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No-Clean Residue Shorts

No-Clean Residue Shorts
We have a PCBA with a QFN 44 using no-clean flux. We have many test failures during testing. Are the failures due to flux reside between terminations?
Board Talk


And welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers, who are also known as ITM Consulting. We are here to discuss your problems with processes related to electronics assembly and SMT, whether it is materials, components, procedures methodology, equipment.

Today we have an interesting question from TY. We have a PCBA with a QFN 44 using no-clean flux. We have encountered many test failures during testing. The test engineer claims that the failures are due to flux reside between terminations that affect the capacitance of the joints. Is this a valid claim?

Good question. And the answer is, yes it could be. Of course, it could be something else too but with regard to the flux, yeah.

First off, let's say that the QFN is a type of bottom-terminated component, BTCs, to keep our terminology straight with the IPC specs.

Yes, I agree that no-clean residues, under bottom-terminated components, QFNs, has been shown to have negative influences on certain assemblies.

Years ago when BTCs first appeared Terry Munson of Foresite, one of the true cleaning gurus of our industry, pulled some QFNs off the board and started looking at the residues underneath and found that no-clean residues were not completely deactivated.

Of course with a no-clean residue you are supposed to be able to leave the residue on the board because during the thermal cycle of the reflow the active flux materials are deactivated and the residues are benign and should not cause influence on the performance of the circuit, which would result in the test failures that are being seen here.

This theory, and a number of other people have advocated this, is that what is happening is that although the flux material under the BTCs reach the proper temperatures because they are confined under the chip complete evaporation does not occur.

Evaporation is one the mechanisms that no-clean materials use to deactivate the paste by evaporating the chemicals, chemically reacting them, and encapsulation.

The feeling and the observation when people pull BTC components off is that the no-clean residue are not as hard as they were before, indicating the presence of non-evaporated solvent type materials.

Also more recently some consortiums, particularly led by Mike Bixenman of Kyzen, are also evaluating and they are taking it a step farther to look at SIR values measured under the residues underneath.

They are also showing, from a quantitative standpoint, it has no-clean residue even when completely reflowed at the proper temperatures still fail SIR test after reflow.

The answer is to clean, even if it is a no-clean. It may be necessary for your parts to actually do a cleaning. That means coming up with a cleaning methodology, a chemistry and a process that will remove those no-cleaned residues from under those parts.

And I just want to add, I believe that I have seen papers along this situation even when you have properly reflowed residue that certain frequency ranges can interfere with the functionality of the circuit. You would have to research that and see if your application falls into these suspect frequencies.

I am sure one of our readers will comment on it. We are typically talking gigahertz range but I don't remember the specifics. Again, if that is the situation as Jim is saying, you may have to go to a water soluble cleaning process. It's a process, not just a material change, it is a process change too.

And if you have type of devices where certain parts can't go through a cleaning operation then we look at primary ops with a water soluble and thermal cleaning and then do a secondary op with a no-clean.

One more possibility. Another plug for current solder paste.

This is an issue that is being addressed by solder paste manufacturers and some of them have flux formulations that perform better in terms of deactivating under BTC components. Something else you might want to investigate to solve this problem.

So we hope that we shed some light on it. It puts you off in a few directions there.

You may also want to take a look at some of the solder paste companies, including your supplier and see if they have done some research on this or ask them about it. They know their formulation. They know their chemistry. Or they better or it is time to switch solder pastes.

You have been listening to Board Talk with Jim Hall and Phil Zarrow from ITM Consulting. We hope we have enlightened your mind in this particular fun subject.

Whatever you do, however you're soldering those, whatever capacitance problem you may have, please don't solder them like my brother.  

Don't solder like my brother.


This sound like a Win-Win for Business, Environment, pricing and availability of Lithium and so on. Could Lithium (and other elements) be recovered from seawater as well?
Calvin Mosher, Nautel Ltd
I suspect that this is a leakage current issue. With wet flux under the QFN that's a low standoff component, the solvents in the flux allow for ions that cause dendritic growth & a path for leakage current. This is a big issue in higher power applications that use QFNs & MOSFETs. Flux chemistries with enhanced SIR & ECM reliability will certainly help.
Karthik Vijay, Indium Corporation
It would be interesting to know which kind of activation has the flux used, if it is an L0 or L1, or others, and if the solder paste is compliant to IPC J-STD-004. Then it would be interesting to know which kind of tests are done on the boards, if they are doing humidity tests or not, and if the PCBA is "as-is" or closed inside a plastic or metallic box. This information can help the discussion. Typically a no-clean flux compliant to J-STD-004 has a 100MOhm minimum impedance, so quite difficult that it makes shorts, unless it hasn't received enough heat. Maybe a temperature profile on this particular device (a probe on the top of the component, a probe under the component) should help to better understand if the soldering profile is OK or not.
Pierpaolo Galli, Meta System SpA
I've generally wondered, being an RF guy, why anyone would use a no-clean flux, since by definition it leaves "crud" behind. It HAS to have some resistive and dielectric properties, and looks like crap, whereas water-soluble flux yields beautiful looking, clean boards. No solvents needed anymore, and water is cheap as a cleaning agent.
Allan Knox II, Knox Associates Design
Solder paste flux residue may indeed change the capacitance of a solder joint. SIR is because of the in-phase, resistive impedance between closely spaced conductors. Capacitance is the out-of-phase impedance. No-clean solder paste flux residue is a dieletric material; it contains a lot of rosin or resin, which is put there to improve SIR. Unfortunately at >GHz frequencies, this additional dielectric material can affect the circuit cpacitance and change signal responses.
Karen Tellefsen, Alpha Assembly Solutions

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