Screaming Circuits: July 2007


Thermal Mass

...or sometimes RoHS is only RoHS in theory.

We recently ran across an interesting situation involving a "RoHS compliant" electrolytic capacitor. The component data sheet claimed RoHS compatibility but the parts came out of reflow slightly bulged on top.

This individual part was a 470uf, 63V and therefor fairly substantial in size for an smt part. The manufacturer's recommended reflow profile did, in fact, specify RoHS level temperatures, but only for a very minimal time period. If we had been soldering one isolated part on a small PCB, the manufacturer's reflow profile would have been adequate and the part would have soldered and lived.

Thermal_mass_2 However - and this is a big "however" - that profile didn't account for the effects of thermal mass. This design has four of these caps in close proximity, along with a smattering of other components close by. The combined thermal mass of the four caps and other parts kept the solder from melting during the short duration spike as specified in the part's reflow profile. If we lengthened the high temperature spike, the capacitors overheated before the solder actually melted. Basically, we had mutually exclusive requirements - quick spike to prevent damage to the caps - longer spike to fully heat soak the solder leads on the part.

The moral of this story is that a RoHS compliant designation is not necessarily an assurance that the part will work in your design and that, even for passive components, you still need to check out the datasheets. In this particular application, we did manage to get the board built up but the right thing to do, and probably the only thing for volume production, is to relayout the board with more space between the big parts or find an equivalent part that is better able to survive a RoHS reflow profile.

Duane Benson
Hot stuff

« June 2007 | Main | August 2007 »