Cu6Sn5 Intermetallic Compounds Research Articles

Laser induced forward transfer of brittle Cu3Sn thin film

Abstract Cu3Sn intermetallic compound as an interconnecting material has great prospect in power electronic device applications due to its high melting temperature. However, the intermetallic compound is essentially brittle, and hard to be transferred. In this study, Cu3Sn thin films have been successfully transferred by means of laser-induced forward transfer (LIFT) process with millisecond pulse length. The morphology of LIFT-processed Cu3Sn thin film was investigated by optical microscopy and Atomic Force Microscopy (AFM). The optimized transfer process parameters were 0.3 ms laser irradiation, 8001100 W laser power and 14 mm defocusing amount. Under the optimized conditions, the deposition pattern of Cu3Sn thin films were complete and uniform. The transfer mechanism was that the solid Cu3Sn films have been pushed and transferred by the gasified part Cu3Sn near the interface of the donor substrate during the LIFT process.

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.

Shear Strength and Aging Characteristics of Sn-3.0Ag-0.5Cu/Cu Solder Joint Reinforced with ZrO2 Nanoparticles

This study investigates the shear strength and aging characteristics of Sn-3.0Ag-0.5Cu (SAC 305)/Cu joints by the addition of ZrO2 nanoparticles (NPs) having two different particle size: 5–15 nm (ZrO2A) and 70–90 nm (ZrO2B). Nanocomposite pastes were fabricated by mechanically mixing ZrO2 NPs and the solder paste. ZrO2 NPs decreased the β-Sn grain size and Ag3Sn intermetallic compound (IMC) in the matrix and reduced the Cu6Sn5 IMC thickness at the interface of lap shear SAC 305/Cu joints. The effect is pronounced for ZrO2A NPs added solder joint. The solder joints were isothermally aged at 175 °C for 24, 48, 144 and 256 h. NPs decreased the diffusion coefficient from 1.74 × 10–16 m/s to 3.83 × 10–17 m/s and 4.99 × 10–17 m/s for ZrO2A and ZrO2B NPs added SAC 305/Cu joints respectively. The shear strength of the solder joints decreased with the aging time due to an increase in the thickness of interfacial IMC and coarsening of Ag3Sn in the solder. However, higher shear strength exhibited by SAC 305-ZrO2A/Cu joints was attributed to the fine Ag3Sn IMC’s dispersed in the solder matrix. Fracture analysis of SAC 305-ZrO2A/Cu joints displayed mixed solder/IMC mode upon 256 h of aging.

Effect of Graphene Nanosheet Addition on the Wettability and Mechanical Properties of Sn-20Bi-xGNS/Cu Solder Joints.

Graphene nanosheets (GNSs) have an extensive application in materials modification. In this study, the effects of graphene nanosheets on the wettability of Sn-20Bi lead-free solder on copper (Cu) substrate and the growth behavior of intermetallic compound (IMC) layers at Sn-20Bi-xGNS/Cu solder joints were investigated. The experimental results indicate that the wettability of Sn-20Bi solder firstly diminished and then increased by the addition of GNSs. Meanwhile, a prism-shaped and scallop-shaped Cu6Sn5 IMC layer was clearly observed at the interface of the solder/substrate system. Moreover, it was found that a small amount of GNS addition can significantly inhibit the growth of the IMC layer at the interface as well as refine the microstructure. Additionally, by nano-indentation apparatus, it can be concluded that the hardness and elastic module of IMCs show the same variation trend, which firstly decreased and then increased. Besides, to intuitively demonstrate the reliability of IMCs, the relationship between the hardness and elastic module was established, and the ratio of hardness/elastic module (H/E) was adopted to characterize the reliability of IMCs. The results show that when the addition of GNSs was 0.02 wt%, the value of H/E is the minimum and the solder joint has the highest reliability.

Study on the thermal treatment conditions for fabricating open-cell structure Cu–Sn alloy foams

Cu–Sn alloy foams, with an open-cell structure, were fabricated by Cu/Sn coelectrodeposition onto a polyurethane (PU) template in an alkaline electrolyte with a citrate complexing agent before undergoing a subsequent thermal treatment for PU removal in a N2 gas environment. These foams were predominantly comprised of a Cu matrix along with Cu6Sn5 and Cu3Sn intermetallic compounds, which are known as active materials in Cu–Sn alloy electrodes used in rechargeable batteries. The chemical phase change of Cu–Sn alloy foams and its influence on the electrical resistance were studied according to the heating temperature and N2 gas flow rate. Assuming the thermal degradation of PU as a first-order reaction, the dependence of the PU removal process on the heating temperature was also theoretically investigated. In addition, a postannealing process of the Cu–Sn alloy foams was employed to reduce their electrical resistance by promoting grain growth.

