Low-Temperature SnBiAg Solder Solution for Flexible Substrates

Flexible substrates experience notable warpage after reflow with SAC305 solder, challenging solder joint reliability. Adopting low-temperature solder (LTS) materials can help lower thermal stresses during reflow, as the package undergoes less warpage at lower peak reflow temperatures. However, solder paste printability of fine-pitch apertures and solder void performance are also critical factors affecting interconnect reliability, aside from the alloy itself. The adoption of LTS in electronic assembly is driven by technology, reliability, and environmental requirements. One key initiative for implementing LTS processes is to minimize dynamic warpage of solder joints and printed circuit boards. Warpage during solder reflow can significantly impact assembly yields and result in unpredictable early field failures due to soldering defects such as head-in-pillow (HiP), non-wet open (NWO), and solder cracks. LTS refers to reflow at a peak temperature below 190°C, with leading solder alloy candidates based on the Bi-Sn binary alloy system, which has a eutectic melting point of 138°C at a composition of 58 wt.% Bi and 42 wt.% Sn. The 42Sn-58Bi alloy was initially considered a viable candidate to replace SAC305 solder, but independent research confirmed its poor mechanical shock resistance and shorter fatigue life due to its brittle two-phase lamellar microstructure. The development of LTS pastes has improved the reliability limitations of the binary 42Sn-58Bi alloy. For instance, the ductility of the Bi-Sn alloy can be enhanced by incorporating elemental additions, such as Sn42Bi57Ag1 in this study. This alloy demonstrates comparable thermal cycling performance to SAC305 based on an internal study, with drop characteristics life of over 100 drops.
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PET (polyethylene terephthalate) substrates are a popular choice for flexible electronics due to their excellent mechanical properties, chemical resistance, and flexibility. They are often used in applications like flexible displays and wearable devices. A study on printing and solderability tests on PET flexible substrates using SnBiAg T6 paste achieved good printing results. Reflow was performed at peak temperatures of 165°C and 185°C. Both temperatures showed good solderability without any solder balling or beading. However, substrate warpage was observed at the 185°C peak temperature, while no warpage occurred at the 165°C peak temperature.