Characterization of coined solder bumps on PCB pads

In this study, we have developed organic substrates solder flip chip bumping processes using a stencil printing method, and then followed by coining process performed by specially designed coining machine with controlled gas environment, temperature, and strain rate. The variations of process temperature are 25 (room temperature), 50, 100, 150, 200 degreesC, and coining rate (considered by the process time) are 0.6, 1.2, 6.0, 12.0 mum/sec. Three types of solders, Sn-37Pb, Sn-3.5Ag, and Sn-3.8Ag-0.7Cu, were used to fabricate solder bumps. It is found that 120 mum pad opening size and 230 mum pitch PCB pads can be easily handled using electroformed stencil-printing masks. The diameter of the reflowed solder bumps was 150 mum and the height above the solder mask was 85 mum. Coining has been successfully performed on these bumps until 25 mum height above the solder mask by using a modified tension/compression tester. It is found that the load vs. height plot of coined solder bump shows three stage of coining deformation. The primary stage is characterized by remained elastic effect at initial deformation. The secondary stage of coining deformation is characterized by yield strength for plastic deformation. The final rapid increased stage, more loads needed for same height deformation because not only compressive stress but also shear stress interacts within solder bumps. As a result of comparison with various solders, which has different Young's modulus and yield strength value, it seems that coining loads were affected not only by Young's modulus and yield strength but also by more complex factors such as density and ductility of solders. Values of coining loads for the same height deformation strongly depend on coining rate and coining temperature. Loads needed for same height deformation increase, as applied coining rates increase. At high temperature coning process, solder bumps were deformed with lower load than room temperature at the same coining rate regardless of kinds of solder. The coining loads for the same height deformation were depend rather on temperature changes than on coining rate changes.