Effects of thermal cycling and room-temperature ageing on bismuth precipitates in Sn-Ag-Cu-Bi solder joints

Abstract

The long-term stability of Bi-strengthened Sn–Ag–Cu solders is an important but insufficiently understood factor for the reliability of stored electronic assemblies. In this study, (Bi) precipitates in Sn-2.25Ag-0.5Cu-6Bi (wt.%) solder joints were examined in two ball grid array (BGA) packages after long-term (> 6 years) room-temperature ageing following reflow soldering and after harsh thermal cycling between − 55 and 125 °C. Significant differences are observed between bulk and surface (Bi) precipitates in their distribution, morphology, crystallographic orientation relationships (ORs) with β-Sn, and the formation of interfaces and ORs with Cu₆Sn₅. Three distinct ORs between (Bi) and β-Sn are identified, each with eight crystallographic variants in good agreement with calculated variants. The precipitates are plate-shaped, with the largest habit planes corresponding to the most coherent interfaces ({100}βSn‖{011¯2}Bi for OR1; {011}βSn‖{011¯2}Bi for OR2), resulting in multiple precipitate alignments within the matrix. Notably, OR2 is preferentially observed for bulk precipitates. Two additional ORs are identified between (Bi) and Cu₆Sn₅. After prolonged room-temperature storage, however, the bulk (Bi) precipitates become coarsened and widely spaced (> 1 μm in size and > 9 μm spacing), and (Bi) is observed to accumulate adjacent to Cu₆Sn₅ and Ag₃Sn particles. These observations indicate over-ageing and significant microstructural evolution even at room temperature, suggesting a potential long-term reliability concern for Bi-strengthened solder joints in stored electronics.