| Electronic pads and interconnects consisting of a passivation layer (hard layer) and a metal layer (ductile layer), e.g. SiN/Al/SiO₂/Si, are very important in many electronic devices. However, they are likely to fail due to cracking of the passivation layers during operation of the devices. This study investigates the failure mechanism through finite-element analysis(FEA). FEA calculations reveal that residual stress is produced in the passivation layer as a result of thermal cycling and increases as cycling continues. During thermal cycling, the multilayer films are deformed by various stresses due to differential thermal expansion of adjacent materials, particularly shear stresses applied to the chip (or die) surface by the large thermal mismatch between substrate and chip (or die). The cyclic shear stress leads to accumulation of asymmetric plastic deformation (called the ratcheting effect) in the metal layers. The residual strain or stress in the passivation layer may be induced by the ratcheting deformation of metal layer. The effects of the thickness and the yield stress of the passivation and metal layers were analyzed through FEA and discussed. The residual stress in the passivation layer decreases with passivation layer thickness and metal layer yield strength, which is closely related to the likelihood of plastic yielding in the metal layer. |
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