Growth Of Cu6Sn5 Research Articles

Liquid structure of Sn-Ag-xCu solders and its effect on the formation and growth of interfacial Cu6Sn5

The liquid structure of Sn-3.5Ag and Sn-3.5Ag-0.7Cu (wt%) solders was first studied using high-temperature X-ray diffraction method. Only short-range order (SRO) was detected in the liquid Sn-3.5Ag melts. However, besides SRO, medium-range order (MRO) was also detected in the liquid Sn-3.5Ag-0.7Cu melts at 260 °C and 330 °C while not at 400 °C. The MRO structure was considered to be related to Cu6Sn5-type clusters which can stably exist below 330 °C. The parameters of the ordered SRO and MRO clusters were calculated. Then, Sn-3.5Ag-xCu (x = 0, 0.7 and 1.5) solders were reflowed on Cu substrate for different durations to investigate the effect of liquid structure on the formation and growth behavior of interfacial Cu6Sn5 intermetallic compound (IMC). The Cu6Sn5 layers were found to be thicker with higher initial Cu content in the solders. It is proposed that the Cu6Sn5-type MRO clusters provided potential nuclei for the formation of Cu6Sn5, which promoted the IMC growth rate. The difference in liquid structure of the solders was regarded as a key reason for the different IMC growth rate as well as IMC type at the soldering interfaces.

Effect of adding Ag to the medium entropy SnBiIn alloy on intermetallic compound formation

Wetting reactions of the medium entropy alloy of SnBiIn and SnBiInAg on Cu substrate have been investigated. The melting points of both SnBiIn and SnBiInAg are about 80 °C. The reactions were performed at 120 °C, 140 °C, and 160 °C. The kinetics of interfacial intermetallic compound (IMC) growth of Cu6Sn5 was studied to be ripening-controlled with activation energy about 11.1 kJ/mol for SnBiIn and 10.4 kJ/mol for SnBiInAg. However, the addition of Ag has effectively reduced the IMC growth by 20–30%. We propose that it is because Ag has reduced the pre-factor of diffusivity by increasing the entropy of the alloy.

Effect of Morphological Change of Ni3Sn4 Intermetallic Compounds on the Growth Kinetics in Electroless Ni-P/Sn-3.5Ag Solder Joint

Recently, the growth kinetics of the intermetallic compound (IMC) Ni3Sn4 related to Ni metallization has been investigated in comparison to Cu6Sn5 growth on Cu metallization. In the literature, Ni3Sn4 growth in liquid Sn was analyzed for IMC morphology, grain coarsening, grain boundary (GB) grooving, and GB diffusion. However, the growth kinetics of Ni3Sn4 on electroless (EL) Ni-P metallization has rarely been investigated. In this study, the morphological change of Ni3Sn4 on EL Ni-P and its effects on growth kinetics were analyzed in detail during a reaction with Sn-3.5Ag at 250 °C for up to 30 minutes and compared with electroplated (EP) Ni. For both cases, the IMC growth kinetics was presumed to be governed by fast GB diffusion accompanied with grain coarsening. The morphology of Ni3Sn4 on EP Ni changed little throughout the reaction. Therefore, a 1/3 power-law could be applicable. However, the aspect ratio (thickness/diameter) of the Ni3Sn4 grains on EL Ni-P decreased with the increasing reaction time, which resulted in a larger time exponent of 4.2. The morphological change from needle to chunk induced a high rate of grain coarsening. Consequently, the IMC growth rate declined according to the reduction of fast diffusion paths (GB area).

Mechanism and control of preferred Cu6Sn5 growth on single crystal (001)Cu

Preferred orientation Cu6Sn5 prisms are usually formed on (001)Cu single crystal, making use of which can highly improve the packaging reliability of electronic devices. A practical method for morphology controlling of preferred orientation Cu6Sn5 was put forward preliminarily in this article based on temperature variation. The roof-type Cu6Sn5 prisms actually behaved as scallops of highly preferred orientation relationship with (001)Cu substrate in isothermal heating of reflow process and began to transform into prismatic roofs without grain number and orientation changing following it cooled down to solidification. This new phenomenon undiscovered before can pave the way for understanding the precise mechanism of roof-type Cu6Sn5 formation and provide a scientific viewpoint that orientation, size, and outline control of oriented Cu6Sn5 should be realized in different stages of reflow process. This is quite significant for manufacture of morphology-controllable solder interconnections in electronic industry.

