Sn-Ag和Sn-Zn及Sn-Bi系无铅焊料

随着微电子组装技术的发展，研制新型的和实用的无铅焊料替代传统的Sn-Pb焊料已成为近年的研究热点。介绍了目前最常见的Sn-Ag、Sn-Zn和Sn-Bi为基体的无铅焊料并与传统的Sn-Pb焊料的性能进行了比较。     

Microstructural evolution of lead-free Sn-Bi-Ag-Cu SMT joints during aging

It was reported in a previous study that the Sn-6Bi-2Ag-0.5Cu solder alloy had great potential to replace leaded alloys. This alloy was prepared by mechanical alloying, and had the advantage of providing a high percentage of supersaturate solution of bismuth in tin. In the present paper, the microstructural evolution of surface-mount joints during aging was examined. In the as-soldered joints, small bismuth and Ag₃Sn particles of about 1 mum in size were found to be finely dispersed in a nearly pure tin matrix with a small amount of eta-Cu₆Sn₅ phase in the bulk of solder. During aging, microstructural evolution of solder joints occurred. These include Cu-Sn intermetallic compound (IMC) layer growth at the interface between solder and copper pad on the printed circuit board, as well as bismuth phase and Ag₃Sn phase coarsening. The shear strength of the solder joints decreased parabolically with the increase in IMC layer thickness, such that tau_s=22.22-radic22.05(t-1.88), where tau_s is the shear strength in MPa and t (>1.88) is the total IMC layer thickness in micrometers. The microstructure of solder appeared to be stable under aging at elevated temperatures up to about 160degC. Above this temperature, brittle and porous IMC epsiv-Cu₃Sn appeared at the copper/eta-Cu₆Sn ₅ interface. Fracture was found to occur at the Cu-Sn IMC layer-solder interface and in the bulk of solder     

Joint shape, microstructure, and shear strength of lead-free solder joints with different component terminations

The joint shape, microstructure, and shear strength of 95Sn5Sb solder joints with components with different terminal metallizations (AgPd and Ni/AgPd) were investigated and compared to 62Sn36Pb2Ag solder. The rapid reaction between SnSb solder and AgPd leads to the solder not spreading entirely on the Cu pad but agglomerating on the component termination. The shear strength of the SnSb/AgPd solder joint is very low and fracture occurs at the original interface of the AgPd/ceramic. The Ni layer in the SnSb/Ni/AgPd joint effectively avoids interdiffusion between the AgPd and the solder so that a high-strength solder joint with an ideal shape is achieved. However, the terminations of the components have little effect on the shape and the shear strength of the SnPbAg solder joint. Fracture occurs in the SnPbAg solder after shear testing to failure.     

A dislocation model of shear fatigue damage and life prediction of SMT solder joints under thermal cycles

In this paper, a damage model of low-cycle shear fatigue has been developed in the dislocation theory. On the basis of this model, a formula including the factors of thermal cycle temperature, dwell time and atmospheric oxidation has been established to predict the life of SMT solder joints under thermal cycles, and this has been verified by experiments on the specimens of true SMT assemblies. The results show that the life formula established in this paper coincides with the experimental results.     

Effect of adding Sb on microstructure and adhesive strength of Sn-Ag solder joints

This study investigates the influence of adding Sb on the microstructure and adhesive strength of the Sn3.5Ag solder. Both solidus and liquidus temperatures increase as Sb additions increase. Adding 1.5wt.%Sb leads to the narrowest range (6.6°C) between the solidus and liquidus temperature of the solder. Adding Sb decomposes the as-soldered ringlike microstructure of Sn3.5Ag and causes solid-solution hardening. The as-soldered hardness increases with increasing Sb addition. For long-term storage, adding Sb reduces the size of the rodlike Ag₃Sn compounds. The hardness also increases with increasing Sb addition. Adding Sb depresses the growth rate of interfacial intermetallic compounds (IMCs) layers, but the difference between 1% and 2% Sb is not distinct. For mechanical concern, adding Sb improves both adhesive strength and thermal resistance of Sn3.5Ag, where 1.5% Sb has the best result. However, adding Sb causes a variation in adhesive strength during thermal storage. The more Sb is added, the higher the variation reveals, and the shorter the storage time requires. This strength variation helps the solder joints to resist thermal storage.     

