稀土相表面Sn晶须生长的研究

研究了Sn-3.8Ag-0.7Cu-RE(RE为Ce、Er或Y)焊料在空气中室温与高温时效过程中稀土相CeSn3、ErSn3与YSn3表面Sn晶须的生长情况。结果表明,Sn晶须的开始生长时间、形态及数量与稀土相的种类及时效条件有密切关系。稀土相因氧化产生的体积膨胀提供了Sn晶须生长的驱动力。稀土与氧的化学亲和力参数及时效温度共同影响稀土相表面Sn晶须的生长。室温实效条件下,三种稀土相表面Sn晶须的直径为0.1~2.0μm,长度可达几百微米;150℃时效条件下,Sn晶须的直径为0.1~0.2μm,长度也可达上百微米。     

钎料稀土相表面Sn晶须生长的研究

为了解对钎料可靠性影响极大的Sn晶须的生长机理,系统研究了钎料稀土相CeSn3、LaSn3及ErSn3表面Sn晶须生长的影响因素.结果表明:稀土相的氧化倾向与时效温度共同影响其表面Sn晶须的生长.室温时效条件下,在稀土相CeSn3与LaSn3表面易出现小尺寸的线状Sn晶须,直径为0.1～0.2 μ.m,而在稀土相ErS...     

SnAgCuLa钎料合金表面Sn晶须的生长研究

在钎料中添加微量稀土可以显著改善钎料合金的综合性能。但添加过量的稀土时,将会出现Sn晶须的快速生长。研究发现,如果将Sn3.8Ag0.7Cu1.0La钎料内部的稀土相暴露于空气中,稀土相将发生氧化而产生体积膨胀,钎料基体对体积膨胀的抑制作用将使稀土相内部产生巨大的压应力从而加速Sn晶须的生长。晶须直径约1μm,长度可达300μm。     

时效温度下钎料的稀土相CeSn3表面Sn晶须生长

于Sn3.8Ag0.7Cu钎料中添加过量的稀土Ce会在其内部形成大尺寸的稀土相CeSn3,将稀土相CeSn3暴露于空气中,研究在时效处理过程中时效温度对其表面Sn晶须生长的影响规律.结果表明:时效温度对稀土相CeSn3表面Sn晶须的生长产生显著影响.室温时效20 min后,其表面会形成少量的白色Sn颗粒:75℃时效20...     

Sn0.7Cu-xEr/Cu焊点界面反应及其化合物层生长行为研究

通过回流焊工艺制备了Sn0.7Cu-x Er/Cu(x=0,0.1,0.5)钎焊接头,研究钎焊温度及等温时效时间对接头的界面金属间化合物(IMC)的形成与生长行为的影响。结果表明:Sn0.7Cu钎料中微量稀土Er元素的添加,能有效抑制钎焊及时效过程中界面IMC的形成与生长。在等温时效处理过程中,随着时效时间的延长,界面反应IMC层不断增厚,在相同时效处理条件下,Sn0.7Cu0.5Er/Cu焊点界面IMC层的厚度略小于Sn0.7Cu0.1Er/Cu焊点界面的厚度。通过线性拟合方法,得到Sn0.7Cu0.1Er/Cu和Sn0.7Cu0.5Er/Cu焊点界面IMC层的生长速率常数分别为3.03×10–17 m2/s和2.67×10–17 m2/s。     

Y2O3增强Sn-3Ag-0.5Cu复合无铅钎料

研究了稀土氧化物颗粒增强复合无铅钎料Sn-3Ag-0.5Cu-Y2O3的钎焊接头的显微组织形貌、性能特点及有关影响机制.结果表明:Y2O3增强颗粒均匀分布于Sn-3Ag-0.5Cu钎料基体中,不仅细化了钎焊接头中共晶相Ag3Sn和Cu6Sn5相颗粒尺寸,而且也显著提高了钎焊接头的剪切强度.另外适量Y2O3颗粒的添加对钎料润湿性影响不大.     

