Copper Wire Bonding: R&D to High Volume Manufacturing

During the past two years copper wire bonding has entered high volume manufacturing at a number of leading edge OSATs and IDMs. Usage of copper wire has achieved 20% market share and is expected to exceed 50% within three years. Products spanning the range from low pin count devices with relatively large wire diameter to FPGA's with nearly one thousand wires at 20 μm or even 18 μm wire are now using copper wire. This paper will discuss the requirements for developing a robust copper wire bonding process and then moving it to high volume manufacturing. Process optimization begins with the selection of the appropriate wire diameter, ball diameter, bonding tool and bonding process type. These are functions not only of the bond pad opening, but also of the pad aluminum thickness and relative sensitivity of the pad to damage. Proper optimization depends on the availability of new and modified bond quality metrologies, such as extensive reliance on cross-sectioning and intermetallic coverage measurements. The bonding window of a copper wire bonding process is defined in substantially new terms compared to optimization in gold wire bonding. Once an optimized process has been developed in the lab on a single bonder, it needs to be verified. Copper wire bond processes are much less forgiving with respect to the acceptable variability on the manufacturing floor. To ensure that the process is stable, a low volume pre-manufacturing test is highly recommended. This not only makes sure that the process is stable across multiple bonders, but also ensures the adequacies of manufacturing controls, incoming materials quality and sufficient equipment calibration and maintenance procedures.