Failure Analysis and Assembly of Package-On-Package Module

Hui Xiao, Daojun Luo, Yabing Zou, Tao Lu
China Electronic Product Reliability and Environmental Testing Research Institute
Guangzhou, China

With the never ending drive for smaller, lighter and more advanced features on handheld products, Package on Package (PoP) assembly that integrates logic die in the bottom package and memory die in the top package into a single 3D package is one of the most promising solutions to enable these requirements. However, there are still some challenges to assuring mounting reliability and yield for PoP module during manufacturing processes.

In this paper, both failure analysis and finite element analysis (FEA) were utilized to study the thermally-induced yield loss for POP modules. Failure analysis involves a serial of tests: X-Ray inspection, microsectioning and SEM analysis, package warpage measurement, etc. FEA was conducted to analysis the package warpage evolution and stress/strain evolution behavior of the PoP module during the full assembly process from the package molding to module reflow. Finite element techniques such as element birth and death as well as restart were used to transfer the values of stress and warpage from one process to the next.

The results show that the thermally-induced warpage is an important manufacturing risk and a yield detractor in the fabrication process of mobile electrical products. For the failure PoP module samples in this paper, the top and bottom packages exhibited different warpage evolution behaviors during reflow process and the maximum deformation difference happened at the peak temperature (260 Celsius). And there was mainly tensile stress in the top solder joints and compressional stress in the bottom ones during assembly process. This significant thermal-induced deformation and stress were the main cause of the soldering defects, such as solder incomplete fusion and cracks in solder joints, especially for the top layer. The FEA result was consistent with experimental observation and result. The developed FEA model in this paper can be used to do further parametric studies to optimize the package structure design and help material selection for improving the PoP module reliability.

The thermally-induced warpage is an important manufacturing risk and a yield detractor in the fabrication process of mobile electrical products.

Failure analysis is an effective tool to find out the root cause of related failure modes for PoP module, which involves a serial of tests: X-Ray inspection, microsectioning and SEM analysis, package warpage measurement, etc. For the failure PoP samples in this paper, the failure analysis results show that the top and bottom packages exhibited different warpage evolution behavior during reflow process and the maximum deformation difference happened at the peak temperature (260 Celsius). This remarkable deformation difference is the main cause of the soldering defects, such as solder incomplete fusion and cracks in solder joints, especially for the top layer. In addition, there was initial warpages for the packages before assembly, which corresponds to the residual deformation after molding process for the packages and will influent the coplanarity of the BGA components.

Following the failure analysis tests, finite element simulation was conducted to analysis the stress/strain evolution behavior of the PoP module during the full assembly process from the package molding to module reflow. Finite element techniques such as element birth and death as well as restart were used to transfer the values of stress and warpage from one process to the next. The warpages of both packages constantly changed during the whole process and presented difference patterns. And there was mainly tensile stress in the top solder joints and compressional stress in the bottom ones. This significant thermal-induced stress would result in cracks in corner solder joints.

The FEA result was consistent with experimental observation and result. The developed FEA model in this paper can be used to do further parametric studies to optimize the package structure design and help material selection for improving PoP module reliability. This work will be presented in next step for our research project.