Formation Of Cu6Sn5 Research Articles

Microstructure of ternary Sn-Bi-xCu alloy on mechanical properties, current endurance and corrosion morphology via cycling corrosion test

Low melting temperature of Sn-58 wt% Bi (SB) solders adding 0.5 wt% Cu (SB05C) and 1.7 wt% Cu (SB17C) concentrations were evaluated for the signal transmission in the 3D IC package. Microstructure combined with phase analysis and Pandat simulation were together with to explain the relationship among the mechanical property, electrical endurance and corrosion resistance. By Cu decoration, the presence Cu6Sn5 intermetallic compound (IMC) acts as heterogeneous nucleation sites to reduce surface free energy and generate fine Bi grain that resulted in higher hardness value. The power-law was used to explain the mechanism of interfacial IMC growth, both Cu6Sn5 and Cu3Sn formation were speculated to be volume diffusion control and surface reaction domination, respectively. The fracture morphology by shear testing indicates that earliest plastic deformation accompanied with brittle rupture were major factors due to the intergranular fracture and Cu6Sn5 compound induced bulk fracture. By cycling corrosion test (CCT), the lump shape of SnO2 by-product was found among solders at three cycling. After seven cycling, Sn phase has been completely corroded and Cu6Sn5 IMC began to be corroded forming circle shape of Cu2O but Bi phase still not be corroded among solders. In the electrical endurance, dense Cu6Sn5 dispersed in solder bulk and fine grain size were feasible to induce current crowding and joule heating, thus Cu deteriorated SB solders is easily failed.

The Effect of Bi Addition on the Electromigration Properties of Sn-3.0Ag-0.5Cu Lead-Free Solder

As electronic packaging technology advances towards miniaturization and integration, the issue of electromigration (EM) in lead-free solder joints has become a significant factor affecting solder joint reliability. In this study, a Sn-3.0Ag-0.5Cu (SAC305) alloy was used as the base, and different Bi content alloys, SAC305-xBi (x = 0, 0.5, 0.75, 1.0 wt.%), were prepared for tensile strength, hardness, and wetting tests. Copper wire was used to prepare EM test samples, which were subjected to EM tests at a current density of approximately 0.6 × 104 A/cm2 for varying durations. The interface microstructure of the SAC305-xBi alloys after the EM test was observed using an optical microscope. The results showed that the 0.5 wt.% Bi alloy exhibited the highest ultimate tensile strength and microhardness, improving by 33.3% and 11.8% compared to SAC305, respectively, with similar fracture strain. This alloy also displayed enhanced wettability. EM tests revealed the formation of Cu6Sn5 and Cu3Sn intermetallic compounds (IMCs) at both the cathode and anode interfaces of the solder alloy. The addition of Bi inhibited the diffusion rate of Sn in Cu6Sn5, resulting in similar total IMC thickness at the anode interface across different Bi contents under the same test conditions. However, the total IMC thickness at the cathode interface decreased and stabilized with increasing EM time, with the SAC305-0.75Bi alloy demonstrating the best resistance to EM.

Interfacial reaction of Sn-1.5Ag-2.0Zn low-silver lead-free solder with oriented copper

High density packaging technology reduces the pad size and the number of grains contained in the pad. When the polycrystalline pad turns into a single crystal pad, the grain orientation has an important impact on the formation of the intermetallic compound (IMC) at the interface. The growth of IMC at the interface between the solder and the single-crystal copper substrate is investigated by selecting the prospective Sn-1.5Ag–2Zn as the solder alloy. Sn-1.5Ag-2.0Zn lead-free solder joints soldered with single crystal (111) copper substrate and polycrystalline red copper substrate are reflowed at 250 °C for 5 min. Samples are subsequently aged at 160 °C. The uneven scallop like Cu6Sn5 IMC layer grows rapidly when the alloy solder contacts with the copper substrate. The Cu6Sn5 grain size formed on the surface of single crystal copper is larger than that of polycrystalline copper. Single crystal copper has no grain boundary to block atomic diffusion, which affects grain nucleation and growth. The growth rate of Cu6Sn5 formed by alloy solder and the single crystal (111) copper solder joint after aging treatment at 160 °C for 20 h is about twice that of the polycrystalline copper solder joint. Then it grows slowly with the increase of aging treatment time. The thick layer Cu6Sn5 breaks due to crack diffusion after 600 h of aging treatment, and the thickness of IMC remains at 3.5 μm. Cu5Zn8 generated at the solder and polycrystalline copper solder joint during aging treatment acts as a barrier layer, preventing the solder from contacting the copper substrate and inhibiting the formation of Cu6Sn5. Cu5Zn8 is broken and decomposed after 300 h of aging treatment, and Cu6Sn5 grows rapidly after the barrier layer disappeared. The thickness of Cu6Sn5 is about 2.8 μm. The thickness of IMC of solder joint on single crystal copper is 0.7 μm more than that on polycrystalline copper. After aging treatment, the IMC formed at the interface between alloy solder and single crystal copper has better compactness and basically no pores, while there are obvious pores between IMC grains at the interface between alloy solder and polycrystalline copper, which can predict that the soldering quality of the interface between alloy solder and single crystal copper is better. This will provide application prospects for solder joint interface reliability research.

