Characterization and reliability analysis of wedge wire bonds inside a package subjected to ultrasonic welding and thermal cycling

Relatively little information is available on the mechanical effects of the ultrasonic welding plastic lid attachment process on wire bonds contained in integrated circuit packages. This paper presents a study on the reliability and mechanical effects of the ultrasonic welding lid attachment process on 1 and 1.5 mil gold wedge wire bonds in an integrated circuit package provided by Quantum Leap Packaging. This study also considers the effects of thermal air-to-air cycles on wire bonds. Theoretical modeling of the wire geometries showed that wire mode natural frequencies were significantly different from the operating frequency of the ultrasonic welding process and would not cause failure due to high stresses associated with the excitation of natural frequencies. Through scanning electron microscope inspection and wire bond pull testing, it was shown that the ultrasonic welding process did not have any damaging effects on wire integrity or cause any decrease in wire bond strength. During experimental study, wire bonds exposed to thermal air-to-air cycles of -65°C and +150°C experienced a decrease in wire pull strength. Thermal cycling also caused an increase in the number of wires that separated in the bond between the wire and the bond pad during wire bond pull testing, indicating a decrease in the integrity of the bond. Scanning electron microscope observation during experimentation showed the presence of wire heel cracks after the wire bonding process as well as the development of cracks after thermal cycling. This result is an indication that the bonding process may weaken the bond at the wire heel and be exposed by the thermal cycling process. Such observations that show that wires can be damaged due to thermal cycling suggest that the wire bonding and thermal cycling processes should be examined in future research.