It sounds bad for me to say this, but I have a lot of SMT magazines cross my desk and, honestly, the solder paste articles rarely draw my interest. I work in a world of solder preforms and that’s really where my mind is at, currently. A recent exception is a paper written by Eric Bastow (Assistant Manager, Technical Services, Indium Corporation), entitled Reflow Profiling on Electrical Reliability of No-Clean Solder Paste Flux Residues. I had a few questions, and Eric is a pretty easy guy to talk to, so I approached him. Here are the results…
Jim: I assume the SIR test coupon was bare, would adding a thermal mass similar to SMT components change the outcome of the test (aside from entrapment, as mentioned in your earlier paper).
Eric: A bare SIR coupon was used for the test. Additional thermal mass does slow the rate at which that location on the PCB heats up and would potentially also reduce the peak temperature. Peak temperature can have an impact on SIR performance. Keep in mind that things that take a long time to heat up also take a long time to cool down. I suspect that the cool down rate may also have an impact on the SIR performance because it impacts how much overall heat input the flux will see.
Jim: Adding a soak period to a traditional ramp profile seems to improve SIR performance of these solder pastes. In addition I would assume using a soak instead of increasing peak temperature helps protect the electronic components. Is this a correct assumption?
Eric: Yes it is. And, correctly employed, it can be a useful technique in such scenarios. Keep in mind that introducing a soak in the profile can create/solve other issues not related to SIR performance.
Jim: The conclusion of the article teases us a bit, are there any plans for research regarding partial or incomplete removal of no-clean flux residue?
Eric: Not at this time. Currently, I am hoping to focus on the impact, if any, that particle size has on SIR performance. In other words, all other things being equal, does reducing or increasing the particle have any noticeable impact on SIR performance.
Like I said before, Eric is a pretty easy guy to talk to. If you want to discuss reflow profiling with him, Eric can be reached at ebastow@indium.com.