Conformal Coating Over No-clean Flux

Phil
Welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers, pick and place, or place and pick. Today we have an interesting question. 

Jim
This question comes from D.L. We have a new product and need to apply conformal coating to one circuit board assembly. We have not applied conformal coating before. Is it normally acceptable to apply conformal coating directly over circuit boards assembled with no clean flux?

Phil
Very good question. Going back a few years, when I actively did no clean evaluations - we still do them occasionally - the conventional wisdom was that you tested for this. You built a few boards with your candidate no clean solder paste, had the conformal coating applied in the normal manner, and tested.

One of the simplest tests was the adhesion test, where you would coat the surface with packing tape and let it sit for a while, and then pull it and check under a UV light. Going back a number of years ago, basically most of the no cleans were compatible, both the tests we did, tests that Alpha Solder did and basically there was compatibility, whether it was acrylic, silicone, really any of them. However, now we fast forward to the modern days and, Jim, you have a different take.

Jim
Whatever is under that conformal coating is going to stay there. What could be wrong with a no clean residue? It may not have been completely deactivated during the reflow cycle. Remember, all no clean fluxes start out with some active chemistry. Ideally, through the reflow cycle, that chemistry gets deactivated or encapsulated so that the residue is safe. If that's true, then, as Phil said, putting a conformal coating over no-clean flux residue should not be a problem.

The issue is if you haven't perfectly deactivated all that solder paste flux, now you're capturing it. You can make the potential for damage even worse because, virtually all conformal coatings are not completely impervious to water. They will, over time, allow water to diffuse through them.

If you have some hydroscopic contaminant from your no clean residue, it will actually attract water, and concentrate that water under the conformal coating. So that's where the controversy comes up. But, certainly, many people are conformal coating over no clean residues. That's a very standard practice.

Like everything else, in my opinion, it comes down to what is the reliability level of your product? How critical a product is it? What is the potential, what's the life? Are these service environments going to be in high humidity? Why is it being conformally coated? It the product going to be subjected to a lot of moisture? All those questions will enter into the equation.

Phil
The fact that this issue recently came up is interesting because it used to be a non-issue going back a few years. We touched base with one of our favorite conformal coating gurus, Chris Palin of HumiSeal, and Chris came back and said things have changed in the flux chemistries themselves.

Back in the old days of the RMA and no clean fluxes with the 63/37 solder, there was good adhesion. But the chemistries in the fluxes, and hence, the residues, have changed so radically because of the higher thermal excursions, principally for lead-free. There is a bit of a game changer, if you will.

Jim
There is also the issue of smaller components where you need more flux to hang around so that you're sure you don't re-oxidize the pads and terminations on your ultra fine resistors and capacitors.

Phil
It certainly behooves you when you're doing a solder paste evaluation to evaluate your conformal coating from a reliability standpoint. As Jim says, don't be surprised if you find yourself relegated to cleaning that no clean residue off.

Once again, it's application-driven. And, once again, I have to remind you this is Phil Zarrow and Jim Hall, the Assembly Brothers reminding you. Don't solder like my brother.

Jim
And don't solder like my brother.

Phil
And don't put conformal coating on him, either.

I'm a IPC-610 trainer. If you are looking at this from an IPC-A-610 standpoint, if the entrapped flux bridges lands or adjacent conductive surfaces, or violates minimum electrical clearance it's a defect for all 3 classes.

We apply conformal coating on no-clean residu since some years and with several conformal coating and we have as many result that conformal coating. The first was a acrylique and we didn't see any probleme. The second was a other acrylic and we had a lot of adhesion problem. Since we use a uretyhane conformal coating and adhesion is perfect (but very difficult to remove)

I am surprised that you say conformal coating over low solids residue is acceptable if the residue has been properly activated and as such is inert. It would seem that you are disregarding mechanical adhesion problems. How can you have proper adhesion of the coating over residues in the areas where the residues exist?