全Cu_(3)Sn焊点在高温时效下的组织及力学性能EI北大核心CSCD

对全Cu_(3)Sn焊点进行620℃下不同持续时间的时效处理,研究时效过程中接头微观组织演变,并利用纳米压痕实验及剪切实验表征时效后焊点的力学性能变化。结果表明:在时效过程中,Cu/Cu_(3)Sn界面以平面状析出Cu_(20)Sn_(6)并持续生长,直至Cu_(3)Sn被完全消耗。随后Cu_(20)Sn_(6)向Cu_(20)Sn_(6)和Cu_(13.7)Sn组成的两相层转变,Cu_(13.7)Sn通过消耗两相层在Cu/两相层的界面处以波浪状析出并继续生长,直至占据整个界面区,该过程中伴随着焊缝中间位置孔洞数量和尺寸的生长,最终聚合成微裂纹。Cu_(20)Sn_(6),Cu_(3)Sn,Cu_(13.7)Sn相的硬度分别为9.62,7.15,4.67 GPa,弹性模量分别为146.5,134.0,133.2 GPa。随时效时间的增加,焊点的抗剪强度呈先增大后减小的趋势,在120 min内保持大于20.1 MPa;其断口形貌和断裂路径也随之发生变化。     

Evolution of Ag3Sn compound formation in Ni/Sn5Ag/Cu solder joint

Ag₃Sn compound formation was observed to evolve from the Cu side to the Ni side in Ni/Sn5Ag/Cu solder joint upon reflow process. The transformation of the interfacial compound phase at the Sn5Ag/Ni interface, i.e., from Ni₃Sn₄-based compound to Cu₆Sn₅-based compound, changes the interfacial energy state, which is the main driving force for the evolution of Ag₃Sn compound formation. The evolution consequence of Ag₃Sn compound suggests that Sn5Ag/(Cu,Ni)₆Sn₅ interface is the best energy-preferential heterogeneous nucleation site for the Ag₃Sn compound phase, comparing to the Sn5Ag/Cu₆Sn₅ interface and the Sn5Ag/Ni₃Sn₄ interface. In addition, the lower Cu concentration near the Ni side is another added driving force for the heterogeneous nucleation of Ag₃Sn phase at Sn5Ag/(Cu,Ni)₆Sn₅ interface.     

Suppression effect of Cu and Ag on Cu3Sn layer in solder joints

Cu₃Sn intermetallic compound (IMC) layer is usually formed in solder joints. Since the formation of Cu₃Sn could induce large volume shrinkage, and further cause a lot of reliability issues, many works focused on suppressing the formation or growth of the Cu₃Sn layer. This work explored that Cu and Ag alloying elements also have benefit in suppressing the Cu₃Sn growth during isothermal aging stage. The Cu₆Sn₅ IMC layer seems to be much stable in the Sn/Cu solder joint during aged at 150 and 180 °C, its thickness changed little, while the Cu₃Sn IMC layer grew much quickly. After about 300 h, the thickness of Cu₃Sn layer exceeds that of Cu₆Sn₅ layer. For the Sn-3.5Ag/Cu and Sn-0.7Cu/Cu solder joints, the thickness of Cu₃Sn layer is near half of that of Cu₆Sn₅ layer. According to the relation between interface location and aging time, the reaction generated at the Cu₆Sn₅/Cu₃Sn interface, which is governed by atom fluxes, controls the growth of Cu₃Sn IMC layer. Since Ag and Cu alloying elements suppress the coarsening of Cu₆Sn₅ IMC grains, the diffusion paths for Cu atoms toward the solder are more for Ag or Cu containing solder joints. Therefore, the growth of the Cu₃Sn layer by consuming Cu₆Sn₅ layer is slower in the SnAg/Cu and SnCu/Cu solder joints than that in the Sn/Cu joints.     

Influence of thermal cycling on fatigue strength of Cu/Sn/Cu solder joints

Abstract

The influence of thermal cycling on the fatigue life of Cu/Sn/Cu solder joints has been examined. Copper plates were bonded with tin foil (with a solder thickness of 60 µm) and suffered thermal cycling in a temperature range of 55 or 125 K. Then they were subjected to fatigue testing at a shear stress amplitude of 2 MPa and a frequency of 3.6 Hz. With the increasing number of the thermal cycles, the fatigue life decreased from 3.0×10⁵ to 5.0×10⁴ at thermal cycle 6000. However, the fatigue life did not change so much during thermal cycling in different temperature ranges. When the solder joints suffered the thermal cycling, the η phase at the bonding interface coarsened and elongated, and its arrangement became irregular. After larger numbers of thermal cycles, fine cracks appeared in the η phase parallel to the interface. After fatigue testing, circular patterns were observed inside the bonded region on a fracture surface, and their shape and size became irregular and larger with the increasing number of thermal cycles, respectively. These showed that the reduction in fatigue life was caused by improved propagation of the fatigue crack following changes in the morphology and arrangement of the η phase during thermal cycling.     

