Delay Before Cleaning Partial Assemblies

Delay Before Cleaning Partial Assemblies
We're assembling circuit boards with a water soluble flux. Is it acceptable to leave these partially assembled boards for 3 to 4 days before finishing them?
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Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
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Phil Zarrow
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
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.


Welcome to Board Talk, this is Phil Zarrow and Jim Hall, the Assembly Brothers, coming to you from high atop Mount Rialto. And what's today's dilemma?

This is a cleaning question from J.C. "We're assembling circuit boards with a water soluble flux, but have run out of some parts." J.C. is the first person in electronic assembly to ever run out of parts while assembling a board.

"Is it acceptable to leave these partially assembled boards for 3 to 4 days, then put on the rest of the parts and then finally wash them?"

In a word, no. You have to think in terms of organically activated water soluble fluxes as basically acid fluxes. This stuff is very highly reactive. The benefit is of course you can clean it with water or with deionized water. But the idea is that they are water-soluble.

The downside is you have to clean them, and the wisdom is clean them as soon as possible. Let me give you some anecdotal information. Years ago, there was a client we knew in the Boston area who had ENIG boards, and they were soldering with the OA flux and their inline cleaner bombed out. And so they were waiting for the repairman to come and the boards sat over the weekend.

What they found Monday morning was that in some instances the OA flux residue had actually eaten through the gold. That's how nasty this stuff is.

And another situation with another company where it was the same type of situation. They were assembling boards on Friday and the old quitting bell rang and there were a bunch of boards that hadn't been cleaned.

Again, with water-soluble fluxes and in this case on HASL boards, and lo and behold, come Monday, there was dendritic growth. Where did that come from?

I believe the bias voltage in this case came from some testing they had done.  But it then the alarm went out, were the other boards sent to the customer clean or not. Scary stuff. 

The rule of thumb is clean as soon as possible and make sure everybody knows that they are being cleaned.

On a much less dramatic level, it's also easier to clean them the sooner you clean them, you leave OA  fluxes, harden and dry out for another couple of days and it makes them that much more difficult to clean even if you didn't have any of the horrible effects that my brother has alluded to.  

So the long-term solution here would be- you know, in this case of the parts shortages is if you're going to solder those boards with the OA flux, missing a few parts, clean them as soon as possible, and then when those other parts come in, you can -- depending on whether you're wave or hand-soldering, whether you're going to use an OA flux and clean them again, or you're going to use a no-clean flux, again depending on the application, everything else and not cleaned, so you know, be careful. 

But we feel your pain about missing components, welcome to the club we call electronic assembly. It's happened before, it will happen again.

Hopefully you weren't waiting for an answer before you got our wonderful wisdom on this, but clean them as soon as possible.  And on that note, whether using a OA flux or a no-clean or an RMA or even an RA flux, whatever you do, don't solder like my brother.  

And don't solder like my brother.


Unconditionally, you must clean the boards as soon as reasonable, within hours, not days. The solder manufacturers know that dendrites not only can, but will, occur after 24 hours. Once dendrites start, no subsequent cleaning effort will stop them. On the one hand, as stated above, military specifications say 30 minutes. For commercial products, of very high commercial quality, it is advisable to wash within 2 to 4 hours.
Jaye Waas, Renkus-Heinz
In one word NO!!!! Ever heard the term "Rust never sleeps". Well, nor does corrosion and dendritic growth.
Jerry Karp, JSK
Just an after thought... Are these Surfacemount assemblies? If so, You also need to consider floor life exposure for any Moisture sensitive parts. You do not want these sitting around for several days then do a second reflow pass.
Alan Woodford, NEO TECH
Well, it could be a better solution to have more information about your cleanliness process. Another point is which kind of soldering process did you use? Assuming your training department is doing a great job educating manufacturing employees, no matter what kind of flux we are applying, we always clean after every soldering application. IPC J-STD-004 demonstrates this.

My opinion is never forget clean boards no matter what kind of flux. Even low fluxes are electrically conductive and chemically corrosive. This is without activating it. After activation, water soluble fluxes are electrically conductive and chemically corrosive.
Alberto Garcia, Sinectech
The data sheet for the water soluble flux we use says that it must be washed off, but it does not specify a time limit. I wonder why not, especially if it is so critical.
Steve OBrien, CNS Precision
As is said, no, no, no.

As a general rule, water-soluble fluxes MUST be cleaned ASAP. This means 15-30 minutes at the most. Chemical changes occur within minutes and the vehicle, rheological additives may continue to polymerize, encapsulating all those nasty acids for later release. The polymers are not necessarily water-soluble. Worse, depending on the surface chemistry, they may form hydrogen bonds with epoxy substrates, as well as physically penetrate into the porous surface.

Remember MIL-P-28809? This was written when it was mandatory to use approved RMA fluxes, which many regard as "no-clean". It specified that cleaning must take place within 30 minutes of soldering. If this were so for such benign hydrophobic fluxes, just imagine what it's like for highly activated hydrophilic ones.

Summing it up in one word: CATASTROPHIC!
Brian Ellis

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