剪切蠕变下无铅焊点厚度的尺寸效应

利用自制的电子测试系统,测量分析了试样焊点厚度(0.05～0.50mm)对电阻应变的影响。结果表明:在剪切蠕变条件下,焊点厚度为0.25mm时,电阻应变最小,蠕变寿命最长。利用有限元软件ANSYS对焊点的蠕变应变进行仿真分析。结果显示:随着焊点厚度变化,焊点蠕变应变的变化趋势与实验结果一致。将相同厚度下的电阻应变与蠕变应变进行拟合,得到了电阻应变与蠕变应变之间的定量关系式。     

热输入及掺Ni对Sn-Ag钎料强度的影响

Sn-Ag共晶钎料中添加Ni颗粒,在钎料内部形成强化相,达到强化效果。研究了Ni颗粒的添加配比对强化效果的影响,并提出了热输入概念,通过调节热输入量控制钎料的强度。结果表明,φ(Ni)为5%是最佳配比,当热输入量为5.07时,其强度值最高,达60.71 MPa。比未添加Ni的钎料提高50%以上。     

Characterization of eutectic Sn-Bi solder joints

This report presents experimental results on 58Bi-42Sn solder joints, optical and SEM microstructures of their matrix and of their interface with copper, solidification behavior studied by differential scanning calorimetry, wettability to copper, creep, and low cycle fatigue. These results are discussed in comparison with 60Sn-40Pb solder, and with three low temperature solders, 52In-48Sn, 43Sn-43Pb-14Bi, and 40In-40Sn-20Pb. The 58Bi-42Sn solder paste with RMA flux wets Cu matrix with a wetting angle of 35° and had a 15° C undercooling during solidification. The constitutive equation of the steady state shear strain rate, and the Coffin-Manson relation constants for the low cycle shear fatigue life at 65° C have been determined. The test results show that this solder has the best creep resistance but the poorest fatigue strength compared with the other four solders.     

颗粒增强Sn-Ag基无铅复合钎料显微组织与性能

通过外加法向Sn-3.5Ag焊料中加入体积分数为10%的微米级Cu、Ni颗粒制备了无铅复合钎料,对钎料的显微组织、拉剪及润湿性能进行了研究。结果表明,颗粒周围以及基板界面处的显微组织中生成了金属间化合物,其形态及大小因加入颗粒而不同。颗粒的加入提高了钎料钎焊接头的剪切强度,其中Cu颗粒增强的接头的剪切强度提高了33%,Ni颗粒的提高了20%。两种复合钎料的铺展面积均下降了约15%,其中Cu颗粒增强复合钎料润湿角由11°增加到18°。     

Microstructure evolution of eutectic Sn-Ag solder joints

Laser and infrared reflow soldering methods were used to make Sn-Ag eutectic solder joints for surface-mount components on printed wiring boards. The microstructures of the joints were evaluated and related to process parameters. Aging tests were conducted on these joints for times up to 300 days and at temperature up to 190°C. The evolution of microstructure during aging was examined. The results showed that Sn-Ag solder microstructure is unstable at high temperature, and microstructural evolution can cause solder joint failure. Cu-Sn intermetallics in the solder and at copper-solder interfaces played an important role in both the microstructure evolution and failure of solder joints. Void and crack formation in the aged joints was also observed.     

Thermomechanical fatigue behavior of Sn-Ag solder joints

Microstructural studies of thermomechanically fatigued actual electronic components consisting of metallized alumina substrate and tinned copper lead, soldered with Sn-Ag or 95.5Ag/4Ag/0.5Cu solder were carried out with an optical microscope and environmental scanning electron microscope (ESEM). Damage characterization was made on samples that underwent 250 and 1000 thermal shock cycles between −40°C and 125°C, with a 20 min hold time at each extreme. Surface roughening and grain boundary cracking were evident even in samples thermally cycled for 250 times. The cracks were found to originate on the free surface of the solder joint. With increased thermal cycles these cracks grew by grain boundary decohesion. The crack that will affect the integrity of the solder joint was found to originate from the free surface of the solder very near the alumina substrate and progress towards and continue along the solder region adjacent to the Ag₃Sn intermetallic layer formed with the metallized alumina substrate. Re-examination of these thermally fatigued samples that were stored at room temperature after ten months revealed the effects of significant residual stress due to such thermal cycles. Such observations include enhanced surface relief effects delineating the grain boundaries and crack growth in regions inside the joint.     

