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48 SMT007 MAGAZINE I DECEMBER 2024 Chapter 1 Low-temperature solders (LTS) continue to attract serious interest from the electronics industry. These solder alloys enable the use of lower glass transi- tion 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 sustain- ability targets. Perhaps more significantly, low-tem- perature soldering is an enabling technology that mitigates warpage-induced defects of the ultra-thin, high IO chip-scale packages used in complex elec- tronic devices. Use of higher reflow temper- ature Sn-Ag-Cu (SAC) solders in package-on-pack- age (PoP) bottom and PoP memory technology can potentially result in soldering defects such as non-wet opens, solder bridg- ing, 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 inter- esting as a solder, as bismuth is one of the few ele- ments forming a solid solution in tin. Its binary com- position, lamellar microstructure, and eutectic melt- ing point resemble that of eutectic tin-lead. However, this first-generation LTS alloy lagged behind tradi- tional lead-free alloys in solder joint thermomechan- The Printed Circuit Assembler's Guide to... Low-Temperature Soldering, Vol. 2 BOOK EXCERPT ical 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. Since the first volume of The Printed Circuit Assem- bler's Guide to...™ Low-temperature Soldering was published over five years ago, considerable changes have occurred in low-temperature soldering. In [Volume 2], we examine the perfor- mance and process- ing of a fourth- generation low- temperature solder, HRL3. We review the evolution of solder alloys from traditional eutectic SnBi solder to the fourth-generation HRL3 low- temperature solders, including key properties and solder joint formation. We focus mainly on the thermomechanical performance of third- and fourth-generation low-temperature solders for various package types and sizes. Readers will find a discussion of innovations and challenges for achiev- ing optimal processing and electrical reliability properties in low-temperature solder pastes, as well as what is next in LTS and related research topics. Continue reading... By Morgana Ribas, Pritha Choudhury, Ramakrishna H V, and Rahul Raut MACDERMID ALPHA ELECTRONICS SOLUTIONS