NEXT GENERATION SOLDER-SYSTEMS FOR THERMAL INTERFACE AND INTERCONNECT APPLICATIONS VIA LIQUID PHASE SINTERING

摘要

This paper reports on a new paradigm for highly flexible solder design, proffering high electrical and thermal conductivity, in conjunction with good mechanical compliance, via a novel Liquid Phase Sintering (LPS) approach. The new LPS solders comprise a high melting point phase HMP (e.g., Cu or Sn) with a small amount of a low melting-point phase LMP (e.g., In) at grain boundaries, such that different properties can be controlled by different constituents. In general, conductivity is dominated by the majority HMP constituent, while deformation is controlled by the minority, LMP grain boundary constituent. The LPS solders are suitable for both thermal interface material (TIM) and interconnect applications. As the application space for solders shifts in the future, and requirements for new property-sets emerge, the flexibility of the LPS solder approach will allow integration of different materials into new LPS solder-systems.Development of two LPS solders, Sn-In and Cu-In, is presented in this paper. Starting from a powder mix, green solders (GS) were formed, followed by LPS of the GS resulting in a microstructure of a dense polycrystal of Sn or Cu grains, with In distributed at the grain boundaries. Long sintering times or higher sintering temperature led to absorption of the liquid In into the Sn grains and reaction products between Cu and In, resulting in de-sintering following initial densification and deformation-resistant grain boundary regions (Figure 1).By optimizing the In content of the GS and the process window (~2 mins. At 160-170°C), highly compliant LPS solders with flow stresses close to that of pure In were obtained, where the overall flow behavior is controlled by that of the grain boundary regions (Figure 2). The electrical conductivity (a) of the Sn-In solder was found to be about half that of pure In, while those of Cu-In were considerably greater (Table 1, Figure 3). Assuming that the K-cj correlation for the LPS solders follows the same trend as of pure metals, the thermal conductivity (K) values for the LPS solders can be predicted from Figure 3.Based on the deduced contact thermal resistance values, a previously developed model was utilized to predict solder thermal conductivity values as a function of the LMP (e.g. In) volume fraction, HMP (e.g. Cu) particle size (D_p), Kapitza radius (a_k) and the contact resistance of the HMP/LMP interface (Rc~(th)) . The results indicate that particularly when the HMP phase has high thermal conductivity, LPS solders with very high thermal conductivity may be obtained as long as the grain size of the HMP is large.