Narrow Results

As a lead-free solder, Sn96.5Ag3Cu0.5 has a wide application in electronic packaging. Since the solder materials usually work under cyclic temperature surroundings, creep constitutive relationships and cyclic behaviors are necessary to carry out the thermal stress analysis of a package with such a solder for its strength and life evaluations. This paper has investigated the creep constitutive relationships by constant (non-cyclic) loadings firstly, based on the creep test results at various stress and temperature levels. The complete form of the constitutive relationship containing both the linear viscous and hyperbola-sine creeps is proposed. Secondly, through the tests under cyclic stress loadings, the cyclic stress-strain relationships have been illustrated.

The failure behaviors of flip chip solder joints under various loading conditions of the high-speed shear test (H-SST) were investigated with an experimental and non-linear 3-dimensional finite element modeling study. The solder composition used in this study was Sn-3.0Ag-0.5Cu (in wt.%). The shear forces were far greater by H-SST than by low-speed shear test (L-SST). The shear force further increased with increasing shear speed, mainly due to the high strain-rate sensitivity of the solder alloy. Brittle interfacial fractures were more easily achieved by H-SST, especially at the higher shear speed. This was discussed in terms of the relationship between the strain-rate and work-hardening effect and the resulting stress concentration at the interfacial regions

In the current work, a test scheme to evaluate solder joint interface fracture toughness using double cantilever beam (DCB) test has been successfully demonstrated. The obtained results, in terms of critical energy release rate, predict the joint failure based on the principle of fracture mechanics. The results can be used as a materials property in the reliability design of various types of solder-ball joined packages. DCB specimens made of 99.9 wt% copper were selected in the current work. Eutectic Sn-37Pb and lead-free Sn-3.5Ag-0.5Cu solders were used to join two pieces of the copper beams with controlled solder thickness. The test record showed steady propagation of the crack along the solder / copper interface, which verifies the viability of such a testing scheme. Interface fracture toughness for as-joined, extensively-reflowed and thermally aged samples has been measured. Both the reflow treatment and the thermal aging lead to degradation of the solder joint fracture resistance. Reflow treatment was more damaging as it induces much faster interface reaction. Fractographic analysis established that the fracture has a mixed micromechanism of dimple and cleavage. The dimples are formed as a result of the separation between the hard intermetallic compound (IMC) particles and the soft solder material, while the cleavage is formed by the brittle split of the IMCs. When the IMC thickness is increased due to extended interface reaction, the proportion of IMC cleavage failure increases, and this was reflected in the decrease of the critical energy release rate.

In order to improve the mechanical behavior of the low-temperature ${\mathrm{\text{Sn}}}_{58}\mathrm{\text{Bi}}$ (SnBi) lead-free solder joint, the ${\mathrm{\text{Sn}}}_{0.7}{\mathrm{\text{Ag}}}_{0.5}{\mathrm{\text{Cu}}}_{3.5}{\mathrm{\text{Bi}}}_{0.05}\mathrm{\text{Ni}}$ (SACBN) solder ball with the diameter of 400 $\mu$m was pre-soldered on Cu to obtain the SnBi/SACBN/Cu composite joint. The microstructure and shear behavior of the solder joints were investigated. Experimental results indicate that SnBi solder is well bonded on the SACBN bump due to the elemental diffusion and dissolution between the molten SnBi and solid SACBN bump during the soldering process. The addition of the SACBN bump shows a significant effect on the formation and growth of the $\beta$-Sn grains in the SnBi bulk. Compared with the SnBi/Cu joint, the SnBi bulk in the composite joint shows enlarged $\beta$-Sn dendritic grains. Meanwhile, the interfacial intermetallic compound (IMC) layer transforms from Cu₆Sn₅ into (Cu, Ni)₆Sn₅. Among these three solder joints, the shear strength of the SACBN/Cu joint is the highest, reaching 86.7 MPa. The shear strength of the SnBi/Cu solder joint is enhanced by the SACBN bump from 68.2 MPa to 75.2 MPa. Additionally, the addition of the SACBN bump shows a positive effect on suppressing the brittleness of the SnBi/Cu solder joint.

