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Design007-May2020

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66 DESIGN007 MAGAZINE I MAY 2020 ways sometime between the application of the material and the exposure to radiation, and during this time, the material will still be sub- ject to gravitational and capillary forces, again resulting in some degree of coverage issues. Given that selective coating with an XYZ- capable robot is now the dominant applica- tion technique, and these robots use a valve that is traditionally mounted perpendicular to the board, these issues are exacerbated, as the coating material is also projected with momen- tum perpendicular to the board. This reduc- es the chance of actually landing on a sharp corner or vertical surface and facilitating the gravitational and capillary flow towards the board surface. Tilting the application nozzle to a 45-degree angle increases the chance of ac- tually hitting the vertical surfaces with mate- rial and reduces the vertical momentum and, consequently, the degree to which the material will flow. Modern, more viscous materials will also resist the gravitational and capillary forc- es when sprayed in this fashion to ensure the areas that were coated remain coated. 2. What steps should I take to protect particularly sensitive parts to be coated? This depends on what the parts are sensitive to. If you are trying to protect parts from the ef- fects of humidity, condensation, or immersion in water, then thickness and coverage are the most important factors to consider, along with the choice of conformal coating. Some coat- ings are inherently better at achieving cover- age at higher thicknesses and have inherently greater barrier properties than others. Some- times, parts are sensitive to being coated, and either the component itself or the solder termi- nations can be damaged by conformal coating materials. This is especially true when the glass tran- sition temperature (Tg) of the material falls in the operating range of the board in question. As the coating material passes through its Tg, there is a delay between the decrease in elas- tic modulus and the increase in the coefficient of thermal expansion (CTE). This lag results in maximum stress being applied to the compo- nent or solder termination and can damage the component or solder. Over a sufficient number of cycles, solder can even be extruded by the coating, leading to shorts. Therefore, the Tg of the coating materi- al should ideally lie outside of the operating range of the PCB or at least towards the lower end of the operating range. Occasionally, taller components can be more susceptible to dam- age by vibration, even though coatings can be used to help mitigate the effect of vibration, but this is a very complex, multi-faceted issue and probably a separate column in itself. Usually, a staking compound or adhesive is used to glue the component body to the board to minimise its ability to move with vibration. However, the material has to be carefully se- lected to minimise CTE and needs to be suf- ficiently soft and elastic to dissipate vibration without being either too stiff or too elastic to avoid exacerbating CTE issues over the operat- ing conditions of the assembly. 3. Is coating/board failure more common on higher-profile parts of the PCB, and if so, why? This depends on the design of the assem- bly and the conditions to which it is exposed in the field. As discussed in point 1, there are numerous challenges of actually coating taller devices effectively. A lack of coating thickness and coverage make taller components more susceptible to failures by tin whiskers, arcing, corrosion, condensation, or immersion. How- ever, under conditions of high humidity, the board level terminations are often more sus- ceptible to corrosion due to the presence of flux residues, which exacerbates the tendency for localised condensation due to their hygro- scopic nature and subsequent corrosion due to their ionic nature. 4. Can you name five "enemies" to a successful coating process? Lack of cleanliness or the presence of con- tamination on the board before coating can lead to open defects in the coating due to de- wetting or non-wetting. The contaminants can affect or compromise the ability of the coat- ing to provide long-term protection by increas- ing the rate of moisture uptake and interac-

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