Flux for Cleanable and No-Clean Solder Pastes

Emmanuelle Guene and Celine Puechagut
Inventec Performance Chemicals
Bry sur Marne, France

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 degrees 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 degrees 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.

The development of two low-temperature solder pastes based on a tin-bismuth-silver (Sn42Bi57.6Ag0.4) powder has been conducted successfully. Due to the specificities of the alloy and its powder (melting point 40 degrees C less than eutectic tin-lead, higher oxide content and different nature of oxides), some adaptations were made in the usual internal laboratory testing methods and new criteria were established.

For the first case study, the requirements (Sn42Bi58 or Sn42Bi57.6Ag0.4, thermal shock resistance requirement, type 3, for dispensing, sufficient wetting on ENIG substrate, low microballing and sufficient residue cleanability with solvent process) were clear and precise enough to start internal testing quickly.

Additionally, the full partnership with the customer has enabled a rapid progress: production trials were able to confirm the laboratory tests. The development of the suitable flux medium was based on knowledge of the chemistry and on the know-how, on the screening of ingredients, and was realized step by step to find the suitable product.

For the development of the no-clean solder paste, the requirements were defined partly according to standards and partly according to internal criteria. The second case confirmed the observations done during the first one: short chain acids were necessary to achieve a solderballing with little microballs but short acids also made the pastes unstable.

The use of other rosins, the change of rosin ratios and solvent ratios was necessary to get the desired properties. Simplex method was used several times to improve the paste characteristics: a special attention was given to the surface insulation resistance criterion. At the final stage, the properties of the optimized solder paste were assessed using industrial tools (test board, printer and oven).

Then, a test was performed with a customer and the quality and strength of the joints in the assembly was assessed with positive results. The successful development of a solder paste implies first a rigorous definition of the specifications.

Second, whatever the strategy used, the standardized tests and laboratory tests shall be promptly confirmed by industrial trials. Third, the implementation of an active partnership between supplier and customer accelerates the development of a solder product.