A Review of a Bi-free In-containing Lead-free Mid-temperature Solder
Hongwen Zhang
R&D Director and Principal Research Metallurgist，Indium Corporation
Abstract:
Bismuth–tin (BiSn) low-temperature solders are able to be used in temperature-sensitive assemblies, but their reliability is constrained by poor drop shock performance and limited electromigration resistance. These issues arise primarily from the presence of brittle, poorly conductive Bi-rich phases. To address these challenges, a bismuth-free, indium (In)-containing low-temperature solder paste, designated with the patented Durafuse® Technology platform, has been developed. This material is specifically engineered to support hierarchical assembly while significantly improving both mechanical and electrical reliability. This materials, Durafuse®LT (DFLT) employs a mixed solder paste design, resulting in a postreflow solidus temperature of 189 °C. This design enables processing at lower peak reflow temperatures, typically between 200 °C and 240 °C. The alloy has demonstrated versatility across multiple applications, including step soldering, hot bar soldering, rework, and full board-level assembly. Its adaptability makes it a strong candidate for a wide range of advanced electronic manufacturing processes. Reliability testing highlights substantial performance gains over conventional BiSn-based systems. DFLT exhibits at least two orders of magnitude improvement in drop shock resistance and more than one order of magnitude reduction in electromigration rate compared to BiSnAg eutectic alloys. Notably, its performance is comparable to or even exceeds that of the widely used SAC305 alloy, making it a compelling alternative for high-reliability applications. As high-performance computing continues to drive increased chip integration, package sizes— especially ball-grid array (BGA) formats—have expanded to dimensions of 110 × 110 mm² or larger. This scaling introduces significant dynamic warpage during board-level assembly, leading to defects such as head-in-pillow (HiP), non-wet open (NWO), and joint necking. By enabling lower peak reflow temperatures (around 210 °C), DFLT effectively reduces the defects rate from the thermally induced warpage. When combined with optimized solder paste volume control, it further minimizes the defect formation and improves assembly yield. The resulting solder joints demonstrate strong resistance to subsequent thermal cycling, confirming the robustness of the DFLT system in demanding operating environments. Additionally, integrating DFLT with lowertemperature solder ball designs—featuring similar melting characteristics—offers further potential to reduce reflow temperatures and mitigate warpage-related challenges. In summary, by eliminating brittle Bi-rich phases and delivering superior performance across mechanical and electrical reliability metrics, DFLT represents a high-performance, bismuth-free solder solution. Its advanced design and demonstrated reliability establish it as a promising platform for next-generation electronic assemblies requiring both lower processing temperatures and enhanced durability.
Speaker's Biography:
Dr. Hongwen Zhang is the Principal Research Metallurgist and Research & Development Director at Indium Corporation, with over eighteen years of experience in developing lead-free solder materials. He holds a Ph.D. in Materials Science and Engineering, along with master’s degrees in both Materials Science and Engineering and Mechanical Engineering. Dr. Zhang led the invention of the Durafuse® paste technology and has been awarded more than twenty patents worldwide. He is a certified SMT Process Engineer and a certified specialist in IPC-A-600 and IPC-A-610D standards. In addition, he serves as Chair of the ASM Mohawk Valley Chapter and is an industrial advisory board member for several universities, including Cornell University, SUNY Polytechnic Institute, and Syracuse University. He was recognized as Member of Technical Distinction at 2023 SMTA International.