Cu6Sn5 Grains Research Articles

Microstructure evolution, interfacial reaction and mechanical properties of lead-free solder bump prepared by induction heating method

The formation process and mechanism of the Sn3.5Ag/Cu solder bump during induction heating were investigated to elucidate the basic characteristics of the induction heating soldering process. During induction heating, solder balls quickly melted from the surface to the center, which could be utilized to prepare solder bumps of controlled height. Cu6Sn5 grains with a round scallop shape were generated at the interface as soon as the solder ball was wetted on the Cu substrate. With prolonged heating time, the uniform morphology of the interfacial Cu6Sn5 grains was transformed to a special morphology in which scallop-shaped grains occupy the central area of the interface, surrounded by prismatic grains. Numerous bulk Cu6Sn5 grains were precipitated and regularly distributed in the solder matrix, which affected the hardness and shear strength of the solder bumps.

Effects of Ag content on the interfacial reactions between liquid Sn–Ag–Cu solders and Cu substrates during soldering

The effects of Ag content on the interfacial reactions between Sn–Ag–Cu (SAC) solders and Cu substrates during soldering were studied based on an experiment in which the liquid solder is removed after a long exposure time. This removal of the solder allows for the capture and visualization of the interfacial IMCs formed during soldering and avoids the influence of Cu6Sn5 precipitated from the solder matrix during cooling. The results showed that a Cu6Sn5 scallop type layer with round grains having a strong texture was formed at the interface of the SAC solders/Cu systems. The addition of Ag decreased the IMC/liquid SAC solder interfacial energy and improved the wettability of the solder on the Cu. In addition to the reaction energy, the interfacial microstructure also affected the interfacial IMC growth extent. By changing the growth environment near the interface, Ag affected the orientation of Cu6Sn5 grains formed at the interface, leading to the evolution of a microstructure that generates more Cu6Sn5 grain boundaries at the interface. Consequently, under the same reflow conditions, SAC solders with a higher Ag content in contact with the Cu substrate form thicker IMCs interfacial layers.

Effects of post-reflow cooling rate and thermal aging on growth behavior of interfacial intermetallic compound between SAC305 solder and Cu substrate

The interfacial reactions between Cu and Sn3Ag0.5Cu (SAC305) solder reflowed under various cooling rates were investigated. It is found that the cooling rate is an important parameter in solder reflow process because it influences not only microstructure of solder alloy but also the morphology and growth of intermetallic compounds (IMCs) formed between solder and Cu substrate. The experimental results indicate that only scallop-like Cu6Sn5 IMC layer is observed between solder and Cu substrate in case of water cooling and air cooling, while bilayer composed of scallop-like Cu6Sn5 and thin layer-like Cu3Sn is detected under furnace cooling due to sufficient reaction time to form Cu3Sn between Cu6Sn5 IMC and Cu substrate which resulted from slow cooling rate. Samples with different reflow cooling rates were further thermal-aged at 423 K. And it is found that the thickness of IMC increases linearly with square root of aging time. The growth constants of interfacial IMC layer during aging were obtained and compared for different cooling rates, indicating that the IMC layer thickness increased faster in samples under low cooling rate than in the high cooling rate under the same aging condition. The long prismatic grains were formed on the existing interfacial Cu6Sn5 grains to extrude deeply into solder matrix with lower cooling rate and long-term aging, and the Cu6Sn5 grains coarsened linearly with cubic root of aging time.

Effect of nano-TiO2 addition on microstructural evolution of small solder joints

The influence of TiO2 addition on the interfacial reaction in Sn–3.0Ag–0.5Cu solder joints with the pad size of 200 µm was investigated in this study. The microstructure of the solder joints and the interfacial intermetallic layers were analyzed by scanning electron microscope. Results show that both the thickness and grain size of the intermetallic compound (IMC) decreases when TiO2 is added. The Sn–3.0Ag–0.5Cu–0.1TiO2 solder exhibits the most prominent effect in retarding interfacial IMC growth and refining IMC grain size. It is observed that the scallop morphology became more faceted in shape compared with the large size Cu/solder interface, where the Cu6Sn5 grains appear to be round shape no matter the reflow time is long or short. The cause might be due to the change in interfacial energy between the molten solder and Cu6Sn5 phase, which is highly correlated with the Cu concentration profile near the interface in the side of the liquid solder.

