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. These solders offer cost advantages over traditional silver-bearing and low-silver alloys. Often overlooked, however, in implementing low-temperature alloys is rework and post-assembly soldering processes.
This paper addresses the rework concerns associated with low-temperature solders and provides guidance on rework materials and techniques for success.
The Challenge of Low-Temperature Soldering
Low-temperature solders, especially those containing bismuth, exhibit unique properties that make their implementation challenging. While bismuth-containing alloys offer lower melting temperatures, their brittleness makes the manufacturing of fluxed cored wire difficult. However, flux cored wire is often the solder of choice for rework.
This limitation leads to two approaches for overcoming the challenge. The first involves using a high-bismuth solid wire solder with an external flux. The second involves 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.
- Condition 1 used Sn42/Bi57/Ag1 paste and SAC305 flux cored wire.
- Condition 2 used Sn42/Bi57/Ag1 paste with solid Sn42/Bi57/Ag1 wire and external flux.
- Condition 3 used SAC305 paste and SAC305 flux cored wire for rework.
Cross-section analysis and shear strength testing was conducted to evaluate solder joint quality.
Results and Observations: Assessing Solder Joint Quality and Ductility
The study revealed that both methods were valid. Low-temperature solder alloys used in SMT and PTH processes can be successfully reworked with low-temperature solid wire and liquid flux. They can also be successfully reworked 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 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. This is because results can be variable even with a highly qualified operator.
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 flux residues from low-temperature alloy flux are planned.