Analysis and control of Cu6Sn5 preferred nucleation on single crystal (0 0 1)Cu

Fully oriented-intermetallic interconnects (FOI) of high melting point and service performance have highly considerable potential application in the current generation of 3D electronic packaging. The primary process of fabrication of this structure is the Cu6Sn5 preferred nucleation. In this article, the orientations of 59 successive Cu6Sn5 grains along the radius of (0 0 1)Cu/Sn-3.0Ag solder interface were investigated. Finally, an analysis and control model of Cu6Sn5 preferred nucleation determined by chemical potential gradient of Cu migration was established and applied to address and manage the orientation growth of Cu6Sn5 under different reflow temperatures and solder compositions. The results pave the way for orientation controlling of interfacial intermetallics in 3D packaging technologies.

Activation energy for Cu-Sn intermetallic in CNT-reinforced Sn-1.0Ag-0.5Cu solder

Purpose The purpose of this study is to investigate the addition of 0.05 Wt.% carbon nanotube (CNT) into the Sn-1.0Ag-0.5Cu (SAC) solder on the intermetallic (IMC) growth. Lead-based solders play an important role in a variety of applications in electronic industries. Due to the toxicity of the lead in the solder, lead-free solders were proposed to replace the lead-based solders. Sn-Ag-Cu solder family is one of the lead-free solders, which are proposed and considered as a potential replacement. Unfortunately, the Sn-Ag-Cu solder faces some reliability problems because of the formation of the thick intermetallic compounds. So the retardation of intermetallic growth is prime important. Design/methodology/approach The solder joint was aged under liquid state aging with soldering time from 1 to 60 min. Findings Two types of intermetallics, which are Cu6Sn5 and Cu3Sn were observed under a scanning electron microscope. The morphology of Cu6Sn5 intermetallic transformed from scallop to planar type as the soldering time increases. The addition of carbon nanotube into the SAC solder has retarded the Cu6Sn5 intermetallic growth rate by increasing its activation energy from 97.86 to 101.45 kJ/mol. Furthermore, the activation energy for the Cu3Sn growth has increased from 102.10 to 104.23 kJ/mol. Originality/value The increase in the activation energy indicates that the growth of the intermetallics was slower. This implies that the addition of carbon nanotube increases the reliability of the solder joint and are suitable for microelectronics applications.

Microstructure evolution and growth behavior of Cu/SAC105/Cu joints soldered by thermo-compression bonding

Purpose This paper used a novel technique, which is thermo-compression bonding, and Sn-1.0Ag-0.5Cu solder to form a full intermetallic compound (IMC) Cu3Sn joints (Cu/Cu3Sn/Cu joints). The purpose of the study is to form high-melting-point IMC joints for high-temperature power electronics applications. The study also investigated the effect of temperature gradient on the microstructure evolution and the growth behavior of IMCs. Design/methodology/approach In this paper, the thermo-compression bonding technique was used to form full Cu3Sn joints. Findings Experimental results indicated that full Cu/Cu3Sn/Cu solder joints with the thickness of about 5-6 µm are formed in a short time of 9.9 s and under a low pressure of 0.016 MPa at 450°C by thermo-compression bonding technique. During the bonding process, Cu6Sn5 grew with common scallop-like shape at Cu/SAC105 interfaces, which was followed by the growth of Cu3Sn with planar-like shape between Cu/Cu6Sn5 interfaces. Meanwhile, the morphology of Cu3Sn transformed from a planar-like shape to wave-like shape until full IMCs solder joints were eventually formed during thermo-compression bonding process. Asymmetrical growth behavior of the interfacial IMCs was also clearly observed at both ends of the Cu/SAC105 (Sn-1.0Ag-0.5Cu)/Cu solder joints. Detailed reasons for the asymmetrical growth behavior of the interfacial IMCs during thermo-compression bonding process are given. The compound of Ag element causes a reduction in Cu dissolution rate from the IMC into the solder solution at the hot end, inhibiting the growth of IMCs at the cold end. Originality/value This study used the thermo-compression bonding technique and Sn-1.0Ag-0.5Cu to form full Cu3Sn joints.