Influence of solder thickness on interfacial structures and fatigue properties of Cu/Sn/Cu joints

Abstract

Copper plates were soldered with tin foil of different thicknesses to examine the influence of the solder thickness on formation of secondary phases at the interface and fatigue properties. In the case of an initial solder thickness of 60 μm, the thickness of the η (Cu₆ Sn₅) phase and the ? (Cu₃ Sn) phase linearly increased with the square root of the bonding time. The fatigue strength was 3 MPa and the fatigue life decreased with increasing stress amplitude and had a low scatter. In the fatigue process, fine cracks appeared in the η phase and propagated in the solder layer. This process was different from the case of static shear fracture. In the case of a 5 μm solder thickness, the solder was replaced by secondary phases in a short time, and only the ? phase remained at the interface after a bonding time of more than 300 min. The fatigue strength was 13 MPa and was independent of the interfacial structure, but the fatigue life showed a large scatter. In the fatigue test, unstable fracture occurred along the interfaces, similar to that observed in static shear fracture. From these results, it was concluded that solder joints become brittle with decreasing solder thickness.     

Stress relaxation behavior of Cu/Sn/Cu micro-connect after electrical current

The effect of electromigration on stress relaxation behavior of pure tin solder joints was investigated. It was found that the stress relaxation rate was accelerated significantly after the sample was subjected to current stressing. The accelerating effect increased with the current stressing time. Measurements of the activation energy and stress exponent suggested that the dominant mechanism of the stress relaxation of pure tin solder joint went from dislocation climb to grain boundary diffusion after electromigration. As a result of grain boundary diffusion and sliding, grain boundary grooves were observed on the surface of the tin solder joints after electromigration. The groove was associated with the divergence of vacancy concentration at the grain boundaries. The vacancy concentration at the grain boundaries, which increased with the current stressing time, promoted the atomic diffusion along the grain boundaries, resulting in a higher stress relaxation rate.     

Textured growth of Cu6Sn5 grains formed at a Sn3.5Ag/Cu interface

The growth orientations of Cu₆Sn₅ grains formed at a Sn3.5Ag/polycrystalline Cu interface were investigated. Similar as reported on Cu single crystals, strong textures in Cu₆Sn₅ layers can also form on polycrystalline Cu, but the texture formation mechanisms differ. The texture formation on polycrystalline Cu occurs during the ripening growth and results from the differences in stability of the interfacial grains with various orientations at different temperatures. A reaction temperature of 240 °C causes the Cu₆Sn₅ layer to form [0001] texture in the direction normal to the substrate, and a special morphology of interfacial Cu₆Sn₅ grains can be formed on this layer to reinforce joint properties.     

Epitaxial growth of Cu6Sn5 formed at Sn-based lead-free solder/non-textured polycrystalline Cu plate interface

This study examines the epitaxial growth of the intermetallic compound (IMC) of Cu₆Sn₅ (or (Cu,Ni)₆Sn₅) that forms at the interface between molten Sn-based lead-free solders and non-textured polycrystalline Cu substrates. Sn, Sn–Cu, Sn–Cu–Ni and Sn–Ag–Cu solders were investigated. The dominant growing planes in a hexagonal structure of this IMC on Cu substrates are (101) and (102). Addition of trace Ni into Sn–Cu solders leads to an increase in (101) growth and a decrease in (102) growth. The presence of Ag in Sn–Ag–Cu solders facilitates (102) growth and suppresses (101) growth. Such an epitaxial growth should have a large influence on the mechanical and electrical characteristics of the Sn-based solder/Cu joints.     

Suppression of Cu3Sn and Kirkendall voids at Cu/Sn-3.5Ag solder joints by adding a small amount of Ge

Kirkendall voids (KVs) have disastrous effects on the properties of the solder joints in the integrated circuits, which are formed after the occurrence of the Cu₃Sn intermetallic compound (IMC) layer at the Sn-based solder/Cu interface. In this paper, 0.1 and 0.3 wt% Ge additions were separately added into the Sn-3.5 wt%Ag eutectic solder, to investigate the effects of Ge on the interfacial reaction under thermal aging at 150 °C. It is found that the Cu₆Sn₅ layer was still the original product, regardless of the concentration of Ge. Moreover, Ge was identified to dissolve into the IMC layer. As the aging time was prolonged to 10 days, the concentration of Ge increased to about 3.0 at%, but the Cu₃Sn IMC layer was not obvious. The single Cu₆Sn₅ IMC layer became flat little by little. Meantime, the thickness of the IMC layer increased slowly. And more significant finding is that the KVs were also not obvious at the interface.     

