There is an increasing reliability concern of thermal stress-induced and electromigration-induced failures in multilevel interconnections in recent years. This paper reports our investigations of thinfilm cracking of a multilevel interconnect due to fast temperature cycling and electromigration stresses. The fast temperature cycling tests have been performed in three temperature cycle ranges. The failure times aare represented well by a Weibull distribution. The distributions are relatively well behaved with generally similar slope (shape factor). The failure mechanism is well fitted by the Coffin-Manson equation indicating a uniform acceleration. The observation of cracking in the interlayre dielectric due to fast temperature cycling stress from failure analysis agrees well with the failure mechanism modeling and the calculated Coffin-Manson exponent. Electromigration experiments have shown that devices failed due to extrusion-shorts without increasing of resistance of metal line. The failure times are represented better by the Weibull distribution than by the lognormal distribution (normally used for electromigration data). A simulation of stress buil-up in metal line using an electromigration simulator confirmed that the cracking of interlayer dielectric is the weakest spot and most likely to cause electromigration failure.