波峰焊及再流焊无铅焊点组织演变规律的研究

以电子封装线上的波峰焊无铅焊点Sn-0.7Cu/Cu、回流焊无铅焊点Sn-3Ag-0.5Cu/Cu为对象,研究了150 ℃时效过程中无铅焊点处金属间化合物(IMC)、焊料合金组织的演化规律及界面处金属间化合物生长的动力学.试验结果表明:两种无铅焊点处IMC层的厚度随着时效时间的延长而增加,IMC层的生长基本上符合抛物线规律,因此IMC层的长大受元素扩散控制;且两种无铅焊点处IMC层的生长速率常数相近,但Sn-0.7Cu焊料中Sn的晶粒尺寸较Sn-3Ag-0.5Cu中的大;长期时效后,在试样的IMC层内发现有孔洞产生.     

微量Ni对Sn-3.0Ag-0.5Cu钎料及焊点界面的影响

研究了Ni的含量对无铅钎料Sn-3.0Ag-0.5Cu润湿性、熔点、重熔及老化条件下界面化合物(IMC)的影响。结果表明:微量Ni的加入使SnAgCu润湿力增加6%;使合金熔点略升高约3℃;重熔时在界面形成了(Cu,Ni)6Sn5IMC层,且IMC厚度远高于SnAgCu/Cu的Cu6Sn5IMC厚度。在150℃老化过程中,SnAgCuNi/Cu重熔焊点IMC随着时间的增加,其增幅小于SnAgCu/Cu的增幅,此时Ni对IMC的增长有一定抑制作用。     

SnAgCu系钎料的合金组织和力学性能

研究了Cu含量对SnAgCu系钎料合金显微组织及其与化学镀Ni基板钎焊接头力学性能的影响。结果表明:高Cu含量SnAgCu合金会产生较多的(CuxNi1-x)6Sn5金属间化合物,从而减少镀Ni层的消耗,进一步提高钎焊接头的剪切强度。与Sn-3.0Ag-0.3Cu相比,Sn-3.0Ag-1.0Cu钎焊接头剪切强度提高了6.78%。经过150℃时效1000h后,界面Ni3(P,Sn)层的增长率从Sn-3.0Ag-0.3Cu合金的约66%降低到Sn-3.0Ag-1.0Cu合金的约40%。     

C/SiC与TC4钛合金异质连接过渡层的研究

分别用A95钨合金、A80钨合金、C103铌合金及三者的焊接组合作为C/SiC与TC4钛合金异质连接的过渡层,采用扫描电子显微镜、X射线衍射对过渡层与C/SiC的界面进行微观组织及结构分析,并对相应的封接件进行气密性及拉伸性能测试。结果表明,采用A95,A80钨合金作过渡层时,钨合金与C/SiC界面为冶金结合,界面产物为TiC,TiSi,Cu4Ti和Cu3Ti等;而采用三者的焊接组合作过渡层时,过渡层的热膨胀系数呈梯度变化且界面均为冶金结合,使C/SiC与TC4钛合金封接件具有高的气密性和抗拉强度（62.21 MPa）,实现良好异质连接。     

激光焊接Zr45Cu48A17块体非晶合金

为了研究激光技术焊接块体非晶合金的可行性,采用激光焊接块体非晶合金Zr45Cu48A17.当焊接速度为8m/min、激光输出功率为1.2kW时,焊缝和热影响区均保持了非晶特性,且没有发现裂纹等的缺陷;试样的热循环温度曲线分析表明,焊缝和热影响区急速从熔点冷却到430%以下是该试样保持非晶的原因.结果表明,采用激光焊接技术可以成功连接块体非晶合金.     

La对Sn-Ag-Cu无铅钎料组织与性能的影响

研究了微量稀土La对Sn-3.0Ag-0.5Cu无铅钎料显微组织、力学性能、断口形貌、润湿性能和熔点的影响.结果表明:La的质量分数为0.1%可使钎料合金晶粒细化,并显著提高钎料合金力学性能和润湿性能;添加La的质量分数为0.4%将形成粗大LaSn3初生枝晶相,降低力学性能和润湿性能;微量La使钎料合金的熔点轻微增加.     

