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Gooey Flux Residue Left Behind

Gooey Flux Residue Left Behind
After selective soldering connectors there is a small amount of no-clean flux residue left behind. How can I judge how much flux residue can be left behind?
Board Talk
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
Phil Zarrow
With over 35 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
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.



And welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers, who by day go as ITM Consulting. We’re here on this free resource for you to impart on you advice regarding your surface mount and assembly process questions.

Today Jim, we have a flux question. This is from S.T. S.T. says after selective soldering connectors there is a small amount of no-clean flux residue left behind. The minimum amount of residue is still gooey. Uh oh, uh oh. It cannot be seen with the naked eye. Are there any objective qualities that I can use to judge how much flux residue can be left behind? Is there any evidence supporting the claim that no-clean flux residue degrades reliability in electronics?

Wow, that was a can of worms. First of all, it says selective soldering, that could be a number of things. Are we talking about with a selective soldering machine with a point machine or are we talking about a wave soldering machine with a pallet? In other words, you have the done your double-sided reflow and now you are doing your through-hole and you have the masking pallet, the aperture pallet as we like to call it.

My first thing is if it is later the aperture pallet and you are seeing flux reside gooey, that is the sinister word here, that has seeped through the fixture to the areas that was previous reflowed. That fixture is doing a good job of keeping the solder out but if flux gets in there that flux is not going to see the proper heat cycle. It is not going to polymerize and it is going to remain gooey. That is very dangerous. It is a time bomb basically and we have seen a lot of it.


Phil, I want to go to the last part of the question here. Is there any evidence supporting the claim that no-clean flux residues degrade reliability in electronics? The no-clean residues are only safe if they are properly deactivated through a complete thermal cycle. That means you have to go through a full re-flow cycle, or wave soldering cycle. Which means getting up to the full temperature of the solder.

When they say gooey, that tells me maybe you didn’t get hot enough. You didn’t evaporate all of the solvents in there or you didn’t encapsulate all of the activators so that the residue is not safe to leave on the board and can degrade reliability. Talking about with selective soldering, particularly with point to point, where we are relying on the immersion in the nozzle to do most of your heating.

If you don’t set that up correctly, in terms of your dwells and so forth, you are not going to heat all of the flux that you have applied to that joint adequately. That residue can be unsafe, in terms of long-term reliability. So, you need to look carefully at profiling your selective process to make sure that all of the flux that you have applied, by whatever means you are using and whatever selective process that you have chosen, that all of that no-clean material has been completely heated through a full soldering cycle so that the residue will be deactivated.

That means all of the solvents have been evaporated and the chemical reactions and taken place and the resin has encapsulated any residual active material. If not, then that residue is chemically active and can damage the electronics.


Even if, as S.T. described there, it is a microscopic amount it is there and under the right conditions mainly adding some humidity and voltage may be enough to cause dendritic growth. Those little dendrites don’t take much to cause a short.

We hope that helps S.T. Be aware. It is one of the plagues that we see quite a bit, all of us. Situations like this one. Situations where people haven’t properly thermocoupled and attained the proper temperatures under heat sink that we have talked about in the past. That gooey flux is sinister.


As our good friend, Dr. Mike Bixenman, says a lot of people think that no-clean is okay no matter what you do and that just isn’t true. No-clean only works in terms of a safe residue if you thermally react it properly and put it through a full thermal soldering cycle. If you don’t, there is significant possibility it will be chemically active.


That’s right and if you want to read volumes of horror stories, talk to our friends Terry Munson and Eric Camden at Foresite. They see a lot of this gooey flux situations. We hope we answered S.T.’s question and some of the other people out there wondering. Whatever you are doing, please be aware of the gooey flux syndrome. Whether you’ve got gooey flux or not, make sure also you’re not soldering like my brother.


Don’t solder like my brother.


I am in complete agreement. Furthermore, and in many cases, fluxing is excessive, so the preheat function (Phil and Jim call it dwell time) is woefully insufficient. It cannot preheat that overabundance of flux, so it is partially activated. And leaving on un-activated flux has been proved to lead to dendritic growth when power is applied.
Russell Claybrook, MicroCare, LLC

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