Design rules for fluxless soldering

Design rules are proposed for achieving robust and reliable soldering in a fluxless environment. The associated processes are discussed in relation to the principal solders that are used in electronics manufacture. Essential to the realization of a successful fluxless soldering process is a mutually compatible selection of solder and finishes supplied on the components and circuit board. This combination must ensure that the joint surfaces are ad equately wetted by the solder whilst at the same time that the finishes do not form intermetallic phases with the solder that are deleterious to the m echanical integrity of the joint. On account of its good solderability and resistance to oxidation, gold is frequently the preferred choice as a finis h, particularly where the use of chemical fluxes is excluded. However, when gold is used with a tin-rich solder, care must be taken to limit its thick ness on the joint surfaces to prevent the catastrophic formation of brittle AuSn4. Guidelines for the optimum thickness of the gold finish appropriate to frequently used tin-rich solders are provided. Indium-based solders are more tolerant to gold metallizations and they are a safer option where thi cker electroplated gold coatings are used. Whilst indium forms up to three intermetallic compounds with gold, they do not compromise joint integrity.