Microelectronic Wirebond Evaluation by Laser-Induced Ultrasonic Energy

摘要

A promising new method for assessing wirebond quality is being developed. The method uses transmission of ultrasonic energy through the bond interface to infer the quality of the first (i.e., wire to chip) bond in gold, thermosonic ball bonded die (aluminum alloy pad metallization). Preliminary measurements on the crescent bond (i.e., wire to substrate or lead frame) have been performed. Initial data show a clearly varying acoustic signature as the interface changes with accelerated aging at elevated temperatures, presumably due to increased intermetallic thickness. Initial data show that the real-time ultrasonic signatures correlate well with the ball bond shear strength within the limits of both test methods. The new method involves generation of the ultrasonic signal by thermal expansion in a small area of the ball bond that is rapidly heated by a focused laser beam, followed by detection of the transmitted acoustic signal on the chip surface near the ball bond, using laser interferometric techniques. Both time domain and equivalent frequency domain data show changes with accelerated aging, bonding machine parameters, and bonding pad structure and geometry. While results to date have been extremely promising, further work is underway to fully explore the sensitivity and application range of this new wirebond testing and quality assurance technique.