Relation between inner voids of plastic IC packages and non-Newtonian flow characteristics of resin encapsulant

The relationship between the flow characteristics of resin encapsulant an the number of inner voids in a plastic IC package was investigated. The number of inner voids decreased as the average transfer pressure rose. This indicated that the higher the viscosity of resin encapsulant in the molding die, the fewer the inner voids. However, the spiral flow (EMMI-1-66 standard), a general measure of resin encapsulant flow characteristics, is not proportional to the number of inner voids. This may be because resin encapsulant has non-Newtonian flow characteristics which change the viscosity with share rate. In comparison with the shear rate of an actual molding die, which is as low as 1 to 100/s in a runner or cavity, the shear rate of a molding die with spiral flow is as large as 100 to 1000/s. The viscosities in the low shear rate range were measured to get a good proportionality with the number of voids. In order to make the viscosity of the resin encapsulant higher in the low shear rate range, it is advisable to use fine filler or silicone flexibilizer