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The Printed Circuit Assembler’s Guide to... Low-Temperature Soldering, Vol. 2, by Morgana Ribas, et al, MacDermid Alpha Electronics Solutions
Chapter 1: Introduction to Low-temperature Soldering
Low-temperature solders (LTS) continue to attract serious interest from the electronics industry. These solder alloys enable the use of lower glass transition temperature (Tg) substrates and components due to the lower processing temperature, promote long-term reliability by reducing exposure to thermal excursion, and enable organizations to meet sustainability targets. Perhaps more significantly, low-temperature soldering is an enabling technology that mitigates warpage-induced defects of the ultra-thin, high IO chip-scale packages used in complex electronic devices. Use of higher reflow temperature Sn-Ag-Cu (SAC) solders in package-on-package (PoP) bottom and PoP memory technology can potentially result in soldering defects such as non-wet opens, solder bridging, and head-in-pillow.
The eutectic 42Sn-58Bi alloy melts at 138°C and has been considered for SMT soldering since the transition from tin-lead to lead-free solders. This alloy is quite interesting as a solder, as bismuth is one of the few elements forming a solid solution in tin. Its binary composition, lamellar microstructure, and eutectic melting point resemble that of eutectic tin-lead. However, this first-generation LTS alloy lagged behind traditional lead-free alloys in solder joint thermomechanical and drop shock reliability and limited thermal cycling performance.
The transition to lead-free solders was motivated by regulatory compliance, and SAC305 (96.5% tin, 3% silver, 0.5% copper) was chosen as the best cost-benefit alternative. As the electronics industry evolved, so did solder joint reliability requirements. Several new solders were introduced to meet specific reliability requirements.
