Microscopy studies of the interfacial properties of wire bonds for wirebonding optimization

Abstract

Interfacial properties of ultrasonic wire bonds were studied extensively during this project. Field emission scanning electron microscopy (FESEM) and analytical resolution transmission electron microscopy (ATEM) techniques proved invaluable in the investigation and analysis of the microstructure and chemistry of the wire bonds and their interface structures. In order to study the interfacial properties of the wire bonds, a difficult, but necessary TEM specimen preparation method for the highly delicate ultrasonic wire bonds was developed. Understanding the interplay between the bonding parameters, namely bond power, bond time, bond force and the excitation frequency (~62 kHz) of the ultrasonic transducer and their influence on the mechanism of bond formation and performance were the objectives of the project. Aluminum wire, 31.75 um in diameter, was ultrasonic bonded onto (a) silicon substrate with thin 1 um aluminum metallization, and (b) printed circuit board (PCB) with gold/nickel pad. Those ultrasonic wire bonds were optimized at the manufacturing level under the combination of the optimized bonding parameters of the ultrasonic transducer. SEM was used to perform the metallurgical and morphological investigations for the wire bonds. Debris and granular structures were observed over the top surface of the wire bonds. Moreover, cracks were also observed at certain bond heels by SEM. For the bond interfacial investigation, the wire bonds were needed to be observed in cross-section with the interface parallel to the electron beam direction. A special technique for preparing cross-sectional TEM specimens, which involved embedding the bonds in epoxy, dimpling, and ion milling, were adopted. For the aluminum wires bonded onto the silicon substrate with thin aluminum metallization, the interfacial microstructures, trapped second phases and defects were characterized. However, for the aluminum wires bonded onto the PCB with gold/nickel pad, the cross-sectional TEM specimen preparation methods showed greater sensitivity to ion milling since small thin areas of the aluminum/gold bond interfaces underwent different thinning. The reason was due to the relatively large differences between the atomic numbers of aluminum and gold, that undergoes thinning by ion milling at different rates. A set of wire bonds were fabricated using different values of bonding power, while the bond time, bond force and the excitation frequency (~62 kHz) were kept constant. The reason for this particular study was that earlier experiments showed that the interfacial morphology was highly sensitive to bond power. Through systematic and careful study of the bonding interface of these sets of wire bonds, new and better understanding of the mechanisms of the bond formation and the influence of the bond power on the bond quality were obtained by comparing with the bond width, bond length, pull strength and shear strength.