Undetected mask remnants can upset the final finish.
The arch-nemesis of final finish is solder mask. I am an expert in surface finishing. I understand the need for solder mask, but it is my right to dislike it. Truthfully, it makes my life more difficult, but those challenges help pay the bills. Solder mask brings out the ugliest in a surface finish, or so that is the perception. Research continues on how to build the widest operating window for a final finish, even when the problem falls outside the final finish line. I like to tell our development teams, “If research were easy, everyone would do it.”
Two main issues are associated with the solder mask process that later affect surface finishing: improper exposure and solder mask residues. Improper solder mask cure can affect the foot characteristics at the interface of the trace. If undercured, the solder mask will suffer chemical attack from the plating baths. This could result in one of two defects. Solder mask constituents leaching into the plating bath cause instability or plating-performance defects. Or, the chemical plating bath attacks the adhesion between the copper trace and solder mask. This creates a crevice for chemistry to become trapped. If overcured, solder mask can become brittle and fracture. Solder mask residues cause plating defects that may go undetected until the panel is post-assembly. Overall, improper solder mask application can wreak havoc on the final finish process.
The most infamous example of undercured solder mask effect on surface finishing was discovered in the early 1990s. Industry found a new post-assembly defect called “black pad.” Hot electroless nickel baths are the perfect environment to leach sulfur-bearing materials out of solder mask that has not properly cross-linked. This causes an increase of the “wrong” sulfur into the EN bath, which then co-deposits and alters the desired phosphorous content. The higher the sulfur in the EN deposit, the lower percent phosphorous included. Also, the type of sulfur included in the deposit can result in less corrosion resistance. This leaves a weakened nickel deposit that is more susceptible to corrosion in the electrolyte of the immersion gold bath. As the immersion gold bath corrodes the nickel, it leaves behind a phosphorous rich layer, which has poor solderability characteristics. The solder joints are weak, and the potential for components to pop off the PCB are greater. I still cannot resist saying, “Oh, there goes the BGA,” every time I drop my cellphone.
The second scenario observed when solder mask is undercured at the foot is a galvanic corrosion of the copper trace. This can be observed after immersion tin plating. All immersion tin baths contain thiourea; it is this material that drives the reaction. Without it, tin would not naturally plate on a copper surface. So the thiourea and acid in the immersion tin baths solubilize the areas of mask that are not cross-linked, leaving a negative foot. This crevice has a small geometry, which is difficult for solution exchange. Plating chemistry gets in and quickly depletes the metal, but then cannot get out for fresh replenishment of more metal. In essence, the solution trapped in the crevice has a high acid and thiourea content, which accelerates corrosion. As the uncured solder mask is removed from the foot and the copper trace corrodes, there is no longer an intimate connection between the mask and trace. This leaves a ledge of mask. Tape tests reveal solder mask adhesion issues around the pad edges at this interface. It is suggested to UV-bump panels prior to immersion tinning. This will help to cross-link any areas that did not go to completion in imaging. It should be noted that the darker the solder mask color, the harder they are to properly cure.
The most problematic defect coming out of imaging is residues on the pad surfaces. This is not cured solder mask; it is normally junk from the developer that redeposits on the board’s surface. The culprit is poor control or maintenance of the developer. Unfortunately, once the panels go through post-bake, these residues do cure and become extremely difficult to remove. The fun part is that solder mask residues are normally very difficult to see by eye, so they go unnoticed until after surface finish. Solder mask does not discriminate; it will prevent any non-HASL surface finish from coating properly. Depending on the degree of residues, it can go undetected even after the surface finish. But rest assured, it will cause a solderability problem.
So there are a few ways to address a solder mask residue issue. You can use a good cleaner designed to remove solder mask residues, but depending on the quality of the exposure, if the foot is soft, you will remove it and create the same crevice discussed above. Another option I really like is jet pumice. Unfortunately jet pumice gets a bad name, as it is associated with its cousin pumice (slurry) scrub. Pumice scrubbing utilizes a nylon brush that mechanically abrades the surface with the pumice slurry. The problem associated with this process is that the brushes can embed pumice into the copper. Once in, it’s not coming out. But jet pumice has no mechanical component; cleaning is driven solely by impingement, and it is a great tool for undermining solder mask residues on the copper surface.
As always, proper process control will lead to successful quality product. Don’t discriminate: There are multiple ways to create good product. Understand your options.