Screaming Circuits: More Thermal Mass Issues


More Thermal Mass Issues

Yesterday I wrote about some thermal mass related traps. Here's another one we see now and then.

Over heated caps

The top image shows good flat-topped caps. The bottom and inset has overheated bulged and damaged caps. These caps are RoHS compliant - supposedly. Their data sheet calls them out as RoHS compliant and their temperature specs and recommended reflow profiles indicate RoHS compliance. So what happened?

Well, they are compliant pretty much only in singles. A single of these caps will solder up fine and not be damaged. However, put four in close proximity like this and the solder paste on the inside pads will not melt at the recommended profile. They need a bit more heat because the thermal mass of the four parts close together sinks heat away from the inside solder pads. In the end, we hand soldered these specific parts to solve the problem, but for production, either a more thermally robust part would be needed or the part spacing would need to be changed to compensate for the combined thermal mass.

Duane Benson
"Hot" is a relative term

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Comments

It's interesting that someone else has seen this. We had a similar problem recently but only on dual-reflow boards; i.e. when the larger RoHS SMT caps had gone through twice. Well, that's what our board assembler told us anyway. Nichicon and Panasonic both behave the same way. We've switched (where possible) back to through-hole or just enlarged the toe on the SMT footprints to assist hand soldering (the recommended footprints never seem to consider hand soldering).

Have you noticed any pattern in capacitor manufacturers exhibiting this problem? How can make sure to spec a capacitor that is more robust?

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