低银Sn—Ag—Cu无铅钎料的性能研究

使用银含量较低的Sn-Ag-Cu无铅钎料是降低焊料成本最直接有效的手段之一,但银含量降低后对钎料性能的影响尚缺乏系统报导。通过向Sn-Cu二元体系中加入不同比例的纯银,制备了一系列低银合金。研究了银含量对钎料的熔点、可焊性及溶铜性能的影响。通过拉伸试验研究了合金的强度及杨氏模量。综合考虑上述各项性能指标,Sn-0.5Ag-0.7Cu是具有最佳性价比的合金成分。     

无铅焊料在清华大学的研究与发展

清华大学材料科学与工程系电子材料与封装技术研究室研制了6个系列的无铅焊料:Sn-3.5Ag添加Cu或Bi; Sn-3.5Ag-1.0Cu添加In或Bi; Sn-Ag-Cu-In添加Bi; Sn-Ag-Cu添加Ga; Sn-Zn添加Ga; Sn-Zn添加多种元素。重点介绍了Sn-Zn添加多种元素。对6个系列无铅焊料的研究取得了较好的实验结果,得到比较理想的低温焊料体系,有的合金熔点已非常接近铅锡共晶焊料熔点183℃。     

Sn-Cu、Sn-Ag-Cu系无铅钎料的钎焊特性研究

制备了Sn-0.7Cu、Sn-3.5Ag-0.6Cu钎料,用润湿平衡法测量了钎料对铜的润湿曲线,研究了温度、钎剂活性、钎焊时间对润湿行为的影响,并与Sn-37Pb钎料进行了比较。结果表明:升高温度能显著改善无铅钎料对铜的钎焊性。当温度<270℃时,Sn-0.7Cu的钎焊性明显低于Sn-3.5Ag-0.6Cu钎料;而当温度≥270℃时,两种钎料对铜都会显示较好的润湿性,而Sn-0.7Cu略优于Sn-3.5Ag-0.6Cu钎料。提高钎剂活性能显著增强钎料对铜的润湿性,其卤素离子的最佳质量分数均为0.4%左右。随着浸渍时间的延长,熔融钎料与铜的界面间产生失润现象。无铅钎料的熔点和表面张力较高,是钎焊性较差的根本原因。     

无铅焊料Sn-Zn-In系列合金的研究

对熔化起始温度和终止温度作线性回归进行合金设计,并对其焊料合金进行了熔点、抗剪切强度及微观组织等研究分析。结果表明:当w(In)(质量分数)为3%~5%,w(Zn)为5%~9%时,焊料的熔化温度在170~200℃,接近于焊料Sn-37Pb的熔化温度183℃;焊料与Cu焊合后形成γ-Cu5Zn8化合物;Sn-Zn-In系焊料的抗剪切强度与焊料Sn-37Pb的剪切强度33.73 MPa相当。     

Influence of alloy composition on fillet-lifting phenomenon in Sn-Ag-Bi alloys

The influence of the Bi content on the fillet-lifting phenomenon in Sn-3Ag-Bi alloys was investigated. The peculiar composition dependence on the fillet-lifting exists in Sn-3Ag-xBi alloys. With fillet-lifting, they are 3–20%Bi. Without fillet-lifting, they are 0–1%Bi, 30%Bi, and above. This is related to the alloy solidification characteristics, in particular, the amount of latent heat release near the final solidification temperature (solidus temperature) during solidification. Fillet-lifting is likely to occur in the alloy with a latent heat released which is insufficient for alleviation of the temperature gradient in the solder joint. A new predictable index for the occurrence of fillet-lifting is proposed as the fillet-lifting resistant index (FRI) that correlates well with the fillet-lifting occurrence rate. The fillet-lifting does not occur when a solder alloy has a FRI of 0.75 and above. The magnitude of the solidification temperature range (T) does not necessarily correlate with the fillet-lifting, because there are some alloys without fillet-lifting in spite of their having a wide T. There is also little correlation between the fillet-lifting and the formation of a Bi concentration at the solder/Cu land interface, previously thought to cause fillet-lifting. The Bi concentration at the solder/Cu land interface may occur as a consequence of the Sn consumption by the intermetallic compound formation with the Cu dissolving into the molten solder and existing at the interface rather than the solidification segregation.     

