|How long do I have to cure my coating and why do I need to bother?|
Conformal coating curing is dependent on the material type used. For instance, solvent based acrylic conformal coatings generally have a three stage curing mechanism. This is tack free, cured enough to handle and fully cured. Typically, the timescale's are approximately 10-15 min, 30-90 min and 3-4 weeks, respectively but can vary depending on factors such as coating, extraction rates, temperature of the coating room and thickness of application.
Now, a coating could be applied to a PCB and after a period of two hours look fully cured. However, there will still be trace solvents in the film which have not evaporated at that point and which may be affecting the coating electrical properties enough that the PCB may fail in test. Therefore, the question of when is a coating fully cured is when it stops affecting the performance of the PCB.
Therefore, curing is only critical if the PCB needs to be operated, tested or calibrated and the coating cure level affects these tasks.
If this is found to be the case then the key issue is to accelerate the cure process enough by baking the coating for an extended period at an elevated temperature according to the material TDS instructions. Typically, 12 hours at 70C works effectively to fully cure a coating but it will depend on the coating thickness, the material itself and other factors.
Further, if polyurethane conformal coatings are used there is another cure mechanism of polymerisation where the conformal coating achieves its chemical resistance. This is typically after 24-72 hours depending on the coating type, the temperature, air flow and coating thickness.