The Obstruction Effect of Ni Layer on the Interdiffusion of Cu Substrate and Sn Solder: A Theoretical Investigation

The protective effect of a Ni layer on Cu-Sn solder joints was investigated from the perspective of interface diffusion. The results indicate that temperature is the primary external factor affecting interfacial diffusion, and the effect of external load also has an influence. Atom diffusion is promoted by the increasing temperature and applied compressive load perpendicular to the interface. In contrast, the tensile load perpendicular to the interface inhibited the diffusion behavior. Ni atoms at both the Ni/Cu and Ni/Sn interfaces act as the main diffusion factor and play a vital role in the growth and evolution of the diffusion layer. Because of the high stability of the Ni lattice and the large radii of Cu and Sn atoms, it is difficult for Cu and Sn atoms to migrate to the Ni atoms, which effectively delays the contact and reaction between Cu and Sn. To reveal the micro-mechanism of the diffusion, the diffusion coefficient and activation energy were studied. Ni atoms need more energy than Cu and Sn atoms to drive the diffusion, which delays the consumption of Ni and helps to maintain the Ni layer. The single vacancy formation energy of the Ni lattice is the largest, which means that it is difficult for Cu and Sn atoms to migrate into the lattice. By comparing the atomic diffusion rate, the final formation of Cu-Sn intermetallic compounds in the Ni layer on the side of tin-based solder was predicted at a macroscopic time scale. Using density functional calculation, the slight lattice distortion of the interfaces was measured. The analysis of charge transfer and electronic properties show that a strong electron transfer and energy level overlap occurs between the Ni and Cu atoms, implying that the Ni/Cu interface has a more considerable electronic thermal conductivity and should be more sensitive to the increasing temperature.

Effect of Ni(P) thickness in Au/Pd/Ni(P) surface finish on the electrical reliability of Sn–3.0Ag–0.5Cu solder joints during current-stressing

The effects of Ni(P) layer thickness (5 μm and 0.7 μm) on the microstructural behavior and electrical reliability of electroless-nickel electroless-palladium immersion gold (ENEPIG) (substrate-side) surface-finished printed circuit boards (PCBs) with Sn–3.0Ag–0.5Cu (SAC305) solder joints under current stressing of 9000 A/cm2 have been investigated. An organic solderability preservative (OSP) surface finish was applied on the chip-side. (Cu,Ni)6Sn5 and Cu6Sn5 intermetallic compound (IMC) layers were formed on the chip-side (the interface between SAC305 and the OSP surface finish) and substrate-side (the interface between the ENEPIG surface finish and SAC305) solder joints after reflow. The thicknesses of the (Cu,Ni)6Sn5 and Cu6Sn5 IMC layers of the chip- and substrate-side of the normal- and thin-ENEPIG/SAC305/OSP solder joints increased with increasing current stressing time, regardless of the nickel phosphorous (Ni(P)) layer thickness. The total IMC thicknesses of normal-ENEPIG/SAC305/OSP solder joints were relatively thinner than those of thin-ENEPIG/SAC305/OSP solder joints under current stressing for 50–120 h. This is the reason why only a P-rich Ni layer was formed at the interface between SAC305 solder and the Cu pad in the thin-ENEPIG/SAC305 solder joints under current stressing. Otherwise, the P-rich Ni and Ni(P) layers remained at the interface of the normal-ENEPIG/SAC305 solder joint under current stressing. The Ni(P) layer of the ENEPIG surface finish played an important diffusion barrier role by suppressing IMC growth and movement toward the SAC305 solder under current stressing. In the electrical evaluation, the time to failure at the normal-ENEPIG solder joint was relatively longer (approximately 2.2 times) than that of the thin-ENEPIG solder joint. Therefore, the relatively thick Ni(P) layer contained in the ENEPIG/SAC305/OSP solder joint is expected to attain higher electrical reliability under the electromigration test.