Characterization of liquid-phase sintered materials produced from mechanically alloyed Cu + Sn powder compacts

Mechanical alloying was employed to produce multilayered Cu/Sn particles with cold welded Cu/Sn interfaces capable to enhance wettability of liquid Sn on solid Cu during liquid-phase sintering. Independent nucleation and growth of Cu6Sn5 grains in the melt resulting in the formation of the Cu6Sn5 intermetallic layer upon grain impingement were observed in the liquid-phase sintered specimens. Amorphous Cu–Sn layer was formed between the Cu and Cu6Sn5 layers. Formation of this phase cannot be explained by the interfacial reactions normally occurring in soldering of Sn-base solders on a Cu substrate but the amorphous layer is inherited from the amorphization during mechanical alloying.

The effect of Cu substrate texture on the intermetallic compounds (IMCs) growth at a Sn3.5Ag–Cu interface

The effect of Cu substrates texture on morphology and growth behavior of intermetallic compounds (IMCs) during reflowing and aging time were systematically investigated. The IMCs formed at Sn3.5Ag/(220) Cu and Sn3.5Ag/(200) Cu interface showed significant difference. After reflowing, the Cu6Sn5 grains in the both couples are all rounded, but more protruded and thinner at Sn3.5Ag/(200) Cu interface due to lower IMC/solder interfacial energy and smaller quantity of dissolution Cu atoms for (200) Cu substrate. At aging process, for IMCs formed on two different textured substrates, the Cu6Sn5 and Cu3Sn exhibited reciprocal inhibition within 48 h, and the whole thickness of IMCs (Cu6Sn5+Cu3Sn) increases with aging time prolonged, proportional to the square root of aging time. The whole IMCs at Sn3.5Ag/(220) Cu interface is thicker than that at Sn3.5Ag/(200) Cu interface, with a reaction constant of 1.849 × 10−17 and 5.39 × 10−18 m2/s for IMCs growth respectively.

Growth characteristics and formation mechanisms of Cu6Sn5 phase at the liquid-Sn0.7Cu/(111)Cu and liquid-Sn0.7Cu/(001)Cu joint interfaces

This work investigated the growth characteristics and formation mechanisms of the Cu6Sn5 phase at the liquid-Sn0.7Cu/(111)Cu and liquid-Sn0.7Cu/(001)Cu joint interfaces. As a result of contributions from the interfacial environments, regular arrays of the roof-type Cu6Sn5 grains with fixed intersecting angles were generated on both types of the Cu single crystals after soldering at 250 °C for 1 s–1 h. At the liquid-Sn0.7Cu/Cu6Sn5 interface, a hexagonal-rod-type growth mechanism for Cu6Sn5 phase was proposed on the basis of its anisotropy in surface energy and roughness. According to this mechanism, the Cu6Sn5 roofs formed on the (111) and (001) Cu pads would consistently elongate in the [0001] direction of Cu6Sn5 phase, regardless of whether they belonged to the minimum mismatch direction of Cu6Sn5 and Cu phases; and they would maintain the {112¯0} and {101¯0} planes as the side faces respectively, despite the reaction time being prolonged to 1 h. At the Cu6Sn5/Cu interface, two types of the three-dimensional placement rules for Cu6Sn5 roofs on the Cu single crystals were determined on the basis of the suitable Cu matches of these two phases. Specifically, the junction interfaces between Cu6Sn5 roofs and Cu single crystals were confirmed to be ‖{101¯0} ‖ {111} and {112¯0} ‖ {001}; and the parallel orientation relationships of {0001} ‖ {011} were always present on both types of interfaces. Our study can help to clarify the growth mechanism of Cu6Sn5 phase and to provide a scientific basis of Cu6Sn5 orientation design for three-dimensional integrated circuit interconnect applications.

