Solder process for fluxless solder paste applications

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 wetting of the solder is made possible 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. In this paper, a residue free SnAgCu solder paste and a suitable soldering process is developed and presented. It is applied on high power ceramic LED packages and 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. Different solder joint analysis methods like X-Ray, the transient thermal analysis, cross section and shear strength tests are presented in this paper. Finally a measurement technique to detect contamination on a soldered module is shown. There is a possibility that the concept and process proposed and demonstrated in this paper can reduce the production costs of electronic systems and improve the quality of soldered electronic modules.