【作者】 叶焕；

【导师】 薛松柏；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2013， 博士

【摘要】 随着电子产业无铅化进程的推进和封装技术的发展，有关无铅钎料的研究已成为电子封装领域研究的热点课题。针对无铅钎料性能的不足，国内外学者大都采用合金化的方法来改善无铅钎料的性能。稀土元素被称为金属材料的“维生素”，诸多研究都推荐向无铅钎料中添加稀土元素并取得了一批实质性的进展；但是有关含稀土无铅钎料可靠性问题的研究则相对匮乏，本论文以添加稀土Pr的低熔点Sn-Zn-Ga(SZG)钎料为研究对象，系统研究了Pr含量(0~0.7wt.%)对钎料及其焊点组织、性能和可靠性的影响；发现了稀土相诱发锡须生长问题，并探讨了锡须生长行为与机制。采用润湿平衡法研究了稀土Pr添加对SZG钎料润湿性能的影响。发现适量Pr的引入可以明显改善钎料的润湿性能；其中SZG-0.08Pr钎料的润湿力相对于SZG原始钎料有大幅提升，润湿时间则明显变小，表现出了优异的润湿性能；其原因在于Pr的表面活性作用降低了钎料/助焊剂相界面之间的界面张力并阻止了Zn原子的氧化。组织研究发现Pr的添加可以减小钎料组织中富Zn相的尺寸，有效细化钎料微观组织；这主要是由于Pr的吸附特性降低了钎料晶界自由能和不同晶界间的自由能差，阻止了晶粒的生长，从而得到弥散而均匀的钎料组织。当Pr含量较高时，钎料组织中有块状稀土相PrSn3金属间化合物生成；基于纳米压痕技术连续刚度法测定PrSn3的弹性模量为58.92GPa、硬度值为1.33GPa，均远高于Sn基体；分析认为钎料中少量PrSn3的存在会起到弥散强化和细晶强化作用，有利于钎料力学性能的提高；较多时则会由于其本身的硬脆特性和易氧化而恶化钎料性能。基于微焊点强度测试技术评估了SZG-Pr无铅焊点在焊后和100℃高温存储条件下的连接强度，结果表明添加稀土Pr可以有效提高无铅焊点的剪切强度；经时效处理，虽然焊点强度都有恶化，但含Pr的无铅焊点强度仍要高于SZG原始焊点；当钎料中Pr含量为0.08wt.%时，在焊后和时效后所对应焊点的剪切强度均达最大值。界面组织研究表明0.08wt.%稀土Pr的添加可以抑制SZG-Pr/Cu焊点界面上Cu5Zn8金属间化合物(IMC)的生长，减小界面层(IML)的厚度，这正是焊点强度提高的主要原因。分析高温存储条件下SZG-xPr(x=0,0.08,0.7wt.%)焊点界面组织演化发现，虽然高温下Cu原子的扩散会加速SZG和SZG-Pr焊点金属间化合物层的长大，但SZG-0.08Pr焊点界面层厚度仍明显小于SZG焊点，且界面更为平整；适量Pr的引入还可以抑制长期时效时界面中Cu6Sn5新相的形成和剥离现象的发生。结合有限元数值模拟(FEA)和Weibull可靠性分析方法对比研究了SZG-Pr焊点、Sn-Ag、Sn-Zn无铅焊点和Sn-Pb焊点的热循环可靠性，结果显示SZG-0.08Pr焊点的可靠性最高，得到基于2参数Weibull模型的SZG-0.08Pr焊点失效概率密度分布函数(PDF)组织研究发现在Pr含量较高的SZG-0.7Pr钎料和焊点界面上均有锡须自发生长现象发生。研究总结了在含Pr的Sn-Zn基钎料表面锡须的生长形貌,发现锡须只在稀土相上萌生,但锡须形态复杂,有针状锡须、弯曲的锡须、缠绕的锡须、肿瘤状锡须、花瓣状锡须、搭接的锡须、扭折的锡须、片状锡须和小丘状锡须等多种形态生长。锡须形态机制研究认为这是由于锡须生长受驱动力方向、大小、锡原子供给、锡须萌生点、力矩等多重因素的影响,最终导致各种形态锡须的形成。针对锡须形态的三维特性和现有锡须长度测量方法误差较大的不足,发展了较为精确的锡须长度评估方法：基于扫描电镜两次不同拍摄角度得到的锡须二维形貌和它们之间的几何关系构建锡须的三维几何模型,然后通过空间几何计算得到锡须的三维长度；避免了二维图片由于拍摄角度问题而产生的误差。通过高、低温时效试验,氧化试验和温度循环试验研究钎料表面锡须生长行为和影响因素。发现0℃和100℃时效时锡须长度较短,25℃室温时锡须生长较快,而50℃时效时锡须长度和密度最大；氧化试验结果发现在氧化较快的湿热条件下锡须呈丘状生长,而在氧化缓慢的氮气氛围中仅在钎料表面有锡粒和少许短锡须生长；在温度循环载荷条件下,锡须主要以丘状形态生长。研究结果说明环境温度、氧化速率和热失配等环境因素都极大地影响锡须生长行为。基于锡须生长行为的研究结果,探讨了含稀土钎料表面的锡须生长机制。分析认为稀土相在时效过程中的氧化导致稀土相体积发生膨胀和活性锡原子的生成。由于受周围Sn基体的限制,稀土相膨胀受限,从而导致稀土相内部压应力的产生；应力集中导致稀土相中微裂纹的产生,为锡须生长提供萌生点。压应力的不断释放驱动锡原子通过微裂纹从稀土相内部向表面堆积。随着氧化的深入,Sn原子不断堆积,从而形成了锡须。而在循环温度载荷条件下,由于基体钎料与稀土相材料物理性能之间的差异而导致的热失配会造成稀土相中较高的内应力集中,导致裂纹的大量生长与扩展;较高的热应力和较多的裂纹为丘状锡须的生长提供了驱动力和生长点。更多还原

