7
\$\begingroup\$

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?

\$\endgroup\$
12
  • 3
    \$\begingroup\$ I have never seen or heard of this before, so this is only a guess. If you're sure it's not demonic posession, then maybe moisture got into your solder paste, and that boiling caused this? I have heard of ICs exploding like this during soldering if sufficient moisture was absorbed by the package. This is why some parts come in sealed bags with dessicant. I suppose this mechanism could apply to solder paste too. \$\endgroup\$
    – Olin Lathrop
    Jul 15, 2012 at 23:04
  • 1
    \$\begingroup\$ @OlinLathrop Unfortunately I cannot rule out demonic posession... \$\endgroup\$
    – vicatcu
    Jul 15, 2012 at 23:12
  • \$\begingroup\$ I'm for moisture in the paste too, can't think of what else (mischievous spirits aside) might cause this, apart from possibly some cleaning agent on the PCB. Did you wash your stencil beforehand? Or spray the PCB with something? (e.g. isopropyl alcohol, flux, etc) How old is your paste and where/how have you been storing it? \$\endgroup\$
    – Oli Glaser
    Jul 15, 2012 at 23:20
  • 1
    \$\begingroup\$ A new variant of popcorn effect ? :-). The ~=110C --> > 100C suggests it's almost certainly a water boiling effect. Age of paste, as others note, and how it has been stored, is worth looking at. \$\endgroup\$
    – Russell McMahon ♦
    Jul 16, 2012 at 0:54
  • 3
    \$\begingroup\$ @vicatcu - I just had a look, I think it's referring to a solder bridge type jumper, rather than the other sort. Makes sense as a thinner layer does indeed help prevent this. \$\endgroup\$
    – Oli Glaser
    Jul 16, 2012 at 1:57

1 Answer 1

Reset to default
4
\$\begingroup\$

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.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.