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 cause d by ratcheting in an adjacent ductile layer. For example, on a silicon die directly attached to an organic substrate, cracking often occurs in the si licon nitride film over aluminum pads. The silicon die and the organic subs trate have different thermal expansion coefficients, inducing shear stresse s at the die corners. Aided by cycling temperature, the shear stresses caus e ratcheting in the aluminum pads. Incrementally, the stress relaxes in the aluminum pads and builds up in the overlaying silicon nitride film, leadin g to cracks.