The Research And Development Of Pb-free Solders For Electronic Surface Mount Technology

In viewing of increasing environmental and health concerns of lead and lead containing solders, especially the recent development of surface mount technology(SMT), the lead-free alternatives need to be considered and become the focus of research. This scientific research and development project bases on this starting point, develops the novel lead-free solders, which can meet the requirements of practicability.The development is aimed at the practicability, giving some consideration to the cost and solderability. With the method of uniform and experiment design, the solder component essential design is on the basis of binary eutectic and near-eutectic Sn-Zn solder alloy systems. The properties of the lead-free solders are predicted and improved by the means of alloying method and experiments, then the solder components get optimized. The sample solder alloys are prepared and refined by the flux to the molten metals on the covering, separation and purify are also discussed in the paper. It also points out the relationship between the flux processing properties and the flux surface properties.The component design and optimization proceeds from the respects of wettability, overpotential, resistance, melting point, melting range and so on. It shows that the solderability is up to the wettability to a certain degree, and it also analyses the feasibility of applying the method of spreading area to the wettability. Besides, it explains the relationship between the wetting angle and the spreading area. The wettability of Sn-Pb solders is better than that of Sn-Zn family solder alloys, andreduces surface tension solder to improve the wettability. The additive Bi can decreas the melting point of the solders, and improve the wettability, and it will get the best wettability when the weight percentage of Bi reaches to 5%. The over-potential of the Sn-Zn solders is much lower than the Sn-Pb solder alloys. The additive Bi will change the over-potential of the solders, near the weight percentage of 5%, the over-potential has maximum value. And value the cost of solders through the density.From the analysis of experimental data, the Sn-(4~6.5)Zn- (1~3) Bi-(0.5~ 1.0)Ag/0.3Cu solder system has been proposed.