A look at solder mask contamination on pads.

This month we look at solder mask contamination on pads. FIGURE 1 is a very, very bad example that never should have made it to the customer. Solder mask residues are visible on the surface and around the edge of the pad. There is also a level of solder mask undercutting.

FIGURE 1. Solder mask residues.

This would not be acceptable per any standard and would have shown complete nonwetting during soldering. This would have led to a bad day in the office for the assembly engineer or, more important, the shop floor staff.

We have presented live process defect clinics at exhibitions all over the world. Many of our Defect of the Month videos are available online at youtube.com/user/mrbobwillis. Find out how you can share our new series of Defect of the Month videos to explain some of the dos and don’ts with your customers via CIRCUITS ASSEMBLY: https://bit.ly/3mfunlF. 

BOB WILLIS is a process engineering consultant; bob@bobwillis.co.uk. His column appears monthly.

X-ray can’t catch all failures.

This month we we look at solder joint separation from pads with dye and pry testing, which is of course intended, and in FIGURE 1 shown as a perfectly good solder joint. (Well, until I covered it with dye and broke it, that is.)

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Get agreement on what constitutes “rework” – and a capable operator.

This month we look at etching defects and their removal – or presence, as in the case of FIGURE 1. A customer was surprised to find a batch of bare boards with this level of rework.

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Getting a close-up look at board quality.

A printed board microsection is one of the best ways to examine the quality of boards and any faults or failures.

The microsection (FIGURE 1) shows a plated through-hole that has been soldered with the nickel layer and through-hole copper visible. Normally, customers would accept the plating standards offered by the fabricator, or define their own, which may or may not impact the price. The nickel layer is part of the nickel/gold surface finish with the very thin gold of less than 1µm consumed during soldering and not visible. The remaining nickel is 5µm, and the copper is around 32µm. This is generous on many circuits board produced today and soldered very easily in production.

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