Popcorn Cavity Pressure

The evolution of the moisture concentration profile and cavity pressure in response to sudden exposure to a reflow ambient is shown for the simplest model of a plastic package with cavity. Careful definition of boundary conditions, particularly of the mass balance at the molding compound/cavity interface, shows that a superposition of two classical solutions to the diffusion equation for a specific class of boundary conditions is necessary. The boundary condition analysis also points the way to modeling cavity pressure correctly in more complicated and realistic cases. The zero cavity size limit of the analytical solution is interesting because it shows, contrary to common belief, that a cavity is not necessary for the existence of very high internal pressures to cause mechanical damage.