Quantification of Performance Variation and Crack Evolution of Bond-Wire Interconnects Under Harsh Temperature Environments by S-Parameter Analysis

Abstract

Electrical interconnects play important roles in both the performance and reliability of integrated circuits. As the operating frequency has increased and products have been exposed to harsh environments such as temperature extremes and thermal cycling, it has become essential to quantify the thermal effects on the reliability of such circuits. In particular, the effects on the signal transmission performance of the interconnects and the evolution of cracks caused by thermo-mechanical stresses need to be investigated. This study proposes that both research goals can be accomplished by S-parameter analysis alone. First, the performance variation was experimentally quantified by in situ measurements of S21 (an indicator of signal transmission performance) on bond-wire test vehicles at static temperatures between -60 degrees C and 120 degrees C. An impedance model of the interconnect was developed focusing on thermal expansion effects and it predicted the performance variation of the interconnect successfully at the given temperatures. Second, the results of ex situ measurements of S11 (an indicator of return signal) obtained using a test vehicle under thermal cycling were compared with scanning electron microscopy images of crack propagation in the interconnect. In the crack initiation phase, a resonance peak occurred on the S-parameter pattern and the peak moved toward lower frequencies as the crack propagated until failure. SPICE simulations using a suggested impedance model of the cracked interconnect confirmed the experimental results. In addition, this study demonstrates that monitoring the resonant frequency in the S-parameter pattern as a prognostic factor can predict the initiation and evolution of cracks earlier and is more sensitive than the conventional dc resistance measurement method.