Naomichi Ohashi

The Influence of Resin Coverage on Reliability for Solder Joints Formed by One-Pass Reflow Using Resin Reinforced Low Temperature Solder Paste

SAC305 solder paste is commonly used electronic assembly. This solder alloy consists of 96.5% tin, 3% silver, and 0.5% copper and melts at 219 °C. The peak reflow temperature range is typically 240 - 260 °C. With electronic devices such as smartphones, notebook PCs, and tablets becoming thinner, packaging substrates, such as ultra-thin flip chip ball grid arrays (FCBAs), and the printed circuit boards (PCBs) on which they are mounted are becoming thinner. The growing use of thinner substrates is creating manufacturing and reliability challenges. For example, it is increasingly difficult to control the warpage of CPU packages in notebook PCs during the solder reflow process. The result is greater numbers of solder joint defects, including Non-Wet Open (NWO) and Head-on-Pillow (HOP) defects caused by warpage of package substrates and PCBs. These issues have created a demand for low-temperature solders to help reduce warpage and improve SMT assembly yields by adopting lower soldering temperatures. Tin Bismuth (SnBi) eutectic solders have a desirably low melting point of 139 °C. However, the brittleness of the alloy limits commercial use. This situation prompted us to develop a solder paste material that combines low temperature SnBi solder with epoxy resin. 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 this low temperature joint reinforced solder paste (JRP) developed by us, which consists of SnBi solder compounded with epoxy resin. We evaluated the influence of the epoxy resin component in the developed material (JRP) on solder joint reliability. We compared the joint properties of samples made SAC305 solder paste, unreinforced SnBi solder paste and JRP solder paste. The evaluation revealed that the JRP technology alleviates issues associated with the brittleness of SnBi solder by encasing the formed solder joints with 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.