Overview and Conclusions

This work summarizes the review and analysis of isothermal tensile, compression and shear creep data for Sn-3.5Ag eutectic solder with a melting point of 221°C. First order creep models are developed for possible use in stress/strain analysis, e.g. Finite Element Analysis (FEA), and solder joint life prediction models. The raw data is tabulated in the Appendix for inclusion in a material databases and further analysis by others. Other properties such as the CTE and Young’s modulus of Sn-3.5Ag were also collected.

The intent of this review is to pull together hard data from the existing literature and develop simple, first-order creep models in an attempt to bridge datasets obtained from independent sources. It is also hoped that the gathered data will be of use for others to develop more sophisticated constitutive models. While this review is not exhaustive, the analysis leads to the following conclusions:

• Tensile creep data for bulk Sn-3.5Ag solder shows significant scatter.
• Compression data is limited. Nevertheless, the available data suggests an uneven behavior of solder with higher strength in compression than in tension.
• Shear creep data from lap joint or plug and ring specimens also shows significant scatter.
• Creep data for Sn-3.65Ag and Sn-4Ag appears to follow similar trends as for Sn3.5-Ag, suggesting a small effect of silver contents in the range 3.5% to 4% Ag.
• Shear creep data from Ceramic Chip Carrier (CCC) assemblies correlates well with flip-chip solder joint data.
• Regression of the data also shows some sensitivity to the regression procedure.
• The review also points to lesser data available at stresses less than 10 MPa. Future work should consider these lower stress levels since they are representative of stress conditions experienced by solder joints of electronic assemblies in use.
• Most studies focus on ultimate strength and secondary or steady state creep with very few investigating initial deformations or primary creep. Complete stress/strain curves are rarely published. These curves, obtained from constant strain rate tests, as well as thermal cycling hysteresis loops would be of much use for the validation of material constitutive models.

Since Sn-3.5Ag is a precipitate-strengthened Sn-based alloy, the findings of this review may also be of use in the development of guidelines or test procedures for the characterization of Sn-Ag-Cu alloys.