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The purpose of this paper is to investigate the effect of fullerene (FNS) reinforcements on the microstructure and mechanical properties of 96.5Sn3Ag0.5Cu (SAC305) lead-free solder joints under isothermal ageing and electrical-migration (EM) stressing.

In this paper, SAC305 solder alloy doped with 0.1 Wt.% FNS was prepared via the powder metallurgy method. A sandwich-like sample and a U-shaped sample were designed and prepared to conduct an isothermal ageing test and an EM test. The isothermal ageing test was implemented under vacuum atmosphere at 150°C, whereas the EM experiment was carried out with a current density of 1.5 × 10⁴ A/cm². The microstructural and mechanical evolutions of both plain and composite solder joints after thermal ageing and EM stressing were comparatively studied.

A growth of Ag₃Sn intermetallic compounds (IMCs) in solder matrix and Cu-Sn interfacial IMCs in composite solder joints was notably suppressed under isothermal ageing condition, whereas the hardness and shear strength of composite solder joints significantly outperformed those of non-reinforced solder joints throughout the ageing period. The EM experimental results showed that for the SAC305 solder, the interfacial IMCs formulated a protrusion at the anode after 360 h of EM stressing, whereas the surface of the composite solder joint was relatively smooth. During the stressing period, the interfacial IMC on the anode side of the plain SAC305 solder showed a continuous increasing trend, whereas the IMC at the cathode presented a decreasing trend for its thickness as the stressing time increased; after 360 h of stressing, some cracks and voids had formed on the cathode side. For the SAC305/FNS composite solder, a continuous increase in the thickness of the interfacial IMC was found on both the anode and cathode sides; the growth rate of the interfacial IMC at the anode was higher than that at the cathode. The nanoindentation results showed that the hardness of the SAC305 solder joint presented a gradient distribution after EM stressing, whereas the hardness data showed a relatively homogeneous distribution in the SAC305/FNS solder joint.

The experimental results showed that the FNS reinforcement could effectively mitigate the failure risk in solder joints under isothermal ageing and high-current stressing. Specifically, the FNS particles in solder joints can work as a barrier to suppress the diffusion and migration of Sn and Cu atoms. In addition, the nanoidentation results also indicated that the addition of the FNS reinforcement was very helpful in maintaining the mechanical stability of the solder joint. These findings have provided a theoretical and experimental basis for the practical application of this novel composite solder with high-current densities.

The purpose of this paper is to investigate the effect of nickel-plated graphene (Ni-GNS) on the microstructure and mechanical properties of 96.5Sn3Ag0.5Cu (SAC305) lead-free solder joints before and after an electro-migration (EM) experiment.

In this paper, SAC305 solder alloy doped with 0.1 Wt.% Ni-GNS was prepared via the powder metallurgy method. A U-shaped sample structure was also designed and prepared to conduct an EM experiment. The EM experiment was carried out with a current density of 1.5 × 104 A/cm2. The microstructural and mechanical evolutions of both solder joints under EM stressing were comparatively studied using SEM and nanoindentation.

The experimental results showed that for the SAC305 solder, the interfacial intermetallic compounds (IMC) formulated a protrusion with an average height of 0.42 µm at the anode after 360 h of EM stressing; however, despite this, the surface of the composite solder joint was relatively smooth. During the stressing period, the interfacial IMC on the anode side of the plain SAC305 solder showed a continuous increasing trend, while the IMC at the cathode presented a decreasing trend for its thickness as the stressing time increased; after 360 h of stressing, some cracks and voids had formed on the cathode side. For the SAC305/ Ni-GNS composite solder, a continuous increase in the thickness of the interfacial IMC was found on both the anode and cathode side; the growth rate of the interfacial IMC at the anode was higher than that at the cathode. The nanoindentation results showed that the hardness of the SAC305 solder joint presented a gradient distribution after EM stressing, while the hardness data showed a relatively homogeneous distribution in the SAC305/ Ni-GNS solder joint.

The experimental results showed that the Ni-GNS reinforcement could effectively mitigate the EM behavior in solder joints under high current stressing. Specifically, the Ni particles that plated the graphene sheets can work as a fixing agent to suppress the diffusion and migration of Sn and Cu atoms by forming Sn-Cu-Ni IMC. In addition, the nanoidentation results also indicated that the addition of the Ni-GNS reinforcement was very helpful in maintaining the mechanical stability of the solder joint. These findings have provided a theoretical and experimental basis for the practical application of this novel composite solder with high current densities.

This paper aims to systematically study the effect of reinforcement type, processing methods and reflow cycle on actual retained ratio of foreign reinforcement added in solder joints.

Two kinds of composite solders based on SAC305 (wt.%) alloys with reinforcements of 1 wt.% Ni and 1 wt.% TiC nano-particles were produced using powder metallurgy and mechanical blending method. The morphology of prepared composite solder powder and solder pastes was examined; retained ratios of reinforcement (RRoR) added in solder joints after different reflow cycles were analysed quantitatively using an Inductively Coupled Plasma optical system (ICP-OES Varian-720). The existence forms of reinforcement added in solder alloys during different processing stages were studied using scanning electron microscope, X-ray diffractometry and energy dispersive spectrometry.

