Electrochemical migration study of fine pitch lead free micro bump interconnect

Elelctrochemical migration (ECM) test was conducted for fine pitch flip-chip micro bump interconnect. Two kinds of micro bump. i. e., Cu post with SnAg solder and Cu under bump metallization (UBM) with SnAg solder bumps in 50 and 100 µm pitch with non clean flux were used for chip interconnection. The test was conducted under 85ºC/85H condition with various biases. The results indicated that for micro bump with underfill, test with different bias and pitch showed that the smaller the pitch, the higher the bias, the easier for insulation resistance (IR) value drop. By top-down grinding, dendrites were detected and the main compositions of the dendrites were Cu and Sn. The ECM tests confirmed that even with underfill, ECM failure is a concern for micro bump interconnects with fine pitch down to 50 µm. The mechanism of the dendrite formation was proposed and it was believed that the adsorption of water steam by underfill materials and the existence of residual flux were the main reasons for the dendrites formation.