Sealing your fate: the pros and cons of different coating material

The process of coating electronic assemblies has been around for decades. And it would be pretty useful if CMs didn’t keep doing it wrong.

The process of conformally coating an electronic assembly has been used since before the 1960s. An industry expert who preferred to remain anonymous explained its value this way: “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."

It sounds pretty useful, right? It would be—if contract manufacturers didn’t keep doing it wrong.

Four main types and their little-known drawbacks

There are several types of coating material available. In general, the main types are acrylic, silicone, urethane, and Parylene. There are probably some exotics out there as well, but these are the primary materials we see at Foresite. Consider the end-use environment when deciding which one is best for your product. You also ought to consider the drawbacks associated with each.

Out of those four, the one we see the least amount of issues with is Parylene. Because it’s applied with a vapor deposition process that requires the substrate to be extremely clean for proper adhesion, it performs well. When the 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 its cost. It is expensive and time-intensive to apply. We don’t often see it used in high-volume production, such as consumer electronics; rather, it’s used more often for high-end reliability products, such as medical devices and aerospace applications. While it is effective, most consumer electronics producers can’t justify the cost. Because of that, I’ll focus on the remaining three materials in this article. After all, these other three materials do not require the precleaning of the surface before application. And that 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. Unfortunately, conformal coating isn’t an all-powerful tool that stops all electrical leakage. Foresite has started many conversations (see Foresite’s blog for even more) about the risk of leaving elevated levels of ionic residues from any part of the process, and it’s no different when coating is involved. Coating is good at keeping out most everything, but given enough atmospheric moisture working with active/hydroscopic ionic residues, the debris will eventually penetrate 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.

Dendrite Under Coating

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 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 few as ten minutes to tack dry for further assembly processes while it continues to cure. UV cure is far quicker, but there is a risk that coating in shadowed areas will not cure. This is most commonly found on high-density assemblies. If UV coating migrates under components, it will never fully cure; 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 if you don’t use 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 allows the inspector to determine exactly where the coating is on the assembly and determine if the UV cure process is adequately reaching 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 is. 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 2.4.1.6. 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 it 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 article 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.