Measurement of microbump thermal resistance in 3D chip stacks

The thermal resistance of Pb-free ~25 μm diameter microbumps with pitches of 50, 71, and 100 μm has been measured with and without underfill in four high chip stacks. With underfill, the unit thermal resistance values were 8.0, 15.5, and 19.0 C-mm
²
/W for 50, 71, and 100 μm pitch microbumps, respectively. The average microbump height was 16.1 microns. For the 50 μm pitch case, the thermal conduction through the underfill is roughly equal to that of the microbumps alone.