Alloying effects in near-eutectic Sn-Ag-Cu solder alloys for improved microstructural stability

This study included a comparison of the baseline Sn-3.5Ag eutectic to one near-eutectic ternary alloy, Sn-3.6 Ag-1.0Cu and two quaternary alloys, Sn-3.6Ag-1.0Cu-0.15Co and Sn-3.6Ag-1.0 Cu-0.45 Co, to increase understanding of the beneficial effects of Co on Sn-Ag-Cu solder joints cooled at 1–3 C/sec, typical of reflow practice. The results indicated that joint microstructure refinement is due to Co-enhanced nucleation of the Cu₆Sn₅ phase in the solder matrix, as suggested by Auger elemental mapping and calorimetric measurements. The Co also reduced intermetallic interface faceting and improved the ability of the solder joint samples to maintain their shear strength after aging for 72 hr at 150 C. The baseline Sn-3.5Ag joints exhibited significantly reduced strength and coarser microstructures.     

Phase equilibria and the related properties of Sn-Ag-Cu based Pb-free solder alloys

We have recently developed a thermodynamic database for micro-soldering alloys which consists of the elements Pb, Bi, Sn, Sb, Cu, Ag, Zn, and In. In this paper, the phase equilibria and the related thermodynamic properties of the Sn-Ag-Cu base alloys are presented using this database, alloy systems being one of the promising candidates for Pb-free solders. The isothermal section diagrams of the Sn-Ag-Cu ternary system were experimentally determined by SEM-EDS, x-ray diffraction and metallographic techniques. Based on the present results as well as the previous data on phase boundaries and thermochemical properties, thermodynamic assessment of this system was carried out. The isothermal and vertical section diagrams, liquidus surface, mass fractions of the phase constitution, etc., were calculated. The predictions of surface energy and viscosity were also investigated. Moreover, a non-equilibrium solidification process using the Scheil model was simulated and compared with the equilibrium solidification behavior in some Sn-Ag-Cu base alloys. Calculated results based on the Scheil model were incorporated into a three-dimensional solidification simulation and the prediction of practical solidification procedures was performed.     

Isothermal aging of near-eutectic Sn-Ag-Cu solder alloys and its effect on electrical resistivity

Solder joints were prepared from seven eutectic and near-eutectic Sn-based compositions and characterized for electrical resistivity after 100 h and 1,000 h of isothermal aging at 423 K. The solder joint samples were prepared by hand soldering to copper substrates, and the post-heat treatment resistivity was measured at room temperature in a specially designed, four-point probe fixture. Compositions tested included Sn-3.5Ag, Sn-3.7Ag-0.9Cu, Sn-3.0Ag-0.5Cu, Sn-3.6Ag-1.0Cu, and Sn-3.9Ag-0.6Cu. In addition, the effect of a minor addition of a fourth element, designed to improve high-temperature shear strength, was also evaluated in the compositions Sn-3.7Ag-0.6Cu-0.3Co and Sn-3.7Ag-0.7Cu-0.2Fe. The observed changes in electrical resistivity are discussed in terms of microstructural coarsening, diffusional transport from the substrate, and nucleation of precipitate phases.     

Effect of heat treatment on the electrical resistivity of near-eutectic Sn-Ag-Cu Pb-Free solder alloys

The electrical resistivity of solder joints prepared from Sn-Ag, Sn-Ag-Cu, and Sn-Ag-Cu-X alloys (where X = Co, Fe, or Bi) was characterized by a four-point probe technique and interpreted in terms of microstructure and composition. The resistivity is also reported of drawn solid wires of these alloys. The solder-joint samples were prepared by hand soldering to copper substrates and were electrically characterized over a temperature range from 293–423 K, covering the anticipated range of elevated-temperature operation for Pb-free solders. Selected joint specimens were measured before and after a 72-h heat treatment at 423 K. Metallographic inspection of the solder joints was performed to characterize coarsening effects and to determine the degree to which these changes affected electrical conduction.     

Microstructural development of Sn-Ag-Cu solder joints

The effects of Cu as pad material and of the metallization of pad (with Sn) and component (with Ni) on the evolving microstructure of lead-free solder joints were studied. A solder paste with composition 95.5wt.%Sn-4.0wt.%Ag-0.5wt.%Cu was used. Partial dissolution of the Cu substrate led to a change in the overall composition of the solder, which caused a precipitation morphology different from the one expected regarding the initial composition. Kinetics of growth of the Cu₆Sn₅ phase, as particles in the bulk of the solder and as a reaction layer adjacent to the Cu pad, was studied in the temperature range 125–175°C.     

