Sn-Bi系无铅焊料熔体结构转变及其对凝固组织及润湿性的影响

本文通过电阻法研究了Sn-57%Bi无铅焊料在液相线以上一定温度范围内发生熔体结构转变的可能性及规律性,并以此为切入点研究了熔体结构转变前后Sn-57%Bi焊料的凝固组织和润湿性能的变化规律。结果表明：Sn-57%Bi无铅焊料在液相线以上785~900℃范围内发生了熔体结构转变,而且熔体结构转变细化了凝固组织,改变了微观形貌;经历熔体结构转变而凝固的合金焊料较未发生转变的合金铺展面积增大,润湿角减小。     

Sn-Bi-Cu合金熔体结构变化及对凝固组织影响

为了探索合金熔体结构与凝固组织间的相关性,进一步改进Sn-Bi-Cu无铅焊料合金组织性能,运用四探针电阻法对Sn70Bi10Cu三元合金的熔体过热行为进行了初步探索.结果表明,升温过程中合金熔体电阻率在812～821℃温度范围内发生了突变,且在降温过程中突变不再发生,表明熔体结构在这一温度区间发生了不可逆的结构转变.通过金相组织观察及力学性能测试发现,该不可逆的转变行为使其凝固组织明显细化及力学性能得到一定的提高:压缩强度极限σd提高了约8%,表面维氏硬度提高约18%.     

温度诱导液相结构转变对Pb-Sn合金凝固行为及组织的影响

在前期工作发现的某些合金熔体发生温度诱导非连续液-液结构转变的基础上,通过研究Pb-Sn共晶合金及Pb-Sn80%合金发生液-液结构转变前后熔体的凝固行为及组织,发现后者凝固过冷度较发生熔体结构转变前成倍增大,并导致凝固组织的明显细化.揭示了温度诱导非连续液-液结构转变对合金凝固行为及组织有着明显影响.     

Pb-Sb30%液-液结构转变的不可逆性及其对凝固的影响

在前期研究发现一些二元合金在液相线上数百度温度范围内发生温度诱导液-液结构转变的基础上,进一步研究了Pb-Sb30%合金液-液结构转变的可逆性,结果表明,该合金熔体发生的结构转变是不可逆的;并探讨了液-液结构转变对其凝固行为及组织的影响,发现结构变化后的熔体凝固过冷度增大,初生相和共晶相组织明显细化,且维氏硬度值提高约30%。     

金属熔体结构及其控制技术的研究进展

综述了国内外在熔体结构研究方面的现状与进展,介绍了通过控制合金熔体的预结晶状态来改善合金组织和性能的相关技术及成果。可以得出：温度和压力可使金属熔体发生液／液结构转变,金属熔体结构随温度的变化存在滞后性,利用熔体结构随温度变化的滞后性可控制合金的凝固过程、明显细化合金的凝固组织、大大提高合金的力学性能。     

Sn-70Bi合金熔体结构转变的可逆性及其凝固行为

利用直流四电极法研究了Sn-70Bi合金在液相线以上数百度温度范围内发生的熔体结构转变;在此基础上,进一步探讨了熔体结构转变的可逆性及其对凝固的影响.试验结果表明,Sn-70Bi合金所发生的熔体结构转变部分可逆,分析认为主要与Sn的熔体结构有关,且熔体结构转变对其凝固行为及组织有较大的影响.发现结构变化后的熔体凝固过冷度显著增大,初生相和共晶相组织明显细化.     

Al-6%Cu合金熔体结构变化及对凝固组织的影响

为了探索合金熔体结构与凝固组织的相关性,改善Al-Cu系合金的组织性能,使用四探针电阻法研究了Al-6%Cu合金升温过程中的电阻率行为。结果表明,在熔体过热处理温度区间该合金存在明显的电阻率突变现象,暗示在该阶段合金熔体发生了温度诱导的结构转变。合金金相组织观察和力学性能测试表明,该结构转变使得合金凝固组织晶粒尺寸明显细化、晶粒形貌趋于等轴化;在过热处理后,合金屈服强度提高了约18%,表面洛氏硬度提高了约14%。     

温度诱导的液-液结构转变对Sn-10%Sb合金凝固的影响

Sn-10%Sb合金在856～960℃可以发生温度诱导的液-液结构转变。研究了温度诱导液-液结构转变对Sn-10%Sb合金凝固行为及组织的影响。结果表明,合金熔体经历结构转变后,凝固时所需过冷度增大,并且凝固组织显著细化。     

Bi2Te3合金熔体结构变化对凝固行为及凝固组织的影响

根据Bi2Te3合金熔体的电阻率随温度而发生的异常行为,从熔体结构变化的视角探索了材料凝固行为与熔体热历史的依存关系。实验证明,Bi2Te3合金熔体结构在686～707℃范围内发生了异常转变,这种结构变化导致其熔体凝固过冷度增大,释放凝固潜热更加剧烈,并且凝固组织细化。这一结果表明通过控制熔体热历史,可更为有效地获得理想的凝固组织。     

