Effect of bonding temperature on eutectic interconnections in Chip-On-Film packages

Chip-On-Film (COF) packaging is an attractive solution for the direct attachment of chips onto a polyimide (PI) substrate patterned with copper leads. However, despite its many advantages, the eutectic Au-Sn alloy formed in the high temperature bonding process has a number of unsolved reliability issues. Accordingly, the current study performs a series of experiments to investigate the heat dissipation characteristics of the COF process in order to develop a design tool to predict the optimum eutectic bonding parameters. In the experiments, the temperature is measured at three separate positions on the inner lead bonding (ILB) apparatus using a thermocouple wire. The heat transfer characteristics of the bonding tool surface are also measured using a system comprising a PID controller, a solid state relay and a thermocouple extension wire. An finite element (FE) model is then constructed to simulate the temperature distribution within the COF/ILB system under equivalent temperature conditions. The validity FE model is used to construct a design chart which specifies the temperatures of the bonding tool and chip support stage which ensure that the Au-Sn eutectic temperature is achieved at the bump / inner lead interface. The results indicate that the tool temperature should be no lower than 100degC (with a stage temperature of 312degC) and no higher than 300degC (with a stage temperature of 302degC).