High temperature Ag-In joints between Si chips and aluminum

The operating temperature of packaged devices is often constrained by the melting temperature of the die-attach material. Taking the most popular lead-free solder SAC as an example, its melting temperature is 217°C. Using 80% of the homologous as a guideline of maximum operating temperature, it corresponds to 119°C. Many advanced semiconductor chips can operate at a much higher temperature. It is thus clear that high temperature die-attachment materials are in demand. The challenge is that typical bonding processes require the bonding temperature to be higher than the melting temperature. This temperature is often too high for the chips and other packaging materials to survive. In this research, we looked into the Ag-In system and developed a bonding process that needs only 190°C but produces joints that have melting temperature higher than 600°C at least. The process is entirely fluxless. To demonstrate this new process, silicon chips were bonded to aluminum (Al) substrates. Al was chosen as demonstration substrate because it has the highest CTE, 23×10^-6/°C, among all substrate materials. The Si-Al pair provides the largest CTE mismatch among all electronic bonding structures. This large CTE mismatch allows us to show the strength and durability of the resulting Ag-In joints. Any weak joints would have broken as the samples cooled to room temperature as a result of the severe CTE mismatch. Many samples were produced. Despite the large CTE mismatch, no joint breaks. To evaluate the joint quality, composition and microstructure, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX) is used. Nearly perfect joints are produced. The joint consists of Ag, solid solution (Ag), and Ag₂In in stack of Ag/(Ag)/Ag₂In/(Ag)/Ag. The melting temperature is higher than 695°C. To determine the strength of the joints, six samples went through shear test. They all pas- the 5kg requirement specified in MIL-STD-883H standard. The fracture force ranges from 17kg (6.7MPa) to 94kg (36.9MPa). There are two fracture types. One is fracture within Ag-In joint where it breaks along the Ag₂In region in brittle mode. The other type is Si chip breakage. It is worthwhile to mention that Al is not solderable. A new process was developed to make Al bondable that results in high bonding strength as evidenced by the high shear strength measured.