The effects of cyclic loading conditions (either thermal or mechanical) on the reliability of electronic assemblies are strongly dependent on the performance of solder joints. Most solder joint fatigue models, and the supporting experimental data, do not treat the crack propagation processes that lead to failure. The benefits from a physically based description of crack propagation in solder joints include an accurate representation of the damage produced by cyclic loading and, therefore, a superior basis to evaluate attachment designs and materials. The development of crack growth rate models has been hampered by the lack of experimental capabilities to observe and characterize crack propagation in solder joints and a computational capability to describe the propagation of cracks in solder joints. This paper describes several approaches that have been used to measure crack propagation rates in solders and a new approach to quantify the crack growth rate and its driving force in solder joints by a quantitative analysis of the fracture surface topography.

The low-cycle fatigue behavior of surface-mount solder joints is influenced by the constitutive behavior and the structural response of all the elements that make up the attachment system including the solder joint, the board, and the package. Modifications to the Coffin-Manson low-cycle fatigue model that describe the constitutive behavior and structural response of a surface-mount solder joint on a leadless ceramic chip carrier have been developed that lead to a closed-form relationship between the cycles to failure and the displacement range. The model is shown to describe accurately the fatigue life of mechanically cycled surface-mount solder joints.

12-mil pitch processing is achievable with solder paste. It may however be the limit of solder paste printing technology, mainly due to the scooping problem associated with thin stencils. With decreasing pitch size, both smear and insufficiency rates increase. Tapering of the stencil aperture helps thick stencil prints, but harms thin stencil printing. Apertures with orientation parallel to squeegee movement results in a higher print defect rate. Overall, the use of fine powders is the most effective means to meet most challenges. It helps in achieving high performance in printability, tack, and non-slump, with acceptable trade-off in rheology and tack time. Solder balling may be the primary hurdle. The problem may be resolved by using an inert reflow atmosphere or through flux chemistry improvements. A metal load of 90.5–91% seems to be the optimum for most properties. Besides paste technology, the chemistry of solder alloys is also a focus for future electronics manufacturing technology. Environmental and toxicity concerns related to the use of lead have initiated the search for acceptable, alternative joining materials for electronics assembly. This paper describes a novel lead-free solder designed as a ‘drop in’ replacement for common tin-lead eutectic solder. The physical and mechanical properties are discussed in detail with comparison to tin-lead eutectic solder. The performance of this solder when used for electronics assembly is discussed and compared to other common solders. Fatigue testing results are reported for thermal cycling electronics assemblies soldered with this lead-free composition.

This paper will demonstrate that diamond grains can be bonded to W micro-wires (AD wires) with a specially developed metal solder in a continuous process capable of producing the wire of 1000 m in length. Cutting experiments using a specially designed cutting machine have been conducted. We will demonstrate that AD wire can cut WC (HV2500) and that AD wire performs superior compared to other commercial cutting wires bonded with diamond grains using either Ni plating or resin type. It has also been found that for wire diameter less than ~ 100 µm cutting life of the wire was short and failed in a torsion fracture mode. This will be explained by comparing the friction induced torsion to the twist strength of the wire.

As a lead-free solder, Sn96.5Ag3Cu0.5 has a wide application in electronic packaging. Since the solder materials usually work under cyclic temperature surroundings, creep constitutive relationships and cyclic behaviors are necessary to carry out the thermal stress analysis of a package with such a solder for its strength and life evaluations. This paper has investigated the creep constitutive relationships by constant (non-cyclic) loadings firstly, based on the creep test results at various stress and temperature levels. The complete form of the constitutive relationship containing both the linear viscous and hyperbola-sine creeps is proposed. Secondly, through the tests under cyclic stress loadings, the cyclic stress-strain relationships have been illustrated.