Modeling thermomechanical fatigue behavior of Sn-Ag solder joints

Stresses that develop because of the coefficient of thermal expansion (CTE) mismatch between solder and substrate/components contribute to thermomechanical fatigue (TMF) of the solder joints. However, the relative importance of several processes that contribute to damage accumulation and its role in affecting the reliability of the solder joints are far from being understood. Aging, creep/stress relaxation, and stress/strain reversals are some of the important processes. These processes are affected by service conditions, such as the temperature extremes experienced, rates of heating and cooling, dwell times at the extreme temperatures, and so on. In addition, the elastic and plastic anisotropy of tin could also contribute to the damage accumulation during TMF of Sn-based solders. This preliminary effort to model TMF in Sn-Ag solder joints will consider the role of each of these parameters, with significant emphasis on the anisotropic-elastic behavior of Sn grains.     

热循环加载片式元器件带空洞无铅焊点的可靠性

建立了片式元器件带空洞无铅焊点有限元分析模型,研究了热循环加载条件下空洞位置和空洞面积对焊点热疲劳寿命的影响.结果表明:热循环加载条件下空洞位置和空洞面积显著影响焊点热疲劳寿命.空洞位置固定于焊点中部且面积分别为7.065×10-4,1.256×10-3,1.963×10-3和2.826×10-3mm2时,焊点热疲劳寿...     

基于热疲劳状态监测的PCB无铅焊点可靠性研究

以PCB片式电阻器件无铅焊点为研究对象,开展与有铅焊点剪切力-热疲劳状态比较试验研究。采用了伪失效寿命比较方法,获得了有限周期温度冲击下的焊点剪切力试验数据,采用非线性最小二乘法进行剪切力数据的曲线拟合。结果表明,封装尺寸是影响焊点热疲劳性能的重要因素。在1 500个有限周期内,无铅焊点的热疲劳性能优于有铅焊点。     

Lead contamination of a transient liquid-phase-processed Sn-Bi lead-free solder paste

The influence of Pb contamination on the solidification behavior of a transient liquid-phase powder-processed Sn-Bi solder paste has been studied using differential scanning calorimetry. The development of low-temperature ternary reactions was found to be very sensitive to both the Pb and Bi content of the solder. Solders with high Bi content favored the formation of the ternary eutectic reaction. Solders with high Pb contents favored the formation of a ternary peritectic reaction. These results agree very well with solidification predictions present in the literature for ternary Sn-Bi-Pb alloys. In particular, the dependence of ternary reactions on composition is due to a change in solidification path. Alloy compositions which mark the transition from one path to the next were identified.     

Low-cycle fatigue behavior of Sn-Ag, Sn-Ag-Cu, and Sn-Ag-Cu-Bi lead-free solders

Low-cycle fatigue (LCF) tests on as-cast Sn-3.5Ag, Sn-3Ag-0.5Cu, Sn-3Ag-0.5Cu-1Bi, and Sn-3Ag-0.5Cu-3Bi solders was carried out using a noncontact strain-controlled system at 20°C with a constant frequency of 0.1 Hz. The addition of Cu does not significantly affect the fatigue life of eutectic Sn-Ag solder. However, the fatigue life was significantly reduced with the addition of Bi. The LCF behavior of all solders followed the Coffin-Manson relationship. The fatigue life of the present solders is dominated by the fracture ductility and can be described by the ductility-modified Coffin-Manson’s relationship. Steps at the boundaries of dendrite phases were the initiation sites for microcracks for Sn-3.5Ag, Sn-3Ag-0.5Cu, and Sn-3Ag-0.5Cu-1Bi solders, while for Sn-3Ag-0.5Cu-3Bi solder, cracks initiated along both the dendrite boundaries and subgrain boundaries in the dendrite phases. The linking of these cracks and the propagation of cracks inside the specimen occurred both transgranularly through eutectic phases and intergranularly along dendrite boundaries or subgrain boundaries.     

