| Preparation methods and related fundamental theories for a new type Au-based mid-temperature eutectic solder with the melting point range from 450℃to 500℃were investigated in this paper,supported by the National Military Industry Mating Programme(No.DZ-2002-021).According to quantity of phase data analysis,the alloy constituent was determined to choose from Au-Ag-Ge ternary system,equilibrium phase diagram and the equilibrium phase compositions of Au-Ag-Ge ternary system were calculated by the CALPHAD and THERMO-CALC method then,the composition of the Au-Ag-Ge alloy which is locating at the single-variable line with an eutectic reaction temperature of 500℃was calculated to be 49.3%Au,25.5%Ag,25.2%Ge(at%)(its mass fraction is 67.95%Au,19.25%Ag,12.80%Ge accordingly).DSC curve shows that the alloy with this composition has a solidus temperature of 446.12℃and a liquidus temperature of 497.85℃,which is agree with the calculated result,and the preparation requirement is satisfied too. One-time determination of the alloy composition successfully has saved a lot of time and money,and the preparation of Au-19.25%Ag-12.80%Ge alloy has filled the solder blank with a melting point range from 450℃to 500℃in our country.Accordingly,the ribbon-processing technology of this brittle material was studied.The mid-temperature eutectic Au-19.25Ag-12.80Ge solder ribbons were prepared by coated-rolling,double-roll rapid solidification and single-roll rapid solidification method respectively. Experimental results show that the Au-19.25Ag-12.80Ge solder ribbon with good surface quality can be made by aluminum-coated rolling technology,which includes multi-pass and little-reduction hot-rolling firstly and cool-rolling finally.Because of the brittleness of the alloy, edge cracks were easily to occur during the process,and the minimum thickness of the Au-19.25Ag-12.80Ge solder ribbon was limited,it's difficult to be less than 0.1 mm.Self-made doule-roll and single-roll rapid solidification equipments were used to prepare Au-19.25Ag-12.80Ge solder ribbons successfully. During the preparation of solder ribbon by double-roll rapid soldification method,only when the ratio of nozzle diameter to the roller gap is between 3.0 and 3.9,can the molten pool be stable,and the solder ribbons would have good surface quality with moderate width and thickness.The minimum thickness of the solder ribbon made by double-roll method is 0.12mm with a bad thickness uniformity.There would be surface shortcomings such as pits,herringbone like defects,microcracks,and sea weed like defects when the process parameters were inproper during the ribbon preparation by double-roll rapid soldification method.During the single-roll rapid solidification process,it can be seen that when the line speed is certain,the ribbon thickness will decrease with the decrease of the nozzle slit gap,and the cross thickness difference would be smaller. The Au-19.25Ag-12.80Ge solder ribbon with good surface quality was made by single-roll rapid solidification method with a line speed of 18～24m/s,ejecting pressure of 0.05MPa,nozzle slit gap of 0.5mm and a ejecting distance of 0.5mm,the thickness of the ribbons was between 40 and 72μm.Experimental results indicate that,the Au-19.25Ag-12.80Ge solder ribbons made by rapid solidification technology have better welding properties than that of coated-rolling technology.With the same condition, solder ribbons made by rapid solidification technology have better wettability and spreadability than that of the coated rolled ribbons. Single-roller rapid solidified solder ribbons have the highest joint shear strength of 79.63MPa after welded with Ni at 530℃for 5minutes.The main reason is because the microstructure of the solder alloy were changed greatly by the rapid solidification technology.The rapid solidification technology had lowed the liquidus temperature and narrowed the melting temperature gap,Ge-rich metastable had formed too.Microstructure observation results show that,compared to the coarse dendritic eutectic structure of the coated-rolling solder ribbon,the rapid solidification technology have made the crystal of the solder ribbon refinement and evenly.It's found that there is a critical cooling rate value exists during the single-roll rapid solidified Au-Ag-Ge ribbon preparation which could cause great crystal refinement suddenly.Microstructure observation results show that there has two obvious regions in the thickness direction of the single-roll rapid solidified Au-19.25Ag-12.80Ge solder ribbon,one is microcrystallite region which is next to the roller surface,another is coarse crystalline region which is far from the roller surface.The transition from the microcrystallite to the coarse crystalline is a jump reaction,the crystal size would be ten times different.Because of the difference of the cooling rate in the thickness direction,the mutagenic structure is possibly caused by the critical cooling rate value which could cause the crystal refined greatly.There has formed nanocrystalline with a size of 40～50nm when the solder ribbon were made by single-roll rapid soldification method with a line speed of 24m/s.The rapid solidification technology could make the microhardness of the solder ribbon much higher.Double-roll rapid solidified solder ribbon has a maximum hardness of 309HV,which is 268HV higher than that of the coated-rolling solder ribbon.And the microhardness of the single-roll rapid solidified solder ribbon can get to 312HV,which is 271HV higher than that of coated-rolling solder ribbons.Single-roll quenched Au-19.25Ag-12.80Ge solder ribbons are brittle,they can be toughened greatly by a moderate annealing treatment.The brittleness of the quenched solder ribbon is caused by the rapid solidification technology. There has formed brittle Ge-rich metastable during the rapid solidification process,at the same time,cooling rate gradient has occurred in the thickness direction when the line speed is high,which has made the thermal stress gathering in the crystal.After the annealing treatment,the metastable was stabilized and the crystals were recrystallized,so the thermal stress was removed and the solder ribbon was toughened.Ge phase which was dispersing precipitated by the supersaturated solid solution of the quenched solder ribbon has made the microhardness increased too.Nucleation regularity of the rapid solidified Au-19.25Ag-12.80Ge solder alloy calculated by the nucleation incubation period equations shows that:The nucleation incubation period of the AuAg solid solution is much shorter than that of the Ge phase,AuAg solid solution is the priority precipitated phase during solidification course.Critical nucleation temperature,critical nucleation supercooling degree and critical nucleation number were calculated according to the time-dependence transient nucleation theory,results show that:Increase of the cooling rate would trigger the increase of required beginning nucleation supercooling degree,and the critical nucleation number would increase greatly. Simulating analysis of the temperature field and cross section thermol history by numerical analysis model indicates that,random location of the solder ribbon undergoes the following change with the solidification process:Melt temperature falling down→temperature rising up because of solidification latent heat release→solid alloy forming and cooling down. |
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