A comparative study of Cn/Sn/Cu and Ni/Sn/Ni solder joints for low temperature stable transient liquid phase bonding

Two sandwich structured solder systems (Cu/Sn/Cu and Ni/Sn/Ni) were investigated in terms of interfacial reaction and mechanical reliability for low temperature wafer bonding. The relationship between chemical reaction and mechanical reliability of the solder joints was investigated by varying thickness ratio of Cu/Sn (Ni/Sn). The Cu/Sn/Cu joint was proved to be stronger, revealing very smooth fractured surface through Cu6Sn5 grains (intragranular fracture). Meanwhile, the Ni/Sn/Ni joint showed rough fractured surface through Ni3Sn4 grain boundaries and Ni3Sn4/Ni3Sn2 interphase boundaries (intergranular fracture).

Microstructural and mechanical analysis on Cu–Sn intermetallic micro-joints under isothermal condition

This study focuses on the mechanism of phase transformation from Cu6Sn5 into Cu3Sn and the homogenization process in full intermetallics (IMCs) micro-joints, which were prepared by soldering the initial Cu/Sn/Cu structure through high temperature storage in vacuum environment as the Transient Liquid Phase (TLP) process. From the microstructural observation by electron backscatter diffraction (EBSD), a mixture of IMCs phases (Cu6Sn5 and Cu3Sn) has been found to constitute the sandwich-structured Cu/IMCs/Cu joints. With the dwell time increasing at 533 K, there were two layers of Cu3Sn emerging from both sides of copper substrates with the depletion of Cu6Sn5 layer, toward merging each other in the IMCs interlayer. Then the Cu3Sn grains with various sizes became more homogenous columnar crystallites. Meanwhile, some equiaxial ultra-fine grains accompanied with the Kirkendall voids, were found only in adjacent to the electroplated copper. In addition, a specific type of micropillar with the size ∼5 μm × 5 μm × 12 μm fabricated by focus ion beam (FIB) was used to carry out the mechanical testing by Nano-indentation, which confirmed that this type of joint is mechanically robust, regardless of its porous Cu3Sn IMC interconnection.

Growth behavior of intermetallic compounds at Sn–Ag/Cu joint interfaces revealed by 3D imaging

In this study, the morphologies of intermetallic compounds (IMCs) at the as-soldered and thermal aged Sn–Ag/Cu joint interfaces were observed by SEM and measured using Laser Confocal Microscope, and their three-dimensional (3D) shapes were revealed using 3D imaging technology. The observation reveal that during the soldering process the Cu6Sn5 grains at the joint interface evolve from hemispheroid to a bamboo shoot-shaped body with increasing liquid state reacting time, and their grain size increases sharply. After thermal aging, the Cu6Sn5 grains change into equiaxed grains, while the top of some prominent Cu6Sn5 grains changes little. Due to the higher active energy of the Sn atoms at the grain boundary, the growth rate of IMC grains around the grain boundaries of the solder is higher during the aging process. From the evolution in morphology of the IMC layer, it is demonstrated that the IMC layer grows through grain boundary diffusion of the Cu and Sn atoms during the aging process, and the volume diffusion is very little. The 3D imaging technology is used to reveal the shape and dimension of the IMC grains.

Low temperature bonding for high temperature applications by using SnBi solders

To develop materials and processes enabling low-temperature assembly for applications at higher temperatures, the soldering reactions between Cu substrate and SnBi solders are investigated. Sandwich structures of Cu/Sn5Bi/Cu, Cu/Sn10Bi/Cu, and Cu/Sn58Bi/Cu are prepared by thermal compression bonding. The thickness of the solder layer is controlled at 5 or 10 μm. The results of this study show that to achieve high-temperature applications the initial solder thickness has to be thin and the Bi-content has to be high. It is also found that the presence of Bi interferes with the merging of Cu6Sn5 grains. This interference allows one to deduce that the merging of the Cu6Sn5 grain is through the coarsening mechanism.