Contact angle measurements of Sn-Ag and Sn-Cu lead-free solders on copper substrates

In this study, the contact angles of four lead-free solders, namely, Sn-3.5Ag, Sn-3.5Ag-4.8Bi, Sn-3.8Ag-0.7Cu, and Sn-0.7Cu (wt.%), were measured on copper substrates at different temperatures. Measurements were performed using the sessile-drop method. Contact angles ranging from 30° to 40° after wetting under vacuum with no fluxes and between 10° and 30° with rosin mildly activated (RMA) and rosin activated (RA) fluxes were obtained. The Sn-3.5Ag-4.8Bi exhibited the lowest contact angles, indicating improved wettability with the addition of bismuth. For all soldering alloys, lower contact angles were observed using RMA flux. Intermetallics formed at the solder/Cu interface were identified as Cu₆Sn₅ adjacent to the solder and Cu₃Sn adjacent to the copper substrate. The Cu₃Sn intermetallic phase was generally not observed when RMA flux was used. The effect of temperature on contact angle was dependent on the type of flux used.     

Residual-mechanical behavior of thermomechanically fatigued Sn-Ag based solder joints

Thermomechanical fatigue (TMF) due to the mismatch in coefficients of thermal expansion between solder and substrate gradually degrades the mechanical properties of electronic solder joints during service. This study investigated the role of TMF on the residual-mechanical behavior of solder joints made with eutectic Sn-Ag solder and Sn-Ag solder with Cu or Ni additions. The TMF tests were carried out between −15°C and +150°C with a ramp rate of 25°C/min for the heating segment and 7°C/min for the cooling segment. The hold times were 20 min at the high extreme and 300 min at the low extreme. Residual shear strength was found to drop significantly during the first 250 TMF cycles, although it did remain relatively constant between 250 and 1000 cycles. Alloying elements were found to affect the residual creep strength of solder joints after TMF.     

The influence of dwell time on reliability of SMT solder joints under thermal cycling

The influence of dwell time on mechanical behaviour and fatigue life of SMT solder joints under thermal cycling has been investigated. The dwell time has two effects on the mechanical behaviour of SMT solder joints under thermal cycling: first, in the dwell time of high-temperature part, the stress in SMT solder joints will notably relax, and secondly, as the dwell time increase, the stress in solder joints in the low-temperature part of thermal cycling increases. With the increase of dwell time, the life of SMT solder joints under thermal cycling exponentially decreases.     

Stress relaxation behavior of composite and eutectic Sn-Ag solder joints

Stress relaxation experiments were carried out at 25 C and 150 C on 96.5Sn-3.5Ag eutectic solder and Sn-Ag composite solder joints (Sn-Ag eutectic solder with 20 vol.% Cu₆Sn₅ reinforcements incorporated by in-situ methods). The magnitude of the stress drop during relaxation depends primarily upon the plastic shear strain imposed prior to the stress relaxation process. For sequential stress relaxation experiments that include unloading, the stress drop is nearly independent of the accumulated plastic shear strain. However, for sequential stress relaxation that does not include unloading, the stress relaxation is more dependent upon the cumulative plastic shear strain history. The stress in single shear lap joints does not relax to zero stress, as is observed in stress relaxation of bulk tension specimens, even at 150 C. Creep strain rates extracted from the relaxation data were much lower with smaller pre-strains in both eutectic Sn-Ag and composite solder joints. The stress exponent values (n) calculated from the stress relaxation test data ranged from 7 to 15 for both eutectic and composite solder joints, which were consistent with conventional creep data. These stress-relaxation behaviors can be explained on the basis of dislocation recovery processes that occur during relaxation and when specimens are unloaded.