Thermosonic bonding of an optical transceiver based on an 8×8 vertical cavity surface emitting laser array

This paper reports the results of our thermosonic (T/S) flip-chip bonding process development for the assembly of a smart pixel array (SPA) using an 8×8 vertical cavity surface emitting laser (VCSEL) array. The introduction of ultrasonic energy into the flip-chip bonding process increases the speed of the assembly process while at the same time lowering the physical stresses (temperature and assembly force) applied to bond the components. Many empirical studies have shown that T/S flip-chip bonding is feasible, but there is a lack of detailed understanding of the effects of the ultrasonic energy on the bonding results. We are conducting experiments and developing models that will provide a sound understanding and a rational basis for T/S flip-chip bonding. In particular, we have addressed the problems of the impact of joint bump size, control of the assembly force, and the repeatability of the ultrasonic power. This report details our findings concerning the following aspects important to the development of T/S flip-chip bonding technology: (1) Computer modeling to guide the selection of design parameters and provide a basis to study the effects of the interaction of the critical design and process parameters on process yield. (2) Design of a new end effector for accurately applying and monitoring small assembly force. (3) Monitoring and controlling the impedance of the ultrasonic mechanical and electrical system in order to insure repeatable delivery of acoustic energy to the assembly