Torsional fatigue life prediction method for DRAM module

In this study, shear strain controlled torsional test method was developed for torsional fatigue life prediction of DRAM module. The maximum principal strain and its direction was measured using rosette gage during module inserting user environment, and it was verified that shear strain was the dominant driving factor for solder joint failure. Apart from other studies where torsional tests were controlled by angle or torque, shear strain was adopted as a control parameter, which minimized the distortion resulting from plastic deformation of board. Strain gage was attached near the outermost ball of the FBGA component to get the strain of the test board under torsion. The FBGA used Sn2.5AgO.5Cu solder ball and ENIG for pad finish at the board side. Solder joint reliability under various strain range from plusmn 2000 muepsiv to + 6000 muepsiv was studied in constant strain rate. Daisy chain resistance was monitored to check the solder joint failure and the fatigue life prediction was conducted applying Coffin-Manson equation using scale parameter acquired from weibull distribution of the life cycle at each strain range. SEM was utilized for the failure analysis after the test and dominant failure mode was found to be IMC/bulk solder mixed fracture at the board side. The proposed life prediction model can be applied to predict number of module inserting times as well as life cycle of a module under any torsional user environment.