A study on the interfacial reaction of Sn58Bi/Cu soldered joints under various cooling and aging conditions

As one of the important factors affecting the quality of soldered joints, the interfacial intermetallic compound (IMC) formed at the solder/substrate interface is of increasing concern to researchers. Thus, the growth behavior of interfacial IMC in Sn58Bi/Cu solder joints was investigated as different cooling methods-quenched water, cooling in air, and cooling in a furnace during reflowing were used. In this work, the effect of cooling rate on interfacial IMC growth is studied in Sn58Bi/Cu system. Morphology and growth kinetics of the interfacial Cu–Sn IMC under various cooling rates were studied. It was found that only scallop-like Cu6Sn5 IMC layer was observed between solder and Cu substrate in cases of water cooling and air cooling, while bi-layer composed of scallop-like Cu6Sn5 and planar Cu3Sn was detected under furnace cooling due to sufficient reaction time to form Cu3Sn between Cu6Sn5 IMC and Cu substrate which resulted from slow cooling rate. Samples with different reflow cooling rate were further thermal aged at 373 and 393 K, respectively. The results showed that the morphology of interfacial Cu–Sn IMC layer would transform from scallop-like to planar-like with increasing aging time, and the thickness of IMC increased linearly with square root of aging time. The growth constants of interfacial IMC layer were obtained and compared for different cooling rates and aging temperatures, indicating that the IMC layer thickness increased faster in samples under slow cooling rate than the fast cooling ones under the same aging condition, and higher aging temperature could result in faster growth of IMC layer. The interfacial Cu6Sn5 grains gradually transformed to facet prismatic shape as the cooling rate decreased, and the grain size increased dramatically as the samples were cooled in furnace, comparing with that of samples cooled in water and air.

Effect of alloying Cu substrate on microstructure and coarsening behavior of Cu6Sn5 grains of soldered joints

In order to investigate the effects of alloying Cu substrate on microstructure and coarsening behavior of Cu6Sn5 grains of soldered joints, in the paper, four types of wetting layers (Cu, Cu–8 wt% Zn, Cu–15 wt% Zn and Cu–30 wt% Zn) reaction with Sn–3.0Ag–0.5Cu (SAC305) solder were fabricated. The effects of reflowing and isothermal aging on the interface of soldered joints and the morphology of Cu6Sn5 grains were studied. Experimental results show that the scallop-shaped Cu6Sn5 formed at the interface of SAC/Cu after reflowing. After isothermal aging for 120 h, a bi-layer structure of Cu6Sn5 and Cu3Sn formed at the interface of SAC/Cu and some Ag3Sn grains decorated in Cu6Sn5, and Kirkendall voids were observed inside the Cu3Sn layer after aging for 240 h. However, Cu6Sn5 was the predominant reaction product and Cu3Sn and Kirkendall voids were not observed at the interface of SAC/Cu–Zn irrespective of aging time. The thickness of IMC layer increases with increasing aging time. Compared to SAC/Cu, the additions of Zn can effectively suppress the growth of IMC. During the aging process, the growth rate constants of IMC layers of SAC/Cu, SAC/Cu–8Zn, SAC/Cu–15Zn and SAC/Cu–30Zn were obtained by using fitting method from the experimental data, which were 7.78 × 10−17, 5.42 × 10−17, 2.51 × 10−17 and 1.46 × 10−17 m2 s−1, respectively. The mean diameter of IMC grains increases with the increasing aging time. The relationships of the mean diameter of Cu6Sn5 grains of SAC/Cu, SAC/Cu–8Zn, SAC/Cu–15Zn, SAC/Cu–30Zn interface joints and aging time were given, i.e., d = 1.49t0.237, d = 1.46t0.204, d = 1.43t0.203 and d = 1.71t0.197, respectively.

Size effect model on kinetics of interfacial reaction between Sn-xAg-yCu solders and Cu substrate.