Bi-20.5Te合金熔体结构转变及其对凝固的影响

通过对Bi-20．5Te合金熔体电阻率随温度变化的研究，发现合金熔体在642℃时存在着液一液结构转变。分析Bi-Te合金熔体在不同成分和不同温度的原子结合规律，认为这种结构转变是由于熔体中的原子团簇变化引起的。将Bi-20．5Te合金分别在熔体结构转变前温度（600℃）、结构转变后温度（700℃）的范围内熔炼、保温，测定相应的凝固曲线，并观察凝固组织。结果表明，相对于在结构转变前保温处理的熔体，将合金在700℃保温处理后，凝固时所需过冷度增大，释放凝固潜热更加剧烈，并且凝固组织显著细化。产生这种现象的原因是由于熔体结构内的Bi—Bi同类原子团簇，以及具有共价结合的Bi—Te原子团簇在高温下被打破，熔体趋向均匀，对凝固过程中的原子扩散造成影响，进而影响合金的凝固行为及组织。     

Understanding the Influence of Copper Nanoparticles on Thermal Characteristics and Microstructural Development of a Tin-Silver Solder

This paper presents and discusses issues relevant to solidification of a chosen lead-free solder, the eutectic Sn-3.5%Ag, and its composite counterparts. Direct temperature recordings for the no-clean solder paste during the simulated reflow process revealed a significant amount of undercooling to occur prior to the initiation of solidification of the eutectic Sn-3.5%Ag solder, which is 6.5 °C, and for the composite counterparts, it is dependent on the percentage of copper nanopowder. Temperature recordings revealed the same temperature level of 221 °C for both melting (from solid to liquid) and final solidification (after recalescence) of the Sn-3.5%Ag solder. Addition of copper nanoparticles was observed to have no appreciable influence on melting temperature of the composite solder. However, it does influence solidification of the composite solder. The addition of 0.5 wt.% copper nanoparticles lowered the solidification temperature to 219.5 °C, while addition of 1.0 wt.% copper nanoparticles lowered the solidification temperature to 217.5 °C, which is close to the melting point of the ternary eutectic Sn-Ag-Cu solder alloy, Sn-3.7Ag-0.9Cu. This indicates the copper nanoparticles are completely dissolved in the eutectic Sn-3.5%Ag solder and precipitate as the Cu₆Sn₅, which reinforces the eutectic solder. Optical microscopy observations revealed the addition of 1.0 wt.% of copper nanoparticles to the Sn-3.5%Ag solder results in the formation and presence of the intermetallic compound Cu₆Sn_5. These particles are polygonal in morphology and dispersed randomly through the solder matrix. Addition of microsized copper particles cannot completely dissolve in the eutectic solder and projects a sunflower morphology with the solid copper particle surrounded by the Cu₆Sn₅ intermetallic compound coupled with residual porosity present in the solder sample. Microhardness measurements revealed the addition of copper nanopowder to the eutectic Sn-3.5%Ag solder resulted in higher hardness.     

无铅波峰焊Sn-Bi-Ag-Cu钎料焊点剥离机制

对波峰焊常用的Sn-Bi-Ag-Cu无铅钎料进行了通孔波峰焊焊点剥离现象的模拟实验。分析表明,凝固延迟及铋元素偏析导致焊盘拐角附近钎料区在结晶后期残存液相最终成为缩孔的聚集区,结晶后期该区的低塑性使收缩应变容易超过材料的塑性而发生开裂,且发生机制与结晶裂纹发生的机制相同,强偏析元素铋的存在导致剥离概率极高。通孔拐角附近是应力应变的集中区,沿界面至焊盘外缘,应力应变逐渐降低。Sn-Bi-Ag-Cu系钎料焊点剥离的发生机制是由于Bi元素在焊盘/钎料界面富集使拐角附近钎料的液相线温度降低,结晶后期该区因固液共存而成为相对低塑性区,在应力作用下开裂并向焊盘外缘扩展。     

Sn-Bi-Ag-Cu钎料波峰焊焊点的剥离现象

采用在波峰焊过程中常用的Sn-Bi-Ag-Cu无铅钎料,进行了通孔波峰焊焊点剥离现象模拟实验.剥离的断面形貌和成分分析表明,凝固延后及铋元素偏析导致焊盘拐角附近的钎料区在结晶后期残存液相,并最终成为缩孔的聚集区,结晶后期该区的低塑性使收缩应变容易超过材料的塑性极限而发生开裂.开裂机制与结晶裂纹机制相似.强偏析元素铋的存在导致剥离概率急剧提高,铋元素含量增加,剥离的趋势增加.铅污染也使剥离的概率显著增加.冷却速度增加,剥离趋势减小.大的冷却速度能够抑制偏析,却不能完全抑制剥离,且会导致钎焊圆角表面裂纹增加.避免铅污染、降低铋的含量并控制冷却速度可抑制剥离.     

