Conformal coating thickness can be critical to the proper function of a PWA. If a coating is too thin, proper coverage is impossible; if a coating is too thick, it may create excessive stresses on solder joints and components (particularly glass-bodied components). Controlling coating thickness is of special importance with rigid coating materials (e.g., epoxies and some of the urethanes) because the residual stresses associated with an excessively thick application of these materials are much greater than with flexible coating materials (such as silicones and some urethanes).
Conformal coating thickness requirements are in accordance with IPC J-STD-001 for the following coating types:
|Type AR||Acrylic Resin||0.00118 to 0.00512 in|
|Type ER||Epoxy Resin||0.00118 to 0.00512 in|
|Type UR||Urethane Resin||0.00118 to 0.00512 in|
|Type SR||Silicone Resin||0.00197 to 0.00827 in|
|Type XY||Parylene Resin||0.000394 to 0.00197 in|
Additionally conformal coating thickness is measured on a flat, unencumbered, cured surface of the printed circuit assembly or a coupon that had been process with the assembly. Coupons may be of the same type of material as the printed board or may be of a nonporous material such as metal or glass.
Conformal coating thickness/coverage shall have the following characteristics to ensure that coating thickness specifications are upheld:
Repairs and Rework Performed per IPC-7711 and IPC-7721 Specifications.