Thermomigration and electromigration in Sn8Zn3Bi solder joints

Individual effect of thermomigration (TM) and combined effect of TM and electromigration (EM) on the microstructural variation in Sn8Zn3Bi was investigated by stressing line-type Au/Ni–P/Cu-Sn8Zn3Bi-Au/Ni–P/Ni solder joints with a 5 × 10³ A/cm² alternating current (AC) or direct current (DC) at 110°C. Due to the different thermoelectric characteristics of Cu and Ni wires, a thermal gradient of 196°C/cm could be established across the solder joints according to the finite element simulation. In AC current stressing, there is no EM effect and only TM dominates the migration. Microstructural study shows that Zn atoms migrate towards the lower temperature side during TM. In DC current stressing, it is found that both EM and TM play important roles depending various experimental conditions. And the energy change during the EM and the TM is estimated to be ∆ω_em 3.2 × 10⁻²⁸ Joule and ∆ω_em 2.2 × 10⁻²⁸ Joule, respectively. Upon different current directions in DC current stressing, there is a counteractive or accelerated effect between TM and EM on Zn migration, resulting different microstructures at the cathode side in the solder joints.     

Preparation and characterization of Cu3Sn nanoparticles via a facile ultrasonic irradiation

Cu₃Sn nanoparticles were prepared with ease by allowing bulk tin to react with copper acetate under ultrasonic irradiation. The microstructure and thermal stability of resultant Cu₃Sn nanoparticles were analyzed by means of transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). In the meantime, the possible growing mechanism of Cu₃Sn nanoparticles was presented; and the tribological properties of Cu₃Sn nanoparticles as lubricating additives were evaluated. It has been found that as-synthesized hexagonal Cu₃Sn nanoparticles are of spherical shape and have a narrow size distribution and an average diameter of 90 nm. The growth of Cu₃Sn nanoparticles involves three stages of ultrasonic dispersion, reaction and surface modification. Besides, ultrasonic irradiation in combination with surface-capping by oleic acid contributes to prevent on-growing Cu₃Sn nanoparticles from aggregation, making it feasible for Cu₃Sn nanoparticles to be well dispersed in lubricating base stock and significantly increase the antiwear ability and load-carrying capacity of liquid paraffin.     

Effects of nano-Al2O3 particles on microstructure and mechanical properties of Sn3.5Ag0.5Cu composite solder ball grid array joints on Sn/Cu pads

A Sn3.5Ag0.5Cu (SAC)–XAl₂O₃ nano-composite solder was prepared by adding 100 nm Al₂O₃ to SAC (wt.%) solder. The interfacial microstructures and mechanical properties of SAC–XAl₂O₃ nano-composite solder balls on immersion Sn surface finished BGA joints after multiple reflows was investigated. As a whole, adding Al₂O₃ nanoparticles to SAC solders significantly changed in the interfacial microstructure, and both scallop-type and prism-type modes were observed in the plain SAC solder and SAC–XAl₂O₃ nano-composite solder after reflowing, respectively. The nanoparticles suppressed the growth of the Cu₆Sn₅ layer, significantly improving the shear strength. The fracture surfaces of the plain SAC solder showed a semi-brittle fracture mode, but those of the SAC–XAl₂O₃ nano-composite solder exhibited typical ductile failures.     

Interfacial reaction and properties of Sn0.3Ag0.7Cu containing nano-TiN solder joints

Ultra-low silver Sn0.3Ag0.7Cu (SAC0307) solder is arousing widespread attention because of its low cost. In this paper, the morphology of interfacial intermetallic compounds, microstructure, melting point, wettability and mechanical property of SAC0307 containing nano-TiN solders were investigated using scanning electricity microscope, transmission electron microscopy, micro-joints strength tester and differential scanning calorimetry. Results show that the addition of trace nano-TiN into SAC0307 solder can restrict the growth behavior of interfacial IMC and refine the microstructure of the solder joints. When 0.2 wt% nano-TiN particles were added, the interfacial thickness of SAC0307 solder joint dropped from 2.1 to 1.92 μm. Moreover, the wettability and mechanical property of SAC0307 solder joints were also significantly enhanced, but it has little influence on the melting characteristics of the solder.