Sn-58Bi-0.5Ce/Cu界面金属间化合物的性能研究

研究了Sn-58Bi-0.5Ce/Cu钎焊接头在120℃时效过程中界面组织形貌及金属间化合物层(IMC)的厚度变化。结果表明:在Sn-58Bi-0.5Ce/Cu钎焊接头界面处形成了较为平坦的双层金属间化合物,靠近钎料的上层为Cu6Sn5相,邻近Cu基板的下层为Cu3Sn相。等温时效处理后,IMC层逐渐凸起,且随着时效时间的增加,IMC层不断增厚。通过对实验数据进行拟合,得到钎焊接头界面IMC层的生长速度常数为5.77×10–17m2/s。     

SnBiEr/Cu界面反应及时效过程中IMC的研究

采用SEM观察等手段研究了Sn58BixEr(x=0,0.1,0.5;表示添加质量分数0.01%,0.5%的Er)/Cu钎焊接头界面反应以及在120℃时效过程中金属间化合物(IMC)的生长行为。实验结果表明:Sn58BixEr/Cu钎焊接头IMC层厚度随着钎焊温度的升高而增厚,添加微量的Er能够有效抑制界面IMC的生长。在时效过程中,界面IMC层的厚度随着时效时间的增加而逐渐增厚。通过对实验数据进行拟合,得出120℃时效温度下Sn58Bi0.1Er/Cu和Sn58Bi0.5Er/Cu的IMC层的生长速率常数分别为3.42×10–16 m2/s和2.84×10–16 m2/s。     

无铅波峰焊不锈钢锡炉叶轮和喷嘴溶蚀失效分析

常用无铅钎料熔点的升高以及Sn含量的增加使得300系列不锈钢在使用几个月之后就会出现溶蚀现象。Fe-Sn金属间化合物的生长行为特点以及叶轮、喷嘴接触钎料的特点决定了材料表面的溶蚀形貌特征。     

无铅钎料Sn-0.7Cu机械合金化研究

利用扫描电子显微镜和差热分析仪,研究了Sn-0.7Cu粉末的机械合金化过程;同时将机械合金化获得的粉末制成焊膏,进行铺展实验,并对金相组织进行了分析.研究结果表明,机械合金化是一种制备钎料粉体的方法;粉末的最终颗粒尺寸可通过合适的工艺参数来控制;用机械合金化获得的钎料粉体制成的焊膏,其铺展面积及润湿角方面与传统的熔炼法获得的钎料差别不大,铺展试样的金相组织由富Sn相、Cu6Sn5和少量Cu3Sn相组成.     

Abnormal growth of tin whiskers in a Sn3Ag0.5Cu0.5Ce solder ball grid array package

It has been discovered for the first time that Sn whiskers appeared in Sn3Ag0.5Cu0.5Ce solder joints of ball grid array (BGA) packages after storage at room temperature (natural aging) for less than 3 days and they grew at a high rate of 2.9 ?/sec. In one particular case, whiskers even formed after 1 day of storage at an extremely high growth rate of 8.6 ?/sec. Experimental investigations showed that a number of CeSn₃ clusters existed in the Sn3Ag0.5Cu0.5Ce solder matrix after the reflow process. Further natural aging in air for several days caused the CeSn₃ phases to oxidize rapidly, from which many Sn whiskers sprouted and grew to a length of hundreds of micrometers. The most commonly observed whiskers have been long fiber-shaped ones of 0.1 μm to 0.3 μm in diameter (type I), while short whiskers larger than 1 μm in diameter can also be found (type II). Here in our case, the surface oxide of the CeSn₃ phase possessed a higher content of Ce than of Sn, which implied that a Ce-depleted region (nearly of pure Sn) was left beneath the oxide layer. The abnormal whisker growth was attributed to the compressive stress squeezing the Sn atoms in the Ce-depleted region of CeSn₃ phase out of the oxide layer.     

Observed correlation of Sn oxide film to Sn whisker growth in Sn-Cu electrodeposit for Pb-free solders

Localized cracking of surface oxide has been proposed as a necessary step in the nucleation of Sn whiskers in Sn electrodeposited films. To evaluate the effects of the oxide film on Sn whisker growth, a bright Sn-Cu electrodeposited film was inserted into an ultrahigh vacuum Auger system, cleaned using an Ar⁻ ion beam to remove the oxide film, and aged in the 2×10⁻⁹ Pa Auger system chamber. Whiskers and other features present during Ar⁺ ion cleaning left visible “shadows” on the surface. During aging in the ultrahigh vacuum system, new whiskers, identified by the absence of the telltale shadows, nucleated and grew. Based on these observations, the presence or absence of an oxide film has a minimal effect on Sn whisker nucleation and growth.     