The effect of temperature gradient on interfacial Cu6Sn5 growth during thermal compression bonding

The growth of Cu6Sn5 intermetallic compound (IMC) in Cu/Sn/Cu sandwich system is investigated by thermal compression bonding under a temperature gradient of 800 °C/cm at 400 °C on the hot end. The results show that the interfacial Cu6Sn5 layer shows symmetrical growth at both sides at initial stage, which means that the growth of the Cu6Sn5 layer is controlled by chemical potential gradient. The growth rate of interfacial Cu6Sn5 layer will gradually decrease with increasing Cu6Sn5 layer thickness, which means that the effect of chemical potential gradient on the growth of interfacial Cu6Sn5 layer is undermined and the effect of temperature gradient on the growth of interfacial Cu6Sn5 layer is enhanced. This is due to the fact that the Cu atoms migrate from hot end toward cold end during temperature gradient, the growth rate of Cu6Sn5 layer at cold end is faster than that at hot end. The consumption rate of Cu layer is 0.3 μm/s under temperature gradient of 800 °C/cm. By knowing the calculated consumption rate of Cu layer, the Cu layer thickness on SiC power device can be designed when evaluating the reliability of power device during thermal compression bonding.

Formation Mechanism of Novel Sidewall Intermetallic Compounds in Micron Level Sn/Ni/Cu Bumps

A new kind of intermetallic compounds (IMC) were found around copper pillar in micron level bumps. To investigate the formation mechanism, three different sized Sn/Ni/Cu bumps (10 μm, 20 μm, 50 μm) were electroplated then reflowed at 230 °C for 100 s. After reflow process, a thin layer of IMC was formed around copper pillar, which is attributed to surface wetting behavior. After aging at 170 °C and 200 °C for different times, the growth mechanism of sidewall IMC was observed by scanning electron microscopy combined with electron backscatter diffraction (EBSD) technology. Surface diffusion was considered to be the main driving force for sidewall IMC growth for the activation energy of them was found to be much smaller than that in previous studies. The EBSD results showed a preferred orientation of sidewall Cu3Sn grains being perpendicular to copper periphery, which indicated direction of Cu atoms flux during Cu3Sn growth. Formation mechanism of this novel sidewall IMC was proposed based on surface wetting and surface diffusion. The findings contribute to the failure mechanism study in small size bumps and provide insights into the reliability of 3D electronic packaging.

Influence of Bi Addition on Pure Sn Solder Joints: Interfacial Reaction, Growth Behavior and Thermal Behavior

The effects of different Bi contents on the properties of Sn solders were studied. The interfacial reaction and growth behavior of intermetallic compounds (IMCs) layer (η-Cu6Sn5 + e-Cu3Sn) for various soldering time and the influence of Bi addition on the thermal behavior of Sn-xBi solder alloys were investigated. The Cu6Sn5 IMC could be observed as long as the molten solder contacted with the Cu substrate. However, with the longer welding time such as 60 and 300 s, the Cu3Sn IMC was formed at the interface between Cu6Sn5 and Cu substrate. With the increase of soldering time, the thickness of total IMCs increased, meanwhile, the grain size of Cu6Sn5 also increased. An appropriate amount of Bi element was beneficial for the growth of total IMCs, but excessive Bi (⩾ 5 wt%) inhibited the growth of Cu6Sn5 and Cu3Sn IMC in Sn-xBi/Cu microelectronic interconnects. Furthermore, with the Bi contents increasing (Sn-10Bi solder in this present investigation), some Bi particles accumulated at the interface between Cu6Sn5 layer and the solder.

