Lead-free Solder Contamination Concern

Phil
Welcome to Board Talk. This is Phil Zarrow with Jim Hall of ITM Consulting. Jim, today I believe we have another question on one of our favorite subjects - profiling. What is today's question, Jim?

Jim
How about high-temperature solder, such as Sn5 - 5 percent tin, 95 percent lead. If the solder does not melt in the wave solder because it's high-temperature and not meant to melt, will it contaminate a lead-free solder pot when using it to attach a profile TC, or is it just too risky? Is it okay if you're only attaching the TC to the top and not on the bottom using high-temp lead solder?

Phil
Okay, that's a good question because, in the past, I have used high-temp solder for reflow. Well, that's an interesting question with regard to wave. Grateful the guy's actually profiling, but Jim, weigh in.

Jim
It's a good question, but I do think it's perhaps over-cautious. The TC will not melt. The solder will not melt while passing through the pot, so it will not contribute at all significantly to the amount of lead in the pot. That's what you're concerned about. You're concerned about getting lead contamination in a lead-free pot of either SAC or modified copper or tin/copper, such as SN100 and so forth.

Contrast to where we spoke before of running a tin-lead circuit boards through a lead-free pot, where you've got hundreds - perhaps thousands - of leads all coated with tin-lead, which is going to melt. That's why we say even with a few boards - you don't want to run them through a lead-free pot if they have tin-lead on them.

But a high-temp profiling TC, if you've only got one or two of them on the board, but they're not going to melt. If you went to an atomic level, I imagine a few molecules of lead will dissolve into your solder pot, but since you may have 1.5 to 2.5 cubic feet of your lead-free alloy, it's not a problem.

It's an interesting question on wave soldering because, in our recent experience we've been called to look at wave-soldering processes and we have been shocked at how many people were doing a significant amount of wave soldering on multiple products and not doing any profiling to set up their pre-heat.

What's true for reflow is certainly just as true for wave, and with wave, it's really critical that in your pre-heat you heat enough to activate your flux but not too much so that you've burned it off and consume the activators before you get through the solder pot.

You won't know how much you're heating, what your temperatures are going into the pot, unless you use profile thermal couple. In some cases, you can only look at the top. That's true, but in many cases, I would suggest in all cases, put a TC on the bottom because then you know what the thermal shock is for your PCB as it hits the wave.

There are different opinions, anywhere from 100 degrees to 140 degrees C. You should always profile, and it's not a bad idea to put a bottom-side TC on every board that you're profiling. High-temperature, high-lead solder is a very good way to attach those thermo-couples so that they don't come loose when they come in direct contact with a wave.

Phil
It never ceases to amaze us how many people take this for granted, and I don't know if it's because they consider wave soldering a mature process or one that's very constant, and it's anything but. Everything is application-specific and you can't do this by the seat of your pants. You've got to know you're dealing with a very, very high thermal excursion. Inquiring minds have to know.

Well, that's it for today. We thank you for tuning in and whether you're wave-soldering, hand-soldering or reflow-soldering, whatever you do, do not solder like my brother.

Jim
And don't solder like my brother. And keep those kids away from the flux pot, please.