Hongtao Chen

Inhomogeneous deformation and microstructure evolution of Sn–Ag-based solder interconnects during thermal cycling and shear testing

Abstract Orientation imaging microscopy was adopted to characterize the microstructural changes in Sn–Ag-based solder interconnects during thermal cycling and shear testing. The deformation and microstructure evolution of Sn–Ag-based solder interconnects are inhomogeneous, depending on the orientations of β-Sn grains in the as-solidified microstructure. Recovery or recrystallization can take place even under pure shear stress at room temperature, and it tends to occur at high-angle grain boundaries in multi-grained solder interconnects, while it localizes in near-interface region in solder interconnects with only one grain inside. During thermal cycling, the hardness of recrystallized microstructure decreased significantly due to the segregation of Ag3Sn IMC particles towards the newly-formed recrystallized boundaries, increasing the ease of localized deformation in this weakened microstructure. As a consequence, cracks were propagated intergranularly in the recrystallized microstructure.

Cu@Sn Core–Shell Structure Powder Preform for High-Temperature Applications Based on Transient Liquid Phase Bonding

This paper presents a novel die-attach material for high-temperature applications based on transient liquid phase (TLP) bonding. Cu particles are electroless plated with Sn to achieve Cu@Sn structure, and the fabricated Cu@Sn particles are compressed into preforms for die attachment. This material can be reflowed at a low temperature (<;260°C) due to the low melting temperature of the outer Sn layer. However, after reflow soldering, the resulting interconnections can withstand a high temperature of at least 415°C, with the entire outer Sn layer transforming into Cu-Sn intermetallic compounds (IMCs) with high remelting temperatures. The formed bondlines exhibit good electrical and thermal conductivity due to the low porosity and the embedded Cu particles in the interconnections. A large shear strength at 400°C can be achieved with the high-remelting Cu-Sn IMCs in the interconnections. Furthermore, the interconnections also exhibit excellent reliability under thermal shock cycling from -55 to 200°C. The I-V curve and breakdown voltages of the novel preform-bonded IGBTs are measured to examine the interconnection quality compared with those of Cu-Sn TLP-bonded IGBTs. This die-attach material is suitable for power devices operating under high temperatures or other harsh environments.

Numerical and experimental analysis of the Sn3.5Ag0.75Cu solder joint reliability under thermal cycling

Abstract The Sn3.5Ag0.75Cu (SAC) solder joint reliability under thermal cycling was investigated by experiment and finite element method (FEM) analysis. SAC solder balls were reflowed on three Au metallization thicknesses, which are 0.1, 0.9, and 4.0xa0μm, respectively, by laser soldering. Little Cu–Ni–Au–Sn intermetallic compound (IMC) was formed at the interface of solder joints with 0.1xa0μm Au metallization even after 1000 thermal cycles. The morphology of AuSn 4 IMC with a small amount of Ni and Cu changed gradually from needle- to chunky-type for the solder joints with 0.9xa0μm Au metallization during thermal cycling. For solder joints with 4xa0μm Au metallization, the interfacial morphology between AuSn 4 and solder bulk became smoother, and AuSn 4 grew at the expense of AuSn and AuSn 2 . The cracks mainly occurred through solder near the interface of solder/IMC on the component side for solder joints with 0.1xa0μm Au metallization after thermal shock, and the failure was characterized by intergranular cracking. The cracks of solder joints with 0.9xa0μm Au metallization were also observed at the same location, but the crack was not so significant. Only micro-cracks were found on the AuSn 4 IMC surface for solder joints with 4.0xa0μm Au metallization. The responses of stress and strain were investigated with nonlinear FEM, and the results correlated well with the experimental results.

Microstructure and Grain Orientation Evolution in Sn-3.0Ag-0.5Cu Solder Interconnects Under Electrical Current Stressing

Inxa0situ observation was performed on cross-sections of Sn-3.0Ag-0.5Cu solder interconnects to track the evolution of microstructure and grain orientation under electrical current stressing. Cross-sections of Cu/Ni–Sn-3.0Ag-0.5Cu–Ni/Cu sandwich-structured solder interconnects were prepared by the standard metallographic method and subjected to electrical current stressing for different times. The electron backscatter diffraction technique was adopted to characterize the grain orientation and structure of the solder interconnects. The results show that metallization dissolution and intermetallic compound (IMC) migration have close relationships with the grain orientation and structure of the solder interconnects. Ni metallization dissolution at the cathode interface and IMC migration in the solder bulk can be accelerated when the c-axis of the grain is parallel to the electron flow direction, while no observable change was found when the c-axis of the grain was perpendicular to the electron flow direction. IMC can migrate along or be blocked at the grain boundary, depending on the misorientation between the current flow direction and grain boundary.

Study on the Effects of Adipic Acid on Properties of Dicyandiamide-Cured Electrically Conductive Adhesive and the Interaction Mechanism

A small quantity of adipic acid was found to improve the performance of dicyandiamide-cured electrically conductive adhesive (ECA) by enhancing its electrical conductivity and mechanical properties. The mechanism of action of the adipic acid and its effects on the ECA were examined. The results indicated that adipic acid replaced the electrically insulating lubricant on the surface of the silver flakes, which significantly improved the electrical conductivity. Specifically, one of the acidic functional groups in adipic acid reacted with the silver flakes, and an amidation reaction occurred between the other acidic functional group in adipic acid and the dicyandiamide, which participated in the curing reaction. Therefore, adipic acid may act as a coupling agent to improve the overall ECA performance.

