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Thin Film Cracking and Ratcheting Caused by Temperature Cycling

Layered materials are susceptible to failure upon temperature cycling. This paper describes an intriguing mechanism: cracking in a brittle layer caused by ratcheting in an adjacent ductile layer. For example, on a silicon die directly attached to an organic substrate, cracking often occurs in the silicon nitride film over aluminum pads. The silicon die and the organic substrate have different thermal expansion coefficients, inducing shear stresses at the die corners. Aided by cycling temperature, the shear stresses cause ratcheting in the aluminum pads. Incrementally, the stress relaxes in the aluminum pads and builds up in the overlaying silicon nitride film, leading to cracks.