Max Time Between Soldering and Cleaning

Phil
Welcome to Board Talk. We are the Assembly Brothers, Jim Hall and Phil Zarrow of ITM Consulting, and today we have a question on cleaning. 

Jim
This is from J.T.

What is the maximum staging time permissible for PCBA's between wave soldering and cleaning? I'm aware that ideally it should be ASAP in order to reduce white residue and stain formation, but in many cases, our manufacturing processes are not robust enough for immediate turnaround. Some PCBA's are staged up to 24 hours prior to cleaning. Is there an industry standard specification?

Phil
Well, we're not aware of a standard, at least we haven't come across any, but you are correct about the time factor. Temperature also comes into play, but then again, we're wondering if that's somewhat negated. Right, Jim?

Jim
Well, theoretically, if you've burned off all your solvents in your wave soldering operation, then all you're doing is hardening. So once you reach room temperature, theoretically that should be it. But we know that that's not absolutely true in real life.

One question you have to ask is what kind of flux are you using? Is it water based or is it alcohol based? Obviously, alcohol would evaporate and dry out a lot faster. But you wrote up an interesting historical fact about organic acid fluxes.

Phil
This is going back quite a few years, I'd say about 20 years ago. It was a case here by a company in Boston. What happened was this contact of mine had been running a wave flux on both wave and reflow, and they had run a batch of boards and the surface finish on the boards were gold. What happened was as they finished this batch of boards coming out of the wave solder machine, their cleaner pooped out.

The bottom line is it took about 24 hours to get the cleaner up and running and these boards were sitting there with OA flux residue for 24 hours. After they cleaned the boards my contact told me that there had been degradation of the gold finish attributed to the acid flux.

Jim
That was some powerful acid.

Phil
That was the powerful acid flux, so don't get any on your fingers or hands or God knows what it'll do to you.

Jim
There are a lot of other stuff besides gold finishes on boards that can be degraded by OA fluxes. So if you do have an OA flux in your system, then you may want to reduce that time allowance for concerns about degradation, corrosion, and so forth. But there are no industry standards. The longer you wait, the harder it's going to be to get the stuff off.

Phil
We hopefully answered that question and thank you for tuning in to Board Talk. This is Phil Zarrow and Jim Hall.

Jim
Don't solder like my brother.

Phil
And don't solder like my brother and keep that cleaner up and running.

The NADCAP requirement is 8 hours between soldering and cleaning. This is in play for a lot of MIL/AERO work, which are generally soldered with RxLx fluxes.

Neither of you mention cleaning the board and then storing in Nitrogen until ready for soldering.

The efficiency of cleaning of flux and solder flux residues has been assessed by measuring the ionic contamination removed in an Ionograph 500. Residues were removed from bare boards, and pcbs with through hole and also smt components. This work by the European Space Agency and the UK National Physics Labs was detailed and several conclusions made, most important being that cleaning within 24 hours of assembly should be mandatory (only pure rosin and rosin based RMA-type fluxes in cored wire and solder paste were characterized.)

You definitely want to wait until the post wave/reflow board temperature is below the temperature of the first wash stage. This will prevent porous components taking on moisture during contraction.

C'mon Fellas! Time between soldering and cleaning depends upon more than just the process delays. This has to be considered as a two-part question.

First part is no-cleans. If you're going to clean no-clean fluxes, it depends upon the wash water temperature and the chemistry of the cleaning solution. Saponafiers and other solvents allow you an extended rest period between soldering and wash depending upon the wash temperature and aggressiveness of the cleaning solution. The longer you wait, the harsher and hotter the solution needs to be.

Organic acids on the other hand are a bit more insidious than their no-clean counterparts. The longer they sit, the more corrosion damage they do to the metals on the assembly and the further they creep into porous substrates making them almost impossible to remove completely.

You also forgot to mention that the longer OA's sit, the physically harder the flux gets, changing the chemistry of the wash required to remove the residues. What was once an easy removal in hot DI water becomes a difficult proposition to remove with chemicals, up to the point that they cannot be removed. Did I mention that they stay corrosive even after they harden?

An no mention of cleanliness testing to validate the cleaning process? Omegameter or Ionograph testing will tell you if the boards have been cleaned or if another pass or more aggressive chemistries are required. Ion chromatography is not optimum for routine testing since it is site and ion specific and all you need is a general idea of whether or not the boards meet the specific cleanliness requirements for the assembly.