Shear strength and fracture surface analysis of lead-free solder joints with high fraction of IMCs

The effects of high fraction of intermetallic compounds (IMCs) on the shear strength and fracture mechanism of Cu/Sn3Ag0.5Cu/Cu solder joints were investigated. Reflowing at 250 °C up to 60 min, then isothermal aging was conducted at 150 °C with multiple aging time. The results showed that the thickness of Cu3Sn IMC gradually increased with the extending of reflowing time and aging time. In addition, the shear strength of solder joints decreased with increasing reflowing time and aging time, which was due to the excessive growth of the interfacial IMC layer. From the fractography, the brittle fracture mode was observed because of the Cu6Sn5 and Cu3Sn appeared on each fracture surface in all of shear samples. Furthermore, the area ratio of Cu3Sn on the fractured surfaces increased with the increment of reflowing time and aging time. In addition, the hardness and the Young's modulus of Cu3Sn decreased initially and then increased with increased reflowing time.

Effects of Joint Height on the Interfacial Microstructure and Mechanical Properties of Cu-Cored SAC305 Solder Joints

In order to discuss the effects of Cu-cored solder with pure Ni coating and joint height on the interfacial microstructure and mechanical properties of joints, Cu/Sn-3.0Ag0.5Cu (SAC305)/Cu and Cu/Cu-cored + SAC305/Cu sandwich solder joints were prepared using a reflow process. The interfacial microstructure of the solder joints was investigated by scanning electron microscope with energy-dispersive x-ray spectroscopy. The results showed that, at as-reflowed joints, an intermetallic compound (IMC) of scallop-like Cu6Sn5 was formed at the Cu wire/solder interface, and a plane-like (Cux, &!nbsp;Ni1−x)6Sn5 IMC was formed at the Cu core/solder interface. Compared with SAC305 solder joints, Cu-cored solder joints showed more interface layers, a thicker Cu6Sn5 IMC layer, and higher tensile strength. With increasing joint height, the thickness of the IMC at the two interfaces decreased, and the tensile strength also decreased for Cu-cored solder joints. Mixed brittle/ductile fracture appeared in SAC305 and Cu-cored solder joints with height of 600 μm, but were suppressed and transformed into ductile fractures with increasing height of the solder joints.

Electromigration behavior of Cu/Sn–58Bi–1Ag/Cu solder joints by ultrasonic soldering process

Sn–58Bi–1Ag solder ribbon was prepared by twin-roll rapid solidification technology, and the ultrasonic soldering process was used to prepare Cu/Sn–58Bi–1Ag/Cu linear solder joints. Electron probe microanalysis (EPMA) and energy dispersive X-ray spectroscopy (EDS) were used to study the interface morphology of intermetallic compounds (IMC), Bi segregation, and solder joint matrix microstructure evolution with the current density of 1 × 104A/cm2 (25 °C). The result shows that the morphology of the anode IMC layer changes from scallop-like to hilly-like and then to flat-plate-like with the electrification time, and the thickness of IMC layer increases gradually. Bi is segregated toward the anode to form a Bi-rich layer. The cathode IMC layer is changed from a scallop to a zigzag, and its thickness is firstly increased and then decreased. The Sn is segregated toward the cathode to finally form a β-Sn-rich layer. Coarsening of eutectic microstructure (β-Sn+Bi) in solder joint matrix was caused by prolonged time. Based on linear fitting, the kinetic index n of the IMC layer of the anode and cathode is 0.432 and 0.491, respectively, and the growth mechanism is supposed to be volume diffusion. Ultrasonic soldering techniques can refine the Bi phase and increase the solubility of Bi in β-Sn, which slows down the formation of Bi-rich layers. The addition of Ag forms wedge-shaped and granular Ag3Sn intermetallic compounds, which hinders the growth of the Bi-rich layer and the Cu6Sn5 IMC, thus effectively inhibiting the electromigration process.

