Thermo-mechanical stress analysis and optimization for 28nm extreme low-k large die fcBGA

The pre-solder crack phenomenon after thermal cycling test is observed in 28nm extreme low-k (ELK) large die fcBGA (flip chip ball grid array), which may come from the resulted critical stresses in IMC (intermetallic compounds) on Cu pad layer. For the purpose of realizing the thermal-mechanical stress distributions in 28nm ELK large die fcBGA, a comprehensive study for the effects of package geometry is investigated by using three-dimensional finite element analyses (FEA) in this paper. The effects of under bump metallurgy (UBM) size, solder resistant opening (SRO) size, solder bump dimension, thermal interface material (TIM) thickness, Cu pad diameter, substrate thickness and its coefficient of thermal expansion (CTE) are discussed by Taguchi L16(2⁷) methodology to figure out the most significant factors. Through the statistical results, it is found that the factors of UBM size, SRO size and Cu pad diameter had significant contributions to stress responses. The most important parameters of UBM and SRO size ratio (UBM/SRO) as well as Cu pad diameter and SRO size ratio (pad/SRO) that comprehend the corresponding stress responses are captured by using response surface methodology (RSM). To have further discussions of these significant factors, dissections for UBM, SRO and polyimide (PI) opening size are also illustrated. The simulated results can be good references and effectively served as design guidelines to avoid the critical stresses as well as enhance the reliability in 28nm ELK large die fcBGA.