Constitutive Relationship Development, Modeling and Measurement of Heat Stressing of Micro-SMD Assembly With Sn3.9Ag0.6Cu SAC Alloy

Abstract

The Creep and microstructural changes during creep behaviors of bulk and thin cast forms of Sn3.9Ag0.6Cu were compared. The processing parameters of the thin cast material was selected to result in a very fine microstructure analogous to what occurs in very small size solder electronic interconnections. We found that the thin cast material was less creep-resistant than the bulk material. A comparison of Ag element maps between as crept bulk and thin cast material showed that the relevant climb process occurs in a very different environment in the bulk material as compared to the thin cast material. In the bulk material the relevant climb process occurs within a finely dispersed IMC eutectic which covers broad areas within the material. In the thin cast material the relevant climb process occurs primarily in the beta-Sn grains which continuously surround isolated, coarse IMC particles. This resulted in the activation energy of the bulk material being larger than that for the thin cast material. Finally, it is important to note that the strength deficiency of the thin cast material was persistent, once the material is cast in thin cast form it will remain weak in comparison to the bulk material. Therefore, using data obtained from bulk material samples for the construction of thermo-mechanical models of very small scale solder interconnections is likely to result in significant, intrinsic errors. Second, the thermal-mechanical response of electronic packages was simulated using the commercial finite element code ANSYS coupled with the Garofalo model to represent the solder constitutive creep response. The measured properties for bulk and thin-cast Sn3.9Ag0.6Cu SAC alloy were used in the FE modeling. A 36 I/O micro-surface mount device (SMD) package was used as a test vehicle in this work. Moire´ Interferometry was used to measure the horizontal displacements in the solder joints as a result of cooling the package from 100°C to room temperature. Modeling results were found to have good agreement with moire´ measurements on the actual SAC packages. The bulk properties produced a better correlation with the measurement of the horizontal displacement in the solder joints than the thin-cast properties. However, the assemblies that were tested used the Sn3.8Ag0.6Cu alloy rather than the Sn3.9Ag0.6Cu alloy. It is not known if this difference is significant to the thermo-mechanical response.