Electronics Assembly Knowledge, Vision & Wisdom
Will Typical No Clean Paste Pass an SIR Test?
Will Typical No Clean Paste Pass an SIR Test?
Will a typical no clean paste pass an SIR test? Is it really clean enough for a conformal coating and 20 years of service?
Board Talk
Board Talk is presented by ITM Consulting

Phil Zarrow
Phil Zarrow, ITM Consulting
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, ITM Consulting
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.

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Welcome to Board Talk with Phil Zarrow and Jim Hall of ITM Consulting, the Assembly Brothers. Coming to you today from the ITM Consulting Failure Analysis cave, atop Mount Rialto, and inside Mount Rialto. And we are here to talk about electronics assembly, materials, equipment, components, practices and procedures, among other things. What is today's question, Jim?

The question comes from D.H. Will a typical no clean paste pass an SIR test? Is it really clean enough for a conformal coating and 20 years of service? That is actually two questions, isn't it?

Well, for the SIR test, yes. All no clean fluxes are tested for SIR per IPC spec, I think it is 650.

I think it actually goes back to Bellcorp too, doesn't it?

I'm not sure, Phil. But they will pass. Although there has been some recent experimental data published that shows that the value of the SIR, although it will pass, will vary with the specific reflow profile.

You may want to check that out if you have specific concerns. But the general spec is 1 x 10 to the ninth ohms during the SIR test. And they all have to pass that for a generic profile. What about cleanliness, Phil?

That is a good question because a lot of the wisdom, and it is wisdom being passed by the cleaning experts particularly the guys involved with engineer at Aqueous and some of the other various cleaning agents, will tell you that if you really want to be on the absolute safe side for getting good adhesion of conformal coating that you clean first.

Now of course they are also saying it is not only for adhesion, but also you don't want to do any entrapment. So for a class three product, that might be what you need to do.

However, strictly from adhesion in our experience, and this is going back quite a few years, what we have found as well as experiments done by some of the solder paste companies and things like that that we have resourced, is in general we have found good adhesion between most of the conformal coatings that are out there and most of the no-clean flux residues that are out there.

But again, this is something from both the adhesion situation as well as the other underlying question of long-term reliability. You've got to experiment. With adhesion, you do a design and experiment with the materials that you are working with.

And of course, from a reliability standpoint you do the appropriate highly accelerated life testing and other designed for reliability testing that is apropos to the application and the service and shipping environment that it is going to reside in.

Well, I certainly agree with you technically Phil.  But I look at this question and see 20 years of service, and the back of my mind says clean it.

The idea of getting 20 years service and if you are conformal coating you got to figure that there is something in the service environment that is not going to be too friendly. If I really want 20 years it seems like cleaning is a small price to pay to buy yourself a little extra confidence.

Because accelerating life testing, extrapolating 20 years of service, I'm no expert but it might be difficult to build a very high confidence level for 20 years service life. And make sure that you have highly accelerated all of the possible environmental conditions and failure conditions.

Very sage advice. Very, very good advice. Yeah, you have been listening to Board Talk. And remember nine out of ten Board Talk listeners are current, the other one just impedes. And whatever you do...

Don't solder like my brother.

And please don't solder like my brother.

Today, the need to clean no-clean flux residue is essential for long-term PCB performance, functionality and reliability. When the salt activators in the no-clean flux come in contact with heat or other chemicals, they can leave behind a white residue that may corrode fragile circuits and enable parasitic leakage, electrochemical migration, shorting and dendrite growth. Preventing unpredictable performance, costly board failures, recalls and returns is a big motivator to clean no-clean fluxes before coating.

Plus, removing the flux residue may also help make your board inspection and quality control easier. In addition, other contaminants like ink and fingerprints may also need to be cleaned away for optimal circuit board performance.
You may also want to clean no-clean fluxes simply for improved cosmetic appearance of the boards.
Sheri Pear, MicroCare
AIM has authored a comprehensive paper on the subject of applying coating over no clean flux residue.
Timothy O'Neill, AIM
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