镀层表面锡晶须自发生长现象的研究进展

镀层表面锡晶须自发生长是材料科学中一个受到长期关注的科学现象.随着近年来电子器件无铅化的发展,锡晶须问题日益突出.对于高密度电子封装技术,由晶须自发生长引起的短路和电子故障问题对电子产品的可靠性构成了潜在的威胁.因此,研究锡晶须的生长规律,阐明锡晶须的生长机理,探寻抑制锡晶须生长的技术手段成为当前研究的热点.总结了近年来国内外对锡晶须生长现象的一些相关研究,主要包括锡晶须的生长行为、各种影响锡晶须生长的因素、近年来晶须生长机制方面的新进展、锡晶须生长趋势的评估方法以及工业上抑制晶须生长的一些技术措施等.     

锡晶须生长及抑制试验研究

介绍了锡晶须问题及其危害;总结了晶须产生理论的5种学说以及抑制晶须生长的6种办法;开展了无铅焊点与有铅焊点的晶须生长对比试验研究工作,并在无铅焊点表面观察到了晶须生长现象;开展了敷形涂敷对于晶须生长的抑制试验工作,经过敷形涂敷的无铅焊点未观察到晶须生长现象。     

锡晶须生长机理研究的现状与问题

介绍锡晶须的发现过程以及机制研究和预防策略,总结锡晶须生长过程中已经被一些研究人员发现的特点,如晶须形貌的多样性、生长过程的阶段性以及生长位置的不确定性等。回顾从最初发现锡晶须到现在所提出的用于解释锡晶须生长机理的理论模型,其中主要是位错机制、压应力机制、再结晶机制、氧化膜破裂机制以及活性锡原子机制。在对于这些理论模型的问题进行评述后,对如何进一步探索晶须的生长机理提出一些看法。     

双向脉冲法电沉积低应力晶须抑制型锡镀层

以甲基磺酸锡为镀锡液的主盐,采用Stoney镀层应力测试方法,以及锡晶须生长趋势评价标准(JEDEC标准JESD22Al21.01),研究了双向脉冲电沉积参数对纯锡镀层拉应力大小及其晶须生长特性影响的规律。利用扫描电子显微镜(SEM)表征了锡镀层晶须生长前后的微观形貌,优选出了锡镀层应力低、锡晶须生长趋势小的双向脉冲电沉积参数(平均电流密度为10 A/dm~2,占空比为0.7,逆向脉冲系数为0.5,频率为10 Hz)。结果表明,通过调控双向脉冲参数,可控制纯锡镀层内应力的大小,进而制备出可抑制锡晶须生长的纯锡镀层。     

通过添加POSS颗粒抑制锡基无Pb焊层的晶须生长

在Sn,Sn-3.0Ag-0.5Cu和Sn42-Bi58钎料中添加具有纳米结构的笼形硅氧烷齐聚物(POSS)作为增强相,研究了增强相在恒温恒湿(85℃,相对湿度85%)条件下对锡基无Pb焊层晶须生长行为的影响.结果表明,在恒温恒湿条件下,锡基无Pb焊层晶须生长的驱动力是Sn的氧化物生成引起体积膨胀从而对周围焊层产生的压应力;添加POSS可以有效缓解金属Sn的氧化进程,抑制Sn的氧化物生成,从而减缓晶须生长;在Sn,Sn3.0Ag0.5Cu和Sn58Bi焊层中,Sn焊层晶须生长能力最强,Sn58Bi焊层晶须生长能力最弱.     

铝液原位反应生成TiAl_3晶须的条件和机制分析

分析了液相原位反应生成 Ti B2 颗粒过程中 ,Ti Al3晶须的生成条件和生长机理。结果表明 ,预制块中 Ti与B的摩尔比显著影响 Ti Al3晶须的生成 ,Ti Al3晶须的生长机理为 VL S机制 ,由于晶须生长过程中触媒液滴中的 Ti参加反应 ,使它与一般晶须生长的 VL S机制有所不同     

铝液原位反应生成TiAl3晶须的条件和机制分析

分析了液相原位反应生成TiB2颗粒过程中，TiAl3晶须的生成条件和生长机理。结果表明，预制块中Ti与B的摩尔比显著影响TiAl3晶须的生成，TiAl3晶须的生长机理为VLS机制，由于晶须生长过程中触媒液滴中的Ti参加反应，使它与一般晶须生长的VLS机制有所不同。     

沉积温度对TiC晶须生长的影响

用CVD做了沉积温度对TiC晶须生长影响的实验。在纯Ni基板上,用TiCl_4-CH_4-H_2-Ar混合气氛,1220～1370K较低温度下获得了高产率的TiC晶须。在实际上做了TiC晶须的形貌的检测,并确认了VLS(汽-液-固)生长机制,按照VLS机制讨论和说明了沉积温度对TiC晶须生长的影响。找出了用镍基板生长TiC晶须的合适温度范围。     

