Not seen any precut sheet suited to this but a good chemical alternative is Rosin Lacquer as Rosin naturally repels water/moisture and is about the only material capable of locking away contaminants where as Conformal coatings generally only go over the top of them. Some liquid rosin fluxes offer this protection if left on the joint or PCB.

There are many types of conformal coating and many ways to apply them. I am not aware of any die cut materials that would work in this application. Some solder companies claimed that some specific high residue pastes would act as a protective barrier around component legs but this never became popular.

If it is just local protection required on one specific part of a PCB then I suggest that you investigate selective application systems that can dispense, spray or jet the coatings onto specific parts of the PCB, or a paint brush!

One other alternative would be a hot melt conformal coating applied locally with a heat gun as this is VOC free and cures within seconds. It is also important to make sure that you select the correct conformal coating and one that is compatible with all other chemistries on your PCB.

Some manufacturers use epoxy to pot their products to protect them from any environmental exposure. By creating a cavity and placing the board in the cavity and filling it with epoxy is one way to accomplish this task.

After the potting process however there is no turning back, no rework, no nothing, if it works great and if it doesn't then it is scrapped. I'm mentioning this as this has to have some ROI analysis to compensate for yields and disposal cost.

Die cut sheet metal, would have to be soldered in place such as with microwave components. The sections on the board would have to be designed to have a soldered shield to prevent moisture entrapment and prevent leakage. This would also require the plugging of all via holes to prevent moisture from coming through those openings.

Best of luck

If you think about what your goals are with conformal coating, then you can look at alternative solutions. You are trying to protect a solder joint in your case from moisture. If moisture condenses on the solder joint you can start to have adverse effects such a dendrite growth.

By covering the solder joint with a polymer you are providing a barrier to moisture. Other materials such as encapsulants can provide similar protection. The problem is that you have to start to think about what is in the fluid you are now going to use in place of the conformal coating. Is the encapsulant clean or does it have any chlorine in it as a number of low cost epoxies do.

Remember that this work has been done for you by the conformal coating manufacturer.

Similarly if you try to use an epoxy preform, how does it wet to the solder joint.Does it flow well over the jointand not have any inclusions where moisture can coalesce?

In the end it may be easier to use conformal coating because some of the work has been done for you. The trick may be to use a selective coating system that is easy to use and program to minimize the work load on your operators.