Microstructure and mechanical properties of Sn–1.0Ag–0.5Cu solder with minor Zn additions

Low silver (Ag) solder alloys e.g., SAC 105 (Sn–1.0Ag–0.5Cu) have attracted a great deal of attention recently due to economic concerns and improved in impact resistance as compared to other SAC solder with higher silver content. This work studies the influence of addition of minor zinc (0.1–0.5 wt%) to SAC105 on the interfacial structure between solder and copper substrate during reflow and after aging. Zn has shown significant solubility in Cu–Sn intermetallic compound (IMC) and formed Cu6(Sn,Zn)5 which resided in the bulk microstructure and at the solder/Cu interface. Results reveal that minor Zn addition decreased the thickness of interfacial Cu6Sn5 IMC after reflow and significantly suppressed the growth of interfacial Cu3Sn after thermal aging without changing the IMC’s morphology. It is suggested that Zn exerts its influence by stabilizing Cu6(Sn,Zn)5 and hindering the flow of Sn and Cu atoms at the solder/IMC and IMC/Cu interface. Nanoindentation results showed that Cu6(Sn,Zn)5 exhibited a higher hardness in comparison to Cu6Sn5 and creep performance of SAC + Zn has improved in comparison to that of SAC105.

Abnormal Cu3Sn growth through grain boundary penetration in space-confined Ni-Sn-Cu diffusion couples

An abnormal growth of Cu3Sn through the grain boundaries of Cu6Sn5 formed in Ni/Sn/Cu ternary diffusion couples is reported in this study. The Ni/Sn/Cu diffusion couples are prepared by thermal compression bonding, with the Sn thickness controlled at 10 μm. The diffusion couples are aged at 200 °C for different periods of time. Due to the thinness of the initial Sn layer, the entire Sn layer ware converted into Cu6Sn5 and Cu3Sn only after 52 h or aging. At this stage, both compounds exhibit the layered structure. Afterward, as the reaction continues, Cu3Sn grows thicker at the expense of Cu6Sn5. Interestingly Cu3Sn now preferably grows along the grain boundaries of Cu6Sn5. The rationalization for this abnormal growth pattern is discussed.

Scanning 3DXRD Measurement of Grain Growth, Stress, and Formation of Cu6Sn5 around a Tin Whisker during Heat Treatment.

The 3D microstructure around a tin whisker, and its evolution during heat treatment were studied using scanning 3DXRD. The shape of each grain in the sample was reconstructed using a filtered-back-projection algorithm. The local lattice parameters and grain orientations could then be refined, using forward modelling of the diffraction data, with a spatial resolution of 250 . It was found that the tin coating had a texture where grains were oriented such that their c-axes were predominantly parallel to the sample surface. Grains with other orientations were consumed by grain growth during the heat treatment. Most of the grain boundaries were found to have misorientations larger than 15, and many coincidence site lattice (CSL) or other types of low-energy grain boundaries were identified. None of the grains with CSL grain boundaries were consumed by grain growth. During the heat treatment, growth of preexisting Cu6Sn5 occurred; these grains were indexed as a hexagonal phase, which is usually documented to be stable only at temperatures exceeding 186 . This indicates that the phase can exist in a metastable state for long periods. The tin coating was found to be under compressive hydrostatic stress, with a negative gradient in hydrostatic stress extending outwards from the root of the whisker. Negative stress gradients are generally believed to play an essential role in providing the driving force for diffusion of material to the whisker root.

Unique interfacial reaction and so-induced change in mechanical performance of Sn–3.0Ag–0.5Cu/Cu solder joints formed during undercooled and eutectic liquid soldering processes

A novel technique is proposed to prepare Sn–3.0Ag–0.5Cu/Cu joints at three different processing temperatures, which are called undercooled liquid (UL, 213.0 °C), eutectic liquid (EL, 217.0 °C) and normal liquid (NL, 231.0 °C) soldering temperatures correspondingly, and each of them is lower than, equal to and higher than Sn–3.0Ag–0.5Cu solder’s melting point (217.0 °C), respectively. The interfacial reaction, intermetallic compound growth and mechanical performance of Sn–3.0Ag–0.5Cu/Cu joints formed during UL, EL and NL soldering processes were investigated systematically. Results show that for Sn–3.0Ag–0.5Cu solder as the undercooled liquid melt or eutectic liquid melt or normal liquid melt in the formation of joints, the thickness of interfacial intermetallic compound (IMC) layer in the joints increases with prolonging dwelling time. For three different liquid soldering processes, the change tendency of the interfacial IMC layer thickness is distinct. For joints formed at UL and EL temperatures, the excessive growth of primary Cu6Sn5 and interfacial IMC (mainly Cu6Sn5) can be suppressed during isothermally dwelling in the liquid state for more than 5 min. Ball shear test results show that solder joints formed at UL and EL temperatures have lower value of the maximum shear force (MSF) than those formed during the NL soldering process. For UL and EL types of joints, the fracture occurs either by shear slide fracture totally along the shear tool tip movement plane (i.e., in-plane shear slide fracture) or by shear slide fracture initially along the shear tool tip movement plane and afterwards shear deformation fracture in the solder matrix under the plane (i.e., out-of-plane shear slide and deformation mixed-mode fracture), while the fracture takes place in NL joints only by in-plane shear slide fracture mode.

