Effects of initial Cu grain size and impurity residues on the liquid phase reactions between molten solder and electroplated Cu

Abstract

Copper (Cu) electroplating is a cost-effective and scalable technology used to fabricate interconnects/metallizations in microelectronic products. Liquid-phase soldering reactions between electroplated Cu and Sn-based alloys are essential for constructing heterogeneous joints for mechanical support and electrical conduction applications. In this study, three Cu electroplated layers with different grain sizes and impurity contents are fabricated by formulating additives in the plating solution. The Cu electroplated layers are reacted with molten Sn-3.5 wt% Ag alloy at 260 °C for 20–60 min to investigate the liquid phase reactions. Microstructural characterizations of the electroplated Cu layers and the Cu/solder/Cu joints are carried out using electron microscopy and X-ray diffraction processes. A time-of-flight secondary ion mass spectrometer is used to analyze the impurity contents in the Cu electroplated layers. The Cu/solder/Cu joints after liquid phase reactions are mechanically analyzed to measure the shear strength. The results show that the additive formulas affect the microstructural features and the impurity contents in the Cu electroplated layers. Electroplating of Cu with an ultrafine grain texture accompanies a high impurity content, and the solder joint constructed by the ultrafine-grained Cu exhibits a rapid liquid phase reaction and interfacial voiding propensity, leading to a low shear strength.