CVD法合成SiC晶须的实验研究

利用简单的实验设备，特殊的金属丝作触媒，以ＳｉＯ２和Ｃ为原料，利用碳热还原反应生成的ＳｉＯ和ＣＯ，通过ＣＶＤ的方法快速合成α－ＳｉＣ晶须，用光学显微镜研究了晶须的生长速度，通过ＴＥＭ研究α－ＳｉＣ晶须的结构和生长方式。讨论了这种方法中α－ＳｉＣ晶须生长的热力学条件，机理及生长动力学模型。     

碳热还原合成Ti(C,N)晶须的相转变行为及生长机理

以TiO2和碳黑为主要原料,NaCl为卤化剂,NiCl2作为晶须生长的添加剂,在N2气氛下采用碳热还原法,在不同温度下合成Ti（C,N）晶须。采用X射线衍射研究合成样品的相组成及合成过程中原料的相转变行为,采用扫描电镜观察合成样品的形貌,探讨晶须的生长机理并建立生长模型。结果表明：在1 200℃合成时,主要发生TiO2与碳黑、氮气之间的直接还原反应,形成TiN;在温度为1 250-1 400℃时,NaCl分解产生的Cl原子起卤化作用,从NiCl2中分解出的Ni起催化作用,合成出Ti（C,N）晶须,晶须长度在10-30μm之间,直径大约1μm;合成的晶须表面光滑,顶端残留有催化剂颗粒,具有典型的气-液-固晶须生长机制特征。     

Zinc whisker growth from electroplated finishes – a review

Abstract

Electroplated zinc finishes have been associated with the electronics industry for many years as a result of their excellent corrosion resistance and relatively low cost. They are normally applied onto ferrous products to provide corrosion protection in a range of different environments. However, the formation of spontaneously grown whiskers on zinc-electroplated components, which are capable of resulting in electrical shorting or other damaging effects, can be highly problematic for the reliability of long life electrical and electronic equipment. The growth of zinc whiskers has been identified as the cause of some electrical and electronic failures in telecommunications and aerospace-based applications, with consequences ranging from mild inconvenience to complete system failures. Investigators have been striving to address the problems induced by whisker growth since 1940s. However, most research effort has been focused on tin whiskers, especially following European Union environmental legislation that restricted the use of lead (Pb), which when alloyed with tin (3–10% by weight) provided effective tin whisker mitigation. Compared with tin whisker research, much less attention has been paid to zinc whiskers. A number of mechanisms to explain zinc whisker growth have been proposed, but none of them are widely accepted and some are in conflict with each other. The aim of this paper is to review the available literature in regard to zinc whiskers, to discuss the reported growth mechanisms, to evaluate the effect of deposition parameters and to explore potential mitigation methods. This paper presents a chronologically ordered review of zinc whisker-related studies from 1946 to 2013. Some important early research, which investigated whisker growth in tin and cadmium, as well as zinc, has also been included.     

Effect of direct current and pulse plating parameters on tin whisker growth from tin electrodeposits on copper and brass substrates

Abstract

Electroplated tin finishes are widely utilised in the electronics industry due to their advantageous properties such as excellent solderability, electrical conductivity and corrosion resistance. However, the spontaneous growth of tin whiskers during service can be highly deleterious, resulting in localised electrical shorting or other harmful effects. The formation of tin whiskers, widely accepted as resulting from the formation of compressive stresses within the electrodeposit, has been responsible for a wide range of equipment failures in consumer products, safety critical industrial and aerospace based applications. The numbers of failures associated with tin whiskers is likely to increase in the future following legislation banning the use of lead in electronics, the latter when alloyed with tin, being an acknowledged tin whisker mitigator. Using a bright tin electroplating bath, the effect of process parameters on the characteristic structure of the deposit has been evaluated for deposition onto both brass and copper substrates. The effect on whisker growth rate of process variables, such as current density and deposit thickness, has been evaluated. In addition, the effect of pulse plating on subsequent whisker growth rates has also been investigated, particularly by varying duty cycle and pulse frequency. Whisker growth has been investigated under both ambient conditions and also using elevated temperature and humidity to accelerate the growth of whiskers. Studies have shown that whisker formation is strongly influenced by pulse plating parameters. Furthermore, increasing both current density and thickness of the deposit reduce whisker growth rates. It is also observed that whisker formation is greatly accelerated on brass substrates compared with copper. The basis for this observation is explained.     

Tin whisker growth from electroplated finishes – a review

Abstract

Tin whiskers are filamentary growths that are formed on the surface of electrodeposited tin, which is used extensively in the electronics industry. The presence of whiskers on electroplated finishes has been observed for more than 60 years, but, despite a huge amount of work in this area, a definite mechanism by which whiskers grow remains unidentified. Whiskers pose a significant problem for manufacturers of electrical and electronic equipment, since they are able to grow across and bridge the gap between adjacent electrical components, resulting in short-circuits and other associated failures. For many years, whisker growth was effectively mitigated by the addition of lead to tin electrodeposits. However, recent legislation prohibits the use of lead in new electrical and electronic devices, and thus alternative whisker reduction techniques are being sought. Effective mitigation is critical in ensuring that widespread whisker initiated failures can be avoided. However, since the mechanism, or mechanisms, which cause whisker growth remain unknown, the development of effective mitigation techniques is a significant industrial challenge, but a challenge which must be undertaken. This review examines some of the work undertaken to elucidate the whisker growth phenomenon. A brief history of whisker initiated failures along with key developments in whisker theory is presented. This is followed by a more detailed assessment of the several growth mechanisms hypothesised; dislocation-based, recrystallisation-based and compressive stress-based theories. The structure and properties of tin whiskers, along with factors that affect whisker growth and potential mitigation techniques are discussed.     

