Intermetallic formation in Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with immersion Ag surface finish

The intermetallic compounds formed in Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Ag/Cu pads are investigated. After reflow, scallop-shaped η-Cu₆Sn₅ and continuous planar η-(cu_0.9Ni_0.1)₆Sn₅ intermetallics appear at the interfaces of the Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder joints, respectively. In the case of the Sn3Ag0.5Cu specimens, an additional ε-Cu₃Sn intermetallic layer is formed at the interface between the η-Cu₆Sn₅ and Cu pads after aging at 150°C, while the same type of intermetallic formation is inhibited in the Sn3Ag0.5Cu0.06Ni0.01Ge packages. In addition, the coarsening of Ag₃Sn precipitates also abates in the solder matrix of the Sn3Ag0.5Cu0.06Ni0.01Ge packages, which results in a slightly higher ball shear strength for the specimens.     

Intermetallic reactions in Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Au/Ni surface finishes

The intermetallic compounds (IMCs) formed during the reflow and aging of Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Au/Ni surface finishes were investigated. After reflow, the thickness of (Cu, Ni, Au)₆Sn₅ interfacial IMCs in Sn3Ag0.5Cu0.06Ni0.01Ge was similar to that in the Sn3Ag0.5Cu specimen. The interiors of the solder balls in both packages contained Ag₃Sn precipitates and brick-shaped AuSn₄ IMCs. After aging at 150°C, the growth thickness of the interfacial (Ni, Cu, Au)₃Sn₄ intermetallic layers and the consumption of the Ni surface-finished layer on Cu the pads in Sn3Ag0.5Cu0.06Ni0.01Ge solder joints were both slightly less than those in Sn3Ag0.5Cu. In addition, a coarsening phenomenon for AuSn₄ IMCs could be observed in the solder matrix of Sn3Ag0.5Cu, yet this phenomenon did not occur in the case of Sn3Ag0.5Cu0.06Ni0.01Ge. Ball shear tests revealed that the reflowed Sn3Ag0.5Cu0.06Ni0.01Ge packages possessed bonding strengths similar to those of the Sn3Ag0.5Cu. However, aging treatment caused the ball shear strength in the Sn3Ag0.5Cu packages to degrade more than that in the Sn3Ag0.5Cu0.06Ni0.01Ge packages.     

Wetting and reaction of Sn-2.8Ag-0.5Cu-1.0Bi solder with Cu and Ni substrates

The wettability of newly developed Sn-2.8Ag-0.5Cu-1.0Bi lead-free solder on Cu and Ni substrates was assessed through the wetting balance tests. The wettability assessment parameters such as contact angle (ϑ_c) and maximum wetting force (F_w) were documented for three solder bath temperatures with three commercial fluxes, namely, no-clean (NC), nonactivated (R), and water-soluble organic acid flux (WS). It was found that the lead-free Sn-2.8Ag-0.5Cu-1.0Bi solder exhibited less wetting force, i.e., poorer wettability, than the conventional Sn-37Pb solder for all flux types and solder bath temperatures. The wettability of Sn-2.8Ag-0.5Cu-1.0Bi lead-free solder on Cu substrate was much higher than that on Ni substrate. Nonwetting for Sn-2.8Ag-0.5Cu-1.0Bi and Sn-Pb solders on Ni substrate occurred when R-type flux was used. A model was built and simulations were performed for the wetting balance test. The simulation results were found very close to the experimental results. It was also observed that larger values of immersion depth resulted in a decrease of the wetting force and corresponding meniscus height, whereas the increase in substrate perimeter enhanced the wettability. The wetting reactions between the solder and Cu/Ni substrates were also investigated, and it was found that Cu atoms diffused into the solder through the intermetallic compounds (IMCs) much faster than did the Ni atoms. Rapid formation of IMCs inhibited the wettability of Sn-2.8Ag-0.5Cu-1.0Bi solder compared to the Sn-Pb solder.     

