Study on ultra low loop copper wire bonding process using 0.8mils Cu wire for low profile package applications

In recent years, as IC packaging development has moved towards higher power, smaller size, thinner dimensions, denser circuits and higher reliability, wire bonding is still the most commonly used bonding interconnection technique in first level microelectronic packages [1]. Gold wire is usually used for bonding to connect the chip and lead frame/substrate metallization because of its easy handling and strong bond with the bond pad metal. However, recently copper wire emerged as an alternative to gold wire due to mainly huge merit in cost saving, higher electrical and thermal conductivities than gold wire, which will contribute to higher signal speed, less heat generation and better heat dissipation[2]. Copper wire has been used in low pin count ICs, high pin count products and stacked die. In high I/O applications, copper wire with smaller diameter is required and wire sweep performance is critical as even small wire sweep may cause wire short. Wire short can be prevented with having low loop height specification for the high I/O application and to achieve thinner and smaller packaging. In this paper, heat-affected zone (HAZ) for three types of Cu wire material of 0.8mils wire diameter were studied to achieve the ultra-low loop height range of 75.0μm. The ultra-low loop wire bonding process optimization for three different Cu wires and bond quality were assessed. The advantages and disadvantages of individual 0.8mils Cu wire types were discussed by comparing the wire pull measurements for three types of 0.8mils Cu wire. The surface of silicon chip after bonding was checked to confirm whether peeling or crack on bond pad surface occurred after the wire pull test for each wire types using high power optical microscope.