Microstructure and shear strength evolution of SnAg/Cu surface mount solder joint during aging

The effect of aging at 150°C on the microstructure and shear strength of SnAg/Cu surface mount solder joint has been investigated with comparison to 62Sn36Pb2Ag/Cu. It is found that the diffusion coefficient of intermetallic compounds at SnAg/Cu interface is smaller than that of intermetallic compounds at SnPbAg/Cu interface at 150°. The shear strength of SnAg solder joint is higher and decreases at a smaller rate during aging compared to that of SnPbAg solder joint. The fracture surface analysis shows that as the aging time increases, the fracture takes place along the solder/Cu₆Sn₅ interface with an extension toward the Sn−Cu intermetallic layer.     

Elevated temperature aging of solder joints based on Sn-Ag-Cu: Effects on joint microstructure and shear strength

The shear strength behavior and microstructural effects after aging for 100 h and 1,000 h at 150°C are reported for near-eutectic Sn-Ag-Cu (SAC) solder joints (joining to Cu) made from Sn-3.5Ag (wt.%) and a set of SAC alloys (including Co- and Fe-modified SAC alloys). All joints in the as-soldered and 100-h aged condition experienced shear failure in a ductile manner by either uniform shear of the solder matrix (in the strongest solders) or by a more localized shear of the solder matrix adjacent to the Cu₆Sn₅ interfacial layer, consistent with other observations. After 1,000 h of aging, a level of embrittlement of the Cu₃Sn/Cu interface can be detected in some solder joints made with all of the SAC alloys and with Sn-3.5Ag, which can lead to partial debonding during shear testing. However, only ductile failure was observed in all solder joints made from the Co- and Fe-modified SAC alloys after aging for 1,000 h. Thus, the strategy of modifying a strong (high Cu content) SAC solder alloy with a substitutional alloy addition for Cu seems to be effective for producing a solder joint that retains both strength and ductility for extended isothermal aging at high temperatures.     

Effect of burn-in on shear strength of 63Sn-37Pb solder joints on an Au/Ni/Cu substrate

A study was performed to examine the effect of burn-in and Au-plating thickness on the shear strength of 63Sn-37Pb solder joints in ball grid array (BGA) packages. The Au-plating thicknesses of 0.3 μm, 0.6 μm, 0.9 μm, and 1.4 μm were evaluated. An isothermal aging temperature of 150°C was employed to simulate burn-in conditions. The evolution of the bulk solder microstructure and intermetallic compounds at the solder joint interface were characterized and correlated to the measured shear strength. The strength of the solder joints with 0.3-μm Au plating was approximately three times higher than the thicker platings after aging. Solder joints with 0.3-μm Au plating failed within the solder matrix, and their strength was dependent upon the bulk solder microstructure and composition. The weakness of the solder joints with thicker Au platings was attributed to the formation of a brittle AuSn₄/Ni₃Sn₄ interface and a ductile Pb-rich layer at the interface.     

Shear strength of LED solder joints using SAC-nano Cu solder pastes

The addition of Cu nanoparticles into the solder pastes by mechanical mixing method creates a positive effect on the microstructure refinement of the LED solder joints. The grain size of β-Sn and Cu₆Sn₅ decrease obviously due to the increasing concentration of the nanoparticles in the solder pastes. However, the addition of nanoparticles facilitates the formation of voids in the solder joints, especially when the concentration of nanoparticles is higher than 0.5 wt% in the solder pastes. Both the microstructure refinement and void percentage affect the shear strength of the solder joints. Since the increase of the void percentage is limited when the concentration of nanoparticles increases from 0 to 0.5 wt%, the microstructure refinement shows a dominant effect on the shear performance and thus improves the shear strength of the solder joints from 49.8 to 55 MPa. Further addition of nanoparticles in the solder pastes leads to a sharp increase of the void percentage. Consequently, the shear strength of the solder joints decreases from 55 to 48.8 MPa when the concentration of doped particles increases from 0.5 to 1 wt% in the solder pastes.     