SnAgCu无铅钎料焊点结晶裂纹

针对印刷电路板(PCB)上无铅钎料SnAgCu微小焊点的结晶裂纹,利用设计的试件重现无铅钎料钎焊过程中产生的结晶裂纹,对无铅钎料结晶裂纹进行模拟,同时研究了微量元素的添加对SnAgCu合金焊点结晶裂纹形成的影响。结果表明,在尺寸很小的焊点上仍然存在明显的结晶裂纹。用焊点结晶裂纹的总长度定量地评价无铅钎料结晶裂纹的敏感倾向,添加Ni和Ce元素能够降低无铅钎料结晶裂纹的形成,而P元素的添加却加剧了结晶裂纹的形成,明显增加了焊点处的结晶裂纹。     

不同冷速Ag-5Sc合金的凝固组织

采用感应熔炼钢模凝固、慢速平衡凝固、液态快速凝固制备Ag-5Sc二元合金。借助差示扫描量热仪(DSC)、金相显微镜、电子探针(EMPA)和X射线衍射仪(XRD)研究合金在不同冷速条件下的组织结构、钪的分布和形成的化合物相。结果表明,钢模凝固形成环状Ag4Sc相与共晶组织交替的环形层状组织结构,其它组织为致密细小的共晶组织。慢冷凝固是粗大的胞状共晶组织,胞状间存在块状Ag4Sc相。液态快冷凝固为呈细小蜂窝状结构,组织为等轴晶粒。钢模凝固环形和慢冷凝固块状Ag4Sc相中钪元素分布密度比共晶中的Ag4Sc相高,液态快冷合金中钪元素分布均匀。     

Effect of solidification on solder bump formation in solder jet process： Simulation and experiment

To investigate the influence of the solidification on the solder bump formation in the solder jet process, the volume of fluid （VOF） models of the solder droplets impinging onto the fluxed and non-fluxed substrates were presented. The high speed camera was used to record the solder impingement and examine the validity of the model. The results show that the complete rebound occurs during the process of the solder droplet impinging onto the fluxed substrate, whereas a cone-shaped solder bump forms during the process of the solder droplet impinging onto the non-fluxed substrate. Moreover, the solder solidification results in the lift-up of the splat periphery and the reduction in the maximum spread factor.     

Influence of rapid solidification on Sn–8Zn–3Bi alloy characteristics and microstructural evolution of solder/Cu joints during elevated temperature aging

The effects of rapid solidification on the microstructure and melting behavior of the Sn–8Zn–3Bi alloy were studied. The evolution of the microstructural characteristics of the solder/Cu joint after an isothermal aging at 150 °C was also analyzed to evaluate the interconnect reliability. Results showed that the Bi in Sn–8Zn–3Bi solder alloy completely dissolved in the Sn matrix with a dendritic structure after rapid solidification. Compared with as-solidified Sn–8Zn–3Bi solder alloy, the melting temperature of the rapid solidified alloy rose to close to that of the Sn–Zn eutectic alloy due to the extreme dissolution of Bi in Sn matrix. Meanwhile, the adverse effect on melting behavior due to Bi addition was decreased significantly. The interfacial intermetallic compound (IMC) layer of the solder/Cu joint was more compact and uniform. Rapid solidification process obviously depressed the formation and growth of the interfacial IMC during the high-temperature aging and improved the high-temperature stability of the Sn–8Zn–3Bi solder/Cu joint.     

SnAgCu-xPr钎料组织及性能

研究了不同含量Pr元素（质量分数分别为0,0.05%,0.5%）对Sn-3.8Ag-0.7Cu无铅钎料凝固特性、润湿铺展性能以及微观组织的影响.结果表明,SAC,SAC-0.05Pr以及SAC-0.5Pr的凝固所需过冷度分别为20.6,5.0,5.1℃,说明适量Pr元素的加入能够显著降低SnAgCu钎料凝固所需的过冷度;同时,Pr元素的加入细化了钎料组织,降低钎料组织中初晶β-Sn的尺寸,抑制了SnAgCu/Cu焊点内部不同形貌大块化合物Ag3Sn初晶的形成;当Pr元素的添加量为0.05%时,钎料润湿性能最优、组织最佳;0.5%Pr元素的添加会在钎料以及焊点内部形成PrSn3相,对焊点的性能造成不利的影响.     

Effects of rapid solidification on the microstructure and microhardness of a lead-free Sn-3.5Ag solder

A lead-free Sn-3.5Ag solder was prepared by rapid solidification technology. The high solidification rate, obtained by rapid cooling, promotes nucleation, and suppresses the growth of Ag3Sn intermetallic compounds （IMCs） in Ag-rich zone, yielding fine Ag3Sn nanoparticulates with spherical morphology in the matrix of the solder. The large amount of tough homogeneously-dispersed IMCs helps to improve the surface area per unit volume and obstructs the dislocation lines passing through the solder, which fits with the dispersion-strengthening theory. Hence, the rapidly-solidified Sn-3.5Ag solder exhibits a higher rnicrohardness when compared with a slowly-solidified Sn-3.5Ag solder.