Compositional Effects on the Creep Properties of SnAgCu Solder

Although there seems to be agreement that the SnAgCu-solder is the lead-free alloy of choice, the basic eutectic SnAgCu system does not fully satisfy the requirements of an alternative to the classic SnPb system. Therefore the question is, how the simple SnAgCu eutectic needs to be changed in order to achieve a higher mechanical performance of the solder. Changes include the increase or reduction of the Sn-, Ag- or Cu-content in order to achieve a non eutectic SnAgCu solder, or the addition of other alloying elements like Ni, Au and Co. The scope of the paper is to present creep data, that was gained on non eutectic SnAgCu-solder specimens with different compositions. The experimental program contained the following alloys: Sn98Ag2, Sn97Ag3, Sn96Ag4, Sn97.5Ag2Cu0.5, Sn97.1Ag2Cu0.9, Sn98.8Ag2Cul.2, Sn96.5Ag3Cu0.5, Sn96.1Ag3Cu0.9, Sn95.8Ag3Cu1.2, Sn96.9Ag3Au0.1. Bulk solder specimens were produced from raw solder bars by melting in an aluminum mould. The length of the specimens is 50 mm. The specimens have a rectangular cross section of 4mm times 3mm. Constant-load creep tests were carried out. The specimens were tested in a standard load frame. All creep tests were carried out at temperatures of 30 degC, 60 degC, 100 degC and 150 degC. Creep data was taken for strain rates between 1*10E-9 s -1 and 1*10E-1 s -1 . The specimens were tested in "as cast" condition. In the second step of this study, the microstructural properties of the specimens were examined using metallographic sectioning, optical microscopy techniques, and SEM-microprobe analysis. The intermetallic phase sizes were determined. Finally, the results of the microstructural analysis were related to the investigated mechanical properties of the solders. The contribution will present all the investigated creep data on the different solders. It will point out, how the individual alloying elements of SnAgCu solders change the creep properties of the alloy. A comparison with the microstructural properties will indicate the physics behind the effect on the creep behavior