Maybe not completely narcissistic, but at least self-centered. Or, self-centering. Okay, are you lost now? Am I making any sense at all? Well, I’m going to say that it doesn’t matter, because the world-revolves around me.
But what I am talking about is parts that will more or less center themselves during the reflow process. Some parts like BGAs and QFNs tend to follow the surface tension of the melted solder and tweak themselves into a more centered position on the land. That’s a good thing.
It’s not always a good thing though. Sometimes that same surface tension action can work against you. Take this TO-263 (pictured). When it was placed on the land, before reflow, the leads were centered right in their pads like they should be. The big land for the thermal pad is set up a little too high though and once melted, the surface tension from the big thermal pad sucked the part up, nearly dragging it off of the lands for the leads. Bummer days. Here’s another example.
You probably shouldn’t leave the part like this, so here’s a few suggestions.
You could make the thermal pad smaller so that when the metal tab of the part is centered, the leads will be, too. Cooling needs might dictate that you don’t reduce the size of the pad, though. If that’s the case, you could make the bad bigger by extending it down toward the leads, again so the leads will be centered when the body of the part is. You could also mask off the top part of the pad, or put a thin strip of mask as a solder dam. What you’re doing is making sure that if and when surface tension moves the part, the leads will end up where they are supposed to.
<em>Duane Benson
It is all about me, you know</em>