Yi-Shao Lai

Bio: Yi-Shao Lai is an academic researcher. The author has contributed to research in topics: Chip-scale package & Wire bonding. The author has an hindex of 3, co-authored 3 publications receiving 56 citations.

Transient simulation of wire pull test on Cu/low-K wafers

Abstract: This paper focuses on the numerical analysis of pull test reliability of gold wires bonded on the Cu/low-K wafer. Prior to wire pull, transient analysis of the complete wirebonding process, which involves both impact and ultrasonic vibration stages, is performed to allocate residual stresses within the wire and the Cu/low-K structure. After wirebonding, fracturing of the wire subjected to a pull load is modeled using the eroding technique so that failure patterns and bonding strength of the wire can be investigated. The analysis applies the explicit time integration scheme, which is feasible in dealing with nonlinear transient structural behavior. Parametric studies show that as the yield stress of the low-K material decreases, the pull force reduces significantly. Furthermore, the pull force increases as the bond force increases but not very significantly

Prediction of board-level reliability of chip-scale packages under consecutive drops

Abstract: Transient structural responses of a board-level chip-scale package subjected to consecutive drops are investigated in this paper using a numerical methodology based on the support excitation scheme and incorporated with the implicit time integration scheme. Evolutions of stresses, plastic strains, and plastic strain energies in the solder joints under repetitive drop impacts are examined and correlated with actual experimental observations. Effects of isotropic hardening and kinematic hardening presumed for the solder alloy are examined and compared

Prediction ofBoard-level Reliability ofChip-scale Packages UnderConsecutive Drops

Abstract: Transient structural responses ofaboard-level chipscalepackage subjected to consecutive dropsare investigated inthis paper using anumerical methodology based onthesupport excitation scheme andincorporated withtheimplicit timeintegration scheme. Evolutions of stresses, plastic strains, andplastic strain energies inthe solder joints under repetitive dropimpacts areexamined andcorrelated withactual experimental observations. Effects ofisotropic hardening andkinematic hardening presumed forthesolder alloy areexamined andcompared.

Transient Dynamic Simulation and Full-Field Test Validation for A Slim-PCB Of Mobile Phone under Drop / Impact

Abstract: Product durability due to drop shock is a critical element for assessment of reliability for handheld devices. So far, no simulation model for a board-level drop has been extensively validated by experiments for its predictions of global (full-field) dynamic response. Accelerometers and strain gages which are traditionally utilized to measure the response at selected locations, fail to assess the global strain gradients and complex mode shapes. In this work, a novel non-contact optical technique has been proposed for measurement of full-field impact response. Pair of synchronized high-speed cameras capture the images of board assembly subjected to JEDEC standard impact, at rates up to 15,000 pictures per second. A digital image correlation (DIC) system has been integrated with the cameras to analyze the acquired images to give dynamic deformation, shape and strain over the entire surface of board during impact. A finite element model for the drop test has been developed using ANSYS/LS-DYNA. The numerical solution has been fully validated against experimental measurements of acceleration, strain and warpage at series of instants of time after impact. Effect of tightening torque at PCB mounts has been studied comprehensively with regards to the eigenvalues and mode shapes, and the necessity for accurate modeling of dynamic contact conditions existing at the supports has been demonstrated. Simulation model has been further used to assess the drop impact reliability of components on the board. Excellent correlation of all simulation results with the measured data validates the experimental and numerical propositions made in this work for analyzing a board-level drop impact.

High-speed bend test method and failure prediction for drop impact reliability

Abstract: The objective of this study is to obtain experimental failure models governing solder joint failure during drop impact testing of board assemblies. A high-speed bend test was developed to perform displacement-controlled bend test of board assemblies at the high flexing frequencies of drop impact. These test frequencies and amplitudes are not achievable by conventional universal testers. Experimental data was obtained for various PCB strain amplitudes, flexural frequencies solder alloys and pad finishes. Results from the high-speed bend tests are used to construct constant amplitude power law fatigue curves. Solder joint reliability found to be dependent on the test frequency, and therefore strain rate. The experimental failure data from these high-speed bend tests are a required basis for a drop impact failure criterion which can take into account frequency and amplitude effects and which is general enough to be applied to product level testing.

Study on the Reliability of Application-Specific LED Package by Thermal Shock Testing, Failure Analysis, and Fluid–Solid Coupling Thermo-Mechanical Simulation

Abstract: Reliability is essential for large-scale applications of high-power light-emitting diode (LED) devices, modules, and systems for general illumination. In this paper, the reliability of a novel application-specific LED package (ASLP) is investigated by thermal shock testing, failure analysis, and fluid-solid coupling thermo-mechanical simulation. The reliability of the ASLP modules was validated with a dual-bath liquid thermal shock testing from 233 to 398 K. The non-destructive failure analysis was conducted to the catastrophic failure ASLP samples by fluorescent penetrant inspection. The delaminations at the interfaces within the ASLP module were detected. The failure mechanisms were identified by digital optical microscopy and field emission scanning electron microscope inspection after the decapsulation. The experimental results show that fracture failure occurs at the wedge joint of the bonding wire, which leads to the catastrophic failure of the ASLP module. The stress and strain behaviors of the ASLP module, especially the bonding wire under thermal shock loading, were analyzed through thermo-mechanical modeling with the nonlinear time- and temperature-dependent material properties. Significant thermal gradient within the ASLP module during the thermal shock testing was taken into consideration by the fluid-solid coupling transient thermal transfer analysis. The effects of the delaminations detected by the fluorescent penetrant inspection on the reliability of the bonding wire were also examined. It is found the delaminations existing at the interfaces within the ASLP module induce significant plastic strain to the wedge joint and results in fracture failure. The results from the numerical simulation can make a good prediction of the failure mechanism of the ASLP modules under thermal shock loading.

Fine and ultra‐fine pitch wire bonding: challenges and solutions

Abstract: Purpose – The purpose of this paper is to review recent advances in fine and ultra‐fine pitch wire bonding.Design/methodology/approach – Dozens of journal and conference articles published recently are reviewed.Findings – The problems/challenges such as possible wire sweep and decreased bonding strength due to small wire sizes, non‐sticking, metal pad peeling, narrow process windows, wire open and short tail defects are analysed. The solutions to the problems and recent findings/developments in fine and ultra‐fine pitch wire bonding are discussed.Research limitations/implications – Because of the page limitation, only brief discussions are given in this paper. Further reading is needed for more details.Originality/value – This paper attempts to provide an introduction to recent developments and the trends in fine and ultra‐fine pitch wire bonding. With the references provided, readers may explore more deeply by reading the original articles.