Pseudo lamellae of Cu6Sn5 on the crystal facet of Sn in electrodeposited eutectic Sn-Cu lead-free solder

Eutectic Sn-Cu lead-free solder is developed using the pulse electrodeposition method from a citrate bath at different current densities like 0.1, 0.2, 0.4, and 0.5 A/cm2. The exact composition of all the deposits was confirmed through atomic absorption spectroscopy (AAS) analysis. The coating deposited at 0.2 A/cm2 constitutes 0.7 wt% of Cu which is similar to the eutectic Sn-Cu composition. Microstructural characterization was carried out with the help of a scanning electron microscope (SEM). Sn was the major constituent in all the deposits while a little amount of intermetallic i.e. Cu6Sn5 was present for the deposits at 0.1 and 0.2 A/ cm2. The energy dispersive spectroscopy (EDS) results showed traces of Cu6Sn5 in the deposits. The formation of Cu6Sn5 was also confirmed through X-ray diffraction (XRD). For the deposit at 0.2 A/cm2, the endothermic peak temperature in DSC is exactly the same as that of the melting point of Sn-Cu eutectic. This electrodeposited microstructure resembles the conventional eutectic Sn-Cu cast structure. The top view of the deposited Cu6Sn5 on the steps of Sn creates an illusion of lamellar structure. The effect of current density on the roughness was analyzed through AFM.

In-situ isothermal aging TEM analysis of a micro Cu/ENIG/Sn solder joint for flexible interconnects

Sn/ENIG has recently been used in flexible interconnects to form a more stable micron-sized metallurgical joint, due to high power capability which causes solder joints to heat up to 200 °C. However, Cu6Sn5 which is critical for a microelectronic interconnection, will go through a phase transition at temperatures between 186 and 189 °C. This research conducted an in-situ TEM study of a micro Cu/ENIG/Sn solder joint under isothermal aging test and proposed a model to illustrate the mechanism of the microstructural evolution. The results showed that part of the Sn solder reacted with Cu diffused from the electrode to form η´-Cu6Sn5 during the ultrasonic bonding process, while the rest of Sn was left and enriched in a region in the solder joint. But the enriched Sn quickly diffused to both sides when the temperature reached 100 °C, reacting with the ENIG coating and Cu to form (NixCu1-x)3Sn4, AuSn4, and Cu6Sn5 IMCs. After entering the heat preservation process, the diffusion of Cu from the electrode to the joint became more intense, resulting in the formation of Cu3Sn. The scallop-type Cu6Sn5 and the seahorse-type Cu3Sn constituted a typical two-layered structure in the solder joint. Most importantly, the transition between η and η’ was captured near the phase transition temperature for Cu6Sn5 during both the heating and cooling process, which was accompanied by a volume shifting, and the transition process was further studied. This research is expected to serve as a reference for the service of micro Cu/ENIG/Sn solder joints in the electronic industry.

Electrochemical formation of Sn films on copper by overpotential and underpotential electrodeposition in deep eutectic solvents

The electrodeposition process of tin films on copper electrodes in ethaline and reline deep eutectic solvents (DES) was studied in the 303 – 353 K range. Voltammetric data indicate the presence of underpotential and overpotential electrodeposition processes. While the former occurs under surface-reaction control, the latter proceeds under mass transport control. Electrochemical impedance spectroscopy shows two capacitive contributions during the underpotential process and a single capacitive contribution at high frequencies and a Warburg contribution at low frequencies, when the electrodeposition process takes place in the overpotential region. Tin deposits obtained in ethaline exhibit blunt particles with ordered structures whereas in reline facetted particles with no preferential order are observed as electrodeposition time is increased. The electrocrystallization mechanism under overpotential conditions in ethaline and reline corresponds to an instantaneous nucleation and a 3D growth process. For underpotential conditions in ethaline, an instantaneous nucleation and 2D growth coupled to an adsorption process occurs. XRD spectra shows the formation of Cu3Sn and Cu6Sn5 intermetallics due to the diffusion of Sn atoms into the Cu lattice during the electrodeposition process. From rotating disk electrode measurements, the diffusion coefficient of Sn(II) ions in both DES at different temperatures, and the diffusion activation energy, were determined.

