Monday, February 3, 2014

What's in a sliver?

When discussing my last post with a colleague, I realized that I explained why one would want to set a minimum solder mask, but not why one would need to set a maximum.

While the minimum is based on the PCB vendor's ability to properly register the solder mask when printing it, the maximum is all about the whether or not small-pitch components get solder mask between their pads or not.  In the screen captures below, the component on the right is the Altera MAX V package that has 0.4mm pitch pins.  With a 4 mil solder mask expansion there is no solder mask between the pins.  With a 2 mil expansion some mask is placed between the CPLD's pads.

4 mil solder expansion (bityExpress-B)

2 mil solder expansion
It is possible that the lack of solder mask between the CPLD's (U4's) pads is why I had a few solder bridges.


Kevin Kassner said...

Nice post. Is there a minimum feature width under which the PCB manufacturer will not print?

John Sloan said...

In theory, yes. Tiny features between pads are called 'solder mask slivers' and most PCB tools let you set a minimum rule. I dont know what OSH Park's minimum is but I may be able to measure some PCBs and get an idea.