The obtained experimental results indicated that the RRoR in composite solder joints decreased with the increase in the number of reflow cycles, but a loss ratio diminished gradually. It was also found that the RRoR which could react with the solder alloy were higher than that of the one that are unable to react with the solder. In addition, compared with mechanical blending, the RRoRs in the composite solders prepared using power metallurgy were relatively pronounced.

Present study offer a preliminary understanding on actual content and existence form of reinforcement added in a reflowed solder joint, which would also provide practical implications for choosing reinforcement and adjusting processing parameters in the manufacture of composite solders.

The purpose of this paper is to investigate microstructure and properties of Sn3.0Ag0.5Cu-XAl₂O₃ composite solder which were prepared through powder metallurgy route.

Sn3.0Ag0.5Cu (SAC305)-XAl₂O₃ (X = 0.2, 0.4, 0.6, 0.8 Wt. %) composite solders were prepared through the powder metallurgy route. The morphology of composite solder powders which consists of Al₂O₃ particles and SAC solder powders after ball milling was observed. The retained ratio of Al₂O₃ nanoparticles in composite solder billets and solder joints were also quantitatively measured. Furthermore, the as-prepared composite solder alloys were studied extensively with regard to their microstructures, thermal property, wettability and mechanical properties.

After ball milling, the Al₂O₃ nanoparticles added were observed embedded into the surface of SAC solder powders. Only about 5-10 per cent of the initial Al₂O₃ nanoparticles added were detected in the composite solder joints after reflow. In addition, finer ß-Sn grains were achieved with addition of Al₂O₃ nanoparticles; the Al₂O₃ nanoparticles were found retained in the composite solder matrix. Besides, negligible changes in melting temperature and the considerably reduced undercooling were obtained in composite solder alloys. Wettability was improved by appropriate addition of Al₂O₃ nanoparticles. Microhardness and shear strength of composite solders were both improved after Al₂O₃ nanoparticles addition.

This paper indicated that powder metallurgy route offered a feasible approach to produce nanoparticle reinforced composite solder. In addition, the quantitative analysis of the actual retained ratio of the Al₂O₃ nanoparticles in solder joints provided practical implications for the manufacture of composite solders.

The purpose of this paper is to fabricate a new Cu-Sn-Ni-Cu interconnection microstructure for electromigration studies in 3D integration.

The Cu-Sn-Ni-Cu interconnection microstructure is fabricated by a three-mask photolithography process with different electroplating processes. This microstructure consists of pads and conductive lines as the bottom layer, Cu-Sn-Ni-Cu pillars with the diameter of 10-40 μm as the middle layer and Cu conductive lines as the top layer. A lift-off process is adopted for the bottom layer. The Cu-Sn-Ni-Cu pillars are fabricated by photolithography with sequential electroplating processes. To fabricate the top layer, a sputtered Cu layer is introduced to prevent the middle-layer photoresist from being developed. With the final Cu electroplating processes, the Cu-Sn-Ni-Cu interconnection microstructure is successfully achieved.

The surface morphology of Cu-Sn pillars consists of densely packed clusters which are formed by an ordered arrangement of tetragonal Sn grains. The diffusion of Cu atoms into the Sn phases is observed at the Cu/Sn interface. Furthermore, the obtained Cu-Sn-Ni-Cu pillars have a flat surface with an average roughness of 13.9 nm. In addition, the introduction of Ni layer between the Sn and the top Cu layers in the Cu-Sn-Ni-Cu pillars can mitigate the diffusion of Cu atoms into Sn phases. The process is verified by checking the electrical performance using four-point probe measurements.

The method described in this paper which combined a three-mask photolithography process with sequential Cu, Sn, Ni and Cu electroplating processes provides a new way to fabricate the interconnection microstructure for future electromigration studies.

This paper aims to investigate the microstructural evolution rules of the intermetallic compound (IMC) layers in high‐density solder interconnects with reduced stand‐off heights (SOH).

Cu/Sn/Cu solder joints with 100, 50, 20 and 10 μm SOH were prepared by the same reflow process and isothermally aged at 150°C. The IMC microstructural evolution was observed using scanning electron microscopy.

The whole IMC layer (Cu₃Sn + Cu₆Sn₅) grew faster in the solder joints with lower SOH because of the thinner IMC layer before aging. Also, the IMC proportion increased more rapidly in solder joints with the lower SOH. In all solder joints with different SOH, the growth rates of the Cu₃Sn (ϵ) layers were similar, and slowed down with increasing aging time. The Cu₆Sn₅ (η) was consumed by the Cu₃Sn (ϵ) growth at the beginning of the aging stage; while it turned to thickening after a period of aging. Finally, the Cu₆Sn₅ thickness was similar in all the solder joints. It is inferred that the thickness ratio of Cu₃Sn to Cu₆Sn₅ would maintain a dynamic balance in the subsequent aging. Based on the diffusion flux ratio of Cu to Sn at the ϵ/η interface, a model has been established to explain the microstructural evolution of IMC layers in high‐density solder interconnects with reduced SOH. In the model, interfacial reactions are mainly supposed to occur at the ϵ/η interface.

