Electro- and thermo-migration induced failure mechanisms in Package on Package

Abstract For mobile electronic devices the trend of market leads to a rising number of features while the dimension of the electronic devices keeps constant or even shrinks. On transistor level the miniaturization reaches its limits and more ICs will be required. More in and out contacts of the ICs have to be realized and the increasing contact density leads to rising current densities, due to smaller solder joints. Similar to the reliability problems with interconnects on IC level, the rising current densities lead to migration induced void formation in the solder joints. In consequence migration induced phenomena have a growing influence on the lifetime of the packages. Hence it is necessary to investigate the migration induced material transport in solder joints to enable a prediction of the package lifetime. For the investigation of migration induced void formation in solder joints a finite element model of a common Package on Package structure was built. Thermal–electrical and thermal–mechanical simulations with ANSYS® were carried out. Furthermore the mass flux and mass flux divergences due thermo- and electromigration were calculated with a user routine. For the validation of the simulation results, stress tests with 14 × 14 mm and 12 × 12 mm Amkor® Package on Package structures were made. Oven temperatures of 120 °C and 100 °C and an applied current of 1A were chosen. Agreements between simulation results and measurements were found. The simulation results verify that current crowding is the main reason for void formation in solder joints. Furthermore a sensitivity analysis was made to check the influence of material parameter variations on the simulation results. Based on this, the influence of underfill materials and inhomogeneous temperature fields on the void formation was investigated. Inhomogeneous temperature fields are the results of heat loss in the mounted ICs in consequence Joule heating. A constant heat loss all over the IC surfaces as well as hot spots placed near to the bump chains and in the center of the package was investigated.

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