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。     

Interfacial microstructure and shear strength of Ag nano particle doped Sn-9Zn solder in ball grid array packages

Sn-9Zn solder joints containing Ag nano particles were prepared by mechanically mixing Ag nano particles (0.3, 0.5 and 1 wt%) with Sn-9Zn solder paste. In the monolithic Sn-Zn solder joints, scallop-shaped AuZn₃ intermetallic compound layers were found at their interfaces. However, after the addition of Ag nano particles, an additional uniform AgZn₃ intermetallic compound layer well adhered to the top surface of the AuZn₃ intermetallic compound layer was found. In addition, in the solder ball region, fine spherical-shaped AgZn₃ intermetallic compound particles were observed as well as Zn-rich and β-Sn phases. With the addition of Ag nano particles, the shear strengths consistently increased with an increase in the Ag nano particle content due to the uniform distribution of fine AgZn₃ intermetallic compound particles. The shear strength of monolithic Sn-Zn and 1 wt% Ag nano particle content Sn-Zn solder joints after one reflow cycle were about 42.1 MPa and 48.9 MPa, respectively, while their shear strengths after eight reflow cycles were about 39.0 MPa and 48.4 MPa, respectively. The AgZn₃ IMCs were found to be uniformly distributed in the β-Sn phase for Ag particle doped Sn-9Zn composite solder joints, which result in an increase in the tensile strength, due to a second phase dispersion strengthening mechanism. The fracture surface of monolithic Sn-Zn solder exhibited a brittle fracture mode with a smooth surface while Sn-Zn solder joints containing Ag nano particles showed a typical ductile failure with very rough dimpled surfaces.     

Intermetallic compounds formed during the reflow and aging of Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder ball grid array packages

After reflow of Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder balls on Au/Ni surface finishes in ball grid array (BGA) packages, scallop-shaped intermetallic compounds (Cu_0.70Ni_0.28Au_0.02)₆Sn₅ (IM1a) and (Cu_0.76Ni_0.24)₆(Sn_0.86In_0.14)₅ (IM1b), respectively, appear at the interfaces. Aging at 100°C and 150°C for Sn-3.8Ag-0.7Cu results in the formation of a new intermetallic phase (Cu_0.70Ni_0.14Au_0.16)₆Sn₅ (IM2a) ahead of the former IM1a intermetallics. The growth of the newly appeared intermetallic compound, IM2a, is governed by a parabolic relation with an increase in aging time, with a slight diminution of the former IM1a intermetallics. After prolonged aging at 150°C, the IM2a intermetallics partially spall off and float into the solder matrix. Throughout the aging of Sn-20In-2Ag-.5Cu solder joints at 75°C and 115°C, partial spalling of the IM1b interfacial intermetallics induces a very slow increase in thickness. During aging at 115°C for 700 h through 1,000 h, the spalled IM1b intermetallics in the solder matrix migrate back to the interfaces and join with the IM1b interfacial intermetallics to react with the Ni layers of the Au/Ni surface finishes, resulting in the formation and rapid growth of a new (Ni_0.85Cu_0.15)(Sn_0.71In_0.29)₂ intermetallic layer (IM2b). From ball shear tests, the strengths of the Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder joints after reflow are ascertained to be 10.4 N and 5.4 N, respectively, which drop to lower values after aging. An erratum to this article is available at .     

Corrosion behavior of Sn-3.0Ag-0.5Cu lead-free solder joints

The aim of this study is to evaluate the corrosion behavior of Sn-3.0Ag-0.5Cu (SAC305) lead-free solder joint using salt spray test. The presence of Cu pad accelerates the dissolution of Sn from solder joints into corrosive medium because of galvanic corrosion mechanism. So, the solder joint was easily corroded in corrosive environment than SAC305 solder bar. During salt spray test, pitting corrosion begin from the solidification cracks in the solder joints, which will lead to a decrease of the reliability of solder joints and shorten the life of electronic devices.     

