A novel Ag-Cu lamination process

A novel direct lamination process of Ag foil on Cu substrate is developed successfully. The Ag foil is 280μm in thickness. In electronic packaging, Si chips are preferably joined to Cu substrate to dissipate heat. However, the mismatch in coefficient of thermal expansion between Si (2.7×10^‒6/°C) and Cu (17×10^‒-6/°C) is quite large. A buffer is thus designed to absorb and reduce the shear stress in a bonded pair. Our analysis shows that Ag is an excellent candidate as a buffer. It has low yield strength and can deform up to 50%. Therefore, in this paper, we develop novel laminating process to bond Ag foil to Cu substrate. After appropriate polishing, Ag and Cu surfaces become mirror finish. The Ag foil is then placed over the Cu substrate in a specific graphite fixture. The fixture applies static pressure of 600 psi to ensure Ag and Cu contact in atomic scale. The lamination process is conducted at 400°C for 5 minutes in a 50-millitorr vacuum environment. Nearly void-free joints are produced as confirmed by Scanning Electron Microscope (SEM). Transmission Scanning Acoustic Microscope (TSAM) identifies that approximately 60 % of the area is perfectly bonded. By this laminating process, Ag foil can be bonded to Cu substrate and function as a buffer. Progress is underway to refine the laminating process. Some applications such as flip-chip joint and high-temperature joint are being studied.