Microstructure evolution and mechanical strength evaluation in Ag/Sn/Cu TLP bonding interconnection during aging test

Abstract In this study, the microstructure of Ag/Sn/Cu TLP bonding interconnections was investigated. No defects such as voids were observed at the interface between Ag-Sn IMCs and Cu-Sn IMCs in both as-reflowed and aged Ag/Sn/Cu TLP bondlines. Microstructure evolution and phase transformation during aging test were studied in detail to further evaluate the reliability of Ag/Sn/Cu TLP bondlines during long-term service in high temperature. The phase composition at the interface of as-reflowed interconnects was Ag/Ag3Sn/Cu6Sn5/Cu and transformed to Ag/(Ag)/ζ-Ag/Cu3Sn/Cu after aging test. Different characteristics of diffusion between Cu and Ag atoms were found in Cu/Ag/Sn TLP sample. The average shear strength of the as-reflowed and the aged Ag/Sn/Cu TLP bonding interconnections are 49.57xa0MPa and 50.03xa0MPa, respectively, and this indicated that almost no deterioration occurred during the aging test. The fracture surfaces of the aged Ag/Sn/Cu TLP bonding interconnections after shearing test were characterized in detail. The results show that the substitution of Cu for one side of Ag plates in traditional Ag-Sn TLP joint did not influence its strength and fracture mode. The Cu-rich structure at the fracture surfaces is always irregular while the Ag-rich phases are often accompanied by shear bands and dimples.

Effect of thermal aging on microstructure, shear and mechanical shock failures for solder ball bonding joint

The interfacial microstructure evolution between Sn-3.5Ag-0.75Cu solder ball and Au/Ni/Cu pad was investigated under as-bonded and thermal aging conditions. Moreover, the shear and mechanical shock tests were carried out to study the failure mechanisms. Results showed that only one intermetallic compound, fine needle-like AuSn4 was found at the solder/pad interface under as-bonded condition. After aging, Ag3Sn appeared in the solder bulk as a particle-like or elongated structure. Meanwhile, the intermetallic compounds layer became flat and a quaternary intermetallic compound Cu-Sn-Ni-Au appeared between AuSn4 layer and Ni(NiFe) layer. In addition, parts of AuSn4 spalled into the solder bulk with long hour aging. In the shear test, the solder joint exhibited a ductile failure. While in the mechanical shock test, the cracks initiated at the right end of the horizontal pad and propagate through the interfaces between IMCs. Furthermore, the factors dominating the failure mode were investigated which consists of external force and stress distribution, interfacial reaction and strain rate

Microstructure evolution during reflow and thermal aging in a Ag@Sn TLP bondline for high-temperature power devices

In this paper, we investigated the microstructure evolution and the resulting change in mechanical properties in a Ag@Sn TLP bondline during reflow and thermal aging. A Ag@Sn high-remelting-point bondline was rapidly achieved with thermocompression bonding of Ag@Sn powder in only 5xa0min at 250xa0°C. After reducing the thickness of the Sn coating on the Ag particles, the main phases in the resulting bondlines changed from Ag/Ag3Sn to Ag/ζ-Ag, increasing the remelting temperatures to 480xa0°C and above. The voids were effectively controlled by reducing the thickness of the Sn coating, thereby increasing the shear strength by 38%. The large surface area of the Ag/Sn interface, provided by a high density of core–shell Ag@Sn particles, enabled the rapid formation of an interconnection that is entirely composed of Ag and ζ-Ag. After thermal aging, the main phases transformed from Ag/ζ-Ag to Ag/Ag (Sn) solid solution/ζ-Ag, which causes an increase in the remelting temperature of aged interconnections up to 724xa0°C. The thermal aged samples showed slight decreases in shear strength, but the morphology of the fracture surfaces indicated better ductility.

A reliability study of adhesion mechanism between liquid crystal polymer and silicone adhesive

Abstract Liquid crystal polymer (LCP) and silicone adhesives are widely used in electronics manufacturing. The integrity of the adhesion between the two materials is crucial to the reliability of electronic products, however, the adhesion mechanism and associated reliability has not been thoroughly understood. In this paper, the adhesion mechanism between a commonly used LCP and a silicone adhesive is evaluated by employing the 85xa0°C/85% RH temperature humidity test, autoclave test, boiling water test (BWT), and dry air reflow aging test. The effects of different plasma treatments of the LCP surface are evaluated by the surface analysis methods, namely XPS and FTIR–ATR spectroscopy, as well as the surface roughness and energy measurements. Moreover, the adhesion strength between the LCP and silicone adhesive is measured by a shear strength tester. The shear strength testing process is simulated by the finite element method for a better understanding of the failure mechanism. The experimental results indicate that the adhesion between the LCP and silicone adhesive is based solely on the hydrogen bonds. It is found that the humidity significantly weakens the adhesion strength between the LCP and silicone adhesive. This is related to the breakdown of hydrogen bonds when water molecules are introduced to the system. Furthermore, the reflow test shows that the weakened adhesion strength can be recovered by removing the moisture from the interface.

Effect of Thermal Aging on the Microstructure Evolution and Solder Joint Reliability in Hard Disk Drive Under Mechanical Shock

The effect of thermal aging on the microstructure evolution and solder joint reliability in hard disk drive (HDD) under mechanical shock was investigated. Significant coarsening of Ag3 Sn particles was found in SnAgCu solder, and AuSn4 intermetallic compound (IMC) changed from needle-type to layer-type during aging. For as-soldered SnAgCu solder joints after mechanical shock, the cracks were initiated in AuSn4 at the corner of the solder joints, and mainly propagated along the thin Ni3 Sn4 IMC layer. After aging at 150 degC for 21 days, the cracks were mainly propagated along the solder, Ni3 Sn4, Au-Sn-Ni-Cu, and Au-Cu-Sn. The significant coarsening of microstructure was found in SnPb solder joints, and only microcracks were found on the surfaces of as-soldered and aged solder joints after mechanical shock.