Effects of Sn grain size on intermetallic compounds formation in 5 µm diameter Cu/Sn pillar bumps

Uniform 5 µm diameter Cu/Sn micro-bump array has been fabricated by multilayer electrodeposition, and the effect of Sn grain orientation and size on Cu6Sn5 and Cu3Sn growth have been studied through FIB–SEM and electron backscatter diffraction. The solder layer in Ф5 µm Cu/matte Sn micro-bump we fabricated contains only four to five columnar Sn grains with no obvious preferred orientation and there is no significant difference in the growth of Cu6Sn5 in different oriented Sn grains. However, the Sn grain size has much more effect on intermetallic compound (IMC) growth. At 150 °C, the diffusion coefficient of Cu/bright-Sn system(~ 200 nm Sn) is calculated to be 13.09 × 10−17 m2 s−1, nearly three times that of Cu/matte–Sn system (2–5 µm Sn), about 4.51 × 10−17m2 s−1. Also, a simple model of boundary diffusion controlled IMC growth is proposed to explore the mechanism of grain size effect. Large Sn grain size will slow down the interfacial reaction in micro-bump due to low grain boundary density and large bulk diffusion distance.

Void Formation and Intermetallic Growth in Pulse Electrodeposited Cu-Sn Layers for MEMS Packaging

Electrodeposited copper (Cu)-tin (Sn) based solid–liquid interdiffusion (SLID) bonding is becoming popular in wafer-scale packaging of inertial Micro-Electro-Mechanical-Systems (MEMS) sensors due to its inherent advantages of lower cost, low processing temperatures and less stringent surface uniformity requirements. However, eliminating micron-size voids within intermetallic compounds (IMCs) and bond interfaces has remained a challenging task. The present study focuses upon IMC growth and void formation at varying temperatures and times. Stacks of varying thickness of Cu and Sn were fabricated by electrodeposition, and the samples were annealed at temperature ranging up to 300°C. Scalloped shaped Cu6Sn5 (η-phase) and comparatively uniform Cu3Sn (e-phase) intermetallics were observed. Experimental results show that the growth of metastable Cu6Sn5 dominates IMC formation at lower temperatures but as temperature increases, Cu3Sn dominates over the Cu6Sn5 growth. This IMC growth transition from Cu6Sn5 dominant growth to Cu3Sn dominant growth depends on the annealing temperature and has a critical time duration. The IMC thicknesses are compared with those obtained by numerical simulation models. For given annealing temperatures, intermittent voids formed in the IMC layers show increasing size and decreasing void fraction trends with increasing annealing times. The results suggest that Cu-Sn SLID bonding performed at 275°C yields reliable bonding since the void growth is minimal. Based on these results, a test vehicle containing a kelvin structure and daisy chains (having large number of Cu-Sn bonded structure), was fabricated, resulting in electrical resistances lower than 30 m-ohms and 6 ohms, respectively.

Effects of Static Heat and Dynamic Current on Al/Zn∙Cu/Sn Solder/Ag Interfaces of Sn Photovoltaic Al-Ribbon Modules

This present study applied Cu∙Zn/Al ribbon in place of a traditional Cu ribbon to a photovoltaic (PV) ribbon. A hot-dipped and an electroplated Sn PV ribbon reflowed onto an Ag electrode on a Si solar cell and estimated the feasibility of the tested module (Ag/Solder/Cu∙Zn/Al). After bias-aging, a bias-induced thermal diffusion and an electromigration promoted the growth of intermetallic compounds (IMCs) (Cu6Sn5, Ag3Sn). To simulate a photo-generated current in the series connection of solar cells, an electron with Ag-direction (electron flows from Ag to Al) and Al-direction (electron flows from Al to Ag) was passed through the Al/Zn∙Cu/Solder/Ag structure to clarify the growth mechanism of IMCs. An increase in resistance of the Ag-direction-biased module was higher than that of the Al-direction biased one due to the intense growth of Cu6Sn5 and Ag3Sn IMCs. The coated solder of the electroplated PV ribbon was less than that of the hot-dipped one, and thus decreased the growth reaction of IMCs and the cost of PV ribbon.

