Nanoindentation for measuring mechanical properties of Sn-Ag-Cu-RE BGA solders joints

Abstract

Creep behavior of the Sn-0.3Ag-0.7Cu-RE(rare earth) BGA (ball grid array) solder joints on copper pad containing 0.05La wt.% and 0.05 Ce wt.% respectively was studied by Berkovich nanoindentation tests with different loading rates at room temperature. The results of the two new solder joints were compared with that of BGA solder joints of Sn-0.3Ag-0.7Cu and Sn-3.0Ag-0.5Cu. The load-depth curves were rate dependent. As the rate increased, the creep depth was increased at same hold time. The derived Young's modulus with Oliver-Pharr method from unloading curves was loading rate-independent. The Young's modulus of BGA solder joints of Sn-3.0Ag-0.5Cu, Sn-0.3Ag-0.7Cu, Sn-0.3Ag-0.7Cu-0.5La and Sn-0.3Ag-0.7Cu-0.5Ce are 19.3±1.0 GPa, 17.8±1.2 GPa, 25.23±1.3 GPa and 29.3±1.0 GPa, respectively. In addition, the Young's modulus of Sn-0.3Ag-0.7Cu-0.05Ce low-Ag BGA solder joint is 52 percent higher than that of Sn-3.0Ag-0.5Cu BGA solder joint. The results indicated that RE additions significantly improve Young's modulus of BGA solder joints, whereas RE additions have inconspicuous effect on increasing indention hardness of Sn-0.3Ag-0.7Cu BGA solder joint. The stress exponents of 15.5424, 11.5741, 17.6897, and 20.0401 are obtained for BGA solder joints of Sn-3.0Ag-0.5Cu, Sn-0.3Ag-0.7Cu, Sn-0.3Ag-0.7Cu-0.5La and Sn-0.3Ag-0.7Cu-0.5Ce, respectively. Results shows that adding trace amounts of RE elements into low-Ag solders have notable effort to improve the creep resistance and RE elements will play an important role in providing better electronic interconnections.