Correlation Between Surface Morphology Evolution and Grain Structure: Whisker/Hillock Formation in Sn-Cu

Sn whisker and hillock formation is a reliability risk that has become increasingly important as the electronics industry has moved toward Pb-free manufacturing. To prevent them, we would like to understand what makes specific sites susceptible to deform into whiskers. We have used in situ scanning electron microscopy (SEM)/electron backscattering diffraction (EBSD) to monitor simultaneously the evolution of surface morphology and grain orientation in Sn surfaces in order to correlate whisker/hillock initiation with the underlying microstructure. Because rough films are difficult to measure with EBSD, we developed a unique procedure to make Sn-Cu samples with ultra-flat surfaces so that a large fraction of Sn grains can be indexed over repeated scans. We find that whiskers/hillocks grow from existing grains (not re-nucleated grains) with orientations close to (001). They often rotate from the as-deposited structure so that the orientation after growth does not indicate the orientation from which the whisker initiated. We measured the interface structure after removal of the Sn layer by chemical etching and found that there is no excessive accumulation of intermetallic compound around the whisker/hillock roots. Cross-sectional measurements revealed that a large fraction of the whiskers/hillocks have oblique boundaries underneath the surface, supporting the idea that these allow whiskers/hillocks to grow with lower stress.     

Sn whisker growth in Sn-9Zn-0.5Ga-0.7Pr lead-free solder

The spontaneous growth of Sn whisker in Sn-Zn series solder is newly reported in this work. It is found that during the exposure of Sn-9Zn-0.5Ga-0.7Pr bulk solder to ambient conditions for a few hours, many different lengths of needle-like Sn whiskers originate spontaneously from the Sn-Pr intermetallic compounds of the solder and grow rapidly at a rate of about 3.5 Å/s. It is proposed that the driving force for whisker formation is the compressive stress resulting from the oxidation of Sn-Pr compounds, and that the free Sn atoms released from oxidation reaction feed the whisker growth during exposure.     

Sn-9Zn/Cu无铅焊点ZnO须自发生长

研究了Sn-9Zn/Cu无铅焊点ZnO须自发生长行为.结果表明,在室温条件下Sn-9Zn/Cu无铅焊点因为富锌相的氧化,在富锌相表面自发生长出大量的ZnO须,ZnO须呈现针状、块状、花朵状等不同形态;对ZnO须进行FIB测试,发现在富锌相中出现明显的空洞,主要是因为Zn元素扩散导致的.另外,证实在Zn元素扩散与氧发生反应的过程中,富锌相发生体积收缩,诱使结构中出现明显的拉应力,拉应力为ZnO须生长提供驱动力.     

Mg2B2O5晶须增强镁基复合材料的组织与界面结构研究

采用光学显微镜、透射电子显微镜和X射线衍射仪研究Mg2B2O5晶须增强镁基复合材料的铸态组织及Mg2B2O5/AZ91D界面反应产物。结果表明:Mg2B2O5晶须具有孪晶结构,其孪生面和晶体生长方向分别为(202)和[010];部分Mg2B2O5晶须中存在MgB4O7颗粒相;在基体晶界与Mg2B2O5晶须之间存在等轴状Mg2Si相;Mg2B2O5/AZ91D界面处存在厚度不均匀的MgO和MgB2相界面层。尽管Mg2B2O5、MgO和MgB2之间没有确定的晶体学位相关系,但在特定Mg2B2O5晶须表面观察到(202)Mg2B2O5//(002)MgO,[010]Mg2B2O5//[110]MgO和(002)MgO//(0001)MgB2,[110]MgO//[2110]MgB2取向关系。最后,讨论Mg2B2O5晶须对复合材料组织结构和性能的影响。     

A Predictive Model for Whisker Formation Based on Local Microstructure and Grain Boundary Properties

Whisker and hillock formation in thin films is well known as a highly local mechanism for stress relaxation, where in many cases, only a few whiskers form out of thousands of grains in a film. In this article, the microstructural characteristics for specific grains to form whiskers in β-Sn films are discussed in light of our recent whisker growth model, establishing a relationship among grain boundary sliding limited Coble creep, surface grain geometry, and film stress for different stress conditions, including for thermal cycling. Through our recent finite-element simulations of stresses induced by room-temperature aging and thermal cycling of textured microstructures, the role of elastic and thermoelastic anisotropy in creating preferred whisker formation sites and the general propensity of a film to form whiskers have been proposed for a range of β-Sn film textures. Taken together, these models suggest a strategy for identifying the effects of local microstructure and β-Sn anisotropy on whisker formation. If these predictions are accurate, then whisker growth risk may be effectively reduced by engineering film microstructures and textures for specific applications and stress conditions.     

