SMTA International Conference Proceedings


Authors: M. Nowottnick, W. Scheel, K. Wittke, U. Pape
Company: Fraunhofer IZM
Date Published: 9/26/2004   Conference: SMTA International

Abstract: The idea of this new joining technology is to combine the mechanical properties of heat resistant polymer adhesives with the electrical and thermal function of low melting leadfree solders. By this combination it is possible to minimize the thermo-mechanical stress in the joints because of the softening or melting of the actual solder alloy in the gap.

This is especially advantageous for high temperature application and large solder joints between materials with different thermal expansion. By the use of low melting bismuth tin alloys, with a solidus temperature of 138°C (280 F), it is possible to apply this at operating temperatures up to 200°C or 250°C (390-480 F) in the liquid state. For the mechanical stability, a heat resistant polymer is covering the solder joint. To ensure a reliable function of this combined adhesive and solder joint, it was very important to protect the metallic surfaces of the parent material against diffusion and excessive growing of intermetallic compounds. This protection was realized by a special inhibitor addition in the low melting solder alloy, which can form a very thin and stable compound with the parent material and acts as a barrier to the temporary liquid solder.

Because of the intended change between the liquid and the solid state of the solder joint, the volume transition of the alloy is very important for the reliability. Tin has, like most metals and alloys, a positive volume transition and bismuth is one of the few metals, which shows the opposite characteristics and is expanding for solidification. The combination of both metals shows the possibility to produce an alloy, which has a volume transition close zero. A special focus was the development of suitable testing methods and strategies to show the reliability of assemblies. First available results and some typical applications, e.g. for automotive or power electronics, will be presented in this paper.

Key words: solder joints, adhesive joints, combined joints

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