Rosin Flux or Water Soluble - Which To Use?

Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting. Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes

Phil Zarrow With over 50 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes. Jim Hall A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript

Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall of ITM Consulting. The question for today has to do with fluxes.

"What are the major differences between rosin-based fluxes and water-soluble fluxes? And, assuming we have updated equipment and cleaning systems to handle either process, under what circumstances should we use one process over the other?"

Jim
This is a very interesting question, but I want the listeners to be careful about the statement at end of that question, "We have cleaning systems that can clean either." So, we are not considering a no-clean process. That's important because, if the question is between water-soluble and no-clean, that is a whole different issue.

If we are considering cleaning it is much more desirable to use water-soluble. Why? Because it's designed to be cleaned and in most cases it can be cleaned with heated water, without any additional chemistry.

Notice I said, "Most cases." Also, it gives us a much wider window on the activation that we can put in the flux. So, for getting more robust soldering, handling a wider range of solderability, of your materials, your leads and your circuit boards, water-soluble is typically a choice.

Most high-rel people, traditionally, have used water-soluble fluxes because they're stronger fluxes, they give a more robust soldering process and they're easier to clean. Because with rosin, you need to use some sort of chemistry, either a saponifier in a water-based system or another solvent.

Remember, the electronics world started out with all rosin-based fluxes and we cleaned them with Freons and Tri-Chlor. And that was great, but we can't use Freons and Tri-Chlor anymore, so we have to go to much more sophisticated and expensive solvents. Not that they don't work, but given a choice, we would traditionally use water-soluble.

However, the issue of water-soluble compatibility with lead-free soldering temperatures is something that is clouding the waters.

The basic scenario that many people are finding is that, at the higher soldering temperatures required for your lead-free alloys, most water-soluble formulas are no longer so easy to clean. And that the possibility of cleaning them with plain water, some people argue, is not reliable - that you have to have add some sort of chemistry.

There's a lot of information out there - claims being made - we're not going to support it. We're just reporting that, whereas for tin-lead, most people felt comfortable using water-soluble and getting good cleaning with just water at higher temperatures.

Another thing is low-profile components, such as QFNs. They have very small stand-offs that make them more difficult to clean with just water.

So, those issues are pushing us towards the use of chemistry, which may re-open the issue of using a rosin-based flux. Since you're going to clean, if you're using chemistry anyway, maybe it's more desirable.

I think, from a soldering standpoint, you'll prefer to use a water-soluble flux because of the potential of more, stronger acids.

Phil
Well, I think that answers that question fairly succinctly. Nice job, Jim, as usual.

This is Jim Hall and Phil Zarrow of ITM Consulting saying that no matter what type of flux system you use, don't solder like my brother.

Jim
And don't solder like my brother, either. Keep those kids away from the flux, whatever it is.

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