Issue link: https://iconnect007.uberflip.com/i/1337116
38 DESIGN007 MAGAZINE I FEBRUARY 2021 e main reason why coatings can become milky is largely due to moisture absorption. High levels of atmospheric moisture can create havoc with certain solvent-based materials. As the solvents evaporate from the coating, a cool- ing effect is experienced. If this cooling effect drops the coating below the dew point, then water will condense onto and into the coating as it dries. is can range from a reversible cos- metic effect (heat it up and the discoloration should disappear) to problems with the coat- ing developing adhesion, or its full protective properties. Blushing can be identified when the conformal coating becomes cloudy, hazy, or milky in appearance. Humidity is a big issue, especially if it exceeds 65%. Equally, low tem- peratures can also wreak havoc at lower than 15 o C. Issues that arise from assemblies/com- ponents stored in environments with exces- sive moisture can be addressed by baking the boards. Some of the problems that can arise from blushing include: • Surface tackiness • Poor adhesion • Incomplete cure • Poor adhesion with second coating • Dull, matte finish Blushing can occur when using aerosol coatings. To avoid blushing when coating via aerosol, there are some important points to remember. First, there is no need to shake the aerosol can before use, as this can add bubbles to the mixture which can then be transferred to the board. e board should be at a 45-degree angle to the can, (e.g., if the board is flat, the can should be at 45 degrees) and around 20-30 cm away from the can. Spray the board in a con- tinuous steady Z motion, coating evenly. e board should then be rotated 90 degrees, the coating applied again using the same motion, and the board rotated again, repeating the process until you have rotated the board a full 360 degrees. is allows an even coating of the correct coating thickness and ensures the coating can cover raised components and leads from different angles, thus ensuring the best coverage possible. A turntable can be used to aid this process. Other factors to help prevent blushing include maintaining a stable humidity and temperature in the production environment. At Electrolube, we recommend humidity between 50-65% and an ambient temperature environment between 15 o C and 25 o C to help combat blushing. Ensur- ing adequate time between coating and cure is also advisable to allow solvents to evaporate fully. e use of slower thinners can also assist with the potential reduction of evaporation/ condensation to reduce blushing incidences. Baking assemblies following the cleaning pro- cess can also eliminate moisture, as well as ensuring all storage atmospheres and contain- ers are sufficient against moisture. e quality and performance of a conformal coating material could also be compromised according to the method of application. is is commonly encountered when a product is transferred from one circuit manufacturer to another; for example, a product may be dip- coated in one country but selectively coated in another with the specification requiring that the same material be used at both sites. e problem that arises here, however, is that using a material formulated for dip coating in selective coating equipment can result in poor yield due to it being excessively fast drying, leading to bubble entrapment. You want fun- damentally different behaviour from a material in a dip-coating application than in a selective spray application. One of my customers spent six months trying to solve a bubble issue inter- nally, without realising that the root cause of this problem lay in the material formulation. Working with the customer, it became clear that by changing the solvent blend, the bub- ble entrapment issue could easily be resolved. Moreover, this solution simplified the process and reduced the cycle time significantly. And since the non-volatile formulation remained the same, there was no need to re-qualify.