SnCu Solder Joints Research Articles

Effect of In Addition on Microstructure, Wettability and Strength of SnCu Solder

Abstract Since the restriction on the use of Pb in solders, several Pb-free solders have been developed to replace the traditional Sn-Pb eutectic solder. The Sn-Ag-Cu series are the most commonly used lead-free solders in the electronics industry. However, during isothermal aging, the brittle Ag3Sn phase can strongly reduce the reliability of solder joints. In addition, the relatively high cost of Sn-Ag-Cu solder limited its application in the electronics industry. An attractive alternative is the Sn-0.7Cu alloy, which has shown good wettability and reasonable cost advantage compared to the Sn-Ag-Cu alloy. Despite the advantages, the melting temperature of Sn-0.7Cu alloy is approximately 10°C higher than that of Sn-Ag-Cu alloy and coarse precipitation of Cu6Sn5 during isothermal aging has been observed. This paper reports the investigation on indium addition into commercial Sn-Cu solder (SN100C) to improve its microstructure, wettability and strength performance. The solder alloys used in this work were SN100C (0.7 wt% Cu, 0.05 wt% Ni, 0.01 wt% Ge and bal. Sn) and SN100C added with 0.5, 1.0, 1.5 and 2.0wt% indium. The addition of indium is expected to refine the β-Sn grains and contribute to higher solder strength. The microstructure of bulk solder was observed using SEM equipped with EDX, while single lap joint shear strength is performed to evaluate joint strength. The wettability of solder alloy was improved with addition of In, observed via higher spreading diameter and lower wetting angle. Microstructure observation showed that In addition refined β-Sn structure, and doping of In into Cu-Sn IMC resulted in fine and rounded shape Cu-Sn-Ni-In particles. Shear strength of Sn-Cu solder joints were increased with increasing In content. The increase could be attributed to the grain structure which was substantially refined with increasing amount of In addition. The smaller grains that formed with addition of In lead to higher grain boundary density that can impede dislocation motion resulting in higher shear strength.

Abstracts

Since the restriction on the use of Pb in solders, several Pb-free solders have been developed to replace the traditional Sn-Pb eutectic solder. The Sn-Ag-Cu series are the most commonly used lead-free solders in the electronics industry. However, during isothermal aging, the brittle Ag3Sn phase can strongly reduce the reliability of solder joints. In addition, the relatively high cost of Sn-Ag-Cu solder limited its application in the electronics industry. An attractive alternative is the Sn-0.7Cu alloy, which has shown good wettability and reasonable cost advantage compared to the Sn-Ag-Cu alloy. Despite the advantages, the melting temperature of Sn-0.7Cu alloy is approximately 10°C higher than that of Sn-Ag-Cu alloy and coarse precipitation of Cu6Sn5 during isothermal aging has been observed. This paper reports the investigation on indium addition into commercial Sn-Cu solder (SN100C) to improve its microstructure, wettability and strength performance. The solder alloys used in this work were SN100C (0.7 wt% Cu, 0.05 wt% Ni, 0.01 wt% Ge and bal. Sn) and SN100C added with 0.5, 1.0, 1.5 and 2.0wt% indium. The addition of indium is expected to refine the β-Sn grains and contribute to higher solder strength. The microstructure of bulk solder was observed using SEM equipped with EDX, while single lap joint shear strength is performed to evaluate joint strength. The wettability of solder alloy was improved with addition of In, observed via higher spreading diameter and lower wetting angle. Microstructure observation showed that In addition refined β-Sn structure, and doping of In into Cu-Sn IMC resulted in fine and rounded shape Cu-Sn-Ni-In particles. Shear strength of Sn-Cu solder joints were increased with increasing In content. The increase could be attributed to the grain structure which was substantially refined with increasing amount of In addition. The smaller grains that formed with addition of In lead to higher grain boundary density that can impede dislocation motion resulting in higher shear strength.

In situ measurement of electromigration-induced transient stress in Pb-free Sn–Cu solder joints by synchrotron radiation based x-ray polychromatic microdiffraction

Electromigration-induced hydrostatic elastic stress in Pb-free SnCu solder joints was studied by in situ synchrotron x-ray white beam microdiffraction. The elastic stresses in two different grains with similar crystallographic orientation, one located at the anode end and the other at the cathode end, were analyzed based on the elastic anisotropy of the β-Sn crystal structure. The stress in the grain at the cathode end remained constant except for temperature fluctuations, while the compressive stress in the grain at the anode end was builtup as a function of time during electromigration until a steady state was reached. The measured compressive stress gradient between the cathode and the anode is much larger than what is needed to initiate Sn whisker growth. The effective charge number of β-Sn derived from the electromigration data is in good agreement with the calculated value.

Creep behavior on Ag particle reinforced SnCu based composite solder joints

SnCu solder is one of the most promising substitutes of SnPb solder, but its creep resistance is worse than that of the other lead-free solders. Particle-reinforcement is a way to improve the creep resistance of solder alloys and cause much more attention than before. A novel Ag particles reinforced SnCu based composite solder is formed and the influence of stress on creep behavior of the composite solder is investigated. Results indicate that the creep resistance of solder joints is superior to that of the SnCu solder joints. Creep rupture lifetime of solder joints decreases gradually with stress increasing. And the creep rupture lifetime of the composite solder joints falls down faster than that of the matrix solder joints.