The first few boards that we are building will be prototypes.Should we be under-filling the GaN FETs for the lab asset? We are not expecting to run and temp cycling tests on these prototypes
for lab testing we never underfill. This is because it is difficult to repair a board if it fails and can cause more damage. The key here is to make sure the flux does not form dendrites. For lab boards we clean with a flux cleaner and for production it is either bake for no-clean flux or wash for soluble flux. In production builds we use underfill primarily to prevent damage to the FET.