Method and apparatus for underfilling semiconductor devices

A method and apparatus for underfilling a gap between a multi-sided die and a substrate with an encapsulant material. The die and/or the substrate is heated non-uniformly by a heat source to generate a temperature gradient therein. The heated one of the die and the substrate transfers heat energy in proportion to the temperature gradient to the encapsulant material moving in the gap. The differential heat transfer steers, guides or otherwise directs the movement of the encapsulant material in the gap. The temperature gradient may be established with heat transferred from the heat source to the die and/or the substrate by conduction, convection, or radiation. The temperature gradient may be dynamically varied as the encapsulant material moves into the gap.