Rapid Solidification Of Sn - 3.0 - Ag - 0.5 - Cu Lead-free Solder Microstructure And Properties Of Influence

As the era of electronic information technology coming, the electronic packagingtechnology plays a decisive role. The lead-free solder has been the hot topic long, however,there is still short-coming in the development and application. The main material of thisresearch was the Sn-3.0Ag-5.0Cu lead-free solder. In this paper, the samples were preparedthrough the method of suction casting and melt-spinning. With the aid of SEM, XRD, EDS,DTA, solderability tester, solder joint strength tester and some other instruments, this paperstudied the effects of the rapid solidification rate on the microstructure, meltingcharacteristics, weldability, microhardness and solder shear strength.The result of microstructure analysis shows that the solidification rate obviouslychanges the microstructure of the solder alloy. From the edge to interside, themicrostructure of suction casting samples changes significantly. The edge regionalorganization is the slim branch pointing to the center which is first precipitates phase β-Sn.The crystal particles between the branches are tiny. The central part of internal organizationis the bulky grid shape organization with the existence of Ag₃Sn and Cu₆Sn₅. On thecondition of rapid solidification, the microstructure of melt-spinning sample is more evenand tiny. The high melting phrase Ag₃Sn and Cu₆Sn₅and Sn form pseudo eutecticorganization with the shape of small round state or small stick grain. The boundarybetween primary phase β-Sn and crystal organization is becoming blur. Sole β-Sn can notbe found in the microstructure pictures. With the rising speed of cooling and solidificating,the organization grain constantly decrease.The result of the study on characteristics and thermal stability of the solder indicatesthat with the increasing cooling solidification rate, Sn-3.0Ag0.5Cu lead-free solder ofliquid line reduces, solid phase line slowly rises, and melting process temperature rangesignificantly reduces. The trend of the change of the melt-spinning samples is more obviousthan that of suction casting with solid-phase temperature of N11up to216.94℃, theliquidus temperature drops to218.69℃and the melting process temperature range reducesto1.75℃. Solder alloy suddenly gets weigh when it’s around liquid phase temperaturewhich indicates that chisel is extremely easy to be oxidated in220℃. Under180℃, themother material and suction casting continues to be gaining weight and the melt-spinningappears to be gaining weight slowly or not gaining weight. Under the work conditions of electronic products, the rapid solidification solder has better stability and strongerantioxidant capacity. The study on solder force shows that the faster of the cooling andsolidificating speed, the bigger of the force of wetting force. The max wetting force Fmaxfor0.85mN which is the fastest cooling speed N11is far larger than the Fmax for0.48mN.With the increasing speed of cooling and solidifacating of the solder, the solder wettingstability gradually would reduce and the wetting stability of the mother material of0.985reduced to0.847of N11. However, the speed of condensing does not change the wettingtime significantly. The study on the solder micro-hardness shows that with the solder alloycooling solidification rising up, the micro-hardness of solder appears to be a rising lineartrend. Compared with mother material, the hardness of the suction casting andmelt-spinning samples have improved significantly. The hardness of the alloy has closelyrelated with the organization of the alloy. On the condition of rapid solidification, the tinyalloy crystal particles show up.. Rich Cu phase Cu₆Sn₅and rich Ag phase Ag₃Sn particlesare distributed among the separated crystals, which plays a important role in fine-grainstrengthening and dispersion strengthening which are the main reason to improve the solderalloy mechanics properties.The study of the solder joint’s reliability shows that: In the same size of solder joint,rapid solidification solder joint, appearance of weld is better; Along with the increase ofsolidification rate, the faster solidification rate of solder is, the narrower the meltingprocess is, the stronger the wetting ability. During the soldering process, solder show thebetter spreading and wetting ability, the interaction between solder and copper material isbetter, the intermetallic compounds are finer and more distributing in the rapidsolidification solder joint, which makes shear strength of the joint higher.