Integration of machine learning with phase field method to model the electromigration induced Cu6Sn5 IMC growth at anode side Cu/Sn interface

Currently, in the era of big data and 5G communication technology, electromigration has become a serious reliability issue for the miniaturized solder joints used in microelectronic devices. Since the effective charge number (Z*) is considered as the driving force for electromigration, the lack of accurate experimental values for Z* poses severe challenges for the simulation-aided design of electronic materials. In this work, a data-driven framework is developed to predict the Z* values of Cu and Sn species at the anode based LIQUID, Cu6Sn5 intermetallic compound (IMC) and FCC phases for the binary Cu-Sn system undergoing electromigration at 523.15 K. The growth rate constants (kem) of the anode IMC at several magnitudes of applied low current density (j = 1 × 106 to 10 × 106 A/m2) are extracted from simulations based on a 1D multi-phase field model. A neural network employing Z* and j as input features, whereas utilizing these computed kem data as the expected output is trained. The results of the neural network analysis are optimized with experimental growth rate constants to estimate the effective charge numbers. For a negligible increase in temperature at low j values, effective charge numbers of all phases are found to increase with current density and the increase is much more pronounced for the IMC phase. The predicted values of effective charge numbers Z* are then utilized in a 2D simulation to observe the anode IMC grain growth and electrical resistance changes in the multi-phase system. As the work consists of the aspects of experiments, theory, computation, and machine learning, it can be called the four paradigms approach for the study of electromigration in Pb-free solder. Such a combination of multiple paradigms of materials design can be problem-solving for any future research scenario that is marked by uncertainties regarding the determination of material properties.

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.

The Effect of Reflow Temperature on Time at the End of Gravity Zone (Tgz) of Sn-3.8Ag-0.7Cu Solder Alloy

Abstract The reflow time for solder until the end of the gravity zone (Tgz) is considered to be the optimum reflow time for obtaining high mechanical performance from lead-free solders. In the present work, the effect of reflow time and temperature on Tgz of Sn-3.8Ag-0.7Cu (SAC387) lead-free solder alloy reflowed on the copper substrate has been investigated. The evolution of interfacial microstructure and solder bond shear strength under different reflow temperatures and time was assessed. Solder balls weighing 0.08 ± 0.01 g were reflowed at 260°C, 280°C, and 300°C for reflow times of 30 s, 60 s, 120 s, and 240 s. Times at the end of the gravity zone for SAC387 solder were obtained as 110 ± 5 s, 55 ± 5 s, and 23 ± 3 s for reflow temperatures of 260°C, 280°C and 300°C, respectively. The contact angle for SAC387 solder on the copper substrate at Tgz was found to be 25.5° ± 0.2° for all reflow temperatures. Scanning electron microscopy revealed the formation of a Cu6Sn5 intermetallic compound (IMC) layer at the interface. The IMC layer thickness increased with increase in reflow temperature and time. Maximum solder joint strength was obtained at Tgz reflow times for all reflow temperatures. Microstructures of samples reflowed beyond the gravity zone showed secondary Cu6Sn5 precipitation in the solder bulk. The present investigation reveals a reduction in Tgz reflow time for SAC387 lead-free solder at higher operating reflow temperatures.

Phase-Field Study of Thermomigration in 3-D IC Micro Interconnects

We studied the evolution of the Cu6Sn5 and Cu3Sn intermetallic compounds under large thermal gradients for assisted directional solidification in Cu/Sn/Cu micro solder units for fast bonding in 3-D IC packaging. Thermomigration-enhanced growth and dissolution of the phasesare observed and tracked using a CALPHAD-reinforced multiphase-field model that accounts for the thermodynamics and kinetics of the Cu(Sn) reacting system. The results show that the process of bonding follows a reaction-controlled growth regime ( $n{\sim }1$ ) during thermal gradient bonding, proceeding faster than isothermal conditions. In addition, the asymmetric phase evolution, i.e., simultaneous growth in hot side and dissolution in the cold side of the interconnect, is modeled. The developed model can be used for the proactive control of the complex heat treatment processes for the design of multicomponent low-volume solder interconnects in electronic packaging.

Microstructure evolution and hardness of MWCNT-reinforced Sn-5Sb/Cu composite solder joints under different thermal aging conditions

In this work, multi-walled carbon nanotubes (MWCNTs) reinforced Sn-5Sb/Cu composite solder joints were synthesized and the effects of MWCNTs addition on the microstructure evolution and hardness of the Sn-5Sb solder alloy during various thermal aging conditions were investigated. After conducting a thorough microstructural analysis, the SbSn and Cu6Sn5 intermetallic compounds (IMCs) were observed in the β-Sn matrix of the composite solder joints subjected to reflow soldering while the latter was also present at the solder/Cu interface. However, after subjecting the composite solder joints to isothermal aging, the Cu3Sn IMC emerged between the Cu6Sn5 IMC at the solder/Cu interface and the Cu substrate. With the promising properties exhibited by MWCNTs as a reinforcement material, experimental results showed that MWCNTs refined the bulk solder microstructure and inhibited growth of the interfacial IMC layer in the Sn-5Sb-xCNT/Cu samples. In general, the composite sample reinforced with 0.05 wt% MWCNTs showed the least IMC layer thickness and diffusion coefficient in the ranges of 2.6–11.99 μm and 1.07 × 10−14-14.9 × 10−14 cm2/s respectively. Meanwhile, the strengthening mechanism triggered by MWCNTs addition was clearly evident in the MWCNT-reinforced Sn-5Sb/Cu as superior hardness values within a range of 20.6–15.3 HV were registered for the as-soldered and aged composite solder joints with 0.05 wt% MWCNTs reinforcement.