     

Phase growth competition in solid/liquid reactions between copper or Cu3Sn compound and liquid tin-based solder

Interfacial reaction between solid ?-Cu₃Sn compound and liquid Sn at 250 °C is studied for the first time. The reaction product formed at the ?-Cu₃Sn/liquid Sn interface consists of the single η-Cu₆Sn₅ phase. The growth kinetics of the η phase formed at the incremental ?/liquid Sn couple (?/η/Sn configuration) is compared to that of η phase formed at the classical Cu/liquid Sn couple (Cu/?/η/Sn configuration). The experimental method consists first in processing of intimate interfaces by dipping peaces of solid ?-Cu₃Sn compound and Cu in liquid Sn for 1 s at 250 °C. Afterwards, isothermal holding of such pre-performed couples for 10, 30, 120 and 480 min at 250 °C are performed for both couples. A theoretical analysis of the growth kinetics of η phase and comparison of its growth in both configurations are performed.     

Investigation on impact strength of the as-soldered Sn37Pb and Sn3.8Ag0.7Cu solder joints

Charpy impact specimens of eutectic Sn37Pb and Sn3.8Ag0.7Cu solder joints with U-type notch were prepared to investigate the joint impact strength. The gap sizes of the butt joint were selected at 0.3 and 0.8 mm. Compared with the values of 0.3 mm joint gap, the impact absorbed energies of two solder joints were increased at the joint gap of 0.8 mm. The impact strengths of Sn37Pb joints were higher than those of Sn3.8Ag0.7Cu joints in both cases. From the macrographic observation of the fracture path, when the gap was 0.3 mm, the crack initiation of two solder joints located at the root of U-type notch then propagated along one interface of the joint. For the Sn37Pb joints, the fracture path was not changed at 0.8 mm gap size. However, the fracture path of Sn3.8Ag0.7Cu joint was totally changed and the fracture occurred not at the root of pre-U notch but from one side of the solder/Cu interfaces. From the micrographic observation, the crack of the Sn37Pb joints was concentrated on the Pb-rich layer in the vicinity of interfacial intermetallic (IMC) layer and the fracture morphology mainly appeared to be a ductile-like structure. Meanwhile, the fracture of Sn3.8Ag0.7Cu joints propagated along either the interface of IMC/solder or within the IMC layer and showed a brittle failure mode.     

Competitive growth of Cu3Sn and Cu6Sn5 at Sn/Cu interface during various multi-reflow processes

Journal of Materials Science: Materials in Electronics - The competitive growth of Cu3Sn and Cu6Sn5 in the multi-reflow processes induced by temperature, time, and cooling rate was systematically...     

Effects of nano-Al2O3 additions on microstructure development and hardness of Sn3.5Ag0.5Cu solder

This work investigates the effects of nano-Al₂O₃ on the microstructure and microhardness of the Sn3.5Ag0.5Cu composite solder alloy. In comparison with solder without the addition of nano-Al₂O₃ particles, the formation of primary β-Sn phase, the Ag₃Sn phase average size, and the spacing lamellae decreased significantly in the composite solder matrix. In addition, the eutectic areas of the composite solder were wider than that of the Sn3.5Ag0.5Cu solder. This is attributed to the adsorption of nano-Al₂O₃ particles with high surface free energy on the grain surface during solidification. The wettability property was improved by 0.25–0.5 wt.% addition of nano-Al₂O₃ particles into the Sn3.5Ag0.5Cu solder. However, when the nano-Al₂O₃ particles concentration up to over 1.0 wt.% decreased the beneficial influence. Microhardness improved with the addition of nano-Al₂O₃ particles. This improved mechanical property was due to the composite microstructure, which is close to the theoretical prediction from dispersion strengthening theory.     

The effect of 0.5 wt.% Ce additions on the electromigration of Sn9Zn BGA solder packages with Au/Ni(P)/Cu and Ag/Cu pads

It has previously been established that Sn-9Zn-0.5Ce alloy possesses mechanical properties superior to those of undoped Sn-9Zn alloy, and is free of the problem of rapid whisker growth. However, no detailed studies have been conducted on the electromigration behavior of Sn-9Zn-0.5Ce alloy. In this research, Sn-9Zn and Sn-9Zn-0.5Ce solder joints with Au/Ni(P)/Cu and Ag/Cu pads were stressed under a current density of 3.1 × 10⁴ A/cm² at room temperature for various periods of time. Due to finer grain sizes, the electromigration effects were more severe in Sn-9Zn-0.5Ce solder joints than in Sn-9Zn solder joints when joint temperature was around 80 °C. In addition, both solder joints (Sn-9Zn and Sn-9Zn-0.5Ce) with Au/Ni(P)/Cu pads possess longer current-stressing lifetimes than those with Ag/Cu pads because Ni is more resistant than Cu to migration driven by electron flow.