Thermomigration suppression in Sn3.5Ag solder joints by hot-end FeCoNiMn alloy

Temperature gradient is inevitable in solder joints in advanced electronic packaging; it induces thermomigration (TM), critically affecting the reliability of micro solder joints. Although a FeCoNiMn alloy (FCNM) can resist thermal aging in solder joints, its TM resistance has seldom been examined. Thus, this study investigated the TM in Cu/Sn3.5Ag (SA3.5)/FCNM solder joints between a cold end (50 °C) and a hot end (200 °C) at different times. A multielement intermetallic compound (MEIMC), (Cu, Mn, Fe, Co, Ni)6Sn5, is formed at the SA3.5/FCNM interface in the reflowed and aged solder joints. A (Fe, Co, Ni, Mn)Sn2 MEIMC also formed, but it was very thin. The (Cu, Mn, Fe, Co, Ni)6Sn5 was completely decomposed by TM from the hot-end FCNM. In contrast, the (Cu, Mn, Fe, Co, Ni)6Sn5 MEIMC thickened as the rapid TM flux of Cu from the hot-end Cu cap; however, the low chemical flux from FCNM hindered its formation, which was replaced by Cu6Sn5 formation near the Cu/SA3.5 interface. The findings demonstrate that FCNM providing a very low chemical flux is a remarkably promising diffusion barrier for TM in micro solder joints.

Role of Crystallographic Orientation of β-Sn Grain on Electromigration Failures in Lead-Free Solder Joint: An Overview

Due to the miniaturization of electronic devices, electromigration became one of the serious reliability issues in lead-free solder joints. The orientation of the β-Sn grain plays an important role in electromigration failures. Several studies have been carried out to investigate the effect of Sn grain orientation on electromigration. The efforts involve the influence of β-Sn grain orientation on the migration of Cu, Sn, and Ni atoms, on the morphology of the solder joint, and on the formation of Cu6Sn5 and (Cu, Ni)6Sn6 in the lead-free solder joint during electromigration. The current review provides a detailed review of past studies which were conducted to investigate the influence of β-Sn grain orientation on electromigration failures in lead-free solder joints.

CALPHAD-guided optimization of the microstructure of Sn–Sb–Cu bearing alloys

The increasing Cu content of Sn–11Sb-xCu (wt.%, x = 0, 2, 4, 6) bearing alloys promotes the growth of ordered Sn3Sb2, and Sn3Sb2 with the maximum mean diameter (161.3–177.3 μm) is observed in Sn–11Sb–6Cu alloy. Gravity-driven aggregation of Sn3Sb2 occurs in the top region of Sn–11Sb ingot. The inhomogeneous distribution of Sn3Sb2 driven by density difference between Sn3Sb2 and matrix is suppressed with Cu addition due to the formation of Cu6Sn5 with a density of 8.06 g/cm3, and homogeneous microstructure is obtained in Sn–11Sb-xCu alloys (x = 2, 4, 6). Sn3Sb2 changes from regular cube morphology to irregular morphology with Cu content reaching 6 wt%. The relatively high Cu content in liquid leads to the nucleation of Cu6Sn5 around the liquid/Sn3Sb2 interface. Cu6Sn5 inside Sn3Sb2 can thus effectively affect the growth direction of Sn3Sb2, and finally lead to the formation of Sn3Sb2 with irregular morphology. The influence of Cu on microstructure evolution of Sn–Sb–Cu bearing alloys is revealed with the guidance of phase diagram calculation (CALPHAD), which can contribute to the further optimization of Sn–Sb–Cu and other bearing alloy.

