稀土元素对无铅钎料组织和性能的影响

稀土元素以其独特的优势被称为金属材料的维他命,稀土元素的添加可以在不同程度上提高无铅钎料的性能。结合国内外在含稀土元素无铅钎料研究领域的最新研究成果,综合评论稀土元素对无铅钎料组织和性能的影响,阐述含稀土元素的无铅焊点可靠性研究现状,为该钎料的实际应用提供数据支撑,分析过量稀土元素对无铅钎料表面锡须的影响,探讨锡须的生长机制及潜在的问题,最后综合评述含稀土无铅钎料在研究过程中存在的问题以及相应的解决措施,为含稀土元素无铅钎料的研究和应用提供理论依据。     

稀土元素Pr,Nd对无铅钎料组织和性能的影响北大核心

综合了国内外含稀土元素Pr,Nd的无铅钎料最新研究成果,归纳并总结了添加稀土元素Pr,Nd对无铅钎料(Sn-Ag-Cu,Sn-Cu(-Ni)以及Sn-Zn钎料等)的组织和性能的影响规律,分析了稀土元素Pr和Nd对无铅钎料焊点可靠性的影响以及稀土相表面Sn须的形貌与生长机制,并展望了稀土元素改性无铅钎料研究的发展趋势,为进一步开发性能更优良的新型无铅钎料提供了参考依据。     

稀土元素Pr,Nd对无铅钎料组织和性能的影响

综合了国内外含稀土元素Pr,Nd的无铅钎料最新研究成果,归纳并总结了添加稀土元素Pr,Nd对无铅钎料(Sn-Ag-Cu,Sn-Cu(-Ni)以及Sn-Zn钎料等)的组织和性能的影响规律,分析了稀土元素Pr和Nd对无铅钎料焊点可靠性的影响以及稀土相表面Sn须的形貌与生长机制,并展望了稀土元素改性无铅钎料研究的发展趋势,为进一步开发性能更优良的新型无铅钎料提供了参考依据。     

稀土、Ga元素及其协同效应对无铅钎料组织和性能的影响

稀土元素和低熔点的Ga元素常作为合金化元素添加到无铅钎料中以提高钎料的综合性能和可靠性,而向无铅钎料中复合添加稀土元素和Ga元素时,二者的协同作用可以进一步改善钎料组织和性能,同时Ga元素可以有效抑制含稀土Nd的无铅钎料中锡须的生长.对单独添加稀土元素和低熔点元素Ga以及复合添加稀土元素和Ga元素的协同效应对无铅钎料显...     

Pr元素对Sn-Zn-Ga钎料锡须生长的影响

研究了锡须生长与稀土含量之间的关联性,对影响锡须生长的相关因素进行了初步分析,着重研究时效时间与冷却速率对锡须生长的影响规律,并对块状钎料合金表面锡须形成的生长机制进行了初步探讨.结果表明,当Sn-Zn-Ga-xPr钎料中镨的添加量达到0.7%(质量分数)时,钎料表面在室温时效12 h后会生长出锡须.随着时效时间的延长,Sn-Zn-Ga-0.7Pr钎料锡须长度增长,且在时效时间45 d左右达到电子元器件失效的临界值.此外研究发现不同冷却速率对锡须的生长有很大差异,对Sn-Zn-Ga-xPr钎料锡须生长机制进行了初步的理论分析.     

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.     

Sn-Zn-Ga-xPr钎料组织与性能

研究了添加合金元素Ga及稀土元素Pr对Sn-Zn钎料组织与性能的影响.适量添加镨,可以提高钎料的润湿性能;但当镨含量超过0.12%(质量分数)时,镨会在钎料表面形成氧化渣,恶化钎料的润湿性能.镨的添加可以改善焊点的力学性能,抗剪强度在镨含量为0.08%时增加至最大.镨加入后细化了钎料基体中的富锌相,当镨含量不超过0.08%时,界面层平整且厚度变化不大,但若稀土添加过量,则界面层会明显增厚.镨在Sn9Zn0.5Ga钎料中的最佳添加量为0.08%左右.     

稀土元素Er对Bi5Sb8Sn钎料力学性能的影响

通过向Bi5Sb8Sn钎料合金中添加适量的稀土元素铒(Er),研究了不同含量的稀土Er对Bi5Sb8Sn无铅钎料合金力学性能的影响。结果表明,微量稀土Er的添加能显著细化该钎料组织,改善合金的组织分布情况,提高钎料的力学性能;稀土w(Er)0.5%时,Bi5Sb8SnRE合金抗拉强度和钎焊接头的抗剪强度较基体钎料Bi5Sb8Sn分别提高了41.8%和83.0%.;但当稀土w(Er)0.5%时,则组织粗化,钎料力学性能下降。     

稀土相表面特殊形态锡晶须生长现象

在Sn3.8Ag0.7Cu钎料中添加过量的稀土Ce和Er元素会在其内部形成尺寸较大的稀土相CeSn3和ErSn3.将钎料合金Sn3.8Ag0.7Cu1.0Ce/Er的铺展试样沿中心剖开,利用金相水砂纸及专用抛光液抛光后,将制备好的试样在空气中分别进行室温与150℃时效处理.分析了时效处理过程中稀土相CeSn3及ErSn3表面锡晶须的生长行为.结果表明,在稀土相的表面出现了大量规则的针状及线状锡晶须,它们在生长过程中始终保持了恒定的截面.试验中还发现了一些特殊形态的锡晶须,如锡晶须的分枝、合并以及锡晶须的搭接现象.     

稀土Ce加速Sn晶须生长的研究

稀土被认为是金属中的"维他命",在钎料中添加微量的稀土Ce可以显著地改善钎料合金的综合性能.然而,当钎料中添加过量的稀土时,将会发现Sn晶须的快速生长现象.结果表明,如果将Sn3.8Ag0.7Cu1.0Ce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.     

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.     

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.     

Tin whiskers studied by synchrotron radiation scanning X-ray micro-diffraction

A large number of Sn whiskers have been found on the Pb-free solder finish on leadframes used in consumer electronic products. Some of the whiskers on eutectic SnCu finishes are long enough to short the neighboring legs of the leadframe. Tin whisker growth is known to be a stress relief phenomenon. We have performed synchrotron radiation X-ray micro-diffraction analysis to measure the local stress level, the orientation of the grains in the finish around a whisker, and the growth direction of whiskers. The compressive stress in the solder finish is quite low, less than 10 MPa; nevertheless, there exists a stress gradient around the root of a whisker. From the orientation map and pole figure, we found that the growth direction of whiskers is [0 0 1] and there exists a preferred orientation of [3 2 1] grains on the solder finish. In one of the whisker analyzed, we found that the normal orientation of the grain just below the whisker is different; it is [2 1 0].     

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.     

Ni对SnAgCuRE钎料及其钎焊接头性能的影响

以商用Sn3.8Ag0.7Cu为参照系,研究了Ni对Sn2.5Ag0.7Cu0.1 RE钎料合金及其钎焊接头性能的影响。研究结果表明,添加0.05%(质量分数)Ni能在不降低Sn2.5Ag0.7Cu0.1RE钎料合金抗拉强度的同时显著提高其伸长率,是商用Sn3.8Ag0.7Cu的1.4倍;相应地钎焊接头的蠕变断裂寿命最长,为未添加Ni时的13.3倍,远高于现行商用Sn3.8Ag0.7Cu的,满足微电子连接高强韧高可靠性无铅钎料合金系的需求。     

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.     

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.