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.     

The effect of soldering process variables on the microstructure and mechanical properties of eutectic Sn-Ag/Cu solder joints

Fundamental understanding of the relationship among process, microstructure, and mechanical properties is essential to solder alloy design, soldering process development, and joint reliability prediction and optimization. This research focused on the process-structure-property relationship in eutectic Sn-Ag/Cu solder joints. As a Pb-free alternative, eutectic Sn-Ag solder offers enhanced mechanical properties, good wettability on Cu and Cu alloys, and the potential for a broader range of application compared to eutectic Sn-Pb solder. The relationship between soldering process parameters (soldering temperature, reflow time, and cooling rate) and joint microstructure was studied systemati-cally. Microhardness, tensile shear strength, and shear creep strength were measured and the relationship between the joint microstructures and mechani-cal properties was determined. Based on these results, low soldering tempera-tures, fast cooling rates, and short reflow times are suggested for producing joints with the best shear strength, ductility, and creep resistance.     

Microstructure and shear strength of Sn37Pb/Cu solder joints subjected to isothermal aging

The effects of isothermal aging on the microstructure and shear strength of Sn37Pb/Cu solder joints were investigated. Single-lap shear solder joints of eutectic Sn37Pb solder were aged for 1–10 days at 120 °C and 170 °C, respectively, and then loaded to failure in shear with a constant loading speed of 5 × 10⁻³ mm/s. The growth of the interfacial Cu–Sn intermetallic compounds (IMC) layer (Cu₆Sn₅ + Cu₃Sn) of Sn37Pb/Cu solder joints subjected to isothermal aging exhibited a linear function of the square root of aging time, indicating that the formation of Cu–Sn IMC was mainly controlled by the diffusion mechanism. And the diffusion coefficient (D) values of IMC layer were 1.07 × 10⁻¹⁷ and 3.72 × 10⁻¹⁷ m²/s for aged solder joints at 120 °C and 170 °C, respectively. Shear tests results revealed that as-reflowed solder joint had better shear strength than the aged solder joints and the shear strength of all aged solder joints decreased with increasing aging time. The presence of elongated dimple-like structures on the fracture surfaces of these as-reflowed or aged for short time solder joints were indicative of a ductile failure mode. As aging time further increased, the solder joints fractured in the mixed solder/IMC mode at the solder/IMC interface.     

热时效和焊点尺寸对SnAgCu微焊点强度的影响

针对高度为100～300μm的无铅钎料Sn-3.0Ag-0.5Cu微焊点,研究了等温热时效和焊点尺寸对其在100℃下拉伸强度的影响。结果表明,保持焊点直径不变时,高度为100,200和300μm微焊点未经热时效的平均拉伸强度分别为53.75,46.59和44.38MPa;热时效时间延长使微焊点内钎料合金显微组织明显粗化,导致焊点拉伸强度降低,前述三种高度的微焊点96h热时效后平均拉伸强度分别为44.13,38.38和33.48MPa,但96h热时效对IMC厚度无明显影响。     

Suppression of void coalescence in thermal aging of tin-silver-copper-X solder joints

Addressing the potential for drop impact failure of Pb-free interconnects, the shear ductility after extensive aging of Sn-Ag-Cu (SAC) solders has been improved radically by Co or Fe modifications. Several other SAC+X candidates (X=Mn, Ni, Ge, Ti, Si, Cr, and Zn) now have been tested. Solder joint microstructures and shear strength results show that new SAC+X alloys also suppress void formation and coalescence at the Cu (substrate)/Cu₃Sn interface (and embrittlement) after aging at 150°C for up to 1,000 h. Microprobe measurements of 1,000 h aged samples suggest that Cu substitution by X is usually accentuated in the intermetallic layers, consistent with X=Co and Fe results.     

