polymers Article Thermochemical Mechanism of the Epoxy-Glutamic Acid Reaction with Sn-3.0 Ag-0.5 Cu Solder Powder for Electrical Joining Gwang-Mun Choi * , Ki-Seok Jang, Kwang-Seong Choi, Jiho Joo , Ho-Gyeong Yun, Chanmi Lee and Yong-Sung Eom   Citation: Choi, G.-M.; Jang, K.-S.; Choi, K.-S.; Joo, J.; Yun, H.-G.; Lee, C.; Eom, Y.-S. Thermochemical Mechanism of the Epoxy-Glutamic Acid Reaction with Sn-3.0 Ag-0.5 Cu Solder Powder for Electrical Joining. Polymers 2021, 13, 957. https:// doi.org/10.3390/polym13060957 Academic Editor: Emin Bayraktar Received: 15 February 2021 Accepted: 18 March 2021 Published: 20 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). ICT Creative Research Laboratory/Materials & Components Research Division/Emerging Materials Research Section at Electronics and Telecommunications Research Institute (ETRI), 218 Gajeong-ro, Yuseong-gu, Daejeon 34129, Korea; sroka80@etri.re.kr (K.-S.J.); kschoi@etri.re.kr (K.-S.C.); jihojoo@etri.re.kr (J.J.); yunhg@etri.re.kr (H.-G.Y.); hichanmi@etri.re.kr (C.L.); yseom@etri.re.kr (Y.-S.E.) * Correspondence: gmchoi@etri.re.kr Abstract: An epoxy-based solder paste (ESP) is a promising alternative to conventional solder pastes to improve the reliability of fine-pitch electrical joining because the epoxy encapsulates the solder joint. However, development of an appropriate epoxy formulation and investigation of its reaction mechanism with solder powder is challenging. In this study, we demonstrate a newly designed ESP consisting of diglycidyl ether of bisphenol F (DGEBF) resin, Sn-3.0 Ag-0.5 Cu (SAC305) solder powder, and L-glutamic acid (Glu), which is a proteinogenic amino acid for biosynthesis of proteins in living systems. The mechanism of the thermochemical reaction was explored and tentatively proposed, which reveals that the products of the reaction between SAC305 and Glu function as catalysts for the etherification of epoxides and alcohols produced by chemical bonding between DGEBF and Glu, consequently leading to highly crosslinked polymeric networks and an enhancement of impact resistance. Our findings provide further insight into the mechanism of the reaction between various formulations comprising an epoxy, amino acid, and solder powder, and their potential use as ESPs for electrical joining. Keywords: epoxy-based solder paste; L-glutamic acid; tin carboxylate salt; etherification; electri- cal joining 1. Introduction Solder paste, mainly consisting of a solder alloy, a flux, an activator, and a solvent, plays an important role as an electrical joining material in the field of surface mount technology of electronic packaging for interconnection between semiconductor chips and substrates [1]. However, ongoing miniaturization of electronic components such as mini- LEDs, which range in size from 100 to 200 μm and small-sized multilayer ceramic capacitors (MLCCs), has presented considerable technical issues due to the need for new electrical bonding materials [2]. The decrease in chip size has led to a decrease in the joint area, which is largely related to the bonding force for mechanical reliability of the device. In addition, the cleaning process, which is required to completely remove the flux residue for corrosion prevention after the solder reflow bonding process, has become more challenging because of fine-pitch interconnections [3]. Further, generation of harmful fumes during solder reflow, which can be mainly ascribed to the flux and solvent, does not conform to the emerging requirement for eco-friendly manufacturing processes [4]. Over the last few decades, epoxy-based solder pastes (ESPs) have generated consid- erable interest as a leading option to overcome the disadvantages of commercial solder pastes. Eom et al. have conducted extensive research on the mechanism of the reaction between epoxy-based resins and solders [5–8] and their thermomechanical and electrical Polymers 2021, 13, 957. https://doi.org/10.3390/polym13060957 https://www.mdpi.com/journal/polymers