Solder Joint Reliability and Paste Stability Performance of Resin Reinforced Low Temperature Solder Paste

Abstract

ABSTRACT SAC305 solder paste is commonly used in SMT assembly. This lead-free alloy is comprised of 96.5% tin, 3% silver, and 0.5% copper. The peak reflow temperature range is typically 240 - 260 ˚C. Many electronic devices, such as smartphones, notebook PCs, and tablets are becoming thinner. As they do, packaging substrates, such as ultra-thin flip chip ball grid arrays (FCBAs), and the printed circuit boards (PCBs) on which they are mounted are also becoming thinner. The use of progressively thinner substrates is creating manufacturing and reliability challenges. For example, it is increasingly difficult to adequately control the warpage of high I/O CPU packages in notebook PCs during the solder reflow process. This results in a greater numbers of solder joint defects, including Non-Wet Open (NWO) and Head-on-Pillow (HOP) defects resulting from the warpage of package substrates and PCBs. These issues have created a demand for low-temperature solder pastes to help reduce warpage and improve SMT assembly yields. Tin-bismuth (SnBi) eutectic solders have a desirably low melting point of 139 ˚C. However, inteconnects formed with SnBi tend to be brittle and are not as reliable as those made with SAC305. Post-reflow reinforcement with polymer underfills can improve the performance of the solder joints, but the additional materials and processes increase manufacturing costs. These issues have somewhat limited commercial adoption of SnBi solder pastes. This situation prompted us to develop a solder paste material that combines specialized epoxy resins and other organic components with SnBi solder particles to form a low temperature joint reinforced solder paste or JRP. This approach enables the concurrent formation of SiBn solder joints and a reinforcing polymer collar via a one pass reflow process. This paper describes the solder joint properties and reliability of the initial material developed by us, as well as the characteristics of an iterative formulation designed to improve reworkability and paste stability. We evaluated the influence of the epoxy resin component in the first material (JRP1) on solder joint reliability. We compared the joint properties of samples made SAC305 solder paste, unreinforced SnBiAg solder paste and JRP1 solder paste. The evaluation revealed that the JRP technology alleviates issues the associated with the brittleness of SnBi solder by encasing the weakest region of the solder joints in a fully cured epoxy resin. Ball joint shear testing, BGA solder joint strength testing, temperature cycle testing, and drop shock testing revealed that low temperature JRP solder paste demonstrated equivalent or better joint properties than those made with SAC305 solder paste. While the initial formulation exhibited the desired reliability performance, it was noted that this material was somewhat difficult to rework and the -20°C storage requirement was also undesirable. The next iteration (JRP2) incorporated a modified resin system. The new material exhibited improved reworkability, refrigerated storage (5°C) stability and better drop shock reliability, including a 2X improvement in characteristic life performance (number of drops to 63.2% failure) compared to SAC305 solder paste.