Residual free solder process for fluxless solder pastes

For soldering, flux is essential because it enables the wetting of the molten solder. Fluxless soldering, i.e. residue-free soldering with the aid of gaseous activators, has been known for many years, but is only well established in the field of opto- and microwave electronics where the solder is applied as preform. In high-volume SMD applications where solder paste is printed, this technology is rarely used until now. The reducing effect of a gaseous activator like formic acid vapor on certain solder alloys is known in practice. However, the corresponding reactions which occur under soldering conditions in nitrogen atmosphere have so far not been systematically investigated for different solder alloys. This study aims to analyze the different chemical reaction channels which occur on the surface of different solders, i.e. catalytical dissociation of formic acid on the pure or oxidized metal surface and the formation and evaporation of metal formates. Based on this analysis, a residue-free solder process under formic acid is developed for solder paste applications.

In this paper, different solder alloys (SnAgCu, SnPb, BiSn, In) were analyzed with thermal gravimetric analysis (TGA) under formic acid flow. Details on mass change depending on the soldering temperature are presented. Activation temperatures are estimated and correlated to the soldering processes. Based on the analysis, fluxless solder pastes and suitable soldering processes are developed and presented. Major paste properties such as printability are compared to a commercial flux solder paste. High-power flip chip LEDs which can be assembled directly on a printed circuit board are used to demonstrate the fluxless soldering. Likewise, the soldering results of standard paste and fluxless paste systems after a reflow process are evaluated and compared.

The experimental results show that TGA is an efficient way to gain deeper understanding of the redox processes which occur under formic acid activation, i.e. the formation of metal formates and their evaporation and dissociation. It is possible to solder residue-free not only with preforms but also with a fluxless solder paste. The resulting solder joints have the same quality as those for standard solder paste in terms of voids detected by X-ray and mechanical shear strength.

In the fluxless soldering process, the reduction of oxide layers, and therefore the wetting of the solder spheres, is enabled by gaseous formic acid. After the soldering process, no cleaning process is necessary because no corrosive residues are left on the circuit boards and components. Therefore, soldering using solder paste without aggressive chemical ingredients has a high market potential. Expensive preforms could be replaced by paste dispensing or paste printing.