Equi-Axed Grain Formation in Electrodeposited Sn-Bi

Sn is widely used as a coating in the electronics industry because it provides excellent solderability, ductility, electrical conductivity, and corrosion resistance. However, Sn whiskers have been observed to grow spontaneously from Sn electrodeposits and are known to cause short circuits in fine-pitched pre-tinned electrical components. We report here a deposition strategy that produces an equi-axed and size-tunable grain structure in Sn-Bi alloys electrodeposited from a commercial bright Sn electrolyte. An equi-axed grain structure should allow a more uniform creep to relieve compressive stress with no localized surface disturbance. The standard potential for Bi is about 0.45 V more positive than Sn. Pulsed deposition can selectively turn on and off the Sn deposition reaction. During the off cycle, a displacement reaction between metallic Sn on the electrode surface and Bi³⁺ in solution selectively dissolves Sn and deposits Bi, effectively terminating the growth from the previous cycle and forcing the Sn to nucleate a new grain on the Bi-enriched surface. The grain size is tunable by varying the pulsing conditions, and an equi-axed structure can be obtained with as little as 3 at.% Bi. This surface enrichment of Bi by potential modulation is similar to that which occurs naturally in Sn-Pb, and provides an avenue for breaking up the columnar grain structure inherent to pure Sn, thus providing an additional diffusion path for Sn that may prevent whisker growth.     

A physical model and analysis for whisker growth caused by chemical intermetallic reaction

The formation of intermetallic compound Cu₆Sn₅ gives rise to the internal stress in the lead-free coating, which causes the growth of Sn whiskers. This phenomenon is characterized with the expansion of inclusion in a plate perfectly bonded between two infinite solids. Based on the grain boundary diffusion mechanism, a model is established to evaluate the growth rate of Sn whiskers. The results show that the growth rate of whisker varies with the relative site between whisker and inclusion. When the distance between the whisker and inclusion exceeds a critical value, negative growth rate will appear, and it approaches zero as the distance increases. They explain some phenomena observed in experiments.     

Whisker growth on surface treatment in the pure tin plating

Whisker behavior at various surface treatment conditions of pure Sn plating are presented. The temperature cycling test for 600 cycles and the ambient storage for 1 year was performed, respectively. From the temperature cycling test, bent-shaped whiskers were observed on matte and semibright Sn plating, and flower-shaped whisker on bright Sn plating. The bright Sn plating has smaller whiskers than the other types of Sn plating, and the whisker growth density per unit area is also lower than the others. After 1 year under ambient storage, nodule growth of FeNi42 lead frame (LF) was observed in some parts. The Cu LF showed about a 9.0 μm hillock-shaped whisker. This result demonstrated that the main determinant of whisker growth was the number of temperature cycling (TC) in the FeNi42 LF, whereas it was the time and temperature in the Cu LF. Also, whisker growth and shape varied with the type of surface treatment and grain size of plating.     

Can Whiskers Grow on Bulk Lead-Free Solder?

Many possible mechanisms for whisker growth exist, each possible in various scenarios investigated in the literature. This contribution addresses the importance of residual mechanical stress in a solder alloy for providing some of the energy necessary to drive possible whisker growth. We investigate the indentations made on bulk lead-free solder (Sn3.5Ag) to introduce various levels of residual energy associated with localized residual stresses. We confirm that localized residual stresses, in the absence of a thin-film geometry, significant oxide thickness, and interdiffusional stresses from intermetallic Cu-Sn compounds, do not result in the formation of whiskers in bulk Sn3.5Ag. Thus, the combination of stresses associated with thin films (either thermal misfit, plating, or chemical) and the oxidation of Sn at the surface is likely required for continuous whisker growth.     

Tin Whisker Growth Induced by High Electron Current Density

The effect of electric current on the tin whisker growth on Sn stripes was studied. The Sn stripes, 1 μm in thickness, were patterned on silicon wafers. The design of the Sn stripes allowed the simultaneous study of the effect of current crowding and current density. Current stressing was performed in ovens set at 30, 50, or 70°C, and the current density used ranged from 4.5 × 10⁴ A/cm² to 3.6 × 10⁵ A/cm². It was found that the stress induced by the electric current caused the formation of many Sn whiskers. A higher current density caused more Sn whiskers to form. Of the three temperatures studied, 50°C was the most favorable one for the formation of the Sn whiskers. In addition, the current-crowding effect also influenced whisker growth.