Assembly and reliability of PBGA packages on FR-4 PCBs with SnAgCu solder

Plastic-ball-grid-array (PBGA) packages were assembled onto FR-4 printed-circuit-boards (PCBs) using lead-free SnAgCu solder, different solder pad diameters, stencil thicknesses and reflow peak temperatures, and the effects of the design and assembly process conditions on the dimensions and reliability of the solder joints were investigated. The assembled microelectronic devices were subjected to a thermal cycling test, and crack initiation and propagation during the test were studied. The results showed that with increased peak reflow temperature and pad diameter, the average joint diameter increased but the average joint height decreased. Increased stencil thickness resulted in increased joint diameters and heights. The solder joints consisted of two unique structures of Sn-rich and Ag-rich Sn–Ag compounds. The intermetallic thicknesses were less than 3 μm, and the compounds did not affect the reliability of the solder joints. Failures were not found before 5700 thermal cycles and the characteristic lives of all solder joints were more than 7200 thermal cycles, indicating robust solder joints produced with a wide process window.