Microstructure Evolution and Failure Mechanism of Cu-based and Co-based Solder Joints under Thermoelectric Coupling Fields

Abstract

In the advanced packaging technology, the device’s highly powering brings serious Joule heat to the solder joints, while the miniaturization makes the solder joints bear high current density. Therefore, in the post-Moore’s law era, solder joints are more used in the thermoelectric coupling fields. It is of great significance to research the effect of thermoelectric coupling fields on the microstructure of lead-free solder joints. In this paper, the coupling effect of thermomigration (1087 °C/cm) and electromigration (1.27×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> A/cm²) on the microstructure of Cu-based and Co-P based solder joints were studied. For Cu-based solder joints, it was observed that the growth of Cu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> Sn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> IMC on both sides of the interface showed an obvious polarity effect. At the later stage of the hot end (anode), the voids and the dissolution of the Cu matrix appear. Cu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> Sn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> overgrows in the current congested area. For Co-P based solder joints, it is observed that the polarity effect of CoSn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> is weaker than that of Cu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> Sn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> . The thickness change of CoSn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> is much lower than that of Cu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> Sn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> . In the later stage, microcracks occur at the cold end (cathode) corner of Co-P based solder joints. The seriousness of failure of Co-P based solder joints is much lower than that of Cu-based solder joints. Based on the comparison of the evolution of the thickness of CoSn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> and Cu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> Sn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> at the interface, it is found that the atomic transport fluxes caused by thermoelectric stresses counteract each other in the solder joints. This leads to a decrease in the growth and thinning rate of interface IMC. In addition, the distribution of IMC in solder shows that Co possesses better thermoelectric migration resistance than Cu.