【作者】 罗庭碧；

【导师】 李明； 胡安民；

【作者基本信息】 上海交通大学 ， 材料物理与化学， 2014， 博士

【摘要】 目前共晶Sn-Ag-Cu焊料为最常用的无铅焊料。但是由于共晶焊料中容易产生粗大的Ag3Sn金属间化合物（IMC），造成可靠性下降。此外，由于Ag的成本较高也限制了共晶Sn-Ag-Cu焊料的使用。另一方面，商业化的低Ag焊料SAC105比共晶SAC305焊料在抗冲击性能上更可靠，但是降低Ag含量会带来熔融性能降低和强度降低，以及由此引起的可靠性问题，因此SAC105焊料也未得到广泛应用。本文以解决低Ag含量引起的熔融性能和强度降低为目标，首先通过热力学分析确定研究对象体系。通过热力学分析发现，Sn-Ag-Zn体系焊料可以通过调整Zn含量来提高低Ag焊料的熔融性能，而其它体系焊料不具备这一性质。除此之外，相比Sn-Ag-Cu焊料，Sn-Ag-Zn焊料具有其它一些优点例如强度、抗冲击性能等等。目前业界需要一种具有良好熔融性能和力学性能的低Ag焊料，但是对于Sn-Ag-Zn焊料体系的研究主要集中在共晶成分附近，而低Ag含量Sn-Ag-Zn焊料还未见报道。因此，本文后续研究以低Ag焊料Sn-Ag-Zn三元体系作为对象。本文通过DSC分析、扫描电子显微镜（SEM）、X射线衍射（XRD）和室温拉伸来研究焊料合金熔融、微观组织和力学性能及其联系；然后将焊料运用于Cu和Ni/Cu基板制备成焊点，经过不同的热老化环境后，通过剪切力测试研究焊点力学性能和热老化可靠性，并对界面微观形貌和断口进行了研究以确定断裂机理；之后于三元合金体系添加第四组元来研究进一步提高焊料性能的可靠性；最后通过电化学分析来研究焊料的抗腐蚀性能。通过对Sn-Ag-Zn体系焊料熔融性能、微观组织和力学性能的系统研究。最后确定Sn-1.5Ag-2Zn和Sn-2Ag-2.5Zn两个优化配比及形成机理。优化的Ag、Zn配比可以有效抑制焊料中的先共晶相的形成，并在共晶族间形成连续塑性良好的β-Sn界面。因此，优化的配比不仅可以提高焊料的熔融性能和强度，还能大幅提升焊料的塑性，其熔融性能和强度超过共晶SAC305焊料，Sn-2Ag-2.5Zn焊料的塑性优于SAC105焊料。在回流焊接后，优化焊料制备的Sn-1.5Ag-2Zn/Cu和Sn-2Ag-2.5Zn/Cu焊点在同样具有良好的强度和韧性。研究发现，回流过后的Sn-xAg-1Zn/Cu焊点会形成脆性界面，焊点强度和塑性大幅降低。添加焊料中的Zn含量至2wt.%以上可以有效提升焊点界面强度，使得焊点由焊料内部发生断裂。由于优化配比的焊料有良好的力学性能，使得焊点的力学性能也得以提高。但是研究过程中也暴露出Sn-Ag-Zn/Cu焊料的两个缺点：Sn-Ag-Zn焊料润湿性随Ag含量降低而大幅下降；Sn-Ag-Zn/Cu焊点在150oC下长时间老化后发生严重界面反应的问题。150oC下Sn-Ag-Zn/Cu焊点界面反应的问题和Sn-xAg-1Zn焊点界面脆性的问题可以通过Cu焊盘镀Ni的方式来解决。回流焊接后Sn-Ag-Zn/Ni/Cu焊点的韧性要高于Sn-Ag-Zn/Cu焊点。在150oC长时间老化后Sn-Ag-Zn/Ni/Cu界面也会形成颗粒状Ag3Sn IMC。由于Sn-xAg-1Zn焊料内部存在Ag3Sn相，有利于界面Ag3Sn生长，所以Sn-xAg-1Zn/Ni/Cu焊点老化后力学性能下降比较大，而Zn含量高于2wt.%的焊点力学性能下降较小。同样优化配比的Sn-1.5Ag-2Zn/Ni/Cu和Sn-2Ag-2.5Zn/Ni/Cu焊点具有良好的强度和韧性。添加Cr、Cu、Ni可以有效提高Sn-Ag-Zn焊料的润湿性，可有效避免因Sn-Ag-Zn焊料润湿性不足而产生的焊接缺陷，使得焊点强度进一步提高。但是过多添加会造成成分偏析，为了避免偏析，三种元素的添加量为0.05wt.%Cr、0.1wt.%Cu、0.1wt.%Ni。其中，回流焊接后添加Cu的焊点塑性较差，从而韧性较差；而添加Cr和Ni的焊点塑性没有明显变化，韧性较好。而在250oC，4小时回流焊接后添加Cr的焊点强度和韧性明显下降，其原因可能是长时间回流使过饱和固溶的Cr析出，因此添加Cr的焊料不适用于波峰焊和浸焊。而添加Ni和Cu的焊点保持良好的韧性。通过电化学腐蚀分析可以看到，Sn-Ag-Zn体系焊料的抗腐蚀能力介于Sn-Ag-Cu焊料和Sn-Zn焊料之间，而Sn-Ag-Zn焊料之间抗腐蚀能力差别不大。在Sn-xAg-1Zn焊料中，Sn-2Ag-1Zn焊料抗腐蚀性能较好；而Sn-2Ag-xZn焊料中，抗腐蚀性能随Zn含量的上升而下降，但是Zn含量达到3wt.%时抗腐蚀能力改善。本次研究中，由于第四组元添加量较少，因此添加第四组元后对Sn-2Ag-2.5Zn焊料的抗腐蚀性能影响不大。更多还原

