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Thermal Mass Follow Up

Thu, 03/04/2010 - 7:21am
Screaming Circuits

RoHS has been with the electronics manufacturing world for quite a while now but there is still a lot of issues and uncertainty associated with it. As I wrote not long ago, even parts that are supposedly compliant can in some cases not cut it.

Taylor asked in the comments section of that post: "Have you noticed any pattern in capacitor manufacturersClose caps 3 exhibiting this problem? How can make sure to spec a capacitor that is more robust?"

Close caps 1 I can't say that I've seen a real consistent pattern with components from different manufacturers here. It's a case where the design engineer may have to compare the exact thermal specs from different components' data sheets and throw in a good measure of intuition and judgment as well.

In some cases, you might be able to replace a couple of capacitors with a single of a larger value, but in general, if you need multiples, combining them won't do. There are certainly good reasons to parallel up capacitors. You may need a few of different values to cover different frequencies. You may have a clearance issue and not have enough height for a taller cap. Or you may need to keep the ESR (Effective Series Resistance) down. Whatever the reason, if you need a number of caps close together, and they are big SMT electrolytics, you could be setting yourself up for this problem.

Close caps 2 Image A illustrates the issue found in that earlier post. The thermal mass of all of those big metal can caps can slow the solder melt. The most vulnerable pads are the two inside pads for C3 and C4. Keep the heat up long enough to fully melt the solder on all pads and you may destroy the caps, or other components.

You could just spread the two rows apart a bit like in illustration B. This might be enough to allow all pads to solder well or, if nothing else, it would give you enough room to touch up with a soldering iron.

Probably the most common solution though is to take the approach used in illustration C. Just put all the caps in a row so none of the pads are vulnerable.

If you need a compact layout like A, you'll just need to spend some extra time with datasheets to find a specific cap with a bit of extra RoHS temperature margin. Look at the maximum solder temp, the maximum dwell time and the profile curve if available. Don't forget to check your other components too to make sure that the extra reflow time wont harm them either.

Duane Benson

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