Power cycling effects on the structural stability of thermal pastes used in microelectronics

Abstract

Thermal pastes are a class of soft composite materials of great importance to the microelectronics industry. The function of these pastes is two-fold: (i) to transport heat away from the chip; and (ii) to accommodate mechanical stresses arising from the thermal expansion mismatch between the various materials in the package (substrate, chip, and heat spreader/sink). Due to the former requirement, thermal pastes are among some of the most highly-filled composite systems in practice (typical solid filler volume fraction > 70%). These materials are expected to withstand the significant normal forces, lateral forces, and temperature variations associated with chip operation and power on/off transitions. As a consequence of the numerous stresses imparted to the thermal paste during the package lifetime, rigorous characterization of the reliability of these materials over extended time periods is important in evaluating the paste performance in field use. Of equal and arguably increased importance, is the fundamental understanding of the origins of paste structural degradation modes to optimize thermal paste formulation, processing, and assembly-in-package procedures. We use a variety of tools to understand structural changes and degradation of several thermal pastes under simulated package conditions. Two such techniques include IR thermography [1] and optical microscopy. The former is used to obtain in-situ visualization of thermal paste structure with correlations of the paste surface temperature distribution, chip temperature distribution, paste bondline map, heat flow through the paste, and paste compressibility. The latter is used to resolve finer structural changes in the paste that are correlated with the large area IR thermography images. Using these tools, we study the development of common thermal paste structural degradation modes. Three of the most common forms of structural degradation induced by cyclic thermal and mechanical stresses that occur with power cycling are shown in Figure 1. Perhaps the most common of the three degradation modes observed in field failures is large area cracking. This mode is difficult to screen as its development occurs over a much longer time scale than the other two modes. The cracking is shown to stem from air displacement of the binder in binder drainage networks that develop with repetitive confinement of the paste during power cycling. We have successfully identified paste inhomogeneity and poor binder-particle dispersability to be key to the development of this degradation mode. © 2007 Materials Research Society.