A viable tin-lead solder substitute: Sn-Ag-Cu

Rising concern over the use of lead in industry provides a driving force for the development of improved lead-free industrial materials. Therefore, a new lead-free base solder alloy Sn-4.7Agl.7Cu (wt.%) has been developed upon which a family of lead-free solders can be based. This solder alloy exhibits a ternary eutectic reaction at 216.8 ± 1°C (L ↠ η+ ϕ + β-Sn; η = Cu₆Sn₅, θ = Ag₃Sn). Preliminary tests of solderability demonstrate intermetallic phase formation on model solder joint interfaces and good wettability in a fluxed condition suggest technological viability and motivates much more extensive study of this solder alloy.     

The growth of intermetallic compounds at Sn-Ag-Cu solder/Cu and Sn-Ag-Cu solder/Ni interfaces and the associated evolution of the solder microstructure

The evolution of intermetallics at and near SnAgCu/Cu and SnAgCu/Ni interfaces was examined, and compared to the behavior, near PbSn/metal and Sn/metal interfaces. Two different solder compositions were considered, Sn_93.6Ag_4.7Cu_1.7 and Sn_95.5Ag_3.5Cu_1.0 (Sn_91.8Ag_5.1 Cu_3.1 and Sn_94.35Ag_3.8Cu_1.85 in atomic percent). In both cases, phase formation and growth at interfaces with Cu were very similar to those commonly observed for eutectic SnPb solder. However, the evolution of intermetallics at SnAgCu/Ni interfaces proved much more complex. The presence of the Cu in the solder dramatically altered the phase selectivity at the solder/Ni interface and affected the growth kinetics of intermetallics. As long as sufficient Cu was available, it would combine with Ni and Sn to form (Cu,Ni)₆)Sn₅ which grew instead of the Ni₃Sn₄ usually observed in PbSn/Ni and Sn/Ni diffusion couples. This growing phase would, however, eventually consume essentially all of the available Cu in the solder. Because the mechanical properties of Sn-Ag-Cu alloys, depend upon the Cu content, this consumption can be expected to alter the mechanical properties of these Pb-free solderjoints. After depletion of the Cu from the solder, further annealing then gradually transformed the (Cu,Ni)₆Sn₅ phase into a (Ni,Cu)₃Sn₄ phase.     

Growth of Sn and intermetallic compounds in Sn-Ag-Cu solder

The microstructure of the Sn-Ag-Cu solder is examined by optical microscopy and scanning electron microscopy (SEM) for various compositions near the ternary eutectic for different cooling rates from the solder melt. Focus is on the size and orientation of Sn grains as indicated by cross-polarized, light optical microscopy, and pole figures from x-ray diffraction. We find that both composition and cooling rate have strong influences on Sn grain size, with Sn grain size increasing an order of magnitude as Cu concentration increases from 0% to 1.1%. Cyclic growth twinning, with twinning angles near 60°, is observed in Sn-Ag-Cu alloys near the composition Sn-3.9Ag-0.6Cu.     

无铅的锡-银-铜焊料的发展——第二代低银含量SnAgCu体系

概要地评述了无铅焊料中低银含量的锡-银-铜（SnAgCu）体系的发展方向。由于高银含量的锡-银-铜（SnAgCu）体系存在着成本高和耐跌落（摔）性差的问题,它将被低银含量的锡-银-铜（SnAgCu）体系所取代。在低银含量的锡-银-铜（SnAgCu）体系中加入某些微量添加剂可以达到锡-铅焊料的性能水平。     

The solid solubility of Ag and Cu in the Sn phase of eutectic and near-eutectic Sn-Ag-Cu solder alloys

Early studies of Ag-Sn and Cu-Sn binary alloys showed very low values, 0.04 wt.% for Ag and 0.0063 wt.% for Cu, for the solid solubility of these elements in Sn at the eutectic temperature. In recent work on “as-cast” Sn-Ag-Cu solder alloys, much higher values have been reported for the Ag and Cu content of the Sn phase. In the present study, wavelength dispersive x-ray microprobe measurements made on a near-equilibrium sample confirmed the earlier solubility values. It was concluded that higher values, some of which are reported in the current paper, represent nonequilibrium, supersaturated solid solutions.     

