Study on coined solder bumps on micro-via PCBs

In this study, three solders, Sn-37Pb, Sn-3.5Ag, and Sn3.8-Ag-0.7Cu, were screen printed on both electroless Ni/Au and OSP finished micro-via PCBs. The pad opening size and pitch of micro-via PCB were 120 μm and 230 μm respectively. The diameter of the screen printed solder bumps was 150 μm and the height above the solder mask was 85 μm. After that, the solder bumps were coined until 25 μm in height by a newly developed coining machine with variable pressure, temperature, and time. The Sn-37Pb solder formed less amounts of intermetallic compounds than all other Pb-free solders on both electroless Ni/Au and OSP (organic solderability preservatives) finished PCBs during solder reflows because of the lower Sn content and lower reflow temperature. For OSP finish, fracture occurred 100% within the solder regardless of reflow numbers. However, for Ni/Au finish, a brittle fracture at the Ni-Sn IMC layer or the interface between the Ni-Sn intermetallic and the P-rich layer was observed after several reflows. The relation of coining load vs. height of coined solder bump showed three stages of coining deformation. Coining loads needed for same height deformation increased as applied coining rate increased. And the planarity of coined solder bump could be increased as the coining rate decreased. As the coining temperature increased, lower coining loads were needed