Effects of bonding pressures on microstructure and mechanical properties of silver–tin alloy powders synthesized by ball milling for high-power electronics packaging

Silver (Ag) sintering is widely adopted as a die attach method for power electronics packaging. However, oxidation, massive grain growth, and increasing porous structure occur in sintered Ag joints at high temperatures, resulting in decreasing mechanical properties of the die attachment in power devices. To solve the problems of sintered Ag joints, a novel die attach material consisting of silver–tin (Ag–Sn) alloy particles is developed in this study. Ag–Sn alloy powders were newly fabricated by high-energy ball milling technique. The microstructural evolutions of the Ag–Sn alloy joints bonded at different pressures were systematically analyzed. With increasing high-temperature storage time at 300 °C, the reactions between sintered Ag–Sn alloy joints and Cu substrates increased, which resulted in the formation of Cu3Sn or Ag-rich diffusion layer. The relationships between microstructures and the formation of each phase were discussed. In addition, the robust sintered Ag–Sn alloy joints exhibited low porosity without pore coarsening and limit oxidation at high temperatures, indicating a dense and stable microstructure of sintered Ag–Sn alloy joints. As a result, high bonding strength did not deteriorate even after high-temperature storage for 2000 h. It is concluded that the novel Ag–Sn alloy paste produced by high-energy ball milling has long-term high-temperature reliability to be used as a die attach material for the power electronics packaging, thus enabling high-power devices to have higher operating temperatures and thermal stability.

Inhibiting effects of the Ni barrier layer on the growth of porous Cu3Sn in 10-μm microbumps

With the shrinkage of size, porous Cu3Sn has become a new potential threat of the reliability in Cu pillar microbump. The formation of porous Cu3Sn is caused by the decomposition of Cu6Sn5 after Sn atoms in solder depletion. Besides, the stress introduced by the phase transition from η-Cu6Sn5 to η′-Cu6Sn5 will promote the formation of porous Cu3Sn. In this paper, uniform Ф10 µm Cu/Sn and Cu/Ni (~ 0.6 μm)/Sn microbumps have been fabricated by multilayer electrodeposition and the effect of the Ni layer on the growth behavior of porous Cu3Sn was investigated by comparing IMCs’ (Intermetallic Compound) evolution in Cu/Sn and Cu/Ni/Sn bumps aged at 170 °C and 200 °C. The ~ 0.6 μm Ni barrier layer can effectively retard the Cu atoms diffusion, which can hinder IMC from overgrowth. Moreover, the results of X-ray diffraction indicate that the ability of the Ni layer can stabilize Cu6Sn5 phase and weakens the tendency of the porous Cu3Sn formation. Under the conjoint action of retarding the growth of IMC and stabilizing Cu6Sn5 phase, the Ni layer can inhibit the formation of porous Cu3Sn efficaciously.

Near room-temperature formation of Cu3Sn: In-situ synchrotron X-ray diffraction characterization and thermodynamic assessments of its nucleation

The fabrication of Cu3Sn at near room temperatures and atmospheric pressure remains out of reach. We propose a novel concept to address this challenge by exploiting both the classical nano-island model for dealloying and the nanocurvature effect on nucleation, in conjunction with bespoke precursor alloy design. In-situ synchrotron X-ray diffraction (XRD) confirmed significant formation of Cu3Sn at both 70°C and 55°C in 180 min via a dealloying-realloying process in the designed precursor alloy Al67Cu18Sn15, compared with much slower formation at 125°C by conventional approaches. Furthermore, Cu3Sn formed just 10 min after Cu6Sn5 emerged at 70°C, being two orders of magnitude faster than in Cu/Cu6Sn5 diffusion couples at 125°C. The energy barrier to the formation of a lenticular Cu3Sn nucleus on Cu6Sn5 nanocaps was assessed systematically using the CALPHAD approach through the initial driving force (DF) concept and DF-plateau method. The predictions were benchmarked against diffusion couple studies, which fully supported the experimental results. In addition, the in-situ synchrotron XRD data on phase evolution was analysed in detail using the well-established Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, which provided further fundamental support to our conceptual design. The concept and methodology demonstrated are expected to be applicable to the fabrication of other challenging materials.

