Symposium Conference Proceedings


SMT Soldering with Low Temperature Solder Paste

Authors: Tang Kok Kwan, Raiyo F. Aspandiar, Scott Mokler, Olivia Chen, Art Jiang
Company: Intel Corporation
Date Published: 4/14/2015   Conference: Symposium


Abstract: The conversion from Sn/Pb soldering to Lead Free soldering has elevated overall soldering temperature by ~30°C. This increase of soldering temperature has widely impacted Surface Mount Technology (SMT) manufacturing process, material selection, manufacturing yield and cost. Concurrently due to the fierce competitive landscape in the marketing of electronic gadgets such as notebooks, tablets, smart phones and more recently wearables devices, electronic products have undergone slimming in system z-height and miniaturization in their form factors. This slimming and miniaturization of devices is creating new challenges to SMT soldering. The immediate impact from this z height reduction is the increase of high temperature warpage due to Coefficient of Thermal Expansion (CTE) mismatch between key ingredient materials such as Si, organic substrate and Cu in the SoC-PCB. Larger amounts of high temperature warpage has drastically shrunk the manufacturing process window, leading to yield loss caused by open solder or bridging defects. Solutions to this problem typically consist of mechanical reinforcement either to the component, such as over-molds or metal stiffeners on the substrate, or to the PCB, such as customized reflow pallets, but these solutions can be expensive and are unable to totally overcome the compounded effect between high temperature reflow and thinner z-heights.

In this study, we explored SMT soldering with low temperature solder paste consisting of Bi-Sn-Ag or Sn-Bi alloy, reducing the soldering peak temperature to ~180°C vs. ~240°C of Lead Free process that use SnAgCu (SAC) solder paste. Solder alloys of Bi-Sn-Ag or eutectic Sn-Bi composition melt at about ~138°C. Although the peak reflow temperatures significantly reduces the high temperature warpage on FCBGA components and improves the SMT yield, Bi which has been previously proven to contribute to brittleness of solder joint, and significantly reduce the mechanical stress performance of solder joints [1, 2, 3].

This paper describes the yield improvements, defect mechanisms, reliability performance and potential cost saving in soldering with low temperature Bi-Sn-Ag solder pastes.

Key Words: 

Low temperature soldering, low temperature solder paste, SnBiAg, SnBi



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