Screaming Circuits: Monsters Under the Bed

Monsters Under the Bed

TO-263 land with mistakes Monsters, metaphorically speaking, that is. Take a look at this little land pattern for a TO-263 part. Can you tell me the two main things wrong with this land pattern?

I'll give you a hint. One of the problems is an absolute no-no. The other one could be justified with a low-current application. But then, wouldn't you use a smaller package?

Duane Benson
Green Grow the Traces Ho


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My second problem was the lack of solder pad to connect the part's metal tab to. But the via in the middle of the thick trace could be an issue as well. I would certainly suggest a wider annular ring to ensure enough copper there.

Question for those who know more about electron movement than I do: If the edges of the trace split by the via are too thin for the amount of current, will the sides of the via barrel add carrying capacity to compensate for the via hole?

Is the second problem cutting that thick trace with a via? So the low current is ok through the reduced amount of copper, but probably not the amount of current it's designed for (hence the thicker trace)?

Yes, there are a lot of issues here. We made it work for the prototype with a little Kapton tape, but that's a "just make it work" last resort.

1) no way to solder the case to the board
2) no heat-sinking, current path
3) silkscreen across pads
4) circuitry under metallic part (soldermask can't be used as insulator)

I would have said the no-no is not having a via keep out where a heat sink tab might land.

For high current applications you probably want a landing pad for the heatsink. In design review I would have also called out a pad exit violation for the acute angle at pin 3.

Definitely untented vias, as you called out Chris is problem #1.

No-No: Untented vias under the tab. The pad of the part is going to cause all kinds of shorts there.

Second problem: Erm, I'm not sure. The fact that there's a ground loop now with that weird tab pad?

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