I’ve mentioned some cautions with parts substitutions before; wrong V values on barrier or flyback diodes, counterfeit parts and things like that. Here’s another example of something to watch out for if your supply is tight and choices are limited.
One of the things that I’ve run across a couple times, especially when hunting down capacitors, is the package size issue. Say, I need a 16uf, 10V cap on one of my boards. It’s not a critical app. I don’t particularly care about ESR, temperature or even much about tolerance. I just need a little head room in case of minor spikes or power line ripple. I’m not expecting a lot. I just want that safety margin.
But when I run over to my parts supplier, the specific cap I picked two weeks ago, when I started the design, is out of stock or jumped in price. I want to get building, so I just look through my parts drawers for something close. There it is, a 22uf, 50V. It’ll still work just fine. The problem is, of course, that I neglected to realize that the part jumped up a notch in size. Bummer days.
I’ve run across the same problem, not due to a sloppy sub, but also due to picking the wrong footprint in my CAD package. I find that particularly easy to do with SMT electrolytic caps.
The other thing in these examples to watch out for is the open vias next to the pads. Granted, they aren’t in the pads, but they are close and without any kind of a break in the metal before the via. In the left pad of the yellow tantalum cap, I added in an example of a little solder mask dam between the pad and the via. That’s the way you should do it. Even though the vias are off pad, solder can still wick away and down the via – especially with leaded solder. Bad news if that happens.
Duane Benson
Have no fear, Underdog is here…