SMTA International Conference Proceedings

Possibilities and Limits of Bismuth Solders

Authors: Mathias Nowottnick, Andrej Novikov, Joerg Trodler
Company: University of Rostock, Heraeus Deutschland GmbH & Co. KG
Date Published: 9/17/2017   Conference: SMTA International

Abstract: Bismuth alloys have long been known as lead-free alternatives and as solders for low-temperature assembly. The low melting point is both an advantage and a disadvantage. A low soldering temperature allows the processing of sensitive components and low-cost materials. On the other hand, the operating temperature is also limited and flux residues remain rather active after the soldering process. Both factors influence the reliability of the assembly. However, practical tests have shown that the solder joints can be surprisingly temperature stable despite the low melting point. Since the bismuth portion reacts chemically not with the copper, this alloy can be used even at higher temperatures to reduce the diffusion. In certain material combinations, the melting point can even be increased in a targeted manner. In addition, bismuth has some special properties that make it interesting for other applications. A special feature of the bismuth is the negative volume jump during melting. In combination with tin, the volume change of the alloy can be minimized to almost zero. This is of interest for special high-temperature applications in which repeated melting of the solder is permissible. The electrical and thermal connection thus remains stable even in the liquid state. Even the high enthalpy of fusion can be advantageously used for limiting the operating temperature for high-temperature applications. Key words: bismuth, low melting solder, lead free solder, high temperature electronics

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