Estimation method of cracking probability of stacked overhang die during wire bonding

Wire bonding to a thin stacked overhang die causes die cracking. It is experimentally confirmed that cracking starts from one of chippings which are derived from dicing process and there exists correlation between chipping size and fracture load. Under the assumption of the weakest link model, the equation of calculating failure probability is derived as the form of the Weibull function. Die fracture load is measured and converted to die strength by using σ_u and m. σ_w, the driving force of the onset of the fracture during bonding, is expressed as the weighted volume integral of the stress over the chip edge where chippings exist and is obtained by using FEM simulation. Two fracture modes are predicted; “die edge” cracking and “pad vicinity” cracking. The former mode occurs where the bonding position is near the die corner and it is prominent in case of thin dies. The latter occurs where the bonding position is away from the corner. Failure probabilities are calculated to a variety of die thicknesses and overhang lengths. The longer and/or the thinner dies, the higher failure probabilities are obtained.