Wirebonding: reinventing the process for MCMs

Summary form only given. Wirebonding is the mainstay interconnect for current IC products. Due to the standardization of pad metallurgy, wire types, package structures and bond optimization methods, wirebonding of individually packaged parts has achieved significant levels of quality and reliability. The situation in the multichip module (MCM) world is quite different. The introduction of the interconnection substrate with its typically different metallurgy, variable trace and pad geometries, multiple fabrication and attach process, and now in many cases a flexible or compliant (soft) structure has caused significant increases in wirebond failure rates and bondability concerns. We have conducted a systematic study of wirebonding processes (both thermosonic and ultrasonic) for multichip applications. The study included a variety of rigid, semi-rigid and compliant type substrate structures, both Si and GaAs chips, and even some exotic detector chips (CZT) and array structures. In this paper, we describe further results in bonding to soft or compliant substrate structures. Parameters investigated include bonding pad structures and morphology, the effect of ultrasonic delay on allowing compliant materials to recover after application of bonding force, and the influence of changing (increasing) the ultrasonic frequency from the standard 601 Hz. Again, the wirebond pull test and the ball bond shear test are used as measures for improvement. Strength maintenance as a function of aging at elevated temperature is used as an indicator of reliability.