Thermocompression bonding of aluminum bumps in TAB applications

Tape automated bonding (TAB) technology has been widely used in low cost consumer products as well as in high performance computer applications. TAB is an important chip-level interconnection scheme which enables VLSI devices to be packaged as direct chip attach (DCA) or chip-on-board (COB) format. Two key issues of TAB technology are discussed in this paper: wafer bumping and inner lead bonding. Specifically, aluminum bump technology developed for TAB applications is reviewed in comparison with the conventional wafer bumping process. Thermocompression bonding experiment is conducted to define a bonding process window of the pure Al bump. Although the pure Al bump can be bonded for a high bond strength, an excessive plastic deformation is commonly observed, which can dismiss the original purpose of the bump structure. In order to reduce the bump deformation, Al-alloy bumps are fabricated by adding a small amount of an alloying element to the Al matrix, such as Ni, Cr, Cu and others. The harder Al-alloy bumps are bonded with a less bump deformation and a higher pull strength. By this investigation, a general relationship between the pull strength and bump deformation is established for the Al and Al-alloy bumps. A desired bump hardness is recommended for the successful thermocompression bonding of the Al bumps.