Interfacial reactions and mechanical strength of Sn-3.0Ag-0.5Cu/Ni/Cu and Au-20Sn/Ni/Cu solder joints for power electronics applications

The mechanical strength of Sn-3.0Ag-0.5Cu (SAC305) and Au-20Sn solder joints and their interfacial reaction with a Ni-plated ceramic substrate were evaluated to assess their suitability for use as die attach materials in power module applications. The compatibility between the two solder alloys and the Ni substrate was assessed during isothermal long-term aging, while the mechanical strength of the two solder joints was measured by die shear testing. A higher intermetallic compound (IMC) growth rate and Ni consumption rate was observed in the SAC305 solder joint, with the formation of a thick IMC layer and weak interface resulting in brittle fracture. The Au-20Sn solder joint, on the other hand was found to exhibit superior high temperature interfacial stability and joint strength.     

Interfacial microstructure and strength of lead-free Sn-Zn-RE BGA solder bumps

Rare earth (RE) elements, primarily La and Ce, were doped in Sn-Zn solder to improve its properties such as wettability. The interfacial microstructure evolution and shear strength of the Sn-9Zn and Sn-9Zn-0.5RE (in wt%) solder bumps on Au/Ni/Cu under bump metallization (UBM) in a ball grid array (BGA) were investigated after thermal aging at 150 /spl deg/C for up to 1000 h. In the as-reflowed Sn-9Zn solder bump, AuSn/sub 4/ intermetallic compounds (IMCs) and Au-Zn circular IMCs formed close to the solder/UBM interface, together with the formation of a Ni-Zn-Sn ternary IMC layer of about 1 /spl mu/m in thickness. In contrast, in the as-reflowed Sn-9Zn-0.5RE solder bump, a spalled layer of Au-Zn was formed above the Ni layer. Sn-Ce-La and Sn-Zn-Ce-La phases were found near the interface at positions near the surface of the solder ball. Upon thermal aging at 150 /spl deg/C, the concentration of Zn in the Ni-Zn-Sn ternary layer of Sn-9Zn increased with aging time. For Sn-9Zn-0.5RE, the Au-Zn layer began to dissolve after 500 h of thermal aging. The shear strength of the Sn-9Zn ball was decreased after the addition of RE elements, although it was still higher than that of the Sn-37Pb and Sn-36Pb-2Ag Pb-bearing solders. The fracture mode of the Sn-9Zn system was changed from ductile to partly brittle after adding the RE elements. This is mainly due to the presence of the brittle Au-Zn layer.     

Evaluations of Whisker Growth and Fatigue Reliability of Sn-3Ag-0.5Cu and Sn-3Ag-0.5Cu-0.05Ce Solder Ball Grid Array Packages

Sn-Ag-Cu solders doped with rare-earth elements were reported to exhibit beneficial mechanical properties; however, rapid whisker growth could be induced. In order to retain the advantageous effects of rare-earth doping without causing the formation of tin whiskers, a Sn-3Ag-0.5Cu-0.05Ce alloy was suggested. In the present study, metallographic observations indicate that whisker growth is indeed prevented in this alloy. However, tensile tests with the bulk materials show that the ultimate tensile strength and Young’s modulus of this Sn-3Ag-0.5Cu-0.05Ce alloy are only slightly higher than those of undoped Sn-3Ag-0.5Cu. Further reliability tests with actual ball grid array packages indicate that the Sn-3Ag-0.5Cu-0.05Ce solder joints have fatigue lives similar to those of undoped Sn-3Ag-0.5Cu specimens regardless of the various surface finishes, such as electroless nickel/immersion gold (ENIG), immersion tin (ImSn), and organic solderability preservatives (OSP). The results suggest that, although lowering the content of rare-earth elements in solders can inhibit the occurrence of rapid whisker growth, it cannot ensure the improvement of the tensile properties of bulk solders and the fatigue reliability of solder joints in actual packages.     

