De-Shin Liu

Study of wire bonding looping formation in the electronic packaging process using the three-dimensional finite element method

This paper presents a three-dimensional finite element model of the thermosonic wirebond looping process. The gold wire heat affected zone load-displacement relationship was investigated using precision microforce tensile tests. The temperature-dependent elastoplastic behaviors of gold wire were taken into account in the nonlinear finite element model. Both the solid and equivalent shell wire models were used; exhibiting very close agreement, indicating that equivalent model was adequate. The simulation results were verified against the experimental results. The standard triangle loop (STD2-Mode) and trapezoidal loop (LOW2-Mode) were simulated. The finite element model closely predicted the profile of the tested loops. This model was used for an extensive looping factor parameter study. No obvious looping parameter effects were observed for the STD2 Mode. The loop shapes depend primarily on the wire properties, bonding pad distance and tier height. Both the looping and material parameters have significant effects on the loop shape for the LOW2 Mode. Optimizing the reverse angles could reduce the stress on the wire.

An Experimental and Numerical Investigation Into Multilayer Probe Card Layout Design

This paper conducts experimental and numerical investigations into the microforce probing technique used to test the functionality of IC devices. The study commences by considering the case of a single tungsten needle probe and examines the relationship between the contact force and the scrub mark size on aluminum pads at various levels of overdrive and shooting angle. Subsequently, a three-dimensional computational model is developed to facilitate the design of an optimum multilayer needle card layout. The simulation results obtained for the profile and size of the scrub marks on the upper surface of the aluminum pads of an IC device are found to be in good agreement with the experimental observations. The validated model is then applied to analyze the effects of the tip length and beam length on the scrub mark profile and the stress distribution contours within the needle during a wafer level test. The results predicted by the finite-element model (FEM) for the scrub mark length under various beam lengths are used to specify a suitable design for a multilayer needle layout. Taking the case of DDR2 SDRAM of an aluminum pad of dimensions 70 mumtimes70 mum (length by width), the numerical results enable appropriate values to be assigned to the shooting angles, beam lengths, and tip lengths of the individual needles within a four-layer probe card

3D IEM formulation with an IEM/FEM coupling scheme for solving elastostatic problems

In this paper, a detailed three-dimensional infinite element methodology (IEM) formulation with an infinite element (IE)-finite element (FE) coupling scheme for investigating elastostatic problems is presented. This method is equally well suited for a regular perfect domain and a domain with geometric singularity; for example, domains with cracks. In this method, the primary problem domain is subdivided into two sub-domains modeled separately using IEM and finite element method (FEM), respectively. All degrees of freedom related to the IE subdomain, except for those associated with the coupling interface, are condensed and transformed to form a finite master IE with the master nodes on the sub-domain boundary. Finally, a symmetrical IE stiffness matrix containing only master node degrees of freedom is assembled into the system stiffness matrix for the FE sub-domain. A very fine mesh pattern can be established using these efficient numerical techniques without increasing the d.o.fs of the global FEM solution. Numerical examples are presented and compared with the corresponding analytical or numerical solutions to show the performance of the proposed methodology.

A study on the electrical resistance of solder joint interconnections

A BGA package warping produced hybrid deformations in the solder joint interconnections. This situation creates difficult challenges in avoiding certain mechanical failure problems and even maintaining acceptable electrical performance in the electronic package. Accessing electronic circuit simulations for packages design is the major objective of this research. In this paper, an analytical model of equivalent solder joint interconnection resistors was developed to predict the variety in the electrical resistance with various combinations of vertical and shear strains at different environmental temperatures. Compression and shear experiments were employed to confirm the analytical model. This model was then applied to predict the variations in solder joint resistance at ambient and 120 °C temperatures with hybrid deformations. The resistance linearly increased and decreased as the solder joint experienced tension and compression. Once the shearing strain conjugated, the variational tendency akin to an exponential function was performed to demonstrate that the shearing strain dominated the change in resistance. This tendency was aggravated at higher environmental temperatures.

