Microstructure Characterization and Creep Behavior of Pb-Free Sn-Rich Solder Alloys: Part I. Microstructure Characterization of Bulk Solder and Solder/Copper Joints

Abstract

Solders are used as interconnect materials in microelectronic packaging. Traditionally, eutectic and near-eutectic Pb-Sn solder alloys have been used. Due to the toxic nature of lead, environmentally-benign Sn-rich (Pb-free) solders are being developed. In this two part series of articles, we have focused on elucidating the relationship between microstructure and creep behavior of Sn-rich alloys, both in bulk form and at the solder sphere joint level. Part I of our study focuses on a detailed quantitative analysis of the effect of controlled cooling rates on solder microstructures. Bulk solder microstructures from cast bars were compared to smaller solder joint microstructures, reflowed from single solder spheres about 1 mm in diameter. Faster cooling rates resulted in much finer secondary dendrite size and spacing, as well as a much finer distribution of Ag 3 Sn and Cu 6 Sn 5 particles. The companion article, Part II, provides a unified mechanistic understanding of the creep behavior for the Sn-rich solder alloys, intimately linked to the microstructure characterization work reported in Part I.

Microstructure Characterization and Creep Behavior of Pb-Free Sn-Rich Solder Alloys: Part II. Creep Behavior of Bulk Solder and Solder/Copper Joints

“…As shown in Part I [6] of this study, the size of Ag 3 Sn particles for bulk, water-quenched, and solder joints was quite fine, on the order of 100 to 200 nm, while those in furnace-cooled samples were much coarser, on the order of 10 to 17 lm. It is interesting to note that there was no difference in Ag 3 Sn particle size and aspect ratio between Sn-Ag and Sn-Ag-Cu alloys.…”

“…Detail on sample fabrication and reflow for both bulk and solder joints are provided in Part I of this study. [6] Creep testing of the small solder joints was conducted on a micromechanical testing system (TYTRON*) at temperatures of 25°C, 60°C, 95°C, and 130°C. The system consists of an actuator that incorporates a thinfilm air bearing for near friction-free operation.…”

“…As described in the companion article, [6] there is a significant driving force toward developing high-performance, environmentally-benign, Pb-free solder alloys. Among several potential Pb-free solders, Sn-rich solders (Sn-Ag, Sn-Cu, and Sn-Ag-Cu) have been broadly targeted as potential candidates for replacement of the Sn-Pb system.…”

“…Bulk solder creep behavior is compared to small solder sphere joint creep behavior, where the solder joints are analogous in size and geometry to those employed in electronic packages. We refer to the comprehensive microstructure characterization detailed in Part I [6] to draw conclusions regarding the relationship between creep behavior and microstructure. The underlying mechanisms for creep deformation in bulk Sn-rich solders and small solder joints will be determined by invoking a subgrain and threshold stress analysis to account for anomalously high-creep stress exponents.…”

Effects of Thermal Aging on Microstructure and Microhardness of Sn-3.7Ag-0.9Zn-1In Solder

“…9), which were also reported by Chawla and co-authors. 21,22 For the small IMC particles and Zn atoms which dissolved in the b-Sn matrix, the increase of the microhardness of the water-cooled Sn-3.7Ag-0.9Zn-1In solder could be attributed to a solid-solution hardening effect.…”