The downsizing of solder balls results in larger interfacial intermetallic compound (IMC) grains and less Cu substrate consumption in lead-free soldering on Cu substrates. This size effect on the interfacial reaction is experimentally demonstrated and theoretically analyzed using Sn-3.0Ag-0.5Cu and Sn-3.5Ag solder balls. The interfacial reaction between the Sn-xAg-yCu solders and Cu substrates is a dynamic response to a combination of effects of interfacial IMC growth, Cu substrate consumption and composition variation in the interface zone. A concentration gradient controlled (CGC) kinetics model is proposed to explain the combined effects. The concentration gradient of Cu at the interface, which is a function of solder volume, initial Cu concentration and reaction time, is the root cause of the size effect. We found that a larger Cu concentration gradient results in smaller Cu6Sn5 grains and more consumption of Cu substrate. According to our model, the growth kinetics of interfacial Cu6Sn5 obeys a t1/3 law when the molten solder has approached the solution saturation, and will be slower otherwise due to the interfering dissolution mechanism. The size effect introduced in this model is supported by a good agreement between theoretical and experimental results. Finally, the scope of application of this model is discussed.

Size effect on interfacial reactions of Sn–3.0Ag–0.5Cu solder balls on Cu and Ni–P pads

The size (volume) effect of solder balls on the interfacial reactions of Sn–3.0Ag–0.5Cu solders on both Cu and Ni–P pads in electronic packaging was systematically investigated. For the interfacial reaction on Cu pad, the smaller the solder ball is, the larger the interfacial Cu6Sn5 grains are, the thicker the intermetallic compound (IMC) layer is, and the less the Cu pad consumption is. The Cu concentration approached the solution saturation more rapidly in the smaller solder joints through grain boundary diffusion, resulting in smaller Cu outflux at the interface and consequently coarser interfacial Cu6Sn5 grains. For the interfacial reaction on Ni–P pad, the smaller the solder ball is, the earlier the transformation of interfacial IMCs from (Cu,Ni)6Sn5 to (Ni,Cu)3Sn4 occurs. The faster decreasing rate of Cu concentration in smaller molten solder resulted in the less supplying Cu influx and thus the earlier IMC transition. The Cu flux at the interface determines the growth kinetics and the phase transformation of interfacial IMCs on both Cu and Ni–P pads. Accordingly, the variation of Cu fluxes at the interface caused by solder ball volume results in the size effect.

Effects of rare earth Ce addition on the microstructure, wettability and interfacial reactions of eutectic Sn–0.7Cu solder

The effects of trace RE Ce addition on the microstructure, melting point and wettability of eutectic Sn–0.7Cu solder as well as on the interfacial reactions of Sn–0.7Cu–xCe/Cu(Ni) during soldering were investigated. The Ce bearing solders owned slightly higher melting temperatures than eutectic Sn–0.7Cu solder, with small melting intervals. The addition of trace RE Ce had a diverse effect on the wettability of the solders, and better wettability was obtained on Cu substrate than on Ni substrate. Due to the metamorphism of RE elements, the β-Sn dentrites were refined and the eutectic regions became more uniform in the Sn–0.7Cu–xCe solders with the increase of Ce addition. The intermetallic compound (IMC) formed at Sn–0.7Cu–xCe/Cu interfaces during soldering was scallop-like Cu6Sn5, and those formed at Sn–0.7Cu–xCe/Ni interfaces were (Ni,Cu)3Sn4 and (Cu,Ni)6Sn5. With the increase of Ce addition, the average size, the aspect ratio and the thickness of the Cu6Sn5 grains on the Cu substrate decreased, while those of the IMC grains on the Ni substrate increased. The different growth behavior of the interfacial IMCs on Cu and Ni substrates were discussed.

In situ study on dissolution and growth mechanism of interfacial Cu6Sn5 in wetting reaction

Synchrotron radiation real-time imaging technology was used for in situ study of dissolution and growth behavior of interfacial Cu6Sn5 intermetallic compound (IMC) in Sn/Cu solder interconnect during reflow soldering. The pre-formed Cu6Sn5 grains dissolved into the liquid solder with decreasing aspect ratio in the heating stage, maintained a thin layer of scallop-type in the dwelling stage, and re-precipitated on the existing Cu6Sn5 grains at a faster growth rate with increasing aspect ratio in the cooling stage. The Cu concentration gradient at the interface is responsible for the aspect ratio variation (corresponding to dissolution and re-precipitation of interfacial Cu6Sn5 grains), which is also supported by the simulation of atomic diffusion in the solder based on Fick׳s second law. The growth behavior was well explained by a proposed model based on the Cu concentration gradient.

