Electronic – What causes components to “jump” during reflow soldering


I've been using a hot plate technique (with a PID controller / thermocouple / SSR setup) to get into making SMD boards. I had an interesting experience earlier today, and I was hoping some more experienced individuals might be able to help me understand what caused it and what I can do to avoid repeat performances.

I applied (non-lead-free) solder paste to my board using a stencil, put it on the hot plate (with a lid on), and started heating up the plate slowly (maybe 1.5 degrees C / second), to "soak" the board prior to turning it up to the melting point. Way before I got to the melting point (maybe around 110C), I witnessed an incredible phenomenon. A variety (but not all) of my components started jumping off the board like popcorn. Some (e.g. D-PAK voltage regulators) just kind of flipped over, others (e.g. 0603 resistors) literally propelled upward and bounded off the lid.

In my earlier attempts I didn't see anything like this happen, and I'm not really sure what I might have done differently on this particular instance. Can anyone explain the circumstances under which this type of outcome might take place and what one can do to mitigate it?

Best Answer

The flux in solder-paste is indeed hygroscopic.

I have experienced this same problem when assembling prototype boards with old paste. Over time, the paste seems to accumulate moisture, and pop more and more vigorously.

The only solution I have found is to buy new paste. Refrigerating it does seem to extend the shelf-life, but it still goes bad.

It may be possible to gradually warm the board with solder-paste and components to ~100°c and then holt it at that temperature for a while (maybe half an hour?), to try to drive out moisture, and then go directly to the actual reflow heat without letting the board cool. This is how they deal with components that are moisture sensitive, I just don't know if it would work for the solderpaste too.

Really, solder paste is pretty cheap, just buying new paste seems like an easier solution.