Effect of aging on fatigue crack growth at sn-pb/cu interfaces
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INTRODUCTION
IN most modem electronic devices, a solder joint provides both mechanical and electronic connections between and within the many levels of an electronic package. Notable examples are found in surface-mount technology and flip-chip bonding, where solder terminals serve as the necessary leadless interconnections between two separate electronic components. Because of the difference in the thermal expansion coefficient and elastic modulus between the materials on the two sides of the solder, these leadless joints are susceptible to thermomechanical fatigue degradation. With increasing device miniaturization and transmission speed, the reliability of solder contacts, especially in the long term, becomes a serious concem. However, because the long-term exposure of the solder joint to temperature excursions can result in changes in both microstructure and mechanical properties, the life prediction for the solder joint or the reliability control of the electronic device, which often involves time-dependent processes such as diffusion, fatigue, and creep, has become very complicated. Considerable effort has been made in addressing the reliability of Sn-Pb solder joints. Era,31Three approaches have been taken, including fatigue testing of real joints, t4-17] constitutive modeling of stresses and deformation in the joint,Vs,~9,20] and bulk property measurements on solder alloysY ~-25JThe fatigue testing of real joints simulates closely the service condition of the joint and is particularly helpful in sorting out a number of critical issues in the failure of the joint. However, because the deformation and failure of the joint involve several different time-dependent processes, the results of these tests tend to be joint-geometry specific and have poor predictive power. On the other hand, constitutive modeling deals with individual processes such as creep and plasticity separately and provides predictive models that integrate the contributions from individual processes. Its successful implementation so far has been DAPING YAO, Graduate Assistant, and J.K. SHANG, Assistant Professor, are with the Department of Materials Science and Engineering, University of Illinois at Urbana~hampaign, Urbana, IL 61801. Manuscript submitted January 17, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
limited by the lack of property data, uncertainties in failure criterion, and difficulties arising from microstructural evolution and from interactions among individual processes. More recently, studies on bulk solder alloys have begun to address some of these difficulties, e.g., by providing monotonic properties, fatigue properties, and creep parameters under different microstmctures. However, low cycle thermal fatigue studies of solder joints have shown that the failure often occurs at the interface between solder and substrate, which may be modified by the formation of intermetallic phases, t5,6,261The relationship of the bulk studies to such an interfacial failure process yet remains to be established. The intent of this arti
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