Phase equilibria studies of Sn-Ag-Cu eutectic solder using differential cooling of Sn-3.8Ag-0.7Cu alloys

This paper is a study of the phase equilibria of the Sn-3.8Ag-0.7Cu alloy investigated by a differential cooling method. The difficulty in assessing phase equilibria of the Sn-Ag-Cu (SAC) system because of the insufficient resolution of conventional characterization techniques is solved by inducing preferential growth of a solid phase in a melt by holding the alloy at the solid-liquid phase-equilibrium field. Application of the technique to Sn-3.8Ag-0.7Cu with varying holding temperatures yielded results that the alloy is slightly off eutectic composition. The phase-formation sequence of the alloy during solidification was found to be Ag₃Sn, β-Sn, and finally the ternary eutectic microstructure.     

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.     

Intermetallic growth studies on Sn-Ag-Cu lead-free solder joints

Solid-state intermetallic compound (IMC) growth behavior plays an important role in solder joint reliability of electronic packaging assemblies. The morphology and growth of interfacial IMC compounds between 95.5Sn-3.8Ag-0.7Cu Pb-free solders and nickel/gold (Ni/Au) surface finish on BGA solder joint specimen is reported. Digital imaging techniques were employed in the measurement of the average IMC growth thickness. The IMC growth behavior subjected to isothermal aging exposure at 125°C, thermal cycling (TC), and thermal shock (TS) with upper soak temperatures of 125°C are compared. An equivalent isothermal aging time is proposed for comparison of IMC layer growth data. It was noted that IMC layer growth under thermal cycling and thermal shock aging gives an acceleration factor of 1.4 and 2.3 based on the equivalent isothermal aging time.     

Interfacial behaviors of Sn-Pb,Sn-Ag-Cu Pb-free and mixed Sn-Ag-Cu/Sn-Pb solder joints during electromigration

SnPb-SnAgCu mixed solder joints with Sn-Pb soldering Sn-Ag-Cu Pb-free components are inevitably occurred in the high reliability applications. In this study, the interfacial behaviors in Sn-37Pb and Sn-3.0Ag-0.5Cu mixed solder joints was addressed and compared with Sn-37Pb solder joints and Sn-3.0Ag-0.5Cu solder joints with the influence from isothermal aging and electromigration. Considering the difference on the melting point between Sn-3.0Ag-0.5Cu and Sn-37Pb solder, two mixed solder joints: partial mixing and full mixing between Sn-Pb and Sn-Ag-Cu solders were reached with the peak reflowing temperature of 190 and 250 °C, respectively. During isothermal aging, the intermetallic compound (IMC) layer increased with aging time and its growth was diffusion controlled. There was also no obvious affect from the solder composition on IMC growth. After electromigration with the current density of 2.0 × 10³ A/cm², Sn-37Pb solder joints showed the shortest lifetime with the cracks observed at the cathode for the stressing time < 250 h. In Sn-3.0Ag-0.5Cu Pb-free solder joints, current stressing promoted the growth of IMC layer at the interfaces, but the growing rate of IMC at the anode interface was far faster than that at the cathode interface. Therefore, there existed an obvious polarity effect on IMC growth in Sn-Ag-Cu Pb-free solder joints. After Sn-37Pb was mixed with Sn-3.0Ag-0.5Cu Pb-free solder, whether the partial mixing or the full mixing between Sn-Pb and Sn-Ag-Cu can obviously depress both the crack formation at the cathode side and the IMC growth at the anode.     

Wettability of electroplated Ni-P in under bump metallurgy with Sn-Ag-Cu solder

Nickel plating has been used as the under bump metallurgy (UBM) in the microelectronics industry. In this study, the electroplating process was demonstrated to be a good alternative approach to produce the Ni-P layer as UBM. The wettability of several commercial solder pastes, such as Sn-3.5Ag, Sn-37Pb, and Sn-3Ag-0.5Cu solder, on electroplated Ni-P with various phosphorous contents (7 wt.%, 10 wt.%, and 13 wt.%) was investigated. The role of phosphorus in the wettability was probed. The surface morphology and surface roughness in electroplated Ni-P was observed with the aid of both field emission scanning electron microscope (SEM) and atomic force microscope (AFM). The correlation between wettability and phosphorus contents in electroplated Ni-P was evaluated. As the phosphorous contents increased, the surface morphology of the Ni-P deposit was smoother and surface roughness of Ni-P became smaller. The improvement of surface morphology and surface roughness enhanced the wettability of electroplated Ni-P. The interfacial reaction between lead-free solder and electroplating Ni-P UBM was also investigated.     