The findings provide electronic packaging reliability engineers with an insight into IMC microstructural evolution in high‐density solder interconnects with reduced SOH.

The purpose of this paper is to identify the solder joint with optimal mechanical properties among Cu/Sn/Cu, Ni/Sn/Ni and Cu/Sn/Ni solder joints.

Solder joints with the same specimen shape were prepared by reflow. The microstructures were observed and analyzed by scanning electron microscopy and tensile testing was carried out to investigate the mechanical properties.

The mechanical properties of solder joint correlate closely with the intermetallic compounds (IMC) layer structure and the dissociative IMC particles in the solder bulk. Under the influence of the opposite Cu bar, the Cu/Sn/Ni has a duplex IMC layer structure at the Ni side, involving a thin Ni‐Cu‐Sn IMC layer and a faceted (Cu,Ni)₆Sn₅ layer. The mechanical connection of the duplex IMC layers is weak due to the pores in the layers. The Cu/Sn/Ni fractures in the IMC layers in a brittle mode under tensile testing. Comparatively, the Ni/Sn/Ni also has duplex Ni₃Sn₄ layers, and they connect firmly with each other. The tensile fracture of the Ni/Sn/Ni occurs in the solder bulk in a ductile mode, as well as for the Cu/Sn/Cu. Compared with the Cu/Sn/Cu solder bulk, the solder bulk of the Ni/Sn/Ni and the Cu/Sn/Ni have higher ultimate tensile strengths, because the strengthening effect of the dissociative Ni₃Sn₄ and (Cu,Ni)₆Sn₅ particles on the solder bulk is stronger than that of the Cu₆Sn₅ particles. Among Cu/Sn/Cu, Ni/Sn/Ni and Cu/Sn/Ni, Ni/Sn/Ni has the optimal mechanical properties.

The paper offers insights into the significant influence of base material matching on the microstructure and mechanical properties of solder joints.

The purpose of this paper is to investigate the effect of stand‐off height (SOH) on the microstructure and mechanical behaviour of the solder joints in high density interconnection.

Cu/Sn/Cu solder joints with 100, 50, 20 and 10 μm SOH are prepared using a reflow process. The microstructures and compositions of solder joints are observed and analyzed by scanning electron microscopy. Tensile testing is carried out to investigate the mechanical properties of the solder joints.

The SOH has a significant effect on the microstructure and mechanical behaviour of Cu/Sn/Cu solder joints. The thickness of the intermetallic compound (IMC) decreases with the reducing SOH; however, their corresponding IMC proportion increases. Meanwhile, the Cu concentration in the solder bulk experiences a marked increase, and the dissolved Cu exists in the forms of a solid solution and Cu‐rich particles at the grain boundary. Because of the higher strain rate and more dissolved Cu in the solder bulk with the reducing SOH, the ultimate tensile strength of solder joints is enhanced. When the SOH reduces to 10 μm, there is only one grain in height in the bulk, and a fracture in the IMC layer occurs. According to the mass balance of substance, a model is established to semi‐quantitatively calculate the consumed Cu thickness, and it is found that the consumed Cu thickness decreases with the reducing SOH.

The paper offers insights into the microstructural and mechanical property changes of the solder joints with the reducing SOH.

The purpose of this paper is to study the thermal warpage of a plastic ball grid array (PBGA) mounted on a printed circuit board (PCB) during the reflow process.

A thermal-mechanical coupling method that used finite-element method software (ANSYS 13.1) was performed. Meanwhile, a shadow moiré apparatus (TherMoiré PS200) combined with a heating platform was used for the experimental measurement of the warpage of PBGA according to the JEDEC Standard.

The authors found that the temperature profiles taken from the simulated results and experimental measurement are consistent with each other, only with a little and acceptable difference in the maximum temperatures. Furthermore, the maximum warpage measurements during the reflow process are 0.157 mm and 0.149 mm for simulation and experimental measurements, respectively, with a small 5.37 per cent difference. The experimental measurement and simulated results are well correlated. Based on the validated finite element model, two factors, namely, the thickness and dimension of PCB, are explored about their effect on the thermal warpage of PBGA mounted on PCB during the reflow process.

The paper provides a thorough parametrical study of the thermal warpage of PBGA mounted on PCB during the reflow process.

The findings in this paper illustrate methods of warpage study by combination of thermal-mechanical finite element simulation and experimental measurement, which can provide good guidelines of the PCB design in the perspective of thermal warpage during the reflow process.