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? Jim Hall and Phil Zarrow, The Assembly Brothers, discuss this scenario.
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
Phil Zarrow
Phil Zarrow
With over 50 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
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.


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.


I share some of Domingo's concerns. Some but not all military customers have required BGA underfill to attempt to match CTE between a BGA and the substrate. BGAs have some added benefit of having some standoff due to the balls to help with the expansion and contraction just like good old component leads. I cast a wary eye toward all bottom terminated components, especially QFNs that are often critical parts of the circuit. I suppose the fact that they are normally quite small mitigates some of the effects of expansion and contraction as the CTE delta is over a small area.
K. Mobley, Tektone Sound and Signal
You are focused on effect of conformal coating. But what about the effect of the substrate. The CTE of FR4 compared with components or SAC is quite similar, but if you change FR4 for alumina or cupper the effect is completely different and you can have cracks even during validation process.
Domingo Jose Lebron Berdugo, Magneti Marelli - Automotive Lighting
This is definitely an important topic, and you have provided great insight. I would like to build off your comments as well as Nathan's comment. You had already talked about the risks of CTE mismatch, especially if the coating is able to get under the critical components (i.e. BGA, QFN).

There are a couple of other issues that can creep up as well. One thing to be mindful of is the glass transition temperature (Tg) of your potting/coating material. While silicones tend to have fairly low Tg values (roughly -40C to -65C), some of the acrylics and urethanes can have a Tg right around room temperature. If you are constantly crossing over the Tg during operation it can definitely result in reduced time to failure. In this case it is key to understand your field environment when selecting a conformal coating material.

Other conformal coating issues often fall under the topic of board cleanliness and adhesion problems, as entrapping contaminants or coating delamination can lead to a variety of issues. This is not as much related to CTE mismatch.

We have definitely seen several issues related to improper selection or application of pottings and coatings. Below are some slides that give a good overview of various challenges encountered with pottings and coatings:
Josh Akman, DfR Solutions
Yes, excessive conformal coating under a BGA can cause a reduction in reliability. In terms of risk, I would rank the coatings:
  1. Acrylic
  2. Urethane
  3. Silicone

A paper that shows the effect of excessive conformal coatings:

Been seeing a lot of interesting failures associated with conformal coatings and potting
Nathan, DfR Solutions

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