4. Electronic packaging processing
Packaging process is another important factor that impacts the heat dissipation of packaging material. Conventional metal packaging process is mainly comprised of four steps. Firstly, Kovar substrates and Kovar enclosures are brazed with Ag-Cu eutectic alloy at about 830 °C to form a cavity. Secondly, the Au-Si eutectic alloy is utilized in order to attach the die to the cavity. During this process, a gold preform is placed at the top of cavity while heating the package. As the die is mounted over the gold preform, Si from the die backside diffuses into the gold perform, resulting in the formation of Au-Si alloy. A further diffusion of Si into gold preform enhances the Si-to-Au ratio of the alloy until the eutectic ratio is achieved. The Au-Si eutectic alloy contains 2.85% of Si and melts at about 363 °C. Hence, in order to obtain the eutectic melting point, normally 380430 °C, the temperature of attached die must be reasonably high. The third step is wire bonding that supplements the electrical connection between silicon chip and external leads of the semiconductor device by using very fine bonding wires. Finally, Sn-based solder is used to seal a lid to the package at 200–330 °C. For the brazing of advanced composite, the biggest challenges are surface finishing and brazing processes.