Heterogeneous nucleation of Cu6Sn5 in Sn–Cu–Al solders

Cu6Sn5 is a common intermetallic in Pb-free soldering. We explore the influence of dilute aluminium additions on the heterogeneous nucleation and grain refinement of primary Cu6Sn5 in Sn–4Cu–xAl solders. For cooling rates relevant to soldering, it is found that 0.02 and 0.2wt%Al cause significant grain refinement of Cu6Sn5 with 0.2wt%Al increasing the number of Cu6Sn5 grains per unit area by a factor of ∼8. Grain refinement is shown to be due to heterogeneous nucleation of Cu6Sn5 on either delta-Cu33Al17 or gamma1-Cu9Al4, coupled with significant constitutional supercooling ahead of growing Cu6Sn5 crystals. Reproducible orientation relationships are measured between the Cu–Al intermetallics and Cu6Sn5, with a planar lattice mismatch of less than 2.5%. The role of potent nuclei and solutal growth restriction are discussed with reference to the grain refinement of structural casting alloys.

Study of IMC at interfaces of Sn3.0Ag0.5Cu3.0Bi0.05Cr/Cu joints during thermal ageing

Purpose – The purpose of this study was to comparatively investigate interfacial intermetallic compounds (IMCs) in the Sn3.0Ag0.5Cu3.0Bi0.05Cr/Cu (SACBC/Cu) and Sn3.0Ag0.5Cu/Cu (SAC/Cu) solder joints, and to determine any differences. Design/methodology/approach – The samples were annealed after isothermal ageing at 150°C for 0, 168 and 500 hours, and their cross-sections were observed by scanning electron microscopy and energy dispersive spectroscopy. Findings – The interfacial IMC morphology in two joints had significant differences. For the Cu/SAC/Cu joints, the granular and short rod-like Ag3Sn particles attached on the surface and boundary of interfacial Cu6Sn5 grains were detected, and they coarsened observably with ageing time at 150°C, and lastly embedded at the grain boundaries. However, for the Cu/SACBC/Cu joints, there were tiny filamentous Ag3Sn growing on the surface of interfacial Cu6Sn5 grains, and the Ag3Sn had a tendency to break into nanoparticles, which would be distributed evenly and cover the IMC layer, profiting from the Bi and Cr precipitates from solder matrix during ageing. Originality/value – The paper implies that the addition of Bi and Cr could affect the IMCs of joints, thereby delaying interfacial reactions between Sn and Cu atoms and improving the service reliability. The SACBC solder is a potential alloy for electronic packaging production.

Cathodic peeling damage of Cu6Sn5 phase in Cu/SnAg3.0Cu0.5/Cu bridge interconnections under current stressing

The cathodic interfacial damage behavior that has been attributed to electromigration is serious but has often been confused with thermomigration damage in solder interconnections. In this paper, after the effects of the non-uniform temperature distribution of Cu/SnAg3.0Cu0.5/Cu bridge joints are decoupled from the effects of the current stress, the microstructural evolution of the cathodic Cu6Sn5 phase is investigated under an average current density of 7.12 × 107 A·m−2 for 0–350 h. The results show that the interfacial Cu6Sn5 peels rather than dissolving completely at the cathode, because of both adhesion degradation at the Cu6Sn5/Cu interface and sustained cathodic stresses. This unrecorded peeling behavior will contribute to the rapid formation of interfacial voids and will thus dramatically increase the risk of interfacial failure. Fortunately, by restricting the intergranular diffusion and enhancing the bond strengths between adjacent Cu6Sn5 grains, an aging pre-treatment of the solder joints is found to be an effective way to slow down the Cu6Sn5 peeling process and achieve robust solder interconnections.