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58 SMT007 MAGAZINE I MAY 2018 nate surface. If the peelable mask cures very hard, it may rip off components, compo- nent leads or PCB lami- nate during the de-masking process. Conversely, the peel- able mask breaks up easily during removal, so you may leave behind residual mate- rial, which may cause long- term reliability problems. Silicone Boots Silicone boots, customized for different connector and other electronic components, is another conformal coating masking method. They are often used for higher assem- bly volume jobs to greatly reduce the time and effort required for mask- ing. The boots, due to their self-sealing nature, do not leak as frequently as tape dots and masks, thereby eliminating the need for coat- ing re-work. For higher board volumes, these boots are very cost-effective and can reduce the masking time by 60-70%. One drawback to this masking method is that they will not work in conformal coating dip applications. Some boots can be re-used 125-200 times, depend- ing on their care, and thereby driving the cost- per-use down. 3D Printing of Custom Boots Another possibility for masking certain areas of a PCB includes the 3D printing of covers or boots to cover components. These 3D printing covers can be made from a variety of materi- als and offer the same potential benefits as the silicone boots. The material of the boots will determine the seal around the devices as well as temperature that the boots can withstand in the curing process. While the 3D printers are very cost-effective today, their low precision, the length of time to print, the lack of ability of the lower end machines to print heat-resis- tant pliable materials, and the need to have 3D CAD capability within the SMT assembly oper- ation make this option limited. Tape Materials All the tape derivative options may need to be tested. For example, they may or may not be able to prevent uncured coat - ing material from leach- ing underneath the tape parting line. For example, some tape adhesives leave residues behind that may inhibit the long-term reli - ability of the electronics assembly by reducing SIR values or allowing accel- erated dendritic growth to take place. Adhesives used in these tapes must have properties such that they do not peel off the cured coat - ing material, leave unwanted residue on the PCB, or damage components. If the OEM spec- ifies the allowable materials it may limit your options on material choices and type of mask- ing. For example, even the slitting knife-coating material may cause unwanted silicone debris or de-wetting problems. Ensure that when these tape-based masking methods are used, all tape materials are compatible with the coating mate - rials and the coating process being employed. Summary Whatever spraying method is employed in applying the conformal coating, having a handle on what type of masking method to use is critical to making sure the PCB assembly, rework or repair process is done properly and cost-effectively. SMT007 References 1. Kapton is a trademark of EI DuPont. 2. 3M Type 401 is a trademark of 3M. Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago. To contact Wettermann, or read past columns, click here. Figure 2: Peelable mask for conformal coating.

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