Electronics Assembly Knowledge, Vision & Wisdom
Can a CTE Mismatch Cause Reliability Problems?
Can a CTE Mismatch Cause Reliability Problems?
Does usage of BGAs, QFNs or SONs have an increased risk for reliability of solder joints due to the CTE mismatch with the base board material FR4?
Board Talk

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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.

ITM Consulting
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And welcome to the Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers, Pick and Place.

What's our question today?

Okay, it comes from DH. Silicone coating a board has been a standardized process whenever a customer wants protection from moisture or dust.

Lately we have heard it could be detrimental to reliability of solder joints due to CTE mismatch with other materials FR4. Does usage of BGA, bottom terminated parts QFNs or SONs have increased risk? These packages are low profile and likely to have less seepage of silicone under them compared to the channel and caps of discrete parts. Well, the first thing being really technical is that the CTE mismatch concerned about is between the coating material, your silicone or whatever other conformal coating and the solder joints of your balls or your terminations on your bottom terminated components.

And the general answer is that, yes, we have seen problems particularly with bottom terminated components. QFNs and so forth where there is enough seepage flow of the coating material under the parts so that when it goes through heating environment, the material which is now under-filled, at least the edges of the part expands, puts a vertical tensile stress on the solder joints and cracks them. I've seen this on BTCs. I have not seen any examples of that on ball grid arrays, but I can't say that it's impossible.

Yeah, it may happen, just haven't tripped over any lately.

So yes, there is -- there is always a potential of some seepage. And yes, it can be a problem of course. As you were saying the thermal environment is going to be the biggest driver. What is the thermal temperature excursion that your finished product is going to see?

As Jim said the most important thing there is that expansion in the z-axis that there will be out to get you. We actually saw one example where the customer was using a bottom terminated coating and was not soldering the route plane, it didn't call for that, you know, in terms of heat transfer or everything else. And yes, the conformal coating seeped under there and, yes, they had a terrible problem with regard to disconnect, I guess you can say. That CTE is powerful stuff.

It is a concern. And certainly the first thing is to understand well, the thermal environment that you are going to be subjected to.

So hopefully that answers DH's question and he can go back out riding there -- riding out there in the desert and everything else and you guys can too. And thank you for listening to Board Talk. And whatever you do, and whether you're using conformal coating or not, but regarding your solder joints --

Don't solder like my brother.

Oh, please don't solder like my brother.
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