The demand for higher clock speed and higher level of integration, and, at the same time, smaller die size for high-performance electronic packages has accompanied by a drastic increase in package power consumption and dissipation. Large transient thermal gradients are thus experienced upon each power-on. Unlike thermal cycling test which is usually performed to evaluate the fatigue life of solder joints, rapid thermal transient is of major concern for overall package reliability. This rapid transient at short-time scale suggests that dynamic thermal-mechanical phenomenon in packages needs to be fully understood and the associated failure mechanisms identified for improved microelectronic reliability. In this study, how rapid thermal transients affect the generation, spectral characteristics, and interference of the induced stress waves propagating in high-density packaging configurations undergoing power cycling are investigated. The short-time effects facilitated by dispersive waves in current packaging configurations are studied and the possible mechanisms behind mechanical failures including solder cracking and delamination are discussed. Knowledge base thus established enables a set of guidelines to be proposed for developing new types of polymeric underfill material. Materials hence formulated would have optimal characteristics in response to the short-time effect to effectively discourage transient wave phenomena and thus improve the overall package reliability. The presented investigation provides a better understanding of the underlying physics governing the response of a high-performance package subject to rapid thermal-mechanical transients, which could potentially be the foundation for the formulation of new molecularly designed packaging materials and the associated manufacturing processes.