Local creep in SnAg3.8Cu0.7 lead-free solder

In this study, local creep of lead-free solder SnAg3.8Cu0.7 has been investigated for the first time by using surface markers prepared by focused ion beam (FIB). The test setup was optimized with respect to the applied shear stresses. Two different microstructures have been investigated at 30°C, 50°C, and 80°C, respectively. A high sensitivity of the steady-state creep rate to the microstructure has been demonstrated. The shear deformation was observed to be inhomogeneous and concentrated in specific areas. Local dislocation bands dominate the solder deformation.     

The effect of aging on microstructure,room temperature deformation,and fracture of Sn-Bi/Cu solder joints

The effects of isothermal aging on the microstructure and mechanical behavior of Sn-Bi/Cu solder joints are reported. Lap shear solder joints of eutectic Sn-Bi solder were aged for 3 to 30 days at 80°C and then loaded to failure in shear. Changes in the joint microstructure including interphase coarsening, intermetallic growth, and evolution of the intermetallic/solder interface are documented. The aging experiments reveal the segregation of the Bi-rich phase of the solder to the intermetallic/solder interface. The ultimate shear strength and ductility of the joints are reported at strain rates of 4.0 × 10⁻¹ to 4.0 × 10⁻⁵ S⁻¹ for 3 and 30 days aging. The strength of the joints decreases with strain rate for both aging conditions; the ductility is low and independent of strain rate for the joints aged three days and increases considerably with reduced strain rate for joints, aged 30 days. Fractographs and cross sections of the failed joints detail the effect of aging on the fracture mechanism.     

The microstructure of ultrafine eutectic Au-Sn solder joints on Cu

The present work studies the microstructure and microstructural evolution of small volumes of nominally eutectic Au-Sn solder on Cu. The study includes solder bumps 140–145 μm in diameter and 55–65 μm tall deposited on Cu-plated Si, and solder joints 60 μm in diameter and 25 μm in height that join Cu-plated ceramic and polymide substrates. The results show that the microstructure is strongly affected by the addition of Cu from the substrate during reflow, which produces a thick intermetallic layer along the interface. In the case of the joints, normal processing produces a coarse microstructure that includes only a few grains between thick intermetallic coatings. Aging at high temperature causes a further monotonic increase in Cu content, which alters the intermetallic structure at the interfaces and can lead to intermetallic bridging across the joint. Thermal fatigue tests suggest that cyclic deformation breaks up the intermetallic structure, increasing the rate of Cu addition to the joint, but refining the apparent grain size.     

Sn-3.0Ag-0.5Cu/Ni/Cu微焊点剪切强度与断口的研究

用直径为200~500μm的Sn-3.0Ag-0.5Cu无铅焊球分别在Ni和Cu焊盘上制作焊点,并对焊后和时效200h后的焊点进行剪切测试,并采用SEM观察剪切断口形貌。结果表明,焊后和时效200 h后焊点接头的剪切强度都随焊球尺寸增大而减小。焊后断口处韧窝形状为抛物线型,断裂方式为韧性断裂;随着焊球尺寸的增大,剪切断口处的韧窝数量增多,韧窝的变小变浅。时效200 h后,韧窝变浅,趋于平坦,韧窝数量也明显减少,材料的韧性下降,脆性增加,断裂方式由韧性向脆性发生转变。     

Effect of phosphorus content on Cu/Ni-P/Sn-3.5Ag solder joint strength after multiple reflows

Electroless Ni-P layers with three different P contents (6.1 wt.%, 8.8 wt.%, and 12.3wt.%) were deposited on copper (Cu) substrates. Multilayered samples of Sn-3.5Ag/Ni-P/Cu stack were prepared and subjected to multiple reflows at 250°C. A tensile test was performed to investigate the effect of P content on the solder joint strength. The low P samples exhibited the highest joint strength after multiple reflows, while the strength of medium and high P samples decreased more rapidly. From interfacial analysis, the Ni₃Sn₄ intermetallic compound (IMC) formed at the interface of low P sample was found to be more stable, while the one of medium and high P samples spalled into the molten solder. The IMC spallation sped up the consumption of electroless Ni-P, leading to the large formation of Cu-Sn IMCs. Fractographic and microstructural analyses showed that the degradation in solder joint strength was due to the formation of layers of voids and growth of Cu-Sn IMCs between the solder and the Cu substrate.     

