Fluxes for the mechanical soft soldering of heavy metals have been known and are commercially available in various grades depending on the use requirements for the fluxes. The various categories are compiled in German Industry Standard DIN 8511. Fluxes for the mechanical soldering of printed circuits have to meet special requirements, more particularly so, if the printed circuits are intended for use in communication technology, data processing, and defense and space technology. More specifically, corrosion resistance demands, with or without subsequent purification, and In-Line-Circuit-Testing as increasingly applied, set new and higher standards for such fluxes.
The application of fluxes for the mechanical soft soldering of heavy metals is effected by immersing, spraying, or foaming. In immersing and spraying, the holes of perforated printed circuits are also exposed to the flux which causes considerable interference. Therefore, when applying these methods, it is necessary that first the holes are covered or otherwise protected against the action of the flux. Thus, more recently it has been preferred to use foam fluxes. The foam fluxes are foamed, for example, by blowing air thereinto through a porous stone and are applied to the printed circuits in the form of this foam. Thus, according to the state of the art foam fluxes contain a solvent, an activator which is one or more dicarboxylic acids having from 4 to 12 carbon atoms, rosin and/or further conventional additives, and a foaming agent. Fluxes and, more specifically, foam fluxes for corrosion-sensitive printed circuits as well, are preferably prepared free from halogens, since halogen-containing fluxes to a high extent tend to be susceptible to corrosion.
The halogen-free fluxes as so far known have a relatively high total solids and foaming agent contents. Most known foaming agents have concentrations of from about 10 to 30% by weight. Alone the rosin content mostly is 6% (or more) by weight. It is known that the high colophonium content acts to inhibit corrosion, since colophonium coats the metal parts with a thin water-insoluble film; however, since the heavy metal salts of the organic acids have been dissolved or suspended therein, respectively, this layer causes undesirable residual conductivity and interference to occur in high-quality printed circuits. More specifically, the rosin film adversely interferes with the In-Line-Circuit-Testing as conventionally practiced today.
In European patent application No. 83,102,353 there have been described fluxes that are free from halogen and rosin and, thus, appear to be suitable for use in high-quality printed circuits. However, a thorough investigation of these fluxes has revealed that solid coverings comprising ionizable residues are formed also, which either result in malfunctions or have to be subsequently washed off. Another disadvantage of the fluxes as described in the European patent application is that they have to be applied by spraying and cannot be used as foam fluxes.