Sn-Zn-Ga-Pr钎料表面锡须的自发生长

结合扫描电镜与电子能谱分析了稀土元素对无铅钎料显微组织的影响.结果表明,当无铅钎料中的稀土添加过量,会诱发锡须在无铅合金表面的自发生长.在向Sn-9Zn-0.5Ga基体合金中添加0.7%(质量分数)的稀土元素镨(Pr)后,仅需室温时效12 h,合金组织中的稀土相表面即发生了锡须的自发生长.随着时效时间的延长,锡须会继续长大,其最终长度可达100μm.最后对过量稀土元素添加导致锡须生长的力学原因作了初步探讨,分析认为稀土相氧化所产生的微观压应力可能为锡须的生长提供了驱动力.     

Rolling textures in nanoscale Cu/Nb multilayers

Rolling textures in nanoscale multilayered thin films are found to differ markedly from textures observed in bulk materials. Multilayered thin films consisting of alternating Cu and Nb layers with columnar grains were produced by magnetron sputtering, with individual layer thickness ranging from 4 μm to 75 nm and Cu/Nb interfaces locally satisfying the Kurdjumov–Sachs (K–S) orientation relations. After rolling to 80% effective strain, samples with a larger initial layer thickness develop a bulk rolling texture while those with a smaller initial layer thickness display co-rotation of Cu and Nb columnar grains about the interface normal, in order to preserve the K–S orientation relations. The resulting K–S texture has 0 0 1Nb parallel to and 1 1 0Cu approximately 5° from the rolling direction. A crystal plasticity model based on the Principle of Minimum Shear captures the K–S texture approximately and suggests that Nb drags Cu along in the rotation process.     

Sn whisker growth during thermal cycling

Pure Sn plating on ceramic chip capacitors was tested by thermal cycling both in air and in vacuum for up to 3000 cycles and the whisker growth mechanism was clarified. A thin crystalline SnO layer is formed on the surface of Sn plating and whiskers, which exhibits a high level of cracking. Thermal cycling whiskers exhibit two distinct features: fine striation rings on the whisker side face vertical to the whisker growth axis; and deep grooves at the root of the whiskers. One ring of the fine striations corresponds to each thermal cycle. The formation of the grooves can be attributed to thermal cycle cracking along grain boundaries of Sn followed by oxidation and growth of whiskers from the root grains. The characteristic winding feature observed for thermal cycling whiskers can be attributed to the formation of root grooves with severe oxidation. Whisker growth in vacuum is faster than in air. Whiskers grown in vacuum are thinner and longer than whiskers grown in air, while the whisker density is not influenced by atmosphere. The interval of striation rings is wider for vacuum-grown whiskers as compared with air-grown whiskers.     

The Inhibition of Tin Whiskers on the Surface of Sn-8Zn-3Bi-0.5Ce Solders

Through the refinement of the (Ce, Zn)Sn₃ intermetallic phase, the formation of tin whiskers, previously observed on the surface of a Sn-3Ag-0.5Cu-0.5Ce solder, was prevented in a Sn-9Zn-0.5Ce alloy. However, whisker growth can still occur on the surface of Sn-8Zn-3Bi-0.5Ce solder after air storage at room temperature and at 150 °C due to the formation of large (Ce, Zn)Sn₃ intermetallic clusters. Further experiments showed that decreasing the Bi-content in this Sn-8Zn-0.5Ce alloy to 1 and 2 wt.% can recover the beneficial effects of Zn additions on the refinement of the (Ce, Zn)Sn₃ phase and obviously reduce the appearance of tin whiskers. In addition, alloying the Sn-8Zn-3Bi-0.5Ce solder with 0.5 wt.% Ge, which increases the oxidation resistance of the (Ce, Zn)Sn₃ intermetallic clusters, can also effectively inhibit tin whisker growth.     

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.     

