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℃。     

电子元件、组件

TN60 2004030494铜锢钊硫对Sn一Ag基无铅焊料性能的影响/林培豪,刘心宇,成钧(桂林电子工业学院)11电子元件与材料‘一2 003,22(10).一33-34研究了Cu、In、Bi、S元素对Sn一Ag基无铅焊料熔点和铺展性的影响.结果表明:Sn一A片Cu三元合金成分为9 5.5%Sn3.5%Agl%Cu时具有较低熔点(215℃)和好的铺展性;加入适量的In可降低Sn一Ag合金的熔点和改善铺展性能;随二(B劝的增加Sn-Ag一Bi三元合金熔点降低、铺展性变好;Sn一Ag合金熔点随、(S)的增加而升高,加入少量S能改善Sn一Ag合金的铺展性.图8参5(刚)影响,发现随着硫酸浓度增加,温度对隧道…     

Sn—Bi无铅焊料的研究

通过总结传统Sn-Pb焊料的特点和研制无铅焊料的条件,综述Sn-Bi无铅焊料低熔点等优点以及脆性大、易偏析的缺点,分析添加In,Ag,AI,Sb等微量合金元素对Sn-Bi无铅焊料微观组织及性能的影响,提出了此合金系焊料的抗蠕变性、导电性、润湿性、拉伸性能等尚待完善的性能及可采用合金化、开发新型焊剂等新方法使Sn-Bi焊料成为理想的无铅焊料,为无铅焊料的进一步研究提供一定的参考.     

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。     

铜铟铋硫对Sn-Ag基无铅焊料性能的影响

研究了Cu、In、Bi、S元素对Sn-Ag基无铅焊料熔点和铺展性的影响。结果表明:Sn-Ag-Cu三元合金成分为95.5%Sn3.5%Ag1%Cu时具有较低熔点(215℃)和好的铺展性;加入适量的In可降低Sn-Ag合金的熔点和改善铺展性能;随w(Bi)的增加Sn-Ag-Bi三元合金的熔点降低、铺展性变好;Sn-Ag合金的熔点随w(S)的增加而升高,加入少量S能改善Sn-Ag合金的铺展性。     

Sn-Zn-Bi-In-P新型无铅焊料性能研究

新型无铅焊料Sn-4.5Zn-2Bi-In-P、Sn-9Zn-2.5Bi-In-P具有优异的抗氧化、抗腐蚀性能,弥补了Sn-Zn系焊料润湿性能方面的不足,具有极大的实用性。测量了共晶Sn-9Zn、两种新型Sn-Zn系无铅焊料和传统的Sn-37Pb焊料的各项物理性能:密度、熔点、线膨胀系数、电阻率及对铜基体的润湿角。实验结果表明,新型Sn-Zn系无铅焊料的密度约为传统Sn-37Pb焊料的3/4;熔点(依次为194℃,191.9℃)接近Sn-37Pb焊料,熔程仅为8℃;在25~100℃,新型Sn-Zn焊料线膨胀系数依次为20.8×10–6/℃、16.9×10–6/℃,优于Sn-37Pb焊料(21.2×10–6/℃);新型Sn-Zn焊料的电阻率依次为1.73×10–6Ω·m、1.79×10–6Ω·m,优于Sn-37Pb焊料(1.96×10–6Ω·m);新型Sn-Zn焊料对Cu基体的润湿角接近30°,满足实用化的最低要求。     

新型无铅焊料的研制

由于目前使用的Sn3.5AgCu无铅焊料,温度较高、润湿性较差.在Sn3.5AgCu无铅钎料中添加金属元素Ga、Bi,并改变Ag的含量,对其合金进行熔化温度、润湿性、力学性能和微观组织研究.研究表明,添加Ga、Bi并改变Ag含量有利于降低Sn3.5AgCu合金的熔化温度和改善其润湿性,并提高了力学性能.     

Sn-Zn系无铅钎料最新进展

Sn-Zn系钎料熔点与传统Sn-37Pb钎料十分接近,成本低廉,被研究者所推崇。由于Zn的存在导致Sn-Zn钎料润湿性差及抗氧化性不足,阻碍了该钎料的发展。添加合金元素和纳米颗粒是改善Sn-Zn钎料组织和性能行之有效的方法之一,为国内外研究者所推崇。结合国内外Sn-Zn系无铅钎料最新研究成果,探讨添加微量的合金元素In、Ni、Cr、Ga、Bi、Cu、Al、Ag、稀土元素及纳米颗粒对钎料润湿性、抗氧化性、力学性能、显微组织和界面组织的影响,同时简述有关钎剂对Sn-Zn的影响,并对Sn-Zn系钎料的发展趋势进行分析与展望。     

国内外覆铜板文献摘录（11）

电子玻璃纤维介电性能和工艺参数的研究;镀通孔-孔洞产生的根源和调整方法;挠性印制线路板用特殊电解铜箔;Sn-8Zn-3Bi无铅焊料的表面改性;低熔点无铅焊料的研制。     

稀土改性的Sn-58Bi低温无铅钎料

研究了微量稀土对Sn-58Bi低温钎料的改性作用.试验添加质量分数为0.1 ?组混合稀土的无铅材料,并对比Sn-58Bi和Sn-58Bi0.5Ag合金.观察了钎料显微组织的变化并做了定量分析,采用DSC测试了钎料的熔化温度,同时测量了钎料的润湿性能、接头强度与硬度.结果表明,微量稀土添加细化了Sn-58Bi钎料合金的显微组织,对钎料的熔化温度几乎没有影响,能显著改善Sn-58Bi钎料的润湿性能和接头剪切强度,而且改善的程度优于添加微量Ag对Sn-58Bi钎料的作用.     

