Symposium Conference Proceedings

Case Study: Development of a Low Temperature Tin-Bismuth-Silver Solder Paste

Authors: Emmanuelle Guéné and Céline Puechagut
Company: Inventec Performance Chemicals
Date Published: 4/17/2013   Conference: Symposium

Abstract: Tin-bismuth alloys were rarely used in the past because of their poor reliability: one of the major drawbacks was the formation of a bismuth/lead compound with a very low melting temperature (96°C) due to the presence of lead in the components and boards finishes. Due to the implementation of ROHS directive in Europe in 2006, lead was banned in most of the assembly materials.

On the other hand, the introduction of a small quantity of silver (up to 1%) in the tin-bismuth alloy (Sn42Bi58) improves its reliability as far as mechanical and thermal cycling properties are concerned. Regarding assembly process, with its lower melting point (around 139°C), there are several expected benefits from tin-bismuth-silver alloys: better soldering yield for temperature sensitive components, replacement of some selective soldering by reflowing SMT components, shorter reflow cycle times and reduction of energy, lower use of silver (metal price), thus overall cost savings.

However, this relatively low melting point and the high oxidation of the surface of tin-bismuth based powder require the use of more aggressive activators which are efficient at low temperature as well as solvents which evaporate at a lower temperature: the shorter difference between the printing and reflow temperatures decreases the stability of the solder paste.

The purpose of this paper is to present a concrete case study of the development of a suitable flux medium for tin-bismuth-silver according to customer requirements: the methodology, the laboratory tests, the industrial tests and the results obtained will be described.

Key Words: 

lead-free, low temperature, tin-bismuth-silver, solder paste, flux medium

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