This article is a concise summary of a research paper titled “Addressing Low-Temperature Rework Concerns,” authored by Tim O’Neill, Jen Fijalkowski, Carlos Tafoya, Yuan Xu, Steve Hrcek (AIM Solder, Montreal, Quebec, Canada), Leo Lambert (EPTAC Corporation, Manchester, NH, USA), Bob Willis (Chelmsford, Essex, England), and S’ad Hamasha, Ph. D. (Auburn University, Auburn, AL, USA). The full research paper was presented at SMTAI 2022.
The Importance of Addressing Low-Temperature Rework Concerns
Low-temperature (LT), high-bismuth solders have gained significant interest among consumer electronics manufacturers as they offer cost advantages over traditional silver-bearing and low-silver alloys. However, one aspect that has been overlooked in implementing low-temperature alloys is rework and post-assembly attachment processes. This paper aims to address the rework concerns associated with low-temperature solders and provides guidance on rework materials and techniques for successful implementation.
The Challenge of Low-Temperature Soldering
Low-temperature solders, especially those containing bismuth, exhibit unique properties that make their implementation challenging. Bismuth-containing alloys offer lower melting temperatures, but the brittleness of bismuth hinders the manufacturing of fluxed cored wire, which is essential for certain soldering processes. This limitation has led to two approaches for overcoming the challenge: using a high-bismuth solid wire solder with an external flux or employing traditional tin/silver/copper (SAC) flux cored wire solder to rework low-temperature solder joints.
Experimental Methodology: Investigating Rework Performance of Low-Temperature Solders
To investigate the rework performance of low-temperature solders, the study involved three rework conditions.
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- Condition 1 used Sn42/Bi57/Ag1 paste and SAC305 flux cored wire.
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- Condition 2 used Sn42/Bi57/Ag1 paste with solid Sn42/Bi57/Ag1 wire and external flux.
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- Condition 3 used SAC305 paste and SAC305 flux cored wire for rework.
Cross-section analysis and shear strength testing were conducted on various components to evaluate solder joint quality.
Results and Observations: Assessing Solder Joint Quality and Ductility
The study revealed that low-temperature alloys used in SMT and PTH processes can be successfully reworked with low-melting temperature solid wire and external flux, as well as with flux cored SAC305 wire solder. The solder joints met IPC Class 1, 2, and 3 solder joint criteria, indicating their reliability. Shear test results showed that SAC solder joints were more ductile than bismuth-bearing counterparts.
Successful Rework of Low-Temperature Solders
The research demonstrated that rework of low-temperature solders is possible using appropriate techniques and materials. While solid Sn/Bi wire with external flux requires careful consideration of the appropriate flux, using SAC305 flux cored wire simplifies the rework process. The study also emphasizes the importance of operator training for low-temperature rework since even with a highly qualified operator, results were somewhat variable.
Areas for Further Investigation
Further research is planned to evaluate the effect of low-temperature alloys on soldering iron tip life and equipment. Additionally, investigations into the ease of cleaning dark residues from low-temperature alloy flux will be conducted.
To access the full research paper, please click here.
