Copper and Tombstoning
Earlier this week, I wrote about the effect soldermask can have on tombstoning and I referred back to two older posts on the subject of tombstoning. I also wanted to refer back to a third tombstoning-related post that talked about the effect that your copper can have on tombstoning, but I couldn't find the post. Well, apparently, I never actually wrote that post. I meant to but forgot and moved on to other subjects.
Any, way the point I wanted to reference related to the heat-sinking characteristic of copper. With small parts, uneven heating can lead to tombstoning. If one side of the solder paste melts before the other, the surface tension on the melted side can pull the part up.
In the first case illustrated here, (click image to enlarge) one side of the part has a small trace going to it. The other pad has a much wider trace. The eider trace can act as a heat sink and slow the melt of the paste on that side, causing the tombstoning.
In our second illustration, (click image to enlarge) an inner copper layer goes under one pad but not the other. As with the wide trace, the inner layer may sink heat away from that layer and keep the paste from melting at the same time on both sides.
It may not seem like much, but an uneven melt of just a very short time span can lead to tombstoning. That makes it a good practice, especially with small parts like 0402 and 0201 passives, to consider the thermal characteristics of your layout.
Oh! ye'll take the high road and
I'll take the low road...