稀土相ErSn3表面Sn晶须的快速生长

将稀土相ErSn3分别暴露于空气与真空环境中,研究在时效处理过程中ErSn3表面Sn晶须的生长情况。结果表明：大气环境中,在稀土相ErSn3的表面出现了Sn晶须的生长现象,且室温条件下Sn晶须的分布不均,生长速度慢,而高温条件下Sn晶须的分布均匀,生长速度快;真空环境中,在稀土相ErSn3的表面未出现Sn晶须的生长现象。由于在大气环境中稀土相ErSn3发生了氧化,氧原子向ErSn3晶格内部的扩散引起的体积膨胀提供了Sn晶须生长的驱动力,因此,大气环境中在ErSn3的表面会出现Sn晶须的生长现象,而真空环境中则不会出现     

Mechanisms for Sn whisker growth in rare earth-containing Pb-free solders

It has recently been documented that Pb-free solder alloys doped with trace amounts of rare earth (RE) elements show a very strong propensity to grow Sn whiskers. In this work, we have investigated the effect of the addition of 2 wt.% Ce, La or Y on the whiskering behavior of Sn–3.9Ag–0.7Cu. Hillock-type whiskers around particle peripheries were observed in water-quenched alloys with smaller RESn₃ particles, while furnace-cooled alloys with larger RESn₃ particles formed needle-like whiskers from within the particle. Phase separation between Sn and RE oxides occurred during oxidation of the RESn₃ intermetallics. A focused ion beam serial sectioning approach was used to visualize the Sn whiskers and the oxide structure. We show that the driving force for whisker growth is related to the compressive stresses that develop in these particles during the oxidation of the RE intermetallic phases.     

热循环对20％／SiCW／6061Al复合材料尺寸稳定性的影响

通过测量20％（体积分数）SiCW/6061Al复合材料热循环（25℃→300℃→25℃）的尺寸变化,研究了复合材料的热循环尺寸稳定性。结果表明,复合材料尺寸稳定性直接与晶须的限向方式有关。如果晶须取向为混乱分布,热循环后复合材料的尺寸变化不明显。当晶须取向为定向排列时,热循环可造成复合材料的沿晶须方向的尺寸增大,同时垂直晶须方向尺寸减小。     

化学气相沉积方法制备定向W涂层研究

采用化学气相沉积方法在钼基体上制备了具有[110]择优取向的W涂层,采用SEM、XRD、EBSD方法分析了涂层性能,研究表明涂层致密,法向偏离[110]晶向8°范围的晶粒占涂层面积的比例为85.32%,10°范围内[110]晶粒占全部面积的96.86%,[110]择优取向的生成受气体流量H2/Cl2,反应室压强及沉积温度的影响;对截面织构研究表明涂层形成机制为竞争生长机制。在室温～1400℃～室温20次热循环后涂层界面存在互扩散形成的孔洞,但是涂层结合良好。     

GROWTH MECHANISM OF TiC WHISKERS PREMRED BY A MODIFIED CHEMICAL VAPOR DEPOSITION METHOD

1.IntroductionTitaniumcarbideisknowntobenotonlygoodelectricandthermalconductorbutalsosuperhardceramicmaterialwithahighmeltingpointandgreathardness[1--2].Withhighstrength,TiCwhiskerscanbeusedassatisfactoryreinforcingmaterialsinbothmetalmatrixcomposites(MMC)andceramicmatrixcomposites(CMC).Infact,comparedtothecommonlyusedwhiskerssuchasSiCandSt3N4,thenewlydevelopedTiCwhiskerhajsmuchhigherstrength,especiallyatelevatedtemperature,aswellasbetterchemicalstabilityandcompatibilitywithaluminamatrix…     

氧化对含稀土无铅钎料表面锡须生长的影响

通过环境试验(室温、湿热和"无氧"试验)和扫描电镜组织分析对比研究了相同时效时间(30天)、三种不同氧化条件下锡须在Sn-Zn-Ga-Pr无铅钎料表面的生长行为.发现在室温环境下,锡须呈典型的针状生长;在湿热条件下,锡须呈丘状生长;而在氮气氛围的近似无氧条件下仅发现有少量锡粒在稀土相PrSn3表面产生.基于稀土相氧化驱动锡须生长理论,分析认为三种环境条件下锡须生长行为的不同是由于稀土相氧化程度不同造成的.湿热条件下稀土相快速氧化和腐蚀,从而比在室温和"无氧"条件下产生更多的压应力和活性Sn原子,为丘状锡须的生长提供了条件;而在氮气氛围下,稀土相氧化非常缓慢,驱动力和锡源都较少,因而只有少量短锡须和锡粒生长.     

