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78 DESIGN007 MAGAZINE I MAY 2022 What are the main reasons for coating failure? Coating failure can happen for any number of reasons, some common and some not so common. Here are five fundamental reasons which are generally the result of poor product selection and/or application, or some under- lying problem arising from insufficient surface preparation or some chemical activity going on beneath the coating that is entirely unre- lated to the coating chemistry: 1. e formulation of the coating is insuf- ficient for the job. Choosing an appropri- ate material selection at the outset can help avoid problems such as condensation occurring, and the coating fails to maintain an adequate level of insulation when the PCB is subjected to a condensing environ- ment. ere are some coatings that are much more resistant to these sorts of con- ditions; failure could have been prevented. 2. Application is crucial to coating success. If the conformal coating has not cured prop- erly, it will not have a chance to develop its protective properties to the full. By get- ting the application right, you can resolve many issues in one hit. 3. Inadequate coverage or insufficient thick- ness. Sharp edge coverage can be difficult to achieve with many coatings and it can be hard to ensure sufficient thickness in these areas to maintain protection. A com- bination of material selection and applica- tion technique/workmanship will remedy these sorts of issues. 4. Unexpected interaction with other pro- cess materials used to prepare/build the PCB, such as flux residues, are particularly illustrative of this type of problem. In a "no-clean" process, for example, these can inhibit the cure of some types of coating or lead to a loss of insulation of the system, greater than either material in isolation. 5. Something else is going on that isn't directly related to the coating. Unless there has been meticulous attention to preparation or pre-coat cleaning regimes, potentially corrosive residues bridging the PCB's conducting tracks can, over time, cause failures. While the coating may delay failure for many years, at some point failure will inevitably happen. Is the application method important to the reliability of coatings? Application method is probably the num- ber one determinant of success. Oen a poor material, well applied, can be just as good (or sometimes better) than a material with great properties that is applied badly. Coating is about getting sufficient coverage of the sharp edges and metal surfaces without applying the material too thickly elsewhere. Some materi- als "apply better" than others and make this process more straightforward, but in the end, the performance of liquid applied coatings will always be determined by how well they were applied. Would a thicker coating be less likely to fail? To be honest, it depends on the coating chemistry. icker can be better up to a point but at some stage the coating material will be too thick and will either crack itself, or even cause cracking of the coated components themselves, for example, during thermal shock or thermal cycling. And depending on the type of coating material used, solvent entrapment (i.e., the solvent not having enough time to evaporate from the coating film before it hard- ens) can become an issue. On top of this, by adding unnecessary amounts of coating material you are in effect wasting it, adding to costs and, importantly for some applications, adding weight. Com- promised thermal management issues can also

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