Lead-free Sn-Ag and Sn-Ag-Bi solder powders prepared by mechanical alloying

A mechanical alloying (MA) process was used to produce lead-free solder pastes of Sn-3.5Ag and the Sn-3.5Ag-4Bi system. Because of the high energy induced by repeated fracturing and welding, the grinding media played an important role during the MA process. A ceramic container was used to provide stronger impact force, which could induce phase transformation better than a Teflon container. In addition, it was found that 1-cm balls could fracture Bi particles and promote their dissolution into the Sn matrix. On the contrary, the milling process tended to achieve homogeneous mixing when using 3-mm balls. The MA powders, after milling with 3-mm balls, showed a small endothermic peak from the differential scanning calorimetry (DSC) profile at around 138°C, which was the eutectic temperature of Sn-Bi. The melting points of the MA powders in the ceramic container were measured to be 221°C and 203°C, respectively, for Sn-3.5Ag and Sn-3.5Ag-4Bi from the DSC curves. The reduced melting point ensured the complete melting during reflow with a peak temperature of 240°C. The formation of Ag₃Sn was also observed from the x-ray diffraction peaks, indicating successful alloying by MA. The solder pastes could, thus, be produced by adding flux into the MA powders. The wetting property of the solder joint was also evaluated. The as-prepared solder pastes on electroless Ni-P/Cu/Si showed good metallurgical bonding with a contact angle less than 20°.     

The microstructures and mechanical properties of the Sn-Zn-Ag-Al-Ga solder alloys—the effect of Ag

The microstructures and mechanical properties of Sn-8.55Zn-xAg-0.45 Al-0.5Ga (wt.%) lead-free solders were investigated. The x content of the solders investigated were 0.5–3.0 wt.%. The results indicate that Ag plays an important role not only in the structure but also in the mechanically properties. The mechanical properties and differential scanning calorimetry (DSC) behavior has been compared with that of 63Sn-37Pb solder. Small additions of Ag decreased the melting point of the Sn-8.55Zn-xAg-0.45Al-0.5Ga solders while maintaining the same strength and ductility as the 63Sn-37Pb solder.     

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.     

Effects of Co addition in eutectic Sn-3.5Ag solder on shear strength and microstructural development

In this study, the approach of composite solder using eutectic Sn-3.5Ag solder and Co was tried. Co particles and Sn-3.5Ag solder paste were mechanically mixed at Co weight fractions from 0.1% to 2.0%. For the Co-mixed Sn-3.5Ag solder pastes, their melting temperatures and spreading areas were measured. The solder pastes were stencil printed on test substrates and reflowed to form solder bumps. Ball shear test was performed to examine shear strength of Co-reinforced Sn-3.5Ag solder bumps. As a result, Co addition up to 2 wt.% did not alter the melting temperature under heating but reduced undercooling. The maximum shear strength of Co-reinforced Sn-3.5Ag solder bumps increased by 28% compared to normal ones. The increase in shear strength can be attributed to the (Cu,Co)₃Sn₂ intermetallic compounds.     

Interfacial reaction and solder joint reliability of Pb-free solders in lead frame chip scale packages (LF-CSP)

Chip scale packages (CSP) have essential solder joint quality problems, and a board level reliability is a key issue in design and development of the CSP type packages. There has been an effort to eliminate Pb from solder due to its toxicology. To evaluate the various solder balls in CSP package applications, Pb-free Sn-Ag-X (X=In, Cu, Bi) and Sn-9Zn-1Bi-5In solder balls were characterized by melting behavior, phases, interfacial reaction, and solder joint reliability. For studying joint strength between solders and under bump metallurgy (UBM) systems, various UBMs were prepared by electroplating and electroless plating. After T/C (temperature cycle) test, Sn3.5Ag8.5In solder was partially corroded and its shape was distorted. This phenomenon was observed in a Sn3Ag10In 1Cu solder system, too. Their fractured surface, microstructure of solder joint interface, and of bulk solder ball were examined and analyzed by optical microscopy, SEM and EDX. To simulate the real surface mounting condition and evaluate the solder joint reliability on board level, Daisy chain test samples using LF-CSP packages were prepared with various Pb-free solders, then a temperature cycle test (−65∼ 150°C) was performed. All tested Pb-free solders showed better board level solder joint reliability than Sn-36Pb-2Ag. Sn-3.5Ag-0.7Cu and Sn-9Zn-1Bi-5In solders showed 35%, 100% superior solder joint reliability than Sn-36Pb-2Ag solder ball, respectively.     

Properties of quad flat package joints using Sn-Zn-Bi solder with varying lead-plating materials

The effects of plating materials (Sn-10Pb, Sn-3.5Ag, Sn-3Bi, Sn-0.7Cu, and Au/Pd/Ni) on Cu leads on quad flat package (QFP) joints using a Sn-8Zn-3Bi solder were investigated. The joints with Sn-3.5Ag plating and Sn-8Zn-3Bi solder had the slowest growth rate of interfacial reaction layers and the highest strength. The Ag dissolution into the interfacial reaction layers causes this increased strength. The Sn-Ag plating is the best plating material for Cu leads among the five kinds of plating using Sn-8Zn-3Bi solder.     

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.