The size and crystal orientation of Sn grains in Pb-free, near eutectic Sn-Ag-Cu solder joints were examined. A clear dependence of the thermomechanical fatigue response of these solder joints on Sn grain orientation was observed (Sn has a body centered tetragonal crystal structure). Fabricated joints tend to have three orientations in a cyclic twin relationship, but among the population of solder balls, this orientation triplet appears to be randomly oriented. In thermally cycled joints, solder balls with dominant Sn grains having the particular orientation with the c-axis nearly parallel to the plane of the substrate were observed to fail before neighboring balls with different orientations. This results from the fact that the coefficient of thermal expansion of Sn in the basal plane (along the alpha-axis) is half the value along the c-axis; joints observed to be damaged had the maximum coefficient of thermal expansion mismatch between solder and substrate at the joint interface, as well as a tensile stress modes during the hot part of the thermal cycle. Localized recrystallization was observed in regions of maximum strain caused by differential expansion conditions, and its connection with crack nucleation is discussed.

Influence of Sn Grain Size and Orientation on the Thermomechanical Response and Reliability of Pb-free Solder Joints

Experimental investigation of void coalescence in metallic sheets containing laser drilled holes

The effect of isothermal aging on the microstructure and shear strength of Sn3Ag0.5Cu/Cu (SAC305/Cu) solder joints were studied systematically. The single-lap shear samples of SAC305/Cu solder joint were prepared and aged up to 456 h at 150 and 180 °C, respectively. The interfacial intermetallic compound layer of the aged solder joint gradually thickened with increasing both aging time and aging temperature, while, the interfacial intermetallic compound layer morphology transformed from scallop-type to layer-type after the aging treatment. The growth of the interfacial Cu-Sn interfacial intermetallic compounds layer of aged SAC305/Cu solder joints exhibited a linear function of the square root of aging time, indicating that the formation of the interfacial Cu-Sn interfacial intermetallic compounds during aging treatment was mainly controlled by the diffusion mechanism. The diffusion coefficient (D) values of interfacial intermetallic compound layer were 4.64×10−17 and 1.06×10−16 m2 s−1 for aging temperatures of 150 °C and 180 °C, respectively. Single-lap shear tests results revealed that the shear strength of SAC305/Cu solder joints decreased continuously with an increase in interfacial intermetallic compound layer thickness and aging time. The main reason for these characteristics was the excessive increase in the interfacial intermetallic compound thickness of solder joints, causing a change in the stress concentration of the shear load from the protruding region to the inside of the interfacial intermetallic compound layer at the same tested condition. In addition, the shear fractures in as-reflowed and short time aged solder joints were shown to be ductile in nature and confined in the bulk solder rather than through the interfacial intermetallic compound layer. However, the shear fracture locations transferred from the solder bulk to the interfacial Cu-Sn interfacial intermetallic compound layer, and finally to the Cu3Sn/Cu interface with increasing aging time.

Microstructure evolution and shear fracture behavior of aged Sn3Ag0.5Cu/Cu solder joints

Incremental recrystallization/grain growth driven by elastic strain energy release in a thermomechanically fatigued lead-free solder joint

Quantifying the effect of porosity on the evolution of deformation and damage in Sn-based solder joints by X-ray microtomography and microstructure-based finite element modeling

Deformation analysis of lap-shear testing of solder joints

This study is devoted to thermomechanical response and modeling of copper thin films and interconnects. The constitutive behavior of encapsulated copper film is first studied by fitting the experimentally measured stress-temperature curves during thermal cycling. Significant strain hardening is found to exist. Within the continuum plasticity framework, the measured stress-temperature response can only be described with a kinematic hardening model. The constitutive model is subsequently used for numerical thermomechanical modeling of Cu interconnect structures using the finite element method. The numerical analysis uses the generalized plane strain model for simulating long metal lines embedded within the dielectric above a silicon substrate. Various combinations of oxide and polymer-based low-k dielectric schemes, with and without thin barrier layers surrounding the Cu line, are considered. Attention is devoted to the thermal stress and strain fields and their dependency on material properties, geometry, and modeling details. Salient features are compared with those in traditional aluminum interconnects. Practical implications in the reliability issues for modern copper/low-k dielectric interconnect systems are discussed.

Thermomechanical response and stress analysis of copper interconnects

The lap-shear technique is commonly used to evaluate the shear, creep, and thermal fatigue behavior of solder joints. We have conducted a parametric experimental and modeling study, on the effect of testing and geometrical parameters on solder/copper joint response in lap-shear. It was shown that the farfield applied strain is quite different from the actual solder strain (measured optically). Subtraction of the deformation of the Cu substrate provides a reasonable approximation of the solder strain in the elastic regime, but not in the plastic regime. Solder joint thickness has a profound effect on joint response. The solder response moves progressively closer to “true” shear response with increasing joint thickness. Numerical modeling using finite-element analyses were performed to rationalize the experimental findings. The same lap-shear configuration was used in the simulation. The input response for solder was based on the experimental tensile test result on bulk specimens. The calculated shear response, using both the commonly adopted far-field measure and the actual shear strain in solder, was found to be consistent with the trends observed in the lap-shear experiments. The geometric features were further explored to provide physical insight into the problem. Deformation of the substrate was found to greatly influence the shear behavior of the solder.

An evaluation of the lap-shear test for Sn-Rich Solder/Cu couples: Experiments and simulation

A combined experimental and numerical study is undertaken to examine the effects of pre-machined holes on strain localization and overall ductility in eutectic tin-lead alloy. Thin-sheet specimens with equal-sized holes aligned in the tensile loading direction are used. The tensile tests were performed at room temperature with a nominal strain rate of 0.001 s � 1 . The specimen, containing one hole, showed a significant reduction in ductility compared to the control (no-hole) specimen. With an increasing number of holes, however, the overall strain-to-failure increases and fracture tend to follow shear bands generated locally from the hole edges. Finite element analyses, taking into account the viscoplastic response, were carried out to provide a mechanistic rationale to corroborate the experimental findings. The dispersion of plastic deformation and the effect of hole interaction are both found to contribute to the observed behavior. The local maximum equivalent plastic strain decreases with increasing number of holes, resulting in more delayed fracture. Plastic deformation becomes more intense inside the shear band when the holes are spaced more closely, which explains the increasing propensity of fracture along the shear bands in specimens containing more pre-machined holes. # 2003 Elsevier B.V. All rights reserved.

/pdf/cavity-mediated-strain-localization-and-overall-ductility-in-1hye0233k5.pdf

Cavity mediated strain localization and overall ductility in eutectic tin-lead alloy

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Papers

Deformation analysis of lap-shear testing of solder joints

Thermomechanical response and stress analysis of copper interconnects

An evaluation of the lap-shear test for Sn-Rich Solder/Cu couples: Experiments and simulation

Cavity mediated strain localization and overall ductility in eutectic tin-lead alloy