Intermetallic compounds formed in Sn droplet on Cu substrate under the impact of electric currents

The present paper investigates the influence of electric currents on the formation of Sn/Cu intermetallic compounds. Solidification experiments of Sn droplets on Cu substrate were performed under the impact of direct csurrents (DC) and pulsed electric currents (ECP), respectively. The results show that the precipitation of Cu6Sn5 from the Sn/Cu interfacial reaction is significantly enhanced by both DC and ECP. DC promotes the formation of Cu6Sn5 at the interface, whereas ECP triggers the detachment of Cu6Sn5 at the interface and favors the growth of freely floated Cu6Sn5 particles in the bulk Sn melt. Numerical simulations were conducted to calculate the distribution of electric current, induced magnetic field, Lorentz force and forced flow inside the Sn melt. The numerical results show that a strong downwards Lorentz force is generated adjacent electrode bottom owing to the interaction between electric current and induced magnetic field. As a result, a global forced flow is caused inside the Sn melt. It is possible that the forced flow induces the elements transport to promote the precipitation of intermetallic compounds. Moreover, a strong Lorentz force difference between Sn melt and Cu6Sn5 is generated under the application of ECP compared with that of DC, thereby increasing the potential to initiate the detachment of Cu6Sn5.

The fracture mechanism of Cu3Sn-microporous copper composite joint by thermal compression bonding process

The bonding performances of the composite joint that made of microporous copper and Sn were investigated in this paper. It could rapidly generate .The Cu3Sn- microporous copper composite joint with a thickness of 100 μm was successfully obtained after bonding at 360 °C for 5 min under a low pressure of 0.6 MPa.The shear strength of the composite joint reached 166.05 MPa, which was 2.5 times higher than that of full Cu3Sn joint. The fracture mechanism of composite joint was analyzed by fracture morphology.A large amount of copper could be observed in the fractured surface of composite joint.The self-locking structure was generated between the Cu3Sn and microporous copper in the composite joint.The microporous copper provided more nucleation particles during the formation of Cu3Sn and finally resulted in the refinement of the Cu3Sn grains. In addition,the occurrence of fracture was delayed due to the plastic deformation of microporous copper.

Effects of Ni on the nucleation and growth behavior of Cu6Sn5 in Sn–8.5Cu alloy: An in situ observation

Cu6Sn5 and Cu3Sn are important intermetallic compounds, greatly influencing the reliability of the Sn-Cu solder system. Micro-alloying Ni has been proved to be effective in suppressing Cu3Sn, but its influence on the formation of Cu6Sn5 is still unclear. In this paper, the nucleation and growth behavior of Cu6Sn5 in Sn-8.5Cu-xNi (x = 0, 0.05, 0.1, 0.5, 1) alloys are in situ investigated using synchrotron X-ray radiography, X-ray diffraction, and electron probe micro-analyzer. The results show that Ni can significantly increase the nucleation temperature of (Cu, Ni)6Sn5 since (Cu, Ni)6Sn5 has a more negative heat of formation. When Ni ≥ 0.05 wt%, the nucleation temperature of (Cu, Ni)6Sn5 will be higher than that of Cu3Sn. As a result, the peritectic reaction Cu3Sn + L → Cu6Sn5 doesn't occur and instead (Cu, Ni)6Sn5 nucleates from the melt before cooling down to the nucleation temperature of Cu3Sn. During the nucleation and growth of (Cu, Ni)6Sn5, Ni-rich clusters preferentially form stable crystal nuclei, and then low Ni parts grow on them. Raising Ni content increases the number of Ni-rich clusters, so the grain density of (Cu, Ni)6Sn5 increases with Ni addition. With the growth of (Cu, Ni)6Sn5, some Ni-rich clusters are in contact with (Cu, Ni)6Sn5-liquid interface and promote the rapid growth of (Cu, Ni)6Sn5 around the newly formed Ni-rich region. The excessively enlarged parts break down at the weak points because of gravity, further refine the grain. The results based on this study provide an opportunity to optimize the structure of Cu6Sn5 in the Sn-Cu system.

Interface behavior and electrical properties of Zn–Sn–Cu (CTZ) stacking layer films with thermal diffusion and electrically induced crystallization

In this study, a Zn–Sn–Cu sandwich structure was heated and electrically crystallized. The morphology, phase structure, diffusion behavior, and interface electrical properties of the sandwich structure and the microstructure changes in Zn, Sn, and Cu driven by heat and electricity were investigated. The experimental results revealed that thermal effects caused marked diffusion of Cu atoms. Cu6Sn5 IMC was formed at the Cu–Sn interface, and Cu5Sn8 was formed at the Zn–Sn interface. The electron flow drove Sn atoms into Cu with directional diffusion and inhibited Cu6Sn5 formation. Moreover, the overall electrical properties, interlayer thickness, and microstructure differences varied with the diffusion behavior and interface composition caused by thermal and electrical effects.

