| 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. |
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