Improvement of the reliability of the C4 for ultrahigh thermal conduction module with the direct solder-attached cooling system (DiSAC)

In the HITACHI MP6000 (HDS Skyline Trinium TM), the bipolar-CMOS processor dissipates about 600 W, and the new direct solder-attached cooling (named DiSAC) method has been developed for use with it. In this cooling method, the HDM is supported by the 97Sn/Ag C4 (or CCB; controlled collapse bonding) bumps, which are affected by almost all the deformation that occurs in the power on/off cycle. Hence, the fatigue life of the C4 bumps is most important in the application of this cooling method. In this paper, the causes of C4 bump strain are analyzed by the finite element method, and several techniques for reducing strain are simulated. A new method of estimating the fatigue life of the C4 connections, pseudo-elastic plastic creep analysis (EPC), is developed in order to improve the accuracy of fatigue life calculations, and is used to evaluate the creep strain in a 3D model. Using EPC and experimental C4 power cycle damage data, a new strain-fatigue life curve is defined. Process defects in the direct solder attachment are found to markedly shorten the fatigue life of the C4 connections, and the effects are estimated. All the technological developments presented are implemented in the DiSAC model, and the improvement in reliability is verified by experiment.