A Study on the Thermal Reliability of Cu/SnAg Double-Bump Flip-Chip Assemblies on Organic Substrates

The Cu/SnAg double-bump structure is a promising candidate for fine-pitch flip-chip applications. In this study, the interfacial reactions of Cu (60 μm)/SnAg (20 μm) double-bump flip chip assemblies with a 100 μm pitch were investigated. Two types of thermal treatments, multiple reflows and thermal aging, were performed to evaluate the thermal reliability of Cu/SnAg flip-chip assemblies on organic printed circuit boards (PCBs). After these thermal treatments, the resulting intermetallic compounds (IMCs) were identified with scanning electron microscopy (SEM), and the contact resistance was measured using a daisy-chain and a four-point Kelvin structure. Several types of intermetallic compounds form at the Cu column/SnAg solder interface and the SnAg solder/Ni pad interface. In the case of flip-chip samples reflowed at 250°C and 280°C, Cu₆Sn₅ and (Cu, Ni)₆Sn₅ IMCs were found at the Cu/SnAg and SnAg/Ni interfaces, respectively. In addition, an abnormal Ag₃Sn phase was detected inside the SnAg solder. However, no changes were found in the electrical contact resistance in spite of severe IMC formation in the SnAg solder after five reflows. In thermally aged flip-chip samples, Cu₆Sn₅ and Cu₃Sn IMCs were found at the Cu/SnAg interface, and (Cu, Ni)₆Sn₅ IMCs were found at the SnAg/Ni interface. However, Ag₃Sn IMCs were not observed, even for longer aging times and higher temperatures. The growth of Cu₃Sn IMCs at the Cu/SnAg interface was found to lead to the formation of Kirkendall voids inside the Cu₃Sn IMCs and linked voids within the Cu₃Sn/Cu column interfaces. These voids became more evident when the aging time and temperature increased. The contact resistance was found to be nearly unchanged after 2000 h at 125°C, but increases slightly at 150°C, and a number of Cu/SnAg joints failed after 2000 h. This failure was caused by a reduction in the contact area due to the formation of Kirkendall and linked voids at the Cu column/Cu₃Sn IMC interface.     

Effects of microstructural evolution and intermetallic layer growth on shear strength of ball-grid-array Sn-Cu solder joints

The shear strength of ball-grid-array (BGA) solder joints on Cu bond pads was studied for Sn-Cu solder containing 0, 1.5, and 2.5 wt.% Cu, focusing on the effect of the microstructural changes of the bulk solder and the growth of intermetallic (IMC) layers during soldering at 270°C and aging at 150°C. The Cu additions in Sn solder enhanced both the IMC layer growth and the solder/IMC interface roughness during soldering but had insignificant effects during aging. Rapid Cu dissolution from the pad during reflow soldering resulted in a fine dispersion of Cu₆Sn₅ particles throughout the bulk solder in as-soldered joints even for the case of pure Sn solder, giving rise to a precipitation hardening of the bulk solder. The increased strength of the bulk solder caused the fracture mode of as-soldered joints to shift from the bulk solder to the solder/IMC layer as the IMC layer grew over a critical thickness about 1.2 m for all solders. The bulk solder strength decreased rapidly as the fine Cu₆Sn₅ precipitates coarsened during aging. As a consequence, regardless of the IMC layer thickness and the Cu content of the solders, the shear strength of BGA solder joints degraded significantly after 1 day of aging at 150°C and the shear fracture of aged joints occurred in the bulk solder. This suggests that small additions of Cu in Sn-based solders have an insignificant effect on the shear strength of BGA solderjoints, especially during system use at high temperatures.     