Microstructural, compositional and hardness evolutions of 96.5Sn–3Ag–0.5Cu/TiC composite solder under thermo-migration stressing

A 96.5Sn3Ag0.5Cu (SAC305) lead-free composite solder containing 0.1 wt% titanium carbide (TiC) was prepared using the powder metallurgy method and a thermal migration (TM) experiment was then carried out. A temperature gradient generating device was designed and a TM sample with an asymmetric sandwich structure was prepared in order to achieve this. The microstructure, the composition and the mechanical properties of both the SAC305 and SAC305/TiC solder joints were investigated under a temperature gradient of 1070 K/cm that was generated by the TM device that had been designed. The results showed that the addition of a TiC reinforcing phase was able to effectively inhibit both the diffusion and migration of the Cu atoms, and that this affected the distribution of the Cu6Sn5 intermetallic compounds (IMC) under the thermal migration conditions in the solder joint. It has also been shown that, compared with the plain SAC305 solder joint, the interconnect interface of the composite solder joint was relatively intact after 600 h of thermal loading. The results of the nanoindentation test also showed that the addition of the TiC reinforcing phase was able to significantly weaken the distribution gradient of the hardness of the solder joint caused by element migration.

The open-pin failure of power device under the combined effect of thermo-migration and electro-migration

With the miniaturization of electronic products, power devices sustain more and more operations under high power, high frequency, and high integration. Power electronic devices are subjected to high operating temperature and current density, which poses a serious challenge on the packaging materials. An open-pin failure was found on a metal-oxide-semiconductor field-effect transistor (MOSFE) device after operated for 3−4 a under high current density condition. The lead frame material is C194 alloy, belonging to Cu-Fe-P alloy, which is a copper base alloy strengthened by dispersion. The operating temperature of the MOSFET device is in the range of 88 − 120°C at the full working current of about 15 A from the drain to the source, while the gate current is relatively small in a milliampere level. Failure analysis is conducted on the open-pin MOSFET device. According to the investigations by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS), two intermetallic compounds (IMCs), Cu3Sn and Cu6Sn5, were formed at both the C194 alloy/Sn-Pb and the Cu/Sn-Pb solder interfaces. However, the crack is always initiated at the C194 alloy/Sn-Pb solder interface, no matter for the source or the drain. Usually, the crack would initiate at the cathode side due to the sole effect of electro-migration. In this work, the crack initiated at the cathode side for the drain, but at the anode side for the source. According to the working condition of the MOSFET device, this failure crack can be attributed to the combined effects of electro-migration and thermo-migration on multi-element interfacial diffusion and vacancy migration. In detail, for the source of MOSFET device, the direction of Cu thermo-migration flux is opposite to the electro-migration flux, while the thermo-migration flux is large so that it can compensate the effect of electro-migration flux, thus leading to the crack at the anode. For the drain of device, the direction of Cu thermo-migration flux is the same as that of electro-migration, which leads to a severe cracking at the cathode. Moreover, in order to reveal the cracking process, SEM analysis was conducted on the fracture surface where plenty of fine Fe-rich granules and some Cu-Sn IMCs were observed. Furthermore, the electron probe micro analysis (EPMA) demonstrated that these fine Fe-rich granules form a continuous layer adjacent to the crack. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were also conducted in order to characterize the interfacial microstructure with Fe-rich granules/layer. At the C194/IMC interface, the Fe granules with body-centered cubic (bcc) structure accumulated as a continuous layer with an obvious grain growth compared to the uniformly dispersing Fe granules within the C194 alloy matrix. The existence of segregated Fe layer and Kirkendall voids would weaken the original bonding between C194 and IMC, so the crack always initiated along the Fe layer or Kirkendall voids when the thermal stress or mechanical stress applied. Above all, this coupled open-pin failure mode and its mechanism would provide a theoretical guidance for the optimization of product process and the improvement of service life.