Intermetallic reactions in Sn-3.5Ag solder ball grid array packages with Ag/Cu and Au/Ni/Cu pads

During the reflow process of Sn-3.5Ag solder ball grid array (BGA) packages with Ag/Cu and Au/Ni/Cu pads, Ag and Au thin films dissolve rapidly into the liquid solder, and the Cu and Ni layers react with the Sn-3.5Ag solder to form Cu₆Sn₅ and Ni₃Sn₄ intermetallic compounds at the solder/pad interfaces, respectively. The Cu₆Sn₅ intermetallic compounds also appear as clusters in the solder matrix of Ag surface-finished packages accompanied by Ag₃Sn dispersions. In the solder matrix of Au/Ni surface-finished specimens, Ag₃Sn and AuSn₄ intermetallics can be observed, and their coarsening coincides progressively with the aging process. The interfacial Cu₆Sn₅ and Ni₃Sn₄ intermetallic layers grow by a diffusion-controlled mechanism after aging at 100 and 150°C. Ball shear strengths of the reflowed Sn-3.5Ag packages with both surface finishes are similar, displaying the same degradation tendencies as a result of the aging effect.     

Sn-3Ag-0.5Cu/Ni微焊点多次回流焊后IMC厚度及组织分析

采用Sn-Ag-Cu焊球(直径200,300,400和500μm),镀Ni盘,研究1,3,5次回流焊条件下焊点的IMC厚度及显微组织与焊球尺寸间的关系。结果表明:对于同一尺寸的焊球,随着回流焊次数的增加,IMC的厚度增大,形状由平直状逐渐过渡为体钎料一侧凹凸不平;在同一回流焊次数下,随着焊球尺寸的增大,IMC厚度减小,形貌相对没有明显差别。IMC的组成成分随着Ni向体钎料方向的不断扩散而从Sn、Ag、Cu合金变成Sn、Ag、Cu、Ni合金,其主要组成部分为(Cu,Ni)6Sn5。     

Interfacial Reactions of Sn-3.0Ag-0.5Cu Solder with Cu-Mn UBM During Aging

Cu under bump metallurgy (UBM) has been widely used in flip-chip technology. The major disadvantages of Cu UBM are fast consumption of copper, rapid growth of intermetallic compounds (IMCs), and easy formation of Kirkendall voids. In this study we added two different contents of Mn (2 at.% and 10 at.%) to Cu UBM by sputtering to modify the conventional Cu metallization. For the higher Mn concentration in the Cu-Mn UBM, a new Sn-rich phase formed between Cu₆Sn₅ and the Cu-Mn UBM, and cracks formed after aging. For the lower Mn concentration, growth of Cu₃Sn and Kirkendall voids was significantly suppressed after thermal aging. Kinetic analysis and x-ray elemental mapping provided evidence that Mn diffusion into Cu₃Sn slowed diffusion of Cu in the Cu₃Sn layer. The Mn-enriched Cu₃Sn layer may serve as a diffusion barrier to reduce the interfacial reaction rate and Kirkendall void formation. These results suggest that Cu-Mn UBM with low Mn concentration is beneficial in terms of retarding Cu pad consumption in solder joints.     

Microstructural evolutions of Sn-3.0Ag-0.5Cu solder joints during thermal cycling

Temperature-induced solder joint fatigue is a main reliability concern for aerospace and military industries whose electronic equipment used in the field is required to remain functional under harsh loadings. Due to the RoHS directive which eventually will prevent lead from being utilized in electronic systems, there is a need for a better understanding of lead-free thermomechanical behavior when subjected to temperature variations. As solder joints mechanical properties are dependent of their microstructural characteristics, developing accurate solder joint fatigue models means to correctly capture the microstructural changes that undergo the solder alloy during thermal cycling. This study reports the Sn3.0Ag0.5Cu (SAC305) solder joints microstructural evolution during damaging temperature cycles. Electron BackScatter Diffraction (EBSD) analysis was conducted to assess the SAC305 microstructure corresponding to a specific damage level. Investigated microstructural features included the β-Sn grain size and crystallographic orientation, as well as the grain boundary misorientation and Ag₃Sn intermetallic compound (IMC) size. As-reflowed and damaged components were also mechanically characterized using nanoindentation technique. The microstructural analysis of SAC305 solder joints prior to thermal cycling showed a highly textured microstructure characteristic of hexa-cyclic twinning with two β-Sn morphologies consisting of preferentially orientated macrograins known as Kara's beach ball, along with smaller interlaced grains. The main observation is that recrystallization systematically occurred in SAC305 solder joints during thermal cycling, creating a high population of misoriented grain boundaries leading to intergranular crack initiation and propagation in the high strain regions. The recrystallization process is accompanied with a progressive loss of crystallographic texture and twinning structure. Ag₃Sn IMCs coalescence is another strong indicator of SAC305 solder damage since the bigger and more spaced the IMCs are the less dislocation pinning can prevent recrystallization from occurring.     