Enhancement of electrochemical and thermal bonding reliability by forming a Cu3Sn intermetallic compound using Cu and Sn–58Bi

Cu–Cu bonding was successfully achieved through the transient liquid phase sintering (TLPS) bonding method using Cu and an eutectic Sn–Bi alloy. The bonding process was performed for total bonding process time of 2 h under a pressureless condition in air. The bonding temperature was 220 °C considering that the Cu6Sn5 intermetallic compound (IMC) and Bi react at 195.9 °C to transform into Cu3Sn and a liquid. The shear strength of Cu-70(Sn–58Bi) was approximately 22.06 MPa with the formation of the Cu3Sn. The residual eutectic Sn–Bi structure remained at Cu-90(Sn–58Bi), and the voids were largely observed at Cu-50(Sn–58Bi). In case of Cu-70(Sn–58Bi) and Cu-50(Sn–58Bi), the remelting temperature increased to 270 °C by the IMC reaction between Cu and Sn. On the other hand, the remelting temperature of Cu-90(Sn–58Bi) joint remained at 139 °C due to residual eutectic Sn–Bi structure. The salt spray test was conducted at 35 °C under the 5% NaCl condition for 24, 48, and 96 h to evaluate the bonding reliability under the salt spray condition. The corrosion behavior was evaluated using the electrochemical impedance spectroscopy measuring method. In addition, the thermal bonding reliability was investigated through a high-temperature storage test at 200 °C for 100, 500, and 1000 h. Analysis results of the experiment showed that the electrochemical and thermal bonding reliabilities were improved with the Cu3Sn formation owing to its high resistance properties under the salt spray and high-temperature conditions.

Origin of Primary Cu6Sn5 in Hypoeutectic Solder Alloys and a Method of Suppression to Improve Mechanical Properties

This study examines factor(s) behind the formation of primary Cu6Sn5 (in the bulk, rather than at the interface) in solder joints, even though solder alloys are hypoeutectic. To understand the contribution from copper (Cu) dissolution from the substrate a Cu-free alloy, tin-3.5 silver (Sn-3.5Ag), was used as a soldered-on copper organic solderability preservative (Cu-OSP) and electroless nickel immersion gold (ENIG) surface finish substrates. Microstructure observations including in situ synchrotron were used to observe microstructure development real-time and confirm the time and location for nucleation of primary Cu6Sn5. High-speed shear tests were performed to determine the solder joint’s strengths. The results confirm that Cu dissolution during soldering is responsible for the formation of primary Cu6Sn5. The ENIG finish prevented Cu dissolution and the formation of Cu6Sn5 resulting in higher solder joint strength for the Sn-3.5Ag/ENIG solder joints. The findings can be used to understand the evolution of primary Cu6Sn5 and how it can be suppressed to improve joint strength.

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.

Microstructure Influence of SACX0307-TiO2 Composite Solder Joints on Thermal Properties of Power LED Assemblies.

The effect of the microstructure of solder joints on the thermal properties of power LEDs is investigated. Solder joints were prepared with different solder pastes, namely 99Sn0.3Ag0.7Cu (as reference solder) and reinforced 99Sn0.3Ag0.7Cu–TiO2 (composite solder). TiO2 ceramic was used at 1 wt.% and with two different primary particle sizes, which were 20 nm (nano) and 200 nm (submicron). The thermal resistance, the electric thermal resistance, and the luminous efficiency of the power LED assemblies were measured. Furthermore, the microstructure of the different solder joints was analyzed on the basis of cross-sections using scanning electron and optical microscopy. It was found that the addition of submicron TiO2 decreased the thermal and electric thermal resistances of the light sources by 20% and 16%, respectively, and it slightly increased the luminous efficiency. Microstructural evaluations showed that the TiO2 particles were incorporated at the Sn grain boundaries and at the interface of the intermetallic layer and the solder bulk. This caused considerable refinement of the Sn grain structure. The precipitated TiO2 particles at the bottom of the solder joint changed the thermodynamics of Cu6Sn5 formation and enhanced the spalling of intermetallic grain to solder bulk, which resulted in a general decrease in the thickness of the intermetallic layer. These phenomena improved the heat paths in the composite solder joints, and resulted in better thermal and electrical properties of power LED assemblies. However, the TiO2 nanoparticles could also cause considerable local IMC (Intermetallic Compounds) growth, which could inhibit thermal and electrical improvements.