【作者】 李小蕴；

【导师】 祖方遒；

【作者基本信息】 合肥工业大学 ， 材料加工工程， 2013， 博士

【摘要】 焊料的无铅化是国内外电子、电气、仪表及家用电器等行业的共识。然而，与传统PbSn焊料相比，现有无铅焊料在工艺及服役性能等方面仍存在诸种不足。迄今，国内外研究者多从成分选择与配比的优化、微合金化、稀土元素的加入及冷却速度上着手进行研究，却很少有人关注无铅焊料制备过程的熔体结构与状态对焊料本身的凝固组织、焊料施焊过程的工艺性能、接头力学性能以及服役过程中的组织稳定性等方面的影响和规律。本文选取Sn-3.5Ag共晶合金为研究对象，以温度诱导液液结构转变为切入点，通过改变焊料的制备温度，以及添加第三元素Bi来探索熔体结构和性质的变化规律，进而探索熔体结构对无铅焊料凝固组织、润湿性能、接头剪切性能、断裂机理及时效过程IMC生长的影响和规律。本文工作所取得的主要创新性成果和认知如下：1、以两轮升降温过程以及特定温度保温方式，探索了Sn-3.5Ag-xBi（x=0,2,3.5,5,7）焊料电阻率-温度行为，所揭示的现象直观地表明，Sn-3.5Ag-xBi熔体发生了温度诱导的液液结构转变。其具体特征表现为：Sn-3.5Ag-xBi焊料熔体首轮升温过程的结构转变是不可逆的；合金熔体在后续降温及第二轮升降温过程中所发生的液液结构转变具有可逆性；两种转变的温度区间均随成分而有所不同。分析认为，首轮加热熔体转变的物理本质在于，低温熔体原有同类原子团簇（SnN、BiM）及异类团簇（Sn-Ag化学短程序）在一定高温范围被打破并形成新的原子团簇，其相应熔体状态的均匀性及无序度更高；而可逆转变则与具有四面体短程有序结构的Sn-Sn共价键的可逆特征有关。2、凝固热分析及组织检验表明，焊料制备过程的熔体状态对其凝固行为和组织产生显著的影响，与首轮转变前的相比，转变后熔体状态的凝固特点如下：（1）形核过冷度及共晶生长过冷度均明显增大。（2）凝固组织显著细化，表现为初生相及共晶体内间距的尺度均变小，且组织分布更加均匀。（3）共晶生长方式发生了质的改变：一方面，共晶体中Ag3Sn相由原来小平面生长特征的不规则分布，转变为以非小平面生长特征的平行规则分布为主；另一方面，共晶团形貌由原来的粗大树枝状转变为细小的等轴共晶。3、就制备方法对焊料施焊工艺性能及接头强度影响而言，液液结构转变后熔体状态所获得的焊料，与铜基板的润湿性得到改善，即润湿角变小，而且焊接接头的剪切强度也得到明显提高。分析认为，润湿性的改善，一方面得益于焊料凝固组织细化而在施焊过程中熔化更加容易，更为重要的是，熔体首轮不可逆转变致使焊料熔化后其更均匀且更无序的熔体状态，使焊料熔体与Cu基板之间的表面能SL降低；而接头强度的提高，一则是由于焊料本身组织的细化，再则因润湿性改善界面处更加易于形成完美的原子间结合。4、研究表明，焊料中Bi的含量对焊料的工艺性能及接头可靠性也有不可忽略的作用。随着Bi量的增大，Sn-3.5Ag-xBi焊料熔点降低，同时润湿角减小，润湿性能得到显著提高；接头强度随Bi量显著提高，在Bi含量为5%时焊料剪切强度达到最大值。剪切试样的断口分析表明，焊料中Bi含量较少时(＜3.5%)，焊接接头的断裂机制为完全的韧性断裂，而Bi含量较高时(＞3.5%)，接头的断裂机制转变为韧性和脆性断裂的混合断裂机制。5、液液结构转变对所制备焊料的焊接接头界面结构，以及模拟一定服役温度下的界面行为的作用表现为：能够改善焊后界面IMC的形态，使之分布更加均匀平坦，IMC过渡层厚度也有所减小；在特定温度下时效过程中，一方面可减慢接头界面IMC的生长速率，另一方面，可减少界面处柯肯达尔孔洞的数量，抑制焊料中微裂纹的产生。这些作用均有利于提高焊接接头服役过程的可靠性。数据分析表明，液液结构转变提高了SnAgBi/Cu界面IMC的生长激活能，从物理机制上说明了液液结构转变提高组织稳定性的原因。综合上述几方面结论可见，基于SnAgBi无铅焊料在特定温度范围熔体状态发生改变这一重要现象，可有目标地对焊料制备方法进行创新，从而改善焊料本身的凝固组织，进而提高其焊接工艺性能、焊接接头的力学性能，同时改善焊后界面微观结构以及服役过程接头的组织稳定性和可靠性。作者希望并相信，本文系列工作及其所揭示的现象和规律，可为无铅焊料制备工艺方法的创新、新型绿色焊料的研发和生产提供科学与技术依据。更多还原

