The Study On Wafer Warpage And Controlling Strategies For Wlcsp

Wafer Level Chip Scale Package(WLCSP)has been widely used in the field of advanced electronics packaging for its fewer manufacturing processes and higher production efficiency.It meets the requirements of high efficiency,low cost and miniaturization in semiconductor industry.In this paper,8 inch WLCSP wafers were taken as the research objects.Aiming at the warpage problem in packaging process,the warpage of WLCSP wafer in molding process and grinding process was studied by experiment and numerical simulation methods.The process of WLCSP six-side packaging structure was developed,including passivation layer deposition,redistribution layer forming,solder ball dropping,wafer pre-cutting,topside molding,backside grinding,backside molding,final wafer cutting and so on.During wafer packaging process,it is found that wafer warpage is especially serious after the topside molding and backside grinding processes.The maximum warpage values are 1.2 mm and 17.9 mm,respectively.The backside molding has a certain mitigation effect on wafer warpage,but it can not effectively eliminate the warpage problem.The WLCSP finite element model of six-side package was established.The simulation results of wafer warpage evolution are consistent with the measured results after the actual processes.The numerical simulation results show that the warpage deformation increases with the increasing of the elastic modulus and thermal expansion coefficient of molding materials.The effect of thermal mismatch on wafer warpage is higher than that of the elastic modulus.For the choosing of the molding materials,we should firstly consider the thermal expansion coefficient of the materials.The materials with the similar thermal expansion coefficient as the silicon material can reduce the influence of thermal mismatch.With the decreasing of wafer thickness,the bending rigidity of wafer decreases,and the warpage of wafer caused by thermal mismatch increases rapidly.Within the allowable range of process,we should try to choose thinner molding thickness to reduce the influence of thermal mismatch between materials on wafer warpage.The backside molding process can relieve the warpage of the former process to a certain extent,but larger backside molding thickness can cause the wafer reverse warpage.The appropriate topside and backside molding thickness can better control the warpage.According to the results of numerical simulation analysis,the packaging processes were optimized,and a new process scheme of wafer level six-side packaging was proposed and verified by experiments.The maximum wafer warpage value was within 1mm.The chip after wafer final cutting process passed the highly accelerated stress reliability tests,and the wafer warpage problem was effectively solved.It provides a reference for packaging process design and production of wafer level products.In addition,different wafer warpage shapes were observed in the optimization of wafer warpage.The change of wafer warpage shape was studied by numerical simulation and experimental measurement.The results of numerical simulation show that wafer thickness has an important influence on wafer warpage shape.Before wafer grinding,the wafer usually presents spherical warpage.The cylindrical warpage of wafer occurs when the wafer thickness is less than 300 ?m.The change law is consistent with the experimental results.