Be sure your designs reflect the assembler’s soldering methods.

It’s getting very difficult to hand solder many parts these days. Some give it a try, but in general, if you’re dealing with the really tiny parts or leadless parts, it’s just not possible, or at least not practical.

Sometimes a designer will start with the idea of hand soldering the board, and then either decide against it when first looking at the raw PCB, or will build one and then decide it’s too much work. That’s not a bad thing. You can get more reliable assembly, and it keeps me employed. But there are times when a layout designed with hand assembly in mind does not work for machine assembly.

Case in point: Figure 1. There are two things wrong here. The first is that the land pattern is for a smaller part than the actual component. Let’s pretend that problem doesn’t exist. The other problem is that big via hole in the middle of the pad. When hand soldering parts with a solder pad underneath, like QFNs or QFPs, folks will often put a large hole there. They’ll solder the outside connections first, then turn the PCB over and stick a soldering iron and some solder in that big via to solder up the pad.

That works more or less for hand soldering, but it’s a really bad thing to have a big open via like that when machine-assembling parts. The solder will flow down and out the other side. You’ll get a mess on the bottom of the PCB, and you may get little or no solder on the pad.

The moral of this story is that if you’ve designed your PCB for hand soldering and later send it out for automated assembly, go through the layout and make sure you remove things put in there for hand soldering that aren’t conducive to reliable machine assembly. And, coming back to the incorrect footprint, make sure your CAD library land patterns match the actual part being used.

Duane Benson is marketing manager at Screaming Circuits (screamingcircuits.com); dbenson@screamingcircuits.com.

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