Rate-dependent properties of Sn-Ag-Cu based lead-free solder joints for WLCSP

The increasing demand for portable electronics has led to the shrinking in size of electronic components and solder joint dimensions. The industry also made a transition towards the adoption of lead-free solder alloys, commonly based around the Sn-Ag-Cu alloys. As knowledge of the processes and operational reliability of these lead-free solder joints (used especially in advanced packages) is limited, it has become a major concern to characterise the mechanical performance of these interconnects amid the greater push for greener electronics by the European Union.In this study, bulk solder tensile tests were performed to characterise the mechanical properties of SAC 105 (Sn-1%wt Ag-0.5%wt Cu) and SAC 405 (Sn-4%wt Ag-0.5%wt Cu) at strain rates ranging from 0.0088 s⁻¹ to 57.0 s⁻¹. Solder joint array shear and tensile tests were also conducted on wafer-level chip scale package (WLCSP) specimens of different solder alloy materials under two test rates of 0.5 mm/s (2.27 s⁻¹) and 5 mm/s (22.73 s⁻¹). These WLCSP packages have an array of 12 × 12 solder bumps (300 μm in diameter); and double redistribution layers with a Ti/Cu/Ni/Au under-bump metallurgy (UBM) as their silicon-based interface structure.The bulk solder tensile tests show that Sn-Ag-Cu alloys exhibit higher mechanical strength (yield stress and ultimate tensile strength) with increasing strain rate. A rate-dependent model of yield stress and ultimate tensile strength (UTS) was developed based on the test results. Good mechanical performance of package pull-tests at high strain rates is often correlated to a higher percentage of bulk solder failures than interface failures in solder joints. The solder joint array tests show that for higher test rates and Ag content, there are less bulk solder failures and more interface failures. Correspondingly, the average solder joint strength, peak load and ductility also decrease under higher test rate and Ag content. The solder joint results relate closely to the higher rate sensitivity of SAC 405 in gaining material strength which might prove detrimental to solder joint interfaces that are less rate sensitive. In addition, specimens under shear yielded more bulk solder failures, higher average solder joint strength and ductility than specimens under tension.     

Apple iMac MA199LL台式电脑拆解分析

iMacMA199LL是苹果电脑Mac系列中第一个采用Intel处理器的成员。它采用1440×900宽屏17英寸LCD显示器,内置1.83GHzIntelCoreDuo双核处理器,该处理器带有2MB二级缓存,其FSB(前端总线)频率为667MHz。iMacMA199LL的其他特点包括:512MBRAM、ATIRadeon×1600显卡(带128MBGDDR3内存)、160GB串行ATA硬驱,内置以太网、Wi-Fi和蓝牙接口。下面的拆解统计了不同集成部分在总成本中所占的比例,并在下面的列表中给出了部分器件的一些详细信息,包括产品型号、生产厂商、封装形式、引脚及数据手册的网站链接地址等,希望能给国内的电子…     

Microstructural modifications and properties of Sn-Ag-Cu solder joints induced by alloying

Slow cooling (1–3°C/sec) of Sn-Ag-Cu and Sn-Ag-Cu-X (X = Fe, Co) solder-joint specimens, made by hand soldering, simulated reflow in a surface-mount assembly to achieve similar as-solidified joint microstructures for realistic shear-strength testing, using Sn-3.5Ag (wt.%) as a baseline. Consistent with predictions from a recent Sn-Ag-Cu ternary phase-diagram study, either Sn dendrites, Ag₃Sn primary phase, or Cu₆Sn₅ primary phase were formed during solidification of joint samples made from the selected near-eutectic Sn-Ag-Cu alloys. Minor substitution of Co for Cu in Sn-3.7Ag-0.9Cu refined the joint-matrix microstructure by an apparent catalysis effect on the Cu₆Sn₅ phase, whereas Fe substitution promoted extreme refinement of the Sn-dendritic phase. Ambient-temperature shear strength was reduced by Sn dendrites in the joint microstructure, especially coarse dendrites in solute poor Sn-Ag-Cu, e.g., Sn-3.0Ag-0.5Cu, while Sn-3.7Ag-0.9Cu with Co and Fe additions have increased shear strength. At elevated (150°C) temperature, no significant difference exists between the maximum shear-strength values of all of the alloys studied.     

Sn－Zn－In软钎料合金初步研究

对Ｓｎ－Ｚｎ－Ｉｎ钎料合金的性能进行了研究，钎料铺展性和剪切强度试验结果表明，在Ｓｎ－９Ｚｎ－Ｉｎ软钎料合金中，随Ｉｎ含量增加，铺展面积增大，钎焊接头剪切强度降低。钎料熔点和接头组织等性能的综合分析结果表明Ｓｎ－９Ｚｎ－１０Ｉｎ的性能已接近或超过传统的Ｓｎ－Ｐｂ共晶