Temperature Gradient Induced Orientation Change of Bi Grains in Sn-Bi57-Ag0.7 Solder Joint

Sn-Bi-X solders are widely used in electronic packaging industry. However, thermomigration (TM) behaviors of Sn-Bi-X solder joints and the orientations change of Bi grains under the temperature gradient are rarely reported. In this study, Sn-Bi57-Ag0.7/Cu solder joints were used to conduct a TM test under a temperature gradient of 625 °C/cm for 400 h, and an isothermal aging test at 85 °C was also conducted for comparison. The microstructural evolution of Sn-Bi-X solder joints after reflow, TM and isothermal aging were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe microanalysis (EPMA). The results indicated that the Sn/Bi areal ratio after TM did not change significantly whether at the hot end (from 46.78%/52.12% to 50.90%/48.78%) or at the cold end (from 50.25%/49.64% to 48.71%/51.16%) compared with that of as-reflowed samples due to the insufficient thermal energy. The thickness of intermetallic compound (IMC) after TM at hot end (2.49 μm) was very close to that of the IMC at cold end (2.52 μm), which was also close to that of the aged samples. In addition, the preferred orientations of Sn and Bi grains in different Sn-Bi-Ag solder joints resulting from different conditions (reflow, TM and isothermal aging) were characterized by electron backscatter diffraction (EBSD). The obtained results demonstrated that both Sn and Bi grains had no preferred orientation whether after reflow or isothermal aging, while the orientation of Bi grains of the sample after TM changed from random direction to c-axis ([0001] direction) parallel to the heat flow. Ag₃Sn could hinder the change of orientation of Bi grains under the temperature gradient, and the corresponding mechanism was also systematically illuminated. This study firstly revealed the orientation change of Bi grains under the temperature gradient, which would have a profound guiding significance for enhancing the reliabilities of Sn-Bi-Ag solder joints.     

Whisker and hillock growth via coupled localized Coble creep,grain boundary sliding,and shear induced grain boundary migration

A model incorporating the effects of grain geometry and size and grain boundary properties on the growth of whiskers and hillocks has been developed based on coupling between localized Coble creep and grain boundary sliding. For both whiskers and hillocks accretion of atoms by Coble creep on grain boundary planes normal to the growth direction is limited by grain boundary sliding on planes parallel to the direction of whisker growth. If the accretion-induced shear stresses are not coupled to grain boundary migration a whisker forms when sliding occurs. In the case of hillocks an additional coupling between grain boundary sliding and shear-induced grain boundary migration leads to the observed lateral growth. By incorporating grain size and geometry, a structure-dependent grain boundary sliding coefficient and measured film stresses the local conditions for whisker growth, including the growth rates, can be calculated. As described here, other commonly observed whisker and hillock morphologies and geometries are consistent with this model, as are the effects of a surface oxide film and thermal cycling.     

Yielding behavior of aluminum-rich single crystalline γ-TiAl

The yielding behavior of Al-rich single crystalline γ-Ti 55.5 at% Al has been measured along near [0 0 1], [0 1 0] and orientations in both tension and compression and as function of temperature. All three orientations displayed anomalous yielding and a pronounced tension/compression asymmetry. The relative strength of the material, location of the anomalous yielding peak, and tension/compression asymmetry are all orientation dependent. A micromechanical superdislocation model addressing crystal geometry, sense of the applied load, Escaig forces and the Yoo torque is presented to explain the observed experimental results.     

SiC晶须强化树脂的磨损特性及其在磨削加工中的应用

本文使用SiC晶须(细纤维)与酚醛树脂混合，经特殊工艺处理后制成了几种不同晶须含量、排列方向及粉末添加剂的SiC晶须强化树脂。通过对接触材料(模具钢SKD11，HRC60)所进行的磨损特性试验可知，当SiC晶须强化树脂中晶须的含量增高及排列方向与接触材料的摩擦面相垂直时，其磨损量将小于接触材料的磨损量，这表明SiC晶须材料具有磨料的作用。本文利用这一作用将SiC晶须强化树脂制成杯形砂轮，并应用到模具钢SKD11的磨削加工中，能获得高达6000的磨削比和纳米级的加工表面(Ra1．5nm／Ry16nm)，进一步表明SiC晶须作为磨料是可行的、实用的。