remaining in the vias, resulting in what is essen- tially a plugged or partially plugged via. Some of the ink may also be present on the on the pads surrounding the vias (Figure 1). This is not a pleasant situation and there is never an obvious fix, though you may be in- clined to think so. After all, there is ink in the holes and it is not supposed to be there! The key is to not leave it there in the first place. How- ever, it is not that simple. Getting to the Root Cause It would be easy to just start pointing fingers either at the soldermask operation or the metal- lic etch resist process. Clearly something is pre- venting the deposition of the solderable finish into the via and on the pads. There are several causes for the issue shown in Figure 1. As a troubleshooter, one must con- sider the following processes and properties: • Viscosity and solids content of the LPI ink • Excessive tack dry time and temperature • Over exposure and/or under developing • Phototool quality Viscosity and solids content of the ink can be an issue with respect to ink remaining in the holes (especially smaller diameter vias). When adjusting the viscosity of the ink, ensure that a sufficient amount of solvent is added to the ink. Under-adding of the solvent will increase the solids content and adversely affect the vis- cosity. If the material is too viscous, there will be excess ink in the vias, therefore increasing the difficulty of complete removal. Mask sup- pliers will provide a thinner to adjust the ink viscosity. Interestingly, there is a greater chance that holes can remain plugged with double-sid- ed screen printing as opposed to either curtain coating or spray coating of the ink. The principal of double-sided screen print- ing is embodied in the functional aspect of the screen printing equipment that allows for the coating of both sides of the printed circuit board simultaneously (Figure 2). The screens are fixed at the same distance from both sides of the board. There are also squeegees in the same figure 1: Green soldermask ink in via.

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