Aside from these interesting factors, bismuth’s brittleness is what has the greatest impact on its soldering performance. It has one of lowest thermal and electrical conductivity values of all metals, it is denser as a liquid than a solid, it expands on freezing, and is the most naturally diamagnetic element on the periodic table. The physical properties of elemental bismuth are unusual. Given the advantages of a low-melting point alloy, solder manufacturers stay motivated to develop commercially viable products for solder paste, solid alloy, and flux cored wire, but this is not without its challenges. Regulations limiting the use of Pb in electronics have effectively removed it from most of the industry’s material supply chain, reinvigorating the interest in bismuth-bearing solder alloys. However, indium was eliminated for supply and cost issues and bismuth was eliminated because when combined with Sn/Pb, a ternary alloy with a melting temperature of 97˚C is formed causing an unacceptable loss of reliability. Both were attractive as they lower the melting temperature of SAC alloys. Other elements were considered as part of the Sn/Pb solder replacement, such as indium and bismuth. One of the main benefits of SAC305 was its compatibility with tin/lead solders. This change forced the implementation of new equipment and materials due to the higher melting temperature of SAC305 versus the tin/lead materials it replaced. This paper will review the issues associated with using low-temperature alloys in the benchtop setting and how these alloys can be implemented or combined with existing processes and materials.ĭISCUSSION SAC305 is the PCB assembly standard solder since lead (Pb) was banned from many electronic solders for environmental concerns in 2006. Most published studies have overlooked one important consideration when implementing low-temperature alloys: rework and post-assembly attachment processes. Despite these drawbacks, consumer electronics that are not subjected to high thermal or mechanical stress may be candidates for low-temperature materials. Bismuth is strong, but also brittle and has different mechanical properties than the SAC alloys they are targeted to replace. INTRODUCTION Low-temperature (LT), high-bismuth solders are being carefully considered to replace SAC305 for several reasons including minimizing component warpage, reducing energy consumption and lowering the cost of solder, substrate, plastics, and other materials. 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. Shear strength of the resulting solder joints for SAC/SAC, SAC/Sn-Bi and Sn-Bi/Sn-Bi. Solder joint quality of Sn/Bi combined with SAC305, in both through-hole and surface mount applications, will be assessed along with:ī. 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. Rework conditions differ from other reflow processes giving considerable process variability due to operator input. Another approach is to use traditional tin (Sn)/ silver (Ag)/ copper (Cu) (SAC) flux cored wire solder to rework low-temperature solder joints. This approach introduces challenges associated with implementing new materials and training operators on new rework techniques. An approach to overcome this limitation is to use a high- bismuth solid wire solder with an external flux. The inherent brittleness of bismuth inhibits the manufacturing of fluxed cored wire. 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.Īn overlooked application in implementing low-temperature alloys is rework. ABSTRACT Implementation of low-temperature solders has been a subject of considerable interest for many consumer electronics manufacturers.
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