Backside Metallization of Ag–Sn–Ag Multilayer Thin Films and Die Attach for Semiconductor Applications

Backside metallization, which offers the advantages of a thin die-attach layer, excellent ohmic contacts, good thermal dissipation, and electrical conduction, is a reliable die-attach method for high-temperature semiconductor devices. Optimized Ag–Sn–Ag thin film structures have been deposited as a backside metal layer and their mechanical and electrical characteristics evaluated as a replacement for conventional Au-based backside metal. The results confirmed that the Ag/Sn/Ag sandwich structure provided more reliable properties than the Sn/Ag and Ag/Sn structures. After the die-bonding process using the Ag/Sn/Ag backside metal, Cu6Sn5, Cu3Sn, and Ag3Sn intermetallic compounds were observed at the interface between the Si chip and Cu-plated Alloy 42 lead frame. The shear strength of the Ag/Sn/Ag backside metal was superior to that of conventional Au-12Ge backside metal, and its electrical characteristics met the requirement specifications of a commercial discrete semiconductor package.

Microstructural and shear strength properties of RHA-reinforced Sn–0.7Cu composite solder joints on bare Cu and ENIAg surface finish

In this study, the joint effect of rice husk ash (RHA) reinforcement as an alternative silica source and electroless nickel immersion silver (ENIAg) surface finish on the intermetallic compound (IMC) formation and shear strength of the Sn–0.7Cu solder system was investigated. A series of plain and composite lead-free solder systems (Sn–0.7Cu−xRHA; x = 0, 0.01, 0.05 and 0.1 wt%) was successfully developed and subjected to reflow soldering on bare Cu and ENIAg surface finish. After conducting a comprehensive microstructural study using the scanning electron microscopy and energy dispersive spectroscopy techniques, the Cu6Sn5 and Cu3Sn intermetallic compound (IMC) phases were observed at the interface of the Sn–0.7Cu−xRHA/Cu composite solder joints. On the other hand, the (Cu,Ni)6Sn5 and Ni3Sn4 IMC phases dominated the interface of the Sn–0.7Cu−xRHA/ENIAg counterparts. Given the promising potential of the ENIAg surface finish, the Sn–0.7Cu−xRHA/ENIAg exhibited IMC thickness values within a range of 3.81–4.74 µm as compared to the 6.13–9.3 µm range exhibited by the Sn–0.7Cu−xRHA/Cu counterpart. More so, the ENIAg surface finish was effective in improving the shear strength of the plain solder joint, with the Sn–0.7Cu/ENIAg exhibiting 13.44 MPa relative to the 11.5 MPa exhibited by the Sn–0.7Cu/Cu counterpart. Overall, the strengthening effect of the RHA reinforcement was well marked in the Sn–0.7Cu−xRHA/Cu composite solder joints with the composite having 0.1 wt% RHA exhibiting the highest shear strength (14.6 MPa) across the board.

Mechanical Reliability of Epoxy Sn–58wt.%Bi Composite Solder Under Temperature-Humidity Treatment with Organic Solderability Preservatives (OSP) Surface Finish

The microstructures and mechanical reliability of Sn–58Bi solder and epoxy Sn–58Bi composite solder joint were investigated with organic solderability preservative surface finishes. The mechanical reliabilities of Sn58Bi and epoxy Sn58Bi solder were evaluated by the board-level drop test and the 3-point bend test after temperature-humidity storage testing. The microstructure and chemical composition of the solder joints were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. The addition of epoxy in solder paste did not affect the morphology of the intermetallic compound. The thickness of the scalloped-shaped Cu6Sn5 intermetallic compound of solder/OSP joint increased with aging time. The drop number until fail for the epoxy Sn58Bi/OSP joint was higher than that for the Sn–58Bi/OSP joint; the average numbers of drops withstood by the Sn–58Bi/OSP joint and epoxy Sn–58Bi/OSP joint following the reflow process were fewer than 10 drops and 180 drops, respectively. The drop number of solder/OSP joints decreased with increasing aging time. The result of the 3-point bend tests shows that the number of bend cycles for the epoxy Sn–58Bi/OSP joint was 30 times higher than that for the Sn–58Bi/OSP joint. The number of bend cycles for solder/OSP joints was decreased with increasing aging time.