【Abstract】 Lead-free solders are agreed to be adopted by the industries of electronics, electrical,instruments, household appliances etc. in China and foreign countries. However, compared with thetraditional PbSn solder, existing lead-free solders have many deficiencies in technology, service andother performances. So far, most researchers prefer to carry out the research by changing alloycomposition, optimum mixture ratio, microalloying, adding rare earth elements and altering coolingrate. Few people pay attention to the effects and rules of melt structure and status in the preparationof lead-free solders on solidification microstructure of solder, welding technological properties,joint mechanical properties, microstructure stability in the process of service, etc.In this paper, Sn-3.5Ag eutectic alloy was chosen as the investigation object. From theviewpoint of temperature induced Liquid-liquid structure transition(LLST)，the rules of melt statechange were explored through changing preparation temperature of the solders and adding the thirdelement Bi. Besides, the effects of melt state on solidification microstructure, wettability, jointshear performance, fracture mechanism and the growth of intermetallic compound(IMC) duringisothermal aging for Sn-3.5Ag-xBi(x=0,2,3.5,5,7) were studied. The major innovationachievements and cognition of this work are present as follows:1.Through two cycles heating/cooling and specific isothermal experiments,resistivity-temperature behavior of Sn-3.5Ag-xBi（x=0,2,3.5,5,7）solders was investigated. Therevealed phenomena intuitively prompt that temperature induced LLST occurs in Sn-3.5Ag-xBimelts. The concrete features display as: LLST of Sn-3.5Ag-xBi solder melt in first cycle heatingprocess is irreversible; LLST that occurs during subsequent cooling and second cycleheating/cooling process is reversible; Temperature ranges of two kinds of transition are differentaccording to the composition. The analysis shows, the physical nature of LLST in first cycleheating is the same type atomic cluster（SnN、BiM）and different types of clusters (Sn-Ag CSRO) inlow temperature melt are breaking and forming new atomic cluster, the uniformity and disordereddegree of corresponding melt status is higher. The reversible transition is relative to reversiblefeature of Sn-Sn covalent bond with tetrahedral short-range ordered structures.2. Thermal analysis and microstructure test of solidification show melt state of solders duringpreparation process has great effects on solidification behavior and microstructure. Compared withmelt status before first LLST, Solidification characteristics of melt status after LLST is as follow:(1)Nucleation undercooling and eutectic growth undercooling are significantly increased;(2)Solidification microstructure is refined obviously, such as the size of primary phase and eutecticspacing is smaller, distribution of microstructure is more uniform;(3)Eutectic growth pattern undergoes a qualitative change: on the one hand, eutectic growth characteristics of Ag3Sn ineutectic changes from original facets irregular distribution to dominating non-facets parallel ruledistribution; on the other hand, eutectic morphology varies from coarse dendritic eutectic to fineequiaxed eutectic.3.For the effects of preparation method on welding performance and joint strength, thewettability of the solders, which are obtained from melt state after LLST, are significantlyimproved, furthermore, the shear strength of welding joints are also improved. The analysis shows,wettability improvement firstly benefits from refine solidification microstructure melts easierduring welding, most importantly from lower interfacial energy SLof solder melts and Cusubstrate that induced by more uniform and disordered melt state which experienced first LLST.Improvement of shear strength is attributed to refined microstructure and better bonding connectionbetween atoms at the interface because of wettability improvement.4. Experimental studies show, Bi content in solders plays a non-ignorable role on soldertechnology performance and joint reliability. With the increase of Bi content, wettability angle ofSn-3.5Ag-xBi solder decreases, wettability is enhanced; Joint strength is significantly improvedwith the increase of Bi content, shear strength achieves the best when Bi content in solderis5%; Fracture analysis of shear specimens shows fracture mechanism of welding joint istotally ductile fracture when the solder has little Bi element content(＜3.5%); With theincreasing of Bi element content(＞3.5%), the fracture mechanism changes to mixing mechanismof ductile and brittle fracture.5. Effect of LLST on interface structure of welding joints and interface behavior simulating acertain service temperature presents as follows: ameliorate interface IMC morphology afterwelding, make it distribute more uniform and disorder, IMC layer thickness is reduced; At specifictemperature aging process, joint interface IMC growth rate constants are slow down, and thenumber of Kirkendall voids at the interface is reduced, meanwhile, LLST suppresses the generationof micro-cracks in solder. These effects are conducive to improve the the reliability of solder jointsduring service. Date analysis showed LLST reduces growth activation energy of interface IMC，which explains the reason why LLST improves structure stability from physics mechanism.From above conclusions, based on the important phenomenonof melt status change in SnAgBilead-free solder at specific temperature range, researchers can be purposeful to innovate thepreparation methods of solders, to enhance the solder itself solidification microstructure, further toimprove the welding technology performance and the mechanical properties of welded joints,meanwhile, to improve interfacial microstructure, its stability and reliability of joint during serviceprocess. The author hope and believe the series work of this paper and revealing phenomena/laws could provide some scientific and technical basis for the preparation technology innovation oflead-free solders, the research and production of new green lead-free solder.更多还原