Research
Implementing Robust Bead Probe Test Processes
Via-In Pad Plated Over (VIPPO) Design Considerations
Screening of Lower Melting Point Pb-Free Alloys
Condensation Testing - A New Approach
Case Study for Improving the PCB Print Process Using Factory Data
DFX on High Density Assemblies
Improved SMT and BLR of 0.35 mm Pitch Wafer Level Packages
Wetting and Solidification of Pure Tin on Polycrystalline Intermetallic Substrates
MORE RESEARCH
Latest Industry News
MacBook, iPad Production Delayed as Supply Crunch Hits Apple
Microsoft unveils liquid cooling solution for datacenters
A Theory of (Almost) Everything
Toyota unveils new models in advanced driver-assist technology push
Tech Giants Enter Their Chips in the Race for Self-driving Cars
What will self-driving trucks mean for truck drivers?
4 Great Things That Happened When We Went Remote
iPhone 13 is going to be worth the wait: All the major upgrades we're looking forward to
MORE INDUSTRY NEWS

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
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 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
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.

Transcript


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

What's our question today?

Jim
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.

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

Jim
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?

Phil
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.

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

Phil
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 --

Jim
Don't solder like my brother.

Phil
Oh, please don't solder like my brother.

Comments

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:


https://www.dfrsolutions.com/hubfs/Resources/services/Coatings-and-Pottings-Issues-Challenges-and-Consortium.pdf?t=1534877316970
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:

https://www.dfrsolutions.com/the-effect-of-improper-conformal-coating

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

Submit A Comment


Comments are reviewed prior to posting. You must include your full name to have your comments posted. We will not post your email address.

Your Name


Your Company
Your E-mail


Your Country
Your Comments



Board Talk
Solder Paste Volume for BGA Rework
Reflow For Rigid Flex
Delay Before Cleaning Partial Assemblies
Problems With Starved "J" Lead Joints
Solder Paste Transfer Efficiency - What/Why
Can a CTE Mismatch Cause Reliability Problems?
Going Beyond Your Solder Paste Work Life
Issues Mixing Silicone and Acrylic Conformal Coatings
MORE BOARD TALK
Ask the Experts
Bottom Terminated Components and Vias
Is Solder Mask Considered an Insulator
Reduce Glare During Assembly
BGA BAll Sheer Testing
Components Jumping Around During Reflow
Floor Life of MSD Parts
QFN Open Solder Joints
Delamination Causing Scrap
MORE ASK THE EXPERTS