DEVELOPMENT OF A SUITABLE FLUX MEDIUM FOR CLEANABLE AND NO-CLEAN SOLDER PASTES BASED ON TIN-BISMUTH-SILVER ALLOY

摘要

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 properties and thermal cycling are concerned. Regarding assembly process, with its lower melting point (around 139°C), there are several expected benefits of tin-bismuth-silver alloys: better soldering yield for temperature sensitive components, replacement of some selective soldering by reflow, shorter reflow cycle times and saving energy, thus cost. However, the development of a suitable flux medium is a challenge due to its relatively low melting point and its high surface oxidation. The use of aggressive activators which are efficient at low temperature, as well as solvents which evaporate at a lower temperature is required, making the paste more unstable. The purpose of this paper is to present two concrete studies. A general description of flux media and the role of their constituting ingredients will be made first. Then, the development of a tin-bismuth-silver solder paste with cleanable residues according to a specific request will be described. Finally, the methodology followed to develop a suitable flux medium for a no-clean paste will be explained. The development steps will be validated by standardized tests as well as home-made testing, and then by industrial evaluations.