Soldermask and Tombstoning

Tombstoning isn't too much of a problem for us or most of our customers theses days, but it still does happen for a variety of reasons. For example:

• Vias in one smt pad can cause tombstoning or other similar problems
• Uneven pad surfaces or different pad sizes with very small parts

The most common cause these days seems to be solder mask related.

We have found that sometimes, if the solder mask is too thick and rises up above the level of the solder paste deposits, very small parts (0201 and 0402) are more susceptible to tombstoning.

In example A, the solder mask is thin enough so the part can rest on the past and will reflow properly. In example B, the part may rest up on the solder mask and not fully contact the solder paste on one or the other side. This can cause an open or lead to tombstoning - especially if the mask has bumps or variations in the thickness.

Good mask registration will help. Using a board vendor that has tight tolerances and delivers a thin flat mask surface will help. In some cases, with the smallest parts, you may want to keep the soldermask off the pads - use non solder mask defined (NSMD) pads.

Duane Benson
Carry on my wayward capacitor