Properties of Sn-3 wt%Ag-5 wt%Cu alloys with Cu6Sn5 intermetallics grain refined by Mg

Abstract

This paper investigates the influence of Mg additions on the microstructure, thermal properties, electrical properties, and joint strength of bulk and ball grid array (BGA) reflowed Sn-3 wt%Ag-5 wt%Cu solder alloy and the joint intermetallics (IMCs). The cross-sectional microstructure analysis revealed that the addition of Mg effectively triggered the grain refinement of primary Cu6Sn5 IMCs in the bulk solder alloys. In addition, the thickness of the interfacial Cu6Sn5 IMCs layer was suppressed. The optimal addition for refinement is in the vicinity of 0.10 wt%Mg. Furthermore, synchrotron tomography three-dimensional reconstructed imaging reveals that the grain refinement of primary Cu6Sn5 IMCs is also effective even in BGA scale solder joint at normal reflow conditions. These primary Cu6Sn5 IMCs showed a non-faceted morphology. 0.10 wt%Mg addition reduced the number and size of the Cu6Sn5 IMCs. The grain refinement of primary Cu6Sn5 IMCs could be attributed to the presence of Mg, Mg2Sn, and MgO phases that potentially serve as heterogenous nucleation sites. The thermal analysis showed that the degree of undercooling values was significantly reduced to the minimum with 0.10 wt%Mg addition, which increased when the amount of Mg increased to 0.15 wt%. The mechanical properties showed that the ductility of the solder joint at high shear speed was improved.