【Abstract】 Because of concerns about environmental pollution by lead and worldwide Pb-banning legislation,the electronics industry has been moving to adopt lead-free solders to replace the traditional Sn-Pbsolder. A useful approach to improve the performance of lead-free solders is to alloy suitable elementsinto these solders. Rare earth elements (REs), also known as the “vitamins” of metals, have beenwidely recommended by researchers to add into lead-free solders and have shown many beneficialeffects. But the reliability of RE-containing solders has seldom been studied. In this research, effectsof rare earth Pr addition on the microstructure, properties, joint strength and reliability issues of SZGsolder were studied. The phenomenon of spontaneous Sn whisker growth induced by RE addition inthe solders was discovered, the whisker growth behavior and mechanism were discussed.The wetting balance results showed that adding a proper amount of Pr into SZG solder greatlyimproved solder wettability on Cu substrate by increasing the wetting force and decreasing thewetting time, especially when the Pr content is0.08wt.%. This effect is due to the surface-activefeature of RE, which decreases the interfacial surface energy between molten solder and flux andprotects Zn from oxidation. Microstructure analysis results showed that an appropriate amount of Praddition also improved the solder microstructure due to the adsorption effect of RE. However, anexcessive content of Pr can lead to the formation of new RE-rich PrSn3IMCs in the solder.Determined by nanoindentation, it was found that the Young’s Modulus and Hardness of PrSn3were58.92GPa and1.33Gpa respectively, which were much higher than that of Sn matrix, thus anexcessive addition of Pr can worsen the solder microstructure and mechanical properties.Micro-joint strength test results showed that the Pr addition enhanced the shear bond strength ofsolder joints after both soldering and thermal aging at100℃especially when the Pr content is0.08wt.%. The addition of a proper amount of Pr inhibited the growth of Cu5Zn8interfacial IMLs andrefined the as-soldered microstructures during soldering, this is the main reason that joint strengthenhanced. While thermal aging led to the degradation of interfacial microstructures including theformation of micro voids and spalling of IMLs, it was found that the presence of Pr inhibited theformation of Cu6Sn5at the interfaces during aging. By means of FEA method and Weibull analysis,the joint reliabilities of SZG-Pr, Sn-Ag, Sn-Zn and Sn-Pb solders under thermal cycling wereevaluated and it was demonstrated that the SZG-0.08Pr solder showed the highest reliability. Based on 2-parameter Weibull model, the PDF function of solder joint was built asIt was found that an abnormal and spontaneous growth of Sn whiskers took place on the surface of solder and joint interface in high Pr-containing SZG-0.7Pr solder. It was further found that whiskers grew in different morphologies including needle-like, bent, twist, plate-like, hillock-like and many other complex shapes. The morphology mechanism analysis discussed that whisker morphology was governed by many factors such as the orientation and value of driving force, Sn source, initiation site and moment of gravity, any changes of the factors would result in the change in whisker morphologies. Considering the3D feature of whisker, a calculation method for whisker length based on2D measurements taken by SEM from two views and their3D geometry relationship was developed, which can avoid the measuring error effectively.The whisker growth behavior and effect factors were investigated by means of aging experiment, oxidation experiment and thermal cycling experiment. Aging results showed that the50℃aging condition accelerated whisker growth while whisker grew slower under room ambiance (25℃) and slowest under100℃and0℃aging condition. Oxidation results showed that the whiskers grew in hillock-type morphology under temperature humidity and in nodule-type under N2atmosphere while the typical needle-like whisker grew in room ambience. Unlike the needle-like whiskers grown mainly in ambience, it was found that coarse Sn hillocks were the dominant growth morphology under thermal cycling condition.The whisker growth mechanism was discussed based on the whisker growth behavior analysis. It was proposed that when the RE-containing solder is undergoing aging and oxidation processing, RE-induced growth of tin whiskers is governed by the oxidation of RE-rich IMCs, which generates compressive stress within the particles that creates fresh tin atoms and micro cracks on the IMC surfaces. The compressive stress then drives the newly released tin out from the weak points, forming the selective tin whisker growth from the RE-rich IMC surfaces on the bulk Sn-rich alloys. In the case of thermal cycling condition, it was discussed that the CTE mismatch between RE-rich IMC inclusions and surrounding solder matrix induced high thermal mismatch stress within the RE-rich IMCs, leading to the growth and propagation of cracks. The high local thermal stress and cracks ensured enough driving force and initiation sites for Sn hillock growth.更多还原