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How To Verify Cleanliness After Rework and Prior to Re-coating?



How To Verify Cleanliness After Rework and Prior to Re-coating?
Rework on a coated assembly happens often. How do you verify cleanliness of the rework site prior to re-coating?
Board Talk

Transcript


Phil
And welcome to Board Talk with Jim Hall and Phil Zarrow of ITM Consulting, the Assembly Brothers. And we are here to talk about electronic assembly, materials, equipment, components, practices, procedures and many other things.

Today we are coming to you from the ITM conformal coating closet. Fumes are bad in here because I think today we have a conformal coating related question. Don't we Jim?

Jim
That is correct. It comes from C.C.

Phil
Conformal coating! That was clever. Okay, fire away Jim.

Jim
The question is, we use polyurethane coating.

Rework on a coated assembly happens often. I know how to remove the coating and clean the work site, but how do you verify cleanliness of the rework site prior to re-coating?

Phil
That is a good question.

Jim
That is why we have you, Phil.

Phil
Well , yeah right. I have to ponder this. That is a good question.

We are very fond of saying that. At this point in time in this industry there is no such thing as what we call selective cleaning. You should try to do your absolute best with that particular site.

Certainly first of all I would say that when you do the repair, assuming you are using a no-clean because you are saying are we sure it is clean as opposed to obviously if you weren't using a no-clean you would have to clean the whole assembly again, and that would be very difficult with the other conformal coating on there. But basically with no-clean the first objective is, and this is regardless of if you are going to be dealing with conformal coating or not, is to use as little flux as possible.

This is one of the things we see. A lot of people put a lot of flux on and then they try to clean it off and you're not cleaning it off, you're just making things worse.

So right away whatever you do, take away the flux bottles. Don't let operators use the flux bottles.

You should be using either very, very small amount of wire cored solder, flux cored solder or perhaps a little mini syringe. And just using just as much as you really need.

So you are avoiding, as little residue on there as possible. Beyond that, Jim what wisdom do you have to add.

Jim
Well, I agree. First off you have to be comfortable that the residue that your no-clean repair flux, that the residue it is leaving is compatible with your conformal coating.

And that is a debate that goes on, but many people do it. It is a problem that can be dealt with, although many people don't like to do it.

Then in terms of the no-clean, in addition to putting it on, make sure that all of it gets heated, that you are applying adequate heat. That is why using only cored wire is the best.

Because the flux only comes out as you are heating it, so you can be much more confident that all of that flux has been deactivated.

That is the key. Remember no-clean starts out active and it is deactivated by the heating cycle and if that is the repair it is the repair heating cycle.

Oh, another thing maybe preheating the board can help ensure that all of the flux is adequately heated, deactivated. And some people use a post-bake to make sure that any no-clean flux that wasn't heated adequately by the soldering operation, particularly if you are using an iron, you can use a post-bake.

There are different opinions upon what that should be. Those are the basic strategies.

I agree with my brother, spot cleaning, localized cleaning typically doesn't work. You can run the whole board through a batch or an inline cleaner that is great, if that is acceptable to your assembly at that point.

Phil
Right, and if the other remaining conformal coating, polyurethane coating can endure that. And another thing Jim mentioned, the compatibility we are talking about, the debatable area is the adhesion of the spot conformal coating you are putting on to the residue of the repaired joint.

And that is something that you can do a very, very simple experiment with to test that out. Beyond that you might also consult your conformal coating supplier.

They might be able to part some wisdom in that direction as well. But I think we pretty much covered the key concerns there.

Yeah, that should basically do it. Well, good. Thank you.

You have been listening to Board Talk. You should know that nine out of ten Board Talk listeners are very current. The other one just impedes.

Whatever you do and however you're doing it, whether it is with a wave solder machine, selective reflow, soldering iron, laser soldering, whatever you do please don't solder like my brother.

Jim
And don't solder like my brother.

Comments

I would respectfully disagree with you both. Localized cleaning can be done and have a reliable assembly. We do it every day and have tens of millions of flight hours showing it as an acceptable practice. The key is adequate rinsing prior after rework, which is where most people fail. Eric Camden gave a good suggestion for determining localized ionic contamination by IC. You can also do FTIR looking for undesirable residual organics. Most companies don't have an FTIR, but you local university chemistry department is sure to have one and can assist you. There are surface analysis tools that can be used, like water drop contact angle. BTG Labs has some interesting approaches for surface analysis. And for recoating, the key is for the new coating to overlap the old coating by at least 1/8".
Doug Pauls, Collins Aerospace
To answer your question you need to perform a localized extraction using a number of different options. Doug Pauls and myself gave a presentation at the SMTA/IPC High Performance Cleaning and Coating Conference a few years ago that outlined options for localized extraction and testing with Ion Chromatography. It isn't necessarily a viable test for every time you repair CC but it is a good idea to help qualify your process as a first article type qualification.
Eric Camden, Foresite
1. Selective clean can be done with required coating cleaning agents such as recommended strippers. Inspect the coating cleanness in UV scope ensuring cleaning well
2. As much as possible not to use flux during solder touchup, since solder wire has a flux combined.
3. Try to use DI water immediately after soldering touchup since it is No-clean soldering.
4. Then do coating touchup at selected area which is coating removed
5. For coating touchup try to use less thinner mix is added advantage to avoid bubbles and thieving effects in coat.
Parasu, Sanmina, India

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