等温时效对SnSb4.5CuNi/Cu焊接接头力学性能的影响

SnSb4.5CuNi/Cu焊点在175℃进行等温时效,分析了不同时效时间的SnSb4.5CuNi/Cu焊点中金属间化合物(IMC)组织形貌演变,通过纳米压痕法测量SnSb4.5CuNi/Cu焊点界面IMC的硬度和弹性模量,对焊接接头进行拉伸强度和低周疲劳测试。结果表明,时效48 h的焊缝中Cu6Sn5呈曲率半径均匀的半圆扇贝状特征,IMC的弹性模量与铜基板很接近,在恒幅对称应变条件下焊点的抗低周疲劳的性能最佳,焊点的抗拉强度高;当时效时间大于48 h,焊接接口的抗疲劳性能和抗拉伸强度逐渐变差。     

Creep deformation of Sn-3.5Ag-xCu and Sn-3.5Ag-xBi solder joints

Creep properties of lead-free Sn-3.5Ag-based alloys with varying amounts of Cu or Bi were studied by single lap-shear test. Solder balls with five different compositions of Cu (0 wt.%, 0.75 wt.%, 1.5 wt.%) and Bi (2.5 wt.%, 7.5 wt.%) were reflowed on Cu. The Cu-containing alloy had a lower creep rate than the Bi-containing alloy. The Sn-3.5Ag alloy showed the lowest creep rate on Cu, implying that the Cu element already dissolved in the Sn-3.5Ag alloy during reflow. The Cu-containing alloy was strengthened by dispersed small precipitates of Cu₆Sn₅. As the Cu content increased up to 1.5 wt.%, the Cu₆Sn₅ coarsened and platelike Ag₃Sn intermetallics were found, which deteriorated the creep resistance.     

Electromigration study in SnAg3.8Cu0.7 solder joints on Ti/Cr-Cu/Cu under-bump metallization

This paper investigates the electromigration-induced failures of SnAg3.8Cu0.7 flip-chip solder joints. An under-bump metallization (UBM) of a Ti/Cr-Cu/Cu trilayer was deposited on the chip side, and a Cu/Ni(P)/Au pad was deposited on the BT board side. Electromigration damages were observed in the bumps under a current density of 2×10⁴ A/cm² and 1×10⁴ A/cm² at 100°C and 150°C. The failures were found to be at the cathode/chip side, and the current crowding effect played an important role in the failures. Copper atoms were found to move in the direction of the electron flow to form intermetallic compounds (IMCs) at the interface of solder and pad metallization as a result of current stressing.     

Aging study on interfacial microstructure and solder-ball shear strength of a wafer-level chip-size package with Au/Ni metallization on a Cu pad

Eutectic solder balls (63Sn-37Pb) joined to Cu pads with an Au/Ni metallization have been widely used in wafer-level chip-size package (WLCSP) technology for providing electrical and mechanical interconnections between components. However, some reliability issues must be addressed regarding the intermetallic compounds (IMCs). The formation of a brittle IMC layer between the solder/Cu pad interface impacts considerably upon the solder-ball shear strength. In addition, it will degrade the long-term operating reliability of the WLCSP. This study investigates, by means of experiments, the growth of the IMC layer under isothermal aging for the eutectic Sn-Pb solder reflowed on a Cu pad with an Au/Ni metallization. Forming the Cu pad with an Au/Ni metallization was achieved by a simple semiconductor-manufacturing process. The effects of the intermetallic layer on solder-ball shear strength were examined for various parameters, including the thickness of the Au layer, solder-ball size, and the diameter of the Cu pad. Experimental results indicate that two IMC layers, Au_0.5Ni_0.5Sn₄ and Ni₃Sn₄, form at the solder/Cu pad interface after aging. The Au_0.5Ni_0.5Sn₄ intermetallic layer dominates the total thickness of the IMC layer and grows with aging time while the solder-ball shear strength decreases after aging. The degradation of the solder-ball shear strength was found to be caused mainly by the formation of the Au_0.5Ni_0.5Sn₄ layer. The experimental results established that a thinner Au layer on Cu pad can effectively control the degradation of solder-ball shear strength, and this is especially true for smaller ball sizes.