Comparison study on microstructure and mechanical properties of Sn-10Bi and Sn-Ag-Cu solder alloys and joints

The comparison study of Sn-10Bi and Sn-3.0Ag-0.5Cu solder alloys and joints was conducted. The results showed that the liquidus of Sn-10Bi solder alloy was lower than that of Sn-Ag-Cu slightly. The interfacial IMCs layer growth of Sn-10Bi/Cu was slower than that of Sn-Ag-Cu/Cu during liquid/solid reaction. The higher strength and lower creep strain rate of Sn-10Bi comparing with that of Sn-Ag-Cu were contributed by the solid solution strengthening effect of Bi atom in β-Sn phase. The ultimate bending load of Sn-10Bi joint was higher than that of Sn-Ag-Cu joint as the high strength of Sn-10Bi solder alloy. Moreover, the thinner and more flat IMCs layer also ensured the stable maximum bending displacement of Sn-10Bi joint at a loading speed of 1 mm/s compared with that of Sn-Ag-Cu joint.     

Creep and rupture behavior of Cu wire/lead-free solder-alloy joint specimen

Creep and rupture behavior of Cu wire/lead-free solder-alloy joint specimens have been investigated using Sn-3.5% Ag and Sn-0.5% Cu alloys. A Sn-37% Pb solder alloy is also used as a reference material. The present authors have fabricated a creep-rupture testing machine for Cu wire/solder-alloy joint specimens, performed creep and rupture tests at 303 K and 403 K, analyzed the characteristics of the creep and rupture behavior, and compared these to test specimens cut from the same alloy ingots. It is also found that the rupture strength of the joint specimens is related to the rupture strength of the alloys.     

Shear deformation in Sn-3.5Ag and Sn-3.6Ag-1.0Cu solder joints subjected to asymmetric four-point bend tests

The shear deformation behavior of two lead-free solder compositions, Sn-3.5Ag (wt.%) and Sn-3.6Ag-1.0Cu (wt.%), both on copper substrates, was studied using an asymmetric four point bend technique. Four test joints were obtained from one master specimen of each composition, and each joint was subject to progressive loading, up to the maximum shear strength of the joint. One unstressed bar from each composition was retained as a reference. Each sample was metallo-graphically plished and lightly etched, and examined in a field emission scanning electron microscope (SEM) before shearing. Sheared joints were then re-examined in the SEM with no additional surface treatment. Compared with the traditional ring and plug method, the asymmetric four-point bend (AFPB) technique subjects the joints to a condition of pure shear, while providing an opportunity for unambiguous observation of microstructural features before and after shearing, without an intervening mechanical sectioning step. Shear banding in the Sn-rich matrix and crack nucleation in the vicinity of the intermetallic interface were observed at low displacements in the binary alloy. Evidence of non-homogeneous plastic flow in the matrix was seen at higher shear loading. No evidence of brittle fracture was observed in the Sn-3.6Ag-1.0 Cu alloy, with elastic deformation at low stress levels giving rise to plastic deformation at higher loading values. Results show that the AFPB technique is a viable approach to the study of shear loading on solder joints.     

Influence of alloy composition on fillet-lifting phenomenon in tin binary alloys

The influence of alloy composition on the fillet-lifting phenomenon was investigated for Sn−Bi, Sn−Pb, and Sn−In binary alloys. Fillet-lifting occurs in Sn-(1–30%)Bi, Sn-(1–5%)Pb, Sn-(2–15%)In, but does not occur in pure Sn, Sn-(40–62%)Bi, Sn-(10–45%)Pb, and Sn-43 In. Fillet-lifting does not correlate with the formation of a Bi-concentrated layer at the solder/Cu land interface, previously thought to cause fillet-lifting. The solidification temperature range also does not necessarily correlate with fillet-lifting. Fillet-lifting was found to be related to the retention time (t_r) for which solidus temperature (the final solidification temperature) remains by latent heat released during eutectic solidification in the cooling curve. The released latent heat contributes to the alleviation of the temperature gradient causing fillet-fifting in the solder joint. A fillet-fifting resistant index (FRI) is proposed as a new index for fillet-lifting, and correlates well with fillet-lifting occurrence rate in each of the studied tin binary alloys.     