外加载荷对镀锡层锡须生长行为的影响

首先研究了三种不同厚度镀锡层（3,5,13 μm）在相同试验条件（70℃时效24 h后室温放置60天）下的锡须生长情况,并在此基础上首次采用精密动态力学分析仪（DMA Q800）研究了精确控制相同载荷条件下拉、压两种外力对相同厚度镀锡层（3 μm）锡须生长行为的影响.结果表明,相同时效条件下,镀锡层越薄,锡须生长的可能性越大;相同的外加载荷和试验温度作用下,承受压力作用镀锡层,其表面锡须生长比承受拉力时生长更快,并且主要呈柱状生长.     

Effect of strongly oxidizing environment on whisker growth form tin coating

The effect of a strongly oxidizing environment (105 °C/100%RH) on tin whisker formation has been studied with Ni and Ag underlayered samples between the tin surface coating and the bronze base material of the electronic components leads. Using nickel or silver underlayer between the copper substrate and the tin coating can be useful for preventing whiskers. The underlayer blocks the formation of Cu₆Sn₅ intermetallics which is one of the root causes of whisker growth. The silver underlayered samples started extreme whiskering after 2200 h; on the Ni underlayered samples only hillocks have been observed. The shape of the whiskers did not resemble to the usually known whisker appearance. The oxidizing environment creates tin-oxide fast and causes thick layers of SnO_x to appear on the surface of the samples and also on the whiskers. This caused the periodic appearance of large amounts of whiskers; but it blocks the length of them. This combined effect results in only large amounts of short whiskers and even causing the thickness of the tin layer to shrink by 80%. The series of tests has also showed that the nickel underlayered samples resisted whiskering better than the silver underlayered samples.     

Microstructural characteristics of immersion tin coatings on copper circuitries in circuit boards

Immersion tin coating, which is used as a lead-free surface finish, is deposited on the surface of copper circuitries on circuit boards by a replacement reaction. The characteristics of immersion tin coatings and the formation of tin whiskers and the intermetallic compound (IMC) are described. A Cu₆Sn₅ phase forms at the beginning of the immersion-plating process and expands until all of the tin has been transformed into a copper-tin alloy. The IMC layer becomes thicker and the volume of the pure tin layer decreases during storage. The tin involved in the formation of whiskers must originate from the tin layer of the immersion tin coating. Therefore, the formation of tin whiskers stops when all of the tin has been expended. Moreover, relatively larger whiskers grow on thicker coating layers, which contain more tin.     

电迁移诱发镀层锡须生长行为分析

分析了0.3×104 A/cm2恒定电流密度和四种不同加载时间（0，48，144和240 h）电迁移条件对6.5 μm厚镀锡层表面锡须生长行为的影响，以及不同电流密度对阴极裂纹宽度的影响.结果表明，电迁移加速了镀层表面锡须的形成与生长，随着电迁移时间的延长，锡须长度不断增加.此外，电迁移导致在阴极首先出现了圆形空洞，随后在两极均形成了圆形空洞，并且在阴极处还发现有微裂纹存在，随着电流密度增加，阴极裂纹宽度也随之增加，电流密度为0.5×104 A/cm2时，平均最大裂纹宽度约为9.2 μm.     

Microstructure and growth mechanism of tin whiskers on RESn3 compounds

An exclusive method was developed to prepare intact tin whiskers as transmission electron microscope specimens, and with this technique in situ observation of tin whisker growth from RESn₃ (RE = Nd, La, Ce) film specimen was first achieved. Electron irradiation was discovered to have an effect on the growth of a tin whisker through its root. Large quantities of tin whiskers with diameters from 20 nm to 10 μm and lengths ranging from 50 nm to 500 μm were formed at a growth rate of 0.1–1.8 nm s^?1 on the surface of RESn₃ compounds. Most (>85%) of these tin whiskers have preferred growth directions of 〈1 0 0〉, 〈0 0 1〉, 〈1 0 1〉 and 〈1 0 3〉, as determined by statistics. This kind of tin whisker is single-crystal β-Sn even if it has growth striations, steps and kinks, and no dislocations or twin or grain boundaries were observed within the whisker body. RESn₃ compounds undergo selective oxidation during whisker growth, and the oxidation provides continuous tin atoms for tin whisker growth until they are exhausted. The driving force for whisker growth is the compressive stress resulting from the restriction of the massive volume expansion (38–43%) during the oxidation by the surface RE(OH)₃ layer. Tin atoms diffuse and flow to feed the continuous growth of tin whiskers under a compressive stress gradient formed from the extrusion of tin atoms/clusters at weak points on the surface RE(OH)₃ layers. A growth model was proposed to discuss the characteristics and growth mechanism of tin whiskers from RESn₃ compounds.