Characterization of coined solder bumps on PCB pads

Developed organic substrate solder flip chip bumping processes using a stencil printing method, 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°C, and coining rate (considered by the process time) are 0.6, 1.2, 6.0, 12.0 μm/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 μm pad opening size and 230 μm pitch PCB pads can be easily handled using electroformed stencil-printing masks. The diameter of the reflowed solder bumps was 150 μm and the height above the solder mask was 85 μm. Coining has been successfully performed on these bumps until 25 μm 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 stages of coining deformation. 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.