Microstructure, interfacial reactions and mechanical properties of Co/Sn/Co and Cu/Sn/Cu joints produced by transient liquid phase bonding

In this paper, the wettability and growth behaviors of interfacial intermetallic compounds (IMCs) in the Sn/Co and Sn/Cu liquid/solid couples were comparatively investigated. In situ tensile tests were conducted to evaluate the mechanical properties of the Co/Sn(IMCs)/Co and Cu/Sn(IMCs)/Cu joints. The wettability of Sn on Cu substrate was better than that of Co substrate. The lath-like CoSn3 IMCs limited the improvement of wettability of Sn on Co substrate, even if the soldering temperature was increased. The Sn/Co couples showed an acceptable wettability for electronic application. During liquid aging, the rapid growing interfacial CoSn3 were controlled by the combination of chemical reaction and atomic diffusion. Grain boundary diffusion should be responsible for the growth of Cu6Sn5 in the Sn/Cu liquid/solid couples. The tensile strength and elongation of the Co/CoSn3/Co joints were basically equal to that of the Cu/IMCs/Cu full IMCs joints. Interfacial IMCs cracks were more likely to occur in the joints fabricated by Co substrate. The Co/CoSn3/Co sandwich structure can be fabricated within a remarkable short period of time at low temperature without pressure.

All-round suppression of Cu6Sn5 growth in Sn/Cu joints by utilizing TiO2 nanoparticles

Both $$\text {TiO}_{2}$$ and Cu nanoparticles, when added at the interface of the Sn/Cu joints during reflow soldering at 250 °C for 10, 60, 120 s and subsequent air cooling, are observed to reduce the growth of $$\text {Cu}_{6}\text {Sn}_{5}$$ whiskers. Though both reinforcments reduce the vertical growth of intermetallic compound (IMC), only $$\text {TiO}_{2}$$ could suppress the lateral growth of the compound. The solder saturation level with Cu at different reflow duration can influence final dimensions as well as morphology of IMC and occurrence of screw dislocation. The addition of $$\text {TiO}_{2}$$ , by intervening the mechanism of ostwald ripening during reflow can be utilized to enhance the solder-IMC bonding at interface.

Effect of Intermetallic Content on Shear Deformation of Thin Sn-3.0Ag-0.5Cu Solder Micro-joints Between Copper Substrates

In 3D electronic packages, stacked dies are connected vertically using through-silicon vias and solder micro-bumps, which are typically between 1 μm and 50 μm thick. Solder micro-joints undergo significant shear deformation due to various loading conditions, which can occur during usage of microelectronic devices, such as thermal cycling, mechanical bending, and drop impact. A limited amount of work has been done in shear deformation and failure mechanism of these joints. To explore this, 25-μm-thick joints of SAC305 solder between two Cu substrates were tested, containing three different Cu6Sn5-to-Cu3Sn ratios, in shear at strain rates from 1 s−1 to 100 s−1. The joint shear strength is correlated with observed failure mechanisms such as Sn, Cu6Sn5, Cu3Sn and Cu6Sn5/Cu3Sn interface failure. The growth kinetics of intermetallic compounds (IMCs) in thin Sn-3Ag-0.5Cu joints attached to Cu substrates have been analyzed, and empirical kinetic laws for the growth of Cu6Sn5 and Cu3Sn in thin joints are reported. By combining the shear deformation results, we infer that increased IMC content due to heat treatment deteriorates the mechanical properties of the joint due to the presence of disconnected incipient micro-cracks. Deformation and damage are controlled by the intermetallics, and not the strain-rate sensitive solder for the aged samples. Both Cu6Sn5 and the Cu6Sn5/Cu3Sn interface fracture are the dominant mechanisms with increasing aging under the shear deformation.