Effects of Trace Amounts of Rare Earth Additions on Microstructure and Properties of Sn-Bi-Based Solder Alloy

The effect of trace amounts of rare earth additions on the microstructure and properties were studied for the Sn-58Bi and Sn-58Bi-Ag solder alloys. At the same time, the intermetallic compounds (IMCs) in the solder alloys and intermetallic layer (IML) thickness at the solder/Cu substrate interface were investigated, both as-reflowed and after high-temperature aging. The results indicate that adding trace amounts of rare earth (RE) elements has little influence on the melting temperature and microhardness of the solders investigated, but adding RE elements improves the wettability and shear strength of the Sn-58Bi and Sn-58Bi-Ag solder alloys. In addition, it was found that the addition of RE elements not only refines the microstructure and size of the IMC particles, but also decreases the IML thickness and shear strength of the Sn-58Bi solder joint after high-temperature aging. Adding trace amounts of RE elements is superior to adding trace amounts of Ag for improving the properties of the Sn-58Bi solder. The reason may be related to the modification of the microstructure of the solder alloys due to the addition of trace amounts of RE elements.     

Sn-Zn-Bi无铅焊料表面张力及润湿性研究

采用气泡最大压力法对Sn-9Zn-XBi焊料进行了表面张力测试,用平衡法测试焊料的润湿性。结果表明:Bi的添加大大降低了Sn-Zn系焊料熔体的表面张力;然而焊料暴露在空气环境下1min后,表面形成ZnO导致其表面张力增大;Bi的增加提高了焊料在铜片上的润湿力,缩短了润湿时间;Sn-9Zn-XBi焊料润湿力仍低于Sn-40Pb,其原因是焊料–铜界面能偏高。     

无铅焊点在电迁移与高低温冲击下的失效机理

研究了Cu/Sn-3.0Ag-0.5Cu/Cu焊点接头在电迁移与高低温冲击双重耦合作用下的失效机理。结果表明,在高低温冲击条件下,由于铜柱、钎料合金及镶样用环氧树脂的热膨胀系数不匹配,因此,焊点接头在高低温冲击过程中无法自由伸缩,在热疲劳的作用下,焊接接头易于在界面处形成裂纹,且随着时间的延长裂纹会发生扩展,造成焊点接头的有效横截面积减小,使得焊点接头的电阻增大、焦耳热增加,进而导致焊点接头发生熔化而失效。     

Spreading Behavior and Evolution of IMCs During Reactive Wetting of SAC Solders on Smooth and Rough Copper Substrates

The effect of surface roughness of copper substrate on the reactive wetting of Sn-Ag-Cu solder alloys and morphology of intermetallic compounds (IMCs) was investigated. The spreading behavior of solder alloys on smooth and rough Cu substrates was categorized into capillary, diffusion/reaction, and contact angle stabilization zones. The increase in substrate surface roughness improved the wetting of solder alloys, being attributed to the presence of thick Cu₃Sn IMC at the interface. The morphology of IMCs transformed from long needle shaped to short protruded type with an increase in the substrate surface roughness for the Sn-0.3Ag-0.7Cu and Sn-3Ag-0.5Cu solder alloys. However, for the Sn-2.5Ag-0.5Cu solder alloy the needle-shaped IMCs transformed to the completely scallop type with increase in the substrate surface roughness. The effect of Ag content on wetting behavior was not significant.     

Early Interfacial Reaction and Formation of Intermetallic Compounds in the Sn-3.5Ag/Cu Soldering System

The early interfacial reaction in the Sn-3.5Ag/Cu soldering system and the system’s premelting behavior were found and characterized by differential scanning calorimetry incorporated into the reflow process. The results show that the early interfacial reaction occurs by way of melting and wetting of the solder layer adjacent to the Cu substrate at a temperature nearly 4°C below the actual melting point of Sn-3.5Ag solder due to solid-state diffusion of Cu atoms into the Sn-3.5Ag binary solder. Consequently, the early interfacial reaction brings about formation of Cu-Sn intermetallic compounds (IMCs) at a temperature below the melting point of Sn-3.5Ag, and a prolonged early interfacial reaction can lead to change of the Cu-Sn IMC morphology from planar-like to scallop-like and promote excessive growth of IMCs at the interface.