Solder Residues and In-Circuit Test

Phil
Welcome to Board Talk. This is Jim Hall and Phil Zarrow of ITM Consulting, bringing you Board Talk. Let's see, this looks like a solder paste question.

Jim
This question comes from W.O. While running a solder paste comparison through our SMT and ICT processes (that's in circuit test) we found that one solder paste is harder than the other. Pin impact into the solder on test points was less on one type than the other. The two solder pastes use the same powder, a 63/37 Eutectic, but different carriers and actives.

One paste is designed to overcome poor wetting head-in-pillow effects that come with soldering lead-free components in a tin/lead SMT process. The other is a longstanding tin/lead solder paste. What could be causing the difference in solder joint hardness?

Phil
I assume that he's talking about penetrating the flux residue as opposed to making little dents in the solder joints.

Jim
If they're both 63/37, there should be absolutely no difference in penetration just into the metal. So we're obviously talking about a residue.

What jumps out at me is that one paste is designed for a specific thing, overcoming head-in-pillow. The reality is that the chemistry of solder paste is very complex. All of the manufacturers would love to come up with the ultimate solder paste that minimizes all defects and gives you good processing parameters, including ICT test penetration in the residues. But the reality is they're not there yet, and I don't know if they ever will be. Nature doesn't tend to be completely harmonious, so there are tradeoffs.

Manufacturer's optimize paste to get rid of one defect, and you tend to reduce some other parameters. It sounds like one of these pastes, and I’m guessing it's the one that was formulated to avoid head-in-pillow, has the chemistry that  produces a harder residue which is more difficult for the test probes on your ICT, or bed of nails fixtures, to penetrate.

I want to compliment this questioner for doing a solder paste comparison. You are doing the right thing. You're trying to find the best paste for your process, and you're doing the tests that are important to you. That's excellent work. Now, armed with this information, I would talk to this paste vendor, or other paste vendors, and see if they have another formula that gives you a better compromise between head-in-pillow, if that's the defect you're addressing.

Most solder manufacturers will be able to talk to you in a helpful way.

Phil
Maybe someday somebody will come out with a "TempurPedic "solder flux residue that serves all purposes. In any given marketplace, whether we're talking about the Americas, Asia or Europe, there are at least 20 different solder paste manufacturers competing with each other, and they're all trying to out-do each other.

Jim
They are working to give you a better process, to make your life easier.

Phil
So I guess no matter whether you're going to be doing in-circuit testing, whatever you do when you solder those boards ...

Jim
Don't solder like my brother.

Phil
Please don't solder like my brother.