Power Transistor Package Wire-bonding Process Reliability Study

Copper wire bonding has now gradually replaced gold wire bonding in the mass production of power semiconductor devices, esp. power transistors due to the advantage of lower cost and higher performance. However, copper wire bonding is destabilized by the proneness to oxidation and stiffness of copper wire itself. To improve the tolerance of copper wire bonding to process fluctuations and ensure its reliability, work is done based on the study of the principle and technology of copper wire bonding, focusing on process control of copper wire bonding, analysis and design on the Aluminum layer of chip bonding area. For illustration, the study of comparing with gold wire bonding proves the outstanding performance of copper wire bonding in this paper.The present study includes the following:1)The solution of copper wire oxidation. Determine the 0.8 L/min FAB protection gas flow. Through the track of anti-oxidation, make rail temperature 40°c and no oxidation, improve the wire bonding ability. In the process of analysis, the current size FAB and discharge time and the ball is proportional to size, and the current main effect since the size.2)The two main failure modes of copper wire bonding are welding and die crack. The failure mechanism was studied in this two failure modes, which shown that this two failure modes were both associated and mutually contradictory. Appearance Inspection,wire pull test and ball shear test , which outputted the parameters for selection, were used for the comparison of welding and die crack. The conclusion that the high pressure(90) and low-power(40) parameters are the most optimal one to improve the stability of the copper wire bonding.3)In order to design chips for copper wire bonding and enhance the applicability of copper wire bonding, the structure of multi-chip aluminum layer and substrate materials were studied. The best surface condition and thickness data of aluminum layer were obtained by lots of comparison tests and analyses. The tests and analyses also proved that the base material of Si was more impact-resistant and more easily damaged than SiO2. Through the analysis of die crack, the design principles was found that the square area of diode should be bigger than bonding point.4)As the changing rate of Copper– Aluminum INC is more slowly than the Gold– Aluminum one, copper wire bonding technology performs reliably during the aging test within high-temperature condition. By comparing the changes of VFBE, the junction of copper wire and the aluminum layer is more reliable than the gold one.