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。     

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。     

High strain rate compression behavior for Sn-37Pb eutectic alloy, lead-free Sn-1Ag-0.5Cu and Sn-3Ag-0.5Cu alloys

This paper emphasized on studying the high strain-rate compression behavior of the unleaded Sn-3Ag-0.5Cu (SAC305), Sn-1Ag-0.5Cu (SAC105) solders and the traditional Sn-37Pb eutectic solder. The split Hopkinson pressure bar (SHPB) apparatus was used to conduct high strain rate tests in order to characterize the associated high rate mechanical response of these alloys. Specimens were tested at strain rates ranging from 380 to 3030 s⁻¹ to obtain the dynamic stress-strain relationship for the Sn-37Pb, SAC305 and SAC105 alloys. The tested soft and ductile samples experienced a large amount of elastoplastic deformation due to impact test. In the high strain rate range studied, limited strain rate hardening effect was observed for SAC305, SAC105 and Sn-37Pb alloys studied. The strain rate sensitivity parameter (m) related to the power law creep equation was also calculated for the present solder materials at specific strain values. In addition, the saturation stresses for the leaded and lead-free solders at the strain rate range studied are also reported.     

Demonstration and Characterization of Sn-3.0Ag-0.5Cu/ Sn-57Bi-1Ag Combination Solder for 3-D Multistack Packaging

A combination solder of Sn-3.0Ag-0.5Cu (numbers are all in weight percent unless specified otherwise) wrapped by Sn-57Bi-1Ag was tested for application to three-dimensional (3-D) multistack packaging. The experimental variables controlled were the reflow peak temperatures (170, 185, 200, and 230°C), the reflow cycles (up to four times), and the mask which controls the amount of Sn-57Bi-1Ag solder paste (two sizes). We demonstrate and evaluate the combination solder structure, focusing on microstructural changes and the shear strength. The degree of mixing in the combination solder, which is enhanced by an increase in the reflow peak temperature, is independent of the number of reflow cycles. The ball shear strength and the lab shear strength both increased with increases in the reflow peak temperatures. This behavior is explained by the amount of the brittle Bi phase that constitutes the eutectic Sn-Bi phase.     

Interfacial reaction study on a solder joint with Sn-4Ag-0.5Cu solder ball and Sn-7Zn-Al (30 ppm) solder paste in a lead-free wafer level chip scale package

The interfacial reactions of solder joints between the Sn-4Ag-0.5Cu solder ball and the Sn-7Zn-Al (30 ppm) presoldered paste were investigated in a wafer level chip scale package (WLCSP). After appropriate surface mount technology (SMT) reflow process on the printed circuit board (PCB) with organic solderability preservative (Cu/OSP) and Cu/Ni/Au surface finish, samples were subjected to 150°C high-temperature storage (HTS), 1,000 h aging. Sequentially, the cross-sectional analysis is scrutinized using a scanning electron microscope (SEM)/energy-dispersive spectrometer (EDS) and energy probe microanalysis (EPMA) to observe the metallurgical evolution in the interface and solder buck itself. It was found that Zn-enriched intermetallic compounds (IMCs) without Sn were formed and migrated from the presolder paste region into the solder after reflow and 150°C HTS test.