On short-time-scale stress wave phenomena and initiation of mechanical faults in flip-chip configurations

Small scale mechanical faults including microcracks and interfacial delaminations that compromise the integrity of solder balls and bimaterial adhesion are crucial issues impacting the reliability of flip-chip devices of small feature sizes. A small rise in junction temperature in a gigahertz flip-chip ball grid array (BGA) is found to initiate broadband, dispersive stress waves having a main frequency in the 200-800 MHz range. Numerical investigations incorporating a generalized thermoelasticity formulated to account for short-time-scale thermal-mechanical phenomena show that these waves, although fast-attenuating with the short presence of a few microseconds upon power-on, propagate in the bulk and along bonding interfaces with extreme time rate of change of stresses as high as 10/sup 11/ Pascal/s (or 10/sup 11/Watt/m/sup 3/ in equivalent units). The high frequency and high power density associated with the propagating stress waves provide potent mechanisms for the formation of geometric singularities such as microcracks, small scale delaminations, and debond at short time scales (several microseconds). These singularities would eventually lead to mechanical detachment and ultimate electrical failure subject to the coefficient of thermal expansion (CTE) mismatch-induced stress state at operating temperature on a longer time scale (several minutes).