【作者】 崔大田；

【导师】 王志法；

【作者基本信息】 中南大学 ， 材料物理与化学， 2008， 博士

【摘要】 本文结合国家军工重点工程配套项目(No:DZ-2002-021):中温封接共晶焊料研制任务,研制了一种熔点在450～500℃的新型Au基中温共晶焊料及其薄带制备技术,并探讨了所涉及的基础问题。首先根据大量相图资料分析,确定了合金成分应在Au-Ag-Ge三元系内选定,然后利用CALPHAD和THERMO-CALC方法计算出Au-Ag-Ge三元系的平衡相图及平衡相的组成,并计算出Au-Ag-Ge单变量线上共晶温度为500℃的合金成分为Au-25.5Ag-25.2Ge(at%)(为了便于配料称重方便,转换为质量百分含量Au-19.25Ag-12.80Ge),DSC曲线表明该成分合金的固相线温度为446.12℃,液相线温度为497.85℃,其熔化温度与计算结果基本一致,符合设计要求,一次性成功确定了合金成分,避免了“尝试法”多次成分探索试验,大大节省了时间和经费。本合金的研制成功,填补了我国熔点在450～500℃焊料合金的空白。在此基础上,研究了将这种脆性材料加工成薄带的工艺方法。采用包覆轧制法、双辊快速凝固法及单辊快速凝固法三种工艺制备了中温共晶Au-19.25Ag-12.80Ge焊料薄带。实验表明,采用铝合金包覆,先多道次、小变形量结合中间退火工艺热轧,最后冷轧退火的包覆轧制工艺可制备表面品质优良的Au-19.25Ag-12.80Ge焊料合金薄带,但由于合金的固有脆性,加工过程中容易产生边裂现象,且薄带最小厚度有限,很难加工到0.1mm以下。利用自行研制的双、单辊快速凝固装置成功制备了中温共晶Au-19.25Ag-12.80Ge焊料薄带;试验发现,当浇注喷嘴直径与双辊辊缝的比值在3.0～3.9之间时,熔池处于稳定区间,所制备带材的宽度适中、表面品质较好;双辊法制备Au-19.25Ag-12.80Ge焊料合金薄带的最小厚度为0.12mm,但薄带沿宽度方向厚度均匀性较差,双辊快速凝固工艺参数的设置不合理等原因可导致焊料合金薄带出现表面凹坑、鱼骨状表面缺陷、微裂纹及海带状等缺陷。单辊快速凝固法制备焊料薄带时,当辊面线速度一致时,喷嘴狭缝间隙越小,制备的焊料薄带越薄,横向厚差越小。当辊面线速度为18～24m/s,喷射压力为0.05MPa,喷嘴狭缝间隙为0.5mm,喷射距离为0.5mm时,可制备表面质量优良的焊料薄带,薄带为银白色,厚度在40～72μm范围内且较均匀。实验表明,快速凝固工艺制备的焊料薄带比普通铸造加工的焊料薄带具有更加优异的焊接性能,相同条件下,快速凝固工艺制备的焊料合金薄带比包覆轧制工艺制备焊料合金薄带与Ni具有更加优良的润湿性和流动性。单辊快速凝固工艺制备的焊料合金薄带在530℃×5min条件下与Ni焊接后形成的接头剪切强度最高,达79.63MPa。这主要是因为快速凝固工艺使焊料合金的显微组织发生了较大变化。快速凝固工艺使焊料薄带的液相线温度降低,熔化温度区间变窄,且形成了富锗亚稳相;显微组织观察表明,相比于包覆轧制法制备焊料薄带形成了粗大的树枝状共晶组织,快速凝固工艺使得Au-19.25Ag-12.80Ge焊料薄带晶粒显著细化,形成了均匀细小的胞状晶,同时成分更加均匀。本文首次发现单辊快速凝固法制备Au-Ag-Ge合金薄带时,存在一个引起晶粒突然细化的临界冷却速度。显微组织观察结果表明,单辊快速凝固法制备的中温共晶Au-19.25Ag-12.80Ge焊料薄带,沿厚度方向明显分为贴辊面微晶区和自由面粗大等轴晶区,沿厚度方向由细晶区向粗晶区的过渡是突变的,其晶粒尺寸相差10倍以上。由于冷却速度沿薄带厚度方向存在差异,同时冷却速度不可能产生突变,因此这种组织突变可能是存在的一个能引起晶粒突然细化的临界冷却速度造成的。当辊面线速度达到24m/s时,单辊快速凝固法焊料薄带细晶区形成了纳米级晶粒,晶粒尺寸为40～50nm。快速凝固工艺使焊料薄带的维氏显微硬度得以显著提高。双辊快速凝固法所制备的中温共晶Au-19.25Ag-12.80Ge焊料薄带维氏显微硬度最大达309HV,比包覆轧制法焊料薄带高268HV;单辊快速凝固法制备的焊料薄带维氏显微硬度达312HV,比包覆轧制法焊料薄带高271HV。单辊快速凝固法制备的中温共晶Au-19.25Ag-12.80Ge焊料薄带存在淬态脆性,经过一定的退火处理后可大大韧化。焊料薄带的淬态脆性主要是由于快速凝固工艺造成的;快速凝固法制备焊料薄带过程中,合金产生了富锗亚稳相,该相性质较脆,同时快速凝固工艺冷却速度较大时,在薄带厚度方向产生了冷却速度梯度,造成了晶粒内部热应力的产生;退火处理后,亚稳相发生了稳定化转变,晶粒发生了再结晶,热应力得以消除,因而使焊料薄带得到韧化,同时急冷态的过饱和固溶体沉淀析出弥散的Ge相使合金的硬度也同时提高。根据形核孕育期公式对快速凝固Au-19.25Ag-12.80Ge焊料合金的形核规律进行了计算,结果表明:AuAg固溶体的形核孕育期远远小于Ge相的孕育期,AuAg相在凝固过程中作为主要形核相优先析出。根据时间依从瞬态形核理论,对连续冷却条件下临界形核温度、临界形核过冷度和临界形核数的计算结果表明:条带冷却速率的提高,触发熔体形核所需的起始形核过冷度增加,而临界形核数则大幅增加。由数值分析模型对温度场及断面热历史的模拟分析表明:合金薄带内部任意位置随凝固过程的进行经历如下变化:熔体温度下降→凝固潜热释放导致温度回升→固相继续冷却降温。更多还原

【Abstract】 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.更多还原