时效Sn58Bi焊点在低密度电流作用下的强化效应研究

Elevated shear strength of aged Sn58Bi solder joints under low density current stressing

摘要:

虽然特定条件下的时效处理已被证实可提升Sn58Bi焊点的剪切强度，然而经时效处理后电流对焊点力学性能影响的规律尚不明确。为此，本文重点研究低电流密度对时效后Sn58Bi焊点剪切性能影响的规律，并揭示其微观机制。对在120 ℃下不同时效时间（0 h、240 h、480 h、720 h、960 h、1200 h、1440 h、1680 h）处理的Cu/Sn58Bi/Cu焊点在不同电流密度（0 A/cm2、1×103 A/cm2、2×103 A/cm2、3×103 A/cm2）下进行剪切测试。发现随着电流密度的增大，时效Sn58Bi焊点的剪切强度呈现“上升-下降”趋势。其中，时效480 h且电流密度为2×103 A/cm2时，剪切强度较未时效未通电状态下提升12.85 %，提升幅度最大。时效480 h处理后焊点剪切强度上升主要归因于两方面原因:（1）电流促使焊点内部几何必需位错（GND）密度增加；（2）电流密度增大，导致Bi相中亚晶比例升高，亚晶界可以阻碍位错运动，提高焊点剪切强度。焊点剪切强度降低源于焦耳热效应引发界面热失配，导致界面金属间化合物（IMC）层与钎料基体界面结合强度降低，焊点剪切强度降低。该研究为电子封装中低温钎料制焊点的可靠性评估提供了理论参考。

Abstract:

This study investigates the impact of low density current stressing on the shear performance of thermally aged Sn58Bi solder joints and elucidates the underlying mechanisms. Cu/Sn58Bi/Cu joints were subjected to isothermal aging at 120 °C for different durations (0 h, 240 h, 480 h, 720 h, 960 h, 1200 h, 1440 h, 1680 h) and tested under current densities of 0, 1×103, 2×103, and 3×103 A/cm2. The results indicate a non-monotonic "rise-fall" trend in the shear strength of aged joints with increasing current density. Notably, a maximum strength increase of 12.85 % was observed in joint aged for 480 h under a current density of 2×103 A/cm2, comparing with the unaged and current-free ones. This strengthening behavior at 480 h is primarily attributed to two mechanisms: (1) the applied current promotes the multiplication of geometrically necessary dislocations (GNDs), elevating strength via dislocation strengthening; and (2) higher current density facilitates an increase in the subgrain fraction of the Bi phase, which effectively impedes dislocation motion. Conversely, at higher current densities, shear strength decreases due to Joule heating, which induces thermal mismatch and compromises the interfacial bonding between the interfacial intermetallic compounds (IMC) layer and the solder matrix. These findings provide theoretical insights for the reliability assessment of low-temperature solder joints in electronic packaging.