John Hock Lye Pang

A modified Weibull model for tensile strength distribution of carbon nanotube fibers with strain rate and size effects

Fundamental studies on the effects of strain rate and size on the distribution of tensile strength of carbon nanotube (CNT) fibers are reported in this paper. Experimental data show that the mechanical strength of CNT fibers increases from 0.2 to 0.8u2009GPa as the strain rate increases from 0.00001 to 0.1 (1/s). In addition, the influence of fiber diameter at low and high strain rate conditions was investigated further with statistical analysis. A modified Weibull distribution model for characterizing the tensile strength distribution of CNT fibers taking into account the effect of strain rate and fiber diameter is proposed.

Fatigue Reliability Analysis of Sn–Ag–Cu Solder Joints Subject to Thermal Cycling

In this paper, thermal cycling tests and finite-element analysis (FEA) for a plastic ball grid array package with Sn-3.8Ag-0.7Cu lead-free solder joints have been performed. The solder joint fatigue lives were predicted and compared by using different 2-D and 3-D FEA models. The effects of solder constitutive models and fatigue life models on solder fatigue life prediction have been investigated. In order to obtain fatigue parameters, new averaging volumes were proposed for the solder fatigue life prediction. Different reference temperatures were simulated to investigate its effect on the solder fatigue life prediction. The effect of intermetallic compound thickness on solder joint fatigue life prediction was also investigated.

Fracture of Sn-Ag-Cu Solder Joints on Cu Substrates:I. Effects of Loading and Processing Conditions

During service, microcracks form inside solder joints, making microelectronic packages highly prone to failure on dropping. Hence, the fracture behavior of solder joints under drop conditions at high strain rates and under mixed-mode conditions is a critically important design consideration for robust joints. This study reports on the effects of joint processing and loading conditions on the microstructure and fracture response of Sn-3.8%Ag-0.7%Cu (SAC387) solder joints attached to Cu substrates. The impact of parameters which control the microstructure (reflow condition, aging) as well as loading conditions (strain rate and loading angle) are explicitly studied. A methodology based on the calculation of the critical energy release rate, GC, using compact mixed-mode (CMM) samples was developed to quantify the fracture toughness of the joints under conditions of adhesive (i.e., interface-related) fracture. In general, higher strain rate and increased mode-mixity resulted in decreased GC. GC also decreased with increasing dwell time at reflow temperature, which produced a thicker intermetallic layer at the solder–substrate interface. Softer solders, produced by slower cooling following reflow, or post-reflow aging, showed enhanced GC. The sensitivity of the fracture toughness to all of the aforementioned parameters reduced with an increase in the mode-mixity. Fracture mechanisms, elucidating the effects of the loading conditions and process parameters, are briefly highlighted.

Lead free solder : mechanics and reliability

Introduction.- Theory on Mechanics of Solder Materials.-Mechanical Properties and Constitutive Models.- Fatigue Life Prediction Models.- Finite Element Analysis and Design-For-Reliability.- Thermo-Mechanical Reliability Test and Analysis.- Dynamic Mechanical Reliability Test and Analysis.- Thermal Cycling Aging Effects on Board-Level Drop Test Result

Nonlinear stress-strain behavior of carbon nanotube fibers subject to slow sustained strain rate

Nonlinear stress-strain behavior of carbon nanotube (CNT) fibers is studied based on the test data where fiber strength can be modeled by the Weibull distribution. CNT fibers spun from vertically aligned arrays are tensioned at slow sustained strain rate (0.00001u20091/s) to study the tensile strength resulting from sliding-to-failure effects. A model is developed to estimate the Weibull modulus which characterizes the dispersion of fiber strengths in terms of the maximum sustained stress and failure strain of the fibers. The results show that the sliding indeed has great influence on the stress-strain relation of CNT fibers at low strain rate.