Finite element analysis of the effect of motorcycle helmet materials against impact velocity

Abstract A motorcycle helmet is the best protective headgear for the prevention of head injuries caused by direct cranial impact. A finite element model based on realistic geometric features of a motorcycle helmet is established, and explicit finite element LS‐DYNA code is employed to simulate dynamic responses at different impact velocities. Peak acceleration and Head Injury Criterion values derived from the headform are used to assess the protective performance of the helmet. We have concluded that the dynamic responses of the helmet dramatically vary with impact velocity, as well as the mechanical properties of the outer shell and energy‐absorbing liner. At low velocities, e.g., less than 8.3 m/s, the shell stiffness and liner density should be relatively low to diminish head‐contact force. At high velocity, e.g., 11.1 m/s, a stiffer shell and denser liner offer superior protection against head injuries.

Procedure for design optimization of a T-cap flip chip package

Abstract In this study, a 1/4 three-dimensional finite element model of a T-cap flip chip package containing the substrate, underfill, solder bump, silicon die, metal cap and cap attachment was established to conduct thermo-mechanical reliability study during the flip chip fabrication processes. The applied thermal load was cooled from 183 °C to ambience 25 °C to determine the thermal stress and warpage during the curing period of solder ball mounting process. Under fixed geometry, two levels of underfill, metal caps and cap attachments were used to conduct the 2 3 factorial design for determining reliable material combinations. The statistical tests revealed that the significant effects affecting the thermal stress were the underfill, metal cap, cap attachment and the interaction between the underfill and cap attachment. The metal cap, cap attachment and their interaction significantly affected the warpage. The proposed regression models were used to perform the surface response simulations and were useful in selecting suitable materials for constructing the package. This study provides a powerful strategy to help the designer to easily determine reliable packaging structures under various reliability considerations.

Optimization of Bonding Force, Sinking Value, and Potting Gap Size in COF Inner Lead Bonding Process

The thermo-mechanical properties of the polyimide film used in chip-on-film (COF) packaging have a significant effect on the inner lead bonding (ILB) mechanism. Furthermore, specifying an appropriate bonding force is essential in establishing a suitable compromise between a good bonding strength in the ILB process and a suitable gap size for the subsequent resin potting process. This paper commences by performing a series of experimental micro-force tensile tests to investigate the stress-strain relations of two polyimide films, namely Kapton-EN and Espanex-M. The tests are performed using formfitting specimens from actual COF products at various temperatures ranging from 25degC to 200degC . Based on the experimental results, constitutive equations are developed to model the temperature-dependent stress-strain characteristics of the two films. A finite element model of the COF/system comprising the thermo-compression tool, the polyimide film, the copper lead and the gold bump is constructed and is used to simulate the ILB process under various bonding forces. A good agreement is found between the experimental and numerical results obtained for the deformed profile of a single polyimide/copper lead following the bonding process and the variation of the bump sinking value with the bonding force, respectively. The numerical results are used to construct COF/ILB parameter design charts for the Kapton-EN and Espanex-M polyimide films which enable suitable values of the bonding force to be selected for given gap size and bump sinking values. In general, the results show that Kapton-EN has a broader working range than Espanex-M as a result of its higher elastic stiffness. When performing the ILB process using Espanex-M polyimide film, it is necessary to carefully control the bonding force to ensure that the minimum potting gap size is maintained.

A thermo-mechanical study on the electrical resistance of aluminum wire conductors

Abstract From the standpoint of electrical performance, the objective of a wire conductor is to transmit signals efficiently from one point to another. When the clock frequency trends faster, the high frequency signals on the wire conductor exhibit propagation delay, signal distortion and some symbiosis noise problems. These noises are related to the resistance, capacitance and inductance of the wire conductors. The major goal of this research is to propose a mathematical model to predict the unit change in resistance of aluminum wire conductors used in wire bonding technology in various surrounding temperatures and stress–strain states. