The authors of the paper “Epitaxy of advanced nanowire quantum devices”¹ wish to retract this work. When preparing the underlying data for public release², it was discovered that some data had been inappropriately deleted or cropped when preparing the final published figures, and we promptly alerted the editors of Nature. We found unjustified data removal and cropping in Figures 4a and c, and Extended Data Figures 7 and 8, which affect the agreement between the theoretical curves and the experimental data and the claims of ballistic transport. We are accordingly retracting the paper. The authors stand by all the other data, and their contribution to advanced nanowire quantum devices. All authors have agreed to this retraction.

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Sn-Ag-x solders were used as the interfacial layers between SnBi solder and Cu substrate. The effects of Sn-Ag-x layers on the solderability, microstructure, and mechanical properties of SnBi solder joint were investigated. Experimental results indicate that all the barrier layers have positive effects on improving the wettability of SnBi solder. The relative area and grain size of β-Sn was enlarged due to the addition of Sn-Ag-x layers. Meanwhile, the addition of the interfacial layers decreased the hardness of the SnBi solder joint. The addition of Sn-Ag-x layers increased the thickness of the interfacial intermetallic compound (IMC) but had limited effects on the shear force of the SnBi solder joint. Due to the addition of the interfacial layers, the brittleness of the SnBi/Cu solder joints during the shear test was slightly suppressed.

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The Data availability statement of this article has been modified to add the accession link to the raw data. The old Data availability statement read “Materials and data that support the findings of this research are available within the paper. All data are available from the corresponding author upon request”. This has been replaced by “Materials and data that support the findings of this research are available within the paper. The raw data have been deposited at https://zenodo.org/record/4589484#.YEoEOy1Y7Sd”. This has been corrected in both the HTML and PDF version of the article.

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The microstructure, shear behaviour and hardness of the SnBi/SACBN/Cu solder joint before and after isothermal ageing were investigated in comparison with the SnBi/Cu and SACBN/Cu solder joints. The experimental results indicated that the pre-soldered SACBN joint had a significant effect on the formation and growth of the β-Sn grains in the SnBi bulk solder. The brittleness of the SnBi/SACBN/Cu composite solder joint was also suppressed and its failure mode transformed from brittle failure to brittle-ductile failure after reflow. However, the shear strength and failure mode of the SnBi/SACBN/Cu composite solder joint became similar to those of the SnBi/Cu joints after 600 h isothermal ageing. The shear strength of the three kinds of solder joints decreased after isothermal ageing, but the SnBi/SACBN/Cu composite solder joint showed higher shear strength than SnBi/Cu did during ageing. The shear strength of the composite solder joint was 67.1MPa after ageing. Due to the diffusion of elements in the isothermal ageing process, the microstructure of the composite solder joint was significantly coarsened after ageing for 600 hours. This phenomenon further led to the decrease of the hardness and shear strength of the three kinds of solder joints.

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This study investigated the mechanical properties for two types of solder alloy: Sn-0.7Ag-0.5Cu-3.5Bi-0.05Ni (SAC0705BiNi) vs. Sn-0.7Ag-0.5Cu (SAC0705) by using nano-indentation. Two kinds of solder alloy balls with a diameter of 400 μm are soldered to Cu pads on FR-4 substrates, and then formed the ball grid array (BGA) micro solder joints of Cu/SAC0705BiNi/Cu and Cu/SAC0705/Cu. Meanwhile, the combined effect of Bi and Ni elements on the mechanical properties of the bulk of low-Ag Cu/SAC0705/Cu was discussed. Experimental results revealed that the indentation depth and area of the bulk of Cu/SAC0705BiNi/Cu solder joints were smaller than that of Cu/SAC0705/Cu under the same load and strain rate. It was observed that the indentation morphologies of the two kinds of the bulk of micro solder joints have piling-up phenomenon at lower strain rate. Under the maximum load of 20 mN and the strain rate of 2.5 × 10^{− 1} s⁻¹, the indentation hardness of the bulk of Cu/SAC0705BiNi/Cu and Cu/SAC0705/Cu solder joints was 0.449 GPa and 0.200 GPa, respectively. And the strain hardening exponent was 0.302 and 0.159, respectively. Additionally, the stress-strain relationship was developed for the bulk of Cu/SAC0705BiNi/Cu and Cu/SAC0705/Cu micro solder joints. Compare with the bulk of the low-Ag Cu/SAC0705/Cu micro solder joints, the indentation hardness, indentation modulus and strain hardening exponent of the bulk of Cu/SAC0705BiNi/Cu achieve an improved by adding Bi and Ni elements.

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To make the light-emitting diode (LED) more compact and effective, the flip chip solder joint is recommended in LED chip-scale packaging (CSP) with critical functions in mechanical support, heat dissipation, and electrical conductivity. However, the generation of voids always challenges the mechanical strength, thermal stability, and reliability of solder joints. This paper models the 3D random voids generation in the LED flip chip Sn96.5-Ag3.0-Cu0.5 (SAC305) solder joint, and investigates the effect of thermal shock load on its mechanical reliability with both simulations and experiments referring to the JEDEC thermal shock test standard (JESD22-A106B). The results reveal the following: (1) the void rate of the solder joint increases after thermal shock ageing, and its shear strength exponentially degrades. (2) the first principal stress of the solder joint is not obviously increased, however, if the through-hole voids emerged in the corner of solder joints, it will dramatically increase. (3) modelling of the fatigue failure of solder joint with randomly distributed voids utilizes the approximate model to estimate the lifetime, and the experimental results confirm that the absolute prediction error can be controlled around 2.84%.

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The nanoindentation test was conducted in this paper to investigate the indentation hardness, plasticity and initial creep properties of pressure sintered nanosilver joint at various test temperatures. The effects of strain rate on the indentation hardness were first investigated. Then yield stress of nanosilver sintered joint was studied in various pressures sintered joints and the corresponding plastic stress-strain constitutive equations were gained. The maximum indentation depth of nanosilver sintered joint was obviously affected by the test temperature and sintering pressure. The indentation hardness of nanosilver sintered joint decreased with increasing test temperature from 140 to 200°C, which can be attributed to the increased amount of thermal vacancies at high temperatures. However, the indentation modulus exhibited decrease trend as the temperature increased. It is suggested that the distance between adjacent atoms was enlarged at elevated temperatures and furtherly resulted in the decrease of indentation modulus. In addition, the increased sintering pressure from 5 to 30 MPa improved the indentation hardness and modulus of sintered joint. The initial creep was observed in nanosilver sintered joint at temperatures ranged from 140 to 200°C. The increase of sintering pressure improved the resistance to creep of nanosilver sintered joint.

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High power electronics with wide band gap semiconductors are becoming the most promising devices in new energy power suppliers and converters. Highly reliable die attach materials, serving as one of interconnections, play critical roles in power electronic packages and modules. Among which, the nanosilver paste/film has become a promising die attach material with main advantages of a high thermal and electrical conductivity, as well as high temperature stability. Previous works are mostly focusing on the pressure free sintered silver nanoparticles, which has a low bonding strength and high porosity. Alternatively, pressure assisted sintering has exhibited great advantages in enhancing the bonding quality of nanosilver sintered joint. But the sintering properties of pressure sintered silver nanoparticles and the application of this technology in power electronics packaging are still lacking. In this thesis, a comprehensive research is performed on the pressure assisted sintering of silver nanoparticles. The results indicate that the sintering pressure demonstrates significant effect on enhancing the bonding strength of sintered silver nanoparticles. Furthermore, the increase of sintering pressure from 5 MPa to 30 MPa improves the resistance to plastic deformation and creep of nanosilver sintered joint. In addition, the designed nanosilver sintering technology is successfully employed in fabricating the double side sintered power package. Besides, the nanosilver sintering process is designed and employed in the ceramic packaging.@en

Purpose: Crack and stress distribution on dies are key issues for the pressure-assisted sintering bonding of power modules. The purpose of this research is to build a relationship among stress distributions, sintering sequences and sintering pressures during the sintering processes. Design/methodology/approach: Three sintering sequences, S(a), S(b) and S(c), have been designed for the double-side assembly of power module in this paper. Experiments and finite element method (FEM) analysis are conducted to investigate the crack and stress distribution. Findings: The sintering sequence had significant effects on the crack generation in the chips during the sintering process under 30-MPa pressure. The simulation results revealed that the module sintered by S(a) showed lower chip stress than those by the other two sintering sequences under 30 MPa. In contrast, the chip stress is the highest when the sintering sequence follows S(b). The simulation results explained the crack generation and prolongation in the experiments. S(a) was recommended as the best sintering sequence because of the lowest chip stress and highest yield rate. Originality/value: This study investigated the stress distributions of the double-side sintered power modules under different sintering pressures. Based on the results of experiments and FEM analysis, the best sintering sequence design is provided under various sintering pressures.

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Eutectic Sn58Bi (SnBi) solder paste mixed with 0 wt.%, 3 wt.%, 5 wt.%, 8 wt.% and 15 wt.% of Sn-3.0Ag-0.5Cu (SAC) paste was prepared by mechanical mixing. The effects of SAC paste additions on the microstructure evolution of SnBi-SAC/Cu composite solder joints during isothermal aging were investigated. The results indicated that the number of large Bi-rich phases decreased and the relative areas of β-Sn increased with increasing SAC content. Moreover, 1-μm Bi-rich particles were found near the Bi-rich phases. During the isothermal aging process, the diameter of the 1-μm Bi-rich particles in the solder bulk increased by about 50% with aging time by Ostwald ripening. The thickness of the interfacial intermetallic compound in all the solder joints increased slightly during the aging process. The formation of Cu₆Sn₅ was suppressed by the Bi-rich phases above the Cu₆Sn₅ layer with the aging time increasing. In addition, the solder bulk showed many cracks along the β-Sn grain boundaries after isothermal aging when the content of SAC paste was 5 wt.%. With 8 wt.% or 15 wt.% SAC, fractures were more obvious near the interface than away from the interface.

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Modern power electronics has the increased demands in current density and high-temperature reliability. However, these performance factors are limited due to the die attach materials used to affix power dies microchips to electric circuitry. Although several die attach materials and methods exist, nanosilver sintering technology has received much attention in attaching power dies due to its superior high-temperature reliability. This paper investigated the sintering properties of nanosilver film in double-side sintered power packages. X-ray diffraction results revealed that the size of nanosilver particles increased after pressure-free sintering. Compared with the pressure-free sintered nanosilver particles, the 5-MPa sintered particles showed a higher density. When increasing sintering pressure from 5 to 30 MPa, the shear strength of the sintered package increased from 8.71 to 86.26 MPa. When sintering at pressures below 20 MPa, the fracture areas are mainly located between the sintered Ag layer and the surface metallization layer on the fast recovery diode (FRD) die. The fracture occurs through the FRD die and the metallization layer on the bottom molybdenum substrate when sintering at 30 MPa.

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In this study, SAC305 and SAC305-0.3Ni solder balls were soldered onto Cu, high temperature treated Cu (H-Cu) and graphene coated Cu (G-Cu) substrates, respectively. The microstructure, the interfacial reaction, and the hardness of the solder joints were investigated. The interfacial intermetallic compound (IMC) is Cu₆Sn₅ in the solder joints of SAC305/Cu, SAC305/H-Cu, and SAC305/G-Cu. With the addition of 0.3 wt% Ni in the SAC305 solder, the interfacial IMC on Cu, H-Cu, and G-Cu transforms from Cu₆Sn₅ into (Cu, Ni)₆Sn₅. The thickness of Cu₆Sn₅ and (Cu, Ni)₆Sn₅ is the lowest on G-Cu substrate. Meanwhile, smooth (Cu, Ni)₆Sn₅ interfacial IMC layers are obtained in SAC305-0.3Ni/H-Cu and SAC305-0.3Ni/G-Cu solder joints. Both the SAC305 and the SAC305-0.3Ni solder bulks have the highest β-Sn content and the lowest concentration of eutectic phases on G-Cu substrate. Consequently, the hardness of the solder bulks on G-Cu is lower than that on the other two kinds of substrates.

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The microstructure, thickness, porosity, and shear performance of the silver (Ag) sintering layers under different sintering pressures were investigated. Experimental results demonstrated that the thickness and the porosity of the sintering layer decreased when the sintering pressure varied from 5 MPa to 30 MPa. This densification phenomenon facilitated the enhancement of the Ag sintering layers. The shear strength was improved significantly from 44.19 MPa to 69.41 MPa when the sintering pressure increased from 5 MPa to 10 MPa. When the sintering pressure ranged from 10 MPa to 30 MPa, the shear strength presented a slow increase from 69.41 MPa to 73.38 MPa. According to the results of the failure analysis, fracture mode transformation was considered as the basic reason for this phenomenon. The increasing sintering pressure promoted the bonding of the nano-Ag particles during the sintering process. Consequently, the fracture of the sintered-Ag layer transformed from brittle fracture to ductile fracture because of the increasing sintering pressure. The delamination area between Cu and Ni layers coated on the bottom Mo plate was clearly enlarged with the increasing sintering pressure. The delamination between Cu and Ni layers coated on the bottom Mo plate turned to be the main failure mode when the sintering pressure was higher than 10 MPa.@en

High reliable packaging materials are needed for electronics when they work at harsh environments. Among which, the nanosilver material has been widely studied and applied in power electronics due to its low processing temperature and high reliability. This paper investigates the bonding properties of nanosilver sintered hermetic cavity. There are two kinds of lids used in this study, including copper lid and silicon lid. The X-ray and C-Mode Scanning Acoustic Microscopy (C-SAM) results revealed that delamination tended to happen in Cu lid sintered cavity as the recovery of deformed Cu lid was hindered by sintered dense Ag layer. However, no delamination or cracks were found in Si lid sintered cavity. Finite element analysis (FEA) method was used to investigate the effects of lid materials on the stress distribution of lid. The results indicated that the Cu lid sintered cavity showed a much higher stress than the Si lid sintered cavity under the sintering parameters of 250 °C and 10 MPa. There is no obvious change in the stress distribution areas on Cu lid with the increasing of pressures from 5 to 30 MPa. However, the distribution area of stress on Si lid expanded obviously only when the sintering pressure increased to 30 MPa. With the increase of sintering pressures from 5 to 30 MPa, the maximum stresses on Cu lid are almost the same, while increasing trend was found on Si lid.@en

This study investigated the interfacial reaction, the microstructure, and the hardness of the SAC305-xNi solder on both Cu and graphene-coated Cu (G–Cu) substrates. The experimental results indicate that the increase of Ni content in the solder leads to the roughness of the (Cu, Ni)6Sn5 IMC layer on Cu. In contrast, the growth of the (Cu, Ni)6Sn5 interfacial IMC, which results from increasing Ni addition, is significantly suppressed on G–Cu substrates. As the concentration of Ni ranges from 0 to 0.2 wt%, the microstructure of the solder bulks on Cu substrates shows slight changes. The hardness of the solder bulks in SAC305-Ni/Cu is similar to that in the SAC305/Cu solder joint. The amount of β-Sn rises and the eutectic area shrinks due to increasing Ni addition in the solder bulks on G–Cu substrates. Therefore, the solder bulks in the SAC305-Ni/G–Cu show lower hardness than that in the SAC305/G–Cu solder joint.@en

This paper compared the fatigue damage accumulation of the gold-tin eutectic die attach layer in different high electron mobility transistor (HEMT) packages with various types of die attach layers and substrates under thermal cyclic loading. The fatigue damage per cycle used in this study was characterized by the accumulation of plastic work, which was derived by the finite element analysis (FEA). The effects of die attach layer's standoff height and thickness of substrate on fatigue damage accumulation was discussed. The results show that increasing the standoff height of the die attach layer is an effective way to prevent the early crack initiation in gold-tin die attach layer especially for a gallium nitride (GaN) die. It is also indicated that for a GaN die, a thicker die attach layer and a thinner substrate are preferable in order to retain a comparable lifetime with silicon (Si) die and Cu substrate system.@en