A Coupled Thermal-Mechanical Analysis of Ultrasonic Bonding Mechanism
- PDF / 740,939 Bytes
- 12 Pages / 593.972 x 792 pts Page_size
- 64 Downloads / 199 Views
INTRODUCTION
ULTRASONIC welding uses a high-frequency (usually 20 KHz or above) ultrasonic energy source to induce oscillating shears to produce solid-state bonds under a clamping pressure between a wide range of metal sheets,[1,2] thin foils,[3] semiconductors,[4] plastics,[5] glass,[6] and ceramics.[7] In contrast to traditional fusion welding processes, ultrasonic welding has several inherent advantages[3,8] derived from its solid-state process characteristics. Although numerous researchers have been studying the bonding mechanisms of ultrasonic welding for over 50 years, the process is still arguably the least understood welding process. Various mechanisms have been proposed for ultrasonic welding including interdiffusion, recrystallization, plastic deformation, work hardening, breaking of contaminant, generation of heat by friction, and even melting.[9,10] Diffusion has been observed at the interface between copper and aluminum welds for an extended period of welding time.[11,12] It has also been found that diffusion occurs along grain boundaries rather than in the bulk of the material.[13] Kreye et al.[14] examined the microstructure at the weld interface with transmission electron microscopy and claimed that the very small grain size observed in a thin layer could only be explained by melting and solidification. However, Harthoorn[15] and Heymann[16] concluded that neither diffusion nor recrystallization could be responsible for the joint formation of ultrasonic welding after comparing low-frequency vibration welding with ultrasonic welding of aluminum and examining the copper and soft iron ultrasonic welding. A great deal of plastic
CHUNBO (SAM) ZHANG, Doctoral Student, and LEIJUN LI, Associate Professor, are with the Department of Mechanical & Aerospace Engineering, Utah State University, Logan, UT 84322-4130. Contact e-mail: [email protected] Manuscript submitted August 11, 2008. Article published online February 18, 2009. 196—VOLUME 40B, APRIL 2009
deformation and metal flow occurs across the interface, and flow lines, evidence of extensive plastic deformation, are visible in the bond zone.[3,17,18] For the ultrasonic welding of aluminum foil, plastic flow occurs in a narrow interfacial zone about 10 to 20 lm in width.[3] In this region, new subgrain structures form across the bond zone. When the relative motion at the beginning of the welding cycle cleans the surfaces and plastically deforms asperities,[19] microwelds—areas in which the friction exceeds the flow stress level of the material and plastic metal flow has started—occur immediately between points of contact of the adjacent surfaces and spread out until a sufficient weld area is built up.[9,10] Zhou et al. investigated the effects of process parameters on bond formation in thermosonic gold ball bonding on a copper substrate at ambient temperatures with scanning electron microscopy.[20] They concluded that relative motion existed at the bonding interface as microslip at lower powers, transitioning into gross sliding at higher powers. Researchers in a vari
Data Loading...