【Abstract】 Currently, the eutectic Sn-Ag-Cu solder is the most popular lead free solder. But theeutectic Sn-Ag-Cu solder forms coarse Ag3Sn intermetallic compound (IMC), which maycause the decreasing of reliability. The high cost of Ag also limits its use. It is reportedthat the low-Ag content SAC105solder has the advantage in shock resistance comparingwith the SAC305solder. However, to reduce Ag content not only rises liquidus andenlarges pasty range, but also lowers strength and reliability under thermal fatigueconditions. Therefore, the SAC105solder has not been widely used.This study aims to improve melting properties and strength of low Ag content solder.The Sn-Ag-Zn ternary system was chosen by the thermodynamic analysis. The analysisresults show that the melting properties can be improved by adjusting of the third elementin the low-Ag content Sn-Ag-Zn system, while this effect cannot be obtained in otherternary system. Furthermore, the Sn-Ag-Zn system has many other advantages comparedwith Sn-Ag-Cu, such as higher strength and shock resistance. The industry needs alow-Ag content solder with better melting properties and mechanical properties. Themost researches on Sn-Ag-Zn system focus on the eutectic composition, while thelow-Ag content Sn-Ag-Zn solder is rarely reported. Therefore, the Sn-Ag-Zn ternarysystem is chosen in this study.In this study, the selected solders were studied by DSC, SEM and XRD analysis tounderstand their melting properties, microstructures and mechanical properties. Then, theselected solders were applied on Cu and Ni/Cu substrates to prepare solder joints. Themechanical properties of the solder joints were researched before and after the thermalaging. The microsturctures and the fracture surfaces of the solder joints were studied tounderstand the fracture mechanism. To improve wettability of Sn-Ag-Zn solders, thefourth elements were doped. At last, the electrochemical analysis was used to understandthe corrosion resistance of the selected solders. The results show, the Sn-1.5Ag-2Zn and the Sn-2Ag-2.5Zn solders have outstandingmelting properties and mechanical properties. The optimized solder compositions caninhibit the formation of the primary β-Sn and γ-AgZn phases, and form a continuousβ-Sn interfacial layer which has good ductility. Therefore, the optimized Sn-Ag-Znsolders not only have good melting properties and strengths, but also have goodplasticities. Their melting properties and strengths are better than the eutectic SAC305solder, and the plasticity of the Sn-2Ag-2.5Zn solder is better than the SAC105solder.The as-reflowed Sn-1.5Ag-2Zn/Cu and Sn-2Ag-2.5Zn/Cu solder joints also havegood strengths and toughnesses. It was found that these is a brittle Cu6Sn5+Cu5Zn8layerformed on as-reflowed Sn-xAg-1Zn/Cu interface, So that the toughness of the solder jointis decreased. The formation of the brittle layer can be inhibited by increasing of Zncontent, and the solder joints with2wt.%Zn content have good toughnesses. Meanwhile,the disadvantages of Sn-Ag-Zn solders such as lower wettability and the erosion on Cusubstrate at150oC are discovered in this study.The brittleness of the Sn-xAg-1Zn/Cu solder joint and the erosion of Sn-Ag-Znsolder on Cu substrate can be prevented by plating Ni on Cu substrate. The toughnessesof as-reflowed Sn-Ag-Zn/Ni/Cu solder joints are higher than Sn-Ag-Zn/Cu solder joints.After150oC aging for200hours, there is Ag3Sn IMC particle formed on the solder/Niinterface. There is Ag3Sn phase in Sn-xAg-1Zn solders, so the interfacial Ag3Sn is moreconducive to form on the Sn-xAg-1Zn/Ni interface. Therefore, the joint strengths of theSn-xAg-1Zn/Ni/Cu joints were decreased sharply after thermal aging. The strengths ofsolder joints with Zn content more than2wt.%in the solder are not decliningsignificantly.Doping of Cr, Cu and Ni can improve wettability of Sn-Ag-Zn solders obviously, butthe excessive doping leads to segregation. To prevent segregation, there are only0.05wt.%Cr,0.1wt.%Cu and0.1wt.%Ni doped in Sn-2Ag-2.5Zn solder respectively. Afterdoping, the soldering defects are reduced obviously in as-reflowed solder joints. Theplasticity of the solder joint with Cu doping is decreased, so its toughness is low. Whilethe plasticity of joints with Cr and Ni doping are not changed obviously, theirtoughnesses are better than undoped solder joint. After250oC soldering for4hours, thetoughness of solder joint with Cr doping is drop obviously, which may caused byprecipitation of supersaturated Cr. While the toughness of the joints with Cu and Ni doping remain higher than undoped joint.It can be found by the electrochemical Analysis that the corrosion resistance of theSn-Ag-Zn solders are lower than the Sn-Ag-Cu solders but higher than the Sn-Zn solders,while there are not significant differences among the Sn-Ag-Zn solders. The Sn-2Ag-1Znsolder has the best corrosion resistance in the Sn-xAg-1Zn solders. When the Zn contentis lower than2.5wt.%, the corrosion resistance is decreased with the increasing of Zncontent in Sn-2Ag-xZn solders. While Zn content up to3wt.%, the corrosion resistance isturning for the better. This phenomenon is caused by the conversion of the eutectics. Themain eutectic in Sn-2Ag-2.5Zn solder is β-Sn+ζ-AgZn eutectic, while β-Sn+ε-AgZn inthe Sn-2Ag-3Zn solder. The corrosion resistance is not obviously changed after Cr dopedand just little better after Cu and Ni doped, which may be caused by the small dopingmount.更多还原