Residual free solder process for fluxless solder pastes

Fluxless soldering, i.e. residue free soldering with the aid of gaseous activation is known for many years, but only well established in the field of opto-and microwave electronics. In low cost high volume applications this technology has not yet become mainstream. In the fluxless soldering the reducing of oxide layers and therefore the wetting of the solder is enabled by means of an activating process gas. 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. The reducing effect of gaseous activator like formic acid vapor on many solder alloys is known in practice. But a detailed scientific analysis on the corresponding reaction mechanisms, especially at the surfaces, only little is investigated. Different chemical reaction channels can occur on the solder surface, i.e. catalytically dissociation of formic acid on the pure or oxidized metal surface and the formation and evaporation of metal formates. 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 depended to the soldering temperature are presented. Activation temperatures are estimated and correlated to heating processes. Based on the analysis, fluxless solder pastes and suitable soldering processes are developed and presented. It is applied on high power unpackaged flip chip LEDs which can be assembled directly on a substrate. In this paper the main paste properties such as printability of a commercial flux solder paste with those of the fluxless solder paste are compared. Likewise, the soldering results after a reflow process of these different paste systems are evaluated and compared. The experimental results show that the thermogravimetric method is an efficient way to gain deeper understanding of the redox processes which occur under formic acid activation. It is possible to solder residue-free 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, mechanical integrity by shear strength.