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Addressing Low-Temperature Rework Concerns
Analysis Lab |
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Authored By:Tim O’Neill, Jen Fijalkowski, Carlos Tafoya, Yuan Xu, Steve Hrcek AIM Solder Montreal, Quebec, Canada Leo Lambert EPTAC Corporation NH, USA Bob Willis Chelmsford, Essex, England S’ad Hamasha, Ph. D. AL, USA Auburn University SummaryImplementation of low-temperature solders has been a subject of considerable interest for many consumer electronics manufacturers. Low-temperature solders represent a significant material and process cost advantage over traditional silver-bearing and low-silver alloys. Bismuth-containing alloys enable low-melting temperatures; therefore, if the mission profile of an assembly tolerates the limitations of bismuth-containing low-temperature solders, implementation is favorable. An overlooked application in implementing low-temperature alloys is rework. The inherent brittleness of bismuth inhibits the manufacturing of fluxed cored wire. An approach to overcome this limitation is to use a high- bismuth solid wire solder with an external flux. This approach introduces challenges associated with implementing new materials and training operators on new rework techniques. Another approach is to use traditional tin (Sn)/ silver (Ag)/ copper (Cu) (SAC) flux cored wire solder to rework low-temperature solder joints. Rework conditions differ from other reflow processes given considerable process variability due to operator input. This paper will detail the material properties and rework considerations of using a eutectic tin/bismuth (Sn/Bi) solid wire and SAC305 (Sn96.5/Ag3/Cu0.5) flux cored wire in a rework setting. Solder joint quality of Sn/Bi combined with SAC305, in both through-hole and surface mount applications, will be assessed along with: a. Diffusion zone of SAC305 and Sn/Bi b. Shear strength of the resulting solder joint versus, SAC/SAC, SAC/Sn-Bi, Sn-Bi/Sn-Bi. The goal of this study is to provide the end-user guidance on rework materials and techniques that can be successfully implemented during the implementation of low-temperature solders and to identify any trouble spots. ConclusionsLow-temperature alloys used in SMT and PTH processes can be successfully reworked using low-melting temperature solid wire and external flux as well as with flux cored SAC305 wire solder. The low-temperature solder behaves differently in process than the SAC equivalent but was not an obstacle in creating quality solder joints. Solid wire with external flux will require qualification of an appropriate flux for the application requirements. As with any externally applied flux, residue, and reliability characteristics due to processing techniques must be considered. The resulting solder joints met IPC Class 1, 2, 3 solder joint criteria. The shear test results were inconsistent due to the variable nature of rework, but trends are visible. SAC solder joints were more ductile than Bi bearing counterparts. This is not unexpected. SAC+Sn/Bi solder joints shear results reflected the properties of the combined alloys. The operator in this study is an IPC hand soldering certification instructor with decades of experience. Even with a highly qualified operator, results are variable. This highlights the importance of operator training in implementation of low-temperature rework. When using SAC305 flux cored wire solder, the process was like existing SAC/SAC rework processes. No special considerations were noted other than to considering finer wire diameters when reworking small components. Low-temperature flux cored wire solder was NOT included in this study due to the prohibitive cost to the end-user and limited availability. Initially Published in the SMTA Proceedings |
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