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Greetings! In today's column I will be talking about various surface finishes and solder mask issues that can arise due to those surface finishes.

In today's RoHS environment, immersion silver and gold replaces tin-lead. What does this mean to your mask artwork files? Interestingly, when tin-lead finishes were the norm, most solder mask clearances associated with surface mount devices were "gang relieved," which is to say there were no webs of solder mask material between the mounts. This tended to create situations where solder wicking occurred, shorting mounts together.

Today, not only do we have the capability to deal with solder mask webs between surface mounts as little as 0.004 to prevent wicking, the wicking itself no longer occurs with the alternative surface finishes of today, such as immersion gold and immersion silver, or even the lead-free solders.

That's the good news.

The bad news, if there really is any at all, would be that certain solder mask configurations are better suited to the alternative surface finishes of today. Today's fabricators sometimes have to come up with creative process alternatives for solder mask configurations versus surface finishes such as electroless nickel immersion gold (ENIG) and immersion silver.

Let me give you an example: Some customers will provide a solder mask clearance for a given feature on one side and no clearance on the other (say, for a connection/probe point or via-in-pad situation). If left as is, the solder mask feature that is tented or covered on one side creates a cup that does not allow solutions through the barrel of the hole during the ENIG process, trapping the solutions and potentially resulting in blackened or oxidized holes.   

This problem can be approached a number of different ways at the fabrication level, and you may be contacted by your fabricator to consult about acceptable process deviation such as:

1. Add a clearance 1:1 or slightly smaller than the drilled hole size.This allows solution for the ENIG process to flow through the vias and avoid blackened or oxidized holes.
2. Perform a secondary via plugging process from the side with the 1:1 clearances. This accomplishes the customer's original objective of having one side covered with mask and the other side exposed. In certain via-in-pad situations where the vias themselves are in the mount pad and clearances exist for the mount, but you want the via within plugged, fabricators can offer a process (often outsourced) for plugging the vias. This involves using either an epoxy or a conductive fill (if conductive vias are the goal) in which the vias are first plated normally and then outsourced, where they are filled with either epoxy or conductive material and planarized--made flat, so there are no dimples or protrusions on the surface. Upon return to the fabricator, they are cuposited with electroless copper and flash plated using a minimal amount of electroplated copper. Then, normal processing is continued up to mask and the mask configuration is of no consequence; the via-in-pad structures are now plated and made flat so the via itself no longer presents any issues.