SMT007 Magazine

SMT007-Aug2019

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30 SMT007 MAGAZINE I AUGUST 2019 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. Con- formal coating is sometimes thought to be some magical application that stops all electri- cal 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 differ- ent 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 deliv- ery. Any manual method effectiveness is nor- mally 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 assem- blies from being dipped based on keep-out areas that are hard to mask. Automatic spray systems are among the most repeatable meth- ods 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 differ- ent points of view on this process. In general, we see fewer problems with coated boards that have been properly cleaned before coat- ing, like the Parylene application. We have seen adhesion issues with this practice when the flux residues are not fully processed to cre- ate 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 clean- ing 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 impera- tive 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 lit- tle as 10 minutes to tack dry for further assem- bly 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 lumi- nescent property. This will allow the inspec- tor 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 We regularly receive failures in our lab from people who are astonished that coating didn't stop dendrite growth on their product.

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