Effect Of Trace Phosphorus On The Oxidation Resistance Of Lead-Free Solder Used In Wave Soldering

Nowadays in electronic assembly process, wave soldering is changed from lead-contained to lead-free. While, the more tin content in the Pb-free solder will lead to more dross during soldering process. Too many dross will not only waste the solder alloy, but also affect the soldering quality. Therefore, a key problem for Pb-free wave soldering is to control the formation of the solder dross. Although several solder alloys with anti-oxidation ability are available in the market, their real effect is questionable.In this work, the most popular Pb-free solder, i.e., Sn-0.7Cu alloy, is selected as the base alloy. Several elements, In, Ge, Ga, Sb and P, have been added into the base alloy in order to investigate the improvement of the anti-oxidation effect. By comparing the anti-oxidation effect, influence on the wetting ability and the price of the corresponding element, it could be concluded that, for solder alloy, P is the most practical element for the improvement of anti-oxidation ability.As an anti-oxidation element, the best amount of phosphorus in the Sn-0.7Cu solder alloy is from 0.0085wt% to 0.011wt%. During the simulated wave soldering, the dross formation of P-added solder alloy is 60% less than the normal base alloy. Meanwhile, it has been found that such anti-oxidation effect is obvious only when the temperature of solder pot is below 340 centigrade. By means of the X-ray and AES analysis on the solder dross, the anti-oxidation mechanism of the adding element has been studied. The trace adding phosphorus is highly enriched in the surface of the molten solder. Since phosphorus has a higher reactivity with oxygen than Sn or Cu, a dense phosphorus oxide is formed and thus the solder alloy itself is protected from further oxidation. When this layer has been broken, it will form a new layer at once and continue to protect the solder.The phosphorus will be consumed during the lead-free wave soldering and thus the anti-oxidation ability of the solder alloy will be lost gradually. Experimental results showed that, during the simulated wave soldering with a pot temperature of 260°C, the effective time of the adding element is 5 hours.