Investigation On Fabrication Of A Low Melting Point Lead-free Sn-Bi-X Solders And Lead-free Soldering Process

Along with the gradually emergence of Lead-free Proposals by each national governments, adopting lead-free of solders in electronic assembly industry have become an unreverse trend. Among the four acknowledged lead-free solders, Sn-Ag and Sn-Cu series solders made the soldering joints worse because their melting points are 34℃higher than that of Sn-Pb solder. Meanwhile, the soldering equipment, PCB materials and electronic chips have to be changed because they could not stand the higher temperature. Sn-Zn solder's melting point is 198℃which is close to 183℃of Sn-Pb. The solder has good mechnical properties and low price, but it also produces many practical problems because zinc can be easily oxidated. Sn-Bi eutectic solder can only be used in some low temperature field which need requirement for reliability, because its melting point is only 138℃high. Up to now, there is no reports about lead-free solders whose melting points rank from 180℃to 190℃to be used in assembly industry.The melting point of Sn-Bi solder can be controlled to get close to that of Sn-Pb solder by adjusting the content of Bi, however, it is still unable to solve the problems of Bi segregation and brittleness of its soldering joint when Sn-Bi solder was soldered. A novel Sn-Bi-X lead-free solder was firstly fabricated via rapid solidification method by adding a series of microalloying elements which can optimize the properties of Sn-Bi solder. The melting point of the new solder is close to that of the Sn-37Pb solder. The mechanical and soldering properties are also similar to that of the Sn-37Pb solder. Therefore it can be used directly in traditional wave and reflow soldering process without changing current equipment and technologies. This dissertation studied the microstructure, properties, soldering interface problems and soldering process of the Sn-Bi-X siolder, and investigated the problems occured in lead-free soldering. The keys to these problems are also given. The main conclusions are drawn as follows:(1) The effects of microelements on the melting characteristics, microstructure and properties of Sn-Bi solder were systemically studied. The results show that when the added content of Ag is 0.7%, Ga 0.3%, In 0.1%, Sb 0.5%, Ge 0.5% and Ce 0.5%, the microstructure of the solder is fine, the extent of Bi segregation obviously decreases and the mechanical property is close to that of Sn-37Pb solder.(2) When the solder was fabricated by Single Roller Melt Spinning method, the faster the cooling rate is, the better the effect of restraining Bi segregation is. When the melt spinning rate was up to 1000rpm, it had the best effect. The new self-fabricated lead-free solder Sn-20Bi- 0.7Ag-0.1In-0.5Ge-0.5Ce-0.5Sb- 0.3Ga has a melting point sa low as 186.1℃and fine microstructure without forming of Bi-Segregation phase.(3) The effect of heat treatment technology on the microstrutures and mechanical properties of the lead-free solder and its soldering joint was studied. It showed that annealing 16 hours at 125℃removed the coarse grains of the Sn-Bi solder, produced uniform microstructure and improved its mechanical properties. The microstructrue of the solder was stable at 100℃during long thermal exposure. The author firstly propose that the annealing treatment process can be one procedure for surface mounting technology.(4) By analyzing the microstructure of the intermetallic compounds (IMC phase) in soldering interface, the author firstly found the dissolving phenomenon of metal on plated pads and its effect on the microstructure and mechanical properties of the interface. The micro fracture behavior of the Sn-Pb and Pb-free solders was also systemically researched by investigating the relations between cracks with the interface, the second phase, the strain rate and the microstructure. The evolution of the inner nano-scale cracks in the Sn-based alloy was expounded.(5) A lot of process tests of reflow and vapour phase soldering were made under lead-free condition by treating the surface of Printed Circuit Boards (PCB) adopting Organic Solderable Preservative (OSP) and by adjusting different parameters, The arisen problems were analyzed and investigated in detail and the resolving methods were gained. In the end, the optimum profile and process window were obtained. The high gradient heat capacity PCB of 20 layers was successfully soldered.(6) Several kinds of tests of lead-free solders in wave soldering process were designed and the"three factor method"which would be the real mechanism of fillet lifting was concluded. Containing Bi of a solder is one but not the only reason to produce fillet lifting phenomenon. The real reason is from three factors: the melting range, the elongation ratio and the Coefficient of Thermal Expansion (CTE) of the solder. The"three factor method"can explain all the reasons of fillet lifting under any condition and can find countermeasures to restrain fillet lifting. It will produce an important effect to spread lead-free soldering.(7) The new problems in equipment, materials, chips, tin-whisker and the quality of soldering joint brought by using lead-free solders were investigated。The best resolving methods is to use the novel Sn-Bi-X solder fabricated by the author. (8) The new solder has been successfully applied to BGA ball planting technology of the NC chip in our national newest generation high performance huge computer.