According to a quick internet search, the process of conformally coating an electronic assembly goes all the way back to I don’t know when; I couldn’t find that information. Sorry. But I was able to contact an industry colleague that works for one of the major chemical manufacturers. Their company has strict restrictions about being cited in publications, so they will remain nameless, but "the man" can't keep me from telling you what their thoughts are. Here’s what they had to say.
"Conformal coatings being used to protect printed circuits dates back to before the 1960s when the Department of Defense authored specification MIL-I-46058. A general industry standard IPC-CC-830 has more recently carried the industry forward. Conformal coatings can be used to protect printed circuits from current leakage/short circuits (arcing/corona), corrosion, solder joint fatigue, mechanical stresses, such as shock and vibration, along with protection from dust and dirt debris."
So, there's that bit of history that pretty much sums up what a good coating is used for. Now that we've cleared that up, let's talk about my favorite part—how CMs continue to do it wrong. Remember, I work for an analytical lab.
Four Main Types
There are several types of coating material available, and it normally comes down to the end-use environment when deciding which one is best for your product. In general, the choices are acrylic, silicone, urethane, and Parylene. There are probably some exotics out there as well, but for most of what we see, those are the big four.
Out of those four, the one we see the least amount of issues with is Parylene because it is applied with a vapor deposition process that requires the substrate to be extremely clean for proper adhesion. When that process is properly done, the residues that can normally facilitate electrical leakage or electrochemical migration are removed. On top of that, Parylene creates a near hermetic seal when compared to other coating materials.
The drawback for Parylene is the cost and the time it takes to apply it. We don’t see Parylene used in high-volume production, such as consumer electronics, but more often for high-end reliability product, such as medical devices and aerospace applications. The remaining three materials are more of the focus for this month’s column as they do not require the precleaning of the surface before application, which is where we see the problems start.
We regularly receive failures in our lab from people who are astonished that coating didn’t stop dendrite growth on their product. Conformal coating is sometimes thought to be some magical application that stops all electrical leakage, but that’s simply not the case. In the past, I have discussed at length the risk of leaving elevated levels of ionic residues from any part of the process, and that’s no different when coating is involved. Coating is good at keeping out most dust and debris, but given enough atmospheric moisture, in conjunction with active/hydroscopic ionic residues, it will eventually penetrate through and can cause electrical leakage.
Application and Curing
There are multiple ways to apply conformal coating, including manually by brush, dipping, or aerosol cans, and automatically by spray equipment using air pressure and nozzle delivery. Any manual method effectiveness is normally driven by the operator’s competence and experience level. Manual methods may also include proper masking of areas designed to not be coated, which is most often done with polyimide tape. This may prohibit some assemblies from being dipped based on keep-out areas that are hard to mask. Automatic spray systems are among the most repeatable methods for coating application. This is the most prevalent method we see in the industry and is normally used in high-volume processes. Spray systems are the easiest to diagnose and optimize if something should go wrong.
Coating over no-clean flux residue is now a common practice, and there are a few different points of view on this process. In general, we see fewer problems with coated boards that have been properly cleaned before coating, like the Parylene application. We have seen adhesion issues with this practice when the flux residues are not fully processed to create a firm outer shell for the coating to adhere to. If the flux is not properly processed, the soft outer shell can blend with the coating and won’t fully cure. Adhesion is also an issue on assemblies processed with no-clean flux that incorporates a cleaning process. Many times, the bulk of the flux is removed with the cleaning process, but there will be a monolayer of residue left behind that isn’t readily visible. The residues you can’t see can cause as many problems as the ones you can, so it is imperative to qualify the wash process to ensure that all of the residues have been removed.
Then, there are two cure methods: thermal and UV exposure. Thermal exposure curing takes more time than UV but can take as little as 10 minutes to tack dry for further assembly processes while it continues to cure. UV cure is far quicker, but there is a risk of coating in shadowed areas never to cure. This is most commonly found on high-density assemblies. If UV coating migrates under components, it will never fully cure, and it will remain wet for the life of the product. Coating in this state can hold the dust and debris it is intended to block out. If the debris is metallic, you may be at an even higher risk of failure than not using a coating at all.
The good news is that UV coatings are much easier to inspect due to the addition of a luminescent property. This will allow the inspector to determine exactly where the coating is on the assembly and determine if the UV cure process is adequate to reach all the areas of the coating. UV-cured coatings also make it much easier to determine consistency with thickness. In general, the brighter the coating, the thicker the application. Dewetting of coating is common when residues are present, and many times, on sharp edges of leads and component bodies. This may leave some areas more vulnerable to the operating environment.
Determining the level of adhesion can be done using IPC TM-650 220.127.116.11. This method uses a 10 x 10 grid of 1 mm x 1 mm squares etched into the coating with subsequent application of tape and removal of the tape with a steady motion at a 180° angle. Inspection after the tape pull is based on how many of the grid pieces were removed with the tape. The remaining grid area is judged on a scale from zero to five with zero being more than 65% removed and five having none of the coating removed. This is primarily done on test coupons during process evaluation and not on actual product. We recommend this test on the actual product if possible because that will give you a much better idea of what to expect when the coating is combined with the chosen material set.
Now, it's time to circle back to the title of this month’s installation: "Sealing Your Fate." After you apply your coating of choice, that is normally the end of the process with no easy way to make repairs if necessary. You can certainly remove coating with a wide range of chemistries or dry ablation processes, but these are time-consuming and have their own inherent risk of introducing failure opportunities. The important lesson to take away from this column is that coating does not always prevent failures; it is just as important to look at your cleanliness levels just as you would with an assembly that is not bound for coating. If you have a dirty assembly, you might be buying a little time, but ultimately, you’ve sealed your own fate.
Eric Camden is a lead investigator at Foresite Inc.
This article was initially published in the August 2019 issue of SMT007 Magazine.