Screaming Circuits: Surface mount power component footprints

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Surface mount power component footprints

There was a time when the Bison ran free on the plains and power components were easy to design with. Everything, with the exception of an exotic few, used either the TO-220 or TO-3 packages. Even when surface mount came a long and cut the bison off from their grazing lands, most power components came in some derivative of the TO-220, with bent leads.

That's no longer the case. Today, power components come in those TO-220 derivatives, SO-8 packages, QFNs, and down to 0.3 mm pitch wafer scale micro BGAs. It's madness.

The advantage of all of that chaos is that it gives more flexibility for sourcing and sizing of components. Which, of course, brings in a few more potential issues. Take the example below:

Power land original

The footprints were originally created for a package with four 1.27 mm (0.05") pitch leads on one side and a big heat slug on the other. The component selected is a variant in an SO-8 package. It's not an uncommon occurrence.

As long as pins 5 - 8 all share the same internal connection, there isn't anything electrically wrong here. However, with that large open copper pad on top, it's going to be very difficult to get a good solder joint.

The fix is pretty easy. Just add solder mask to separate the pins. Make the mask openings the same size as you would if the pins were on individual pads. You don't need to cover the whole pad with solder mask - just surround the pins so solder will stay where it's needed. The mock-up, below, illustrates what it would look like:

Power land mocked up

You'll need to do the same with your solder paste layer. Unless the component has a heat slug underneath, you'll want to make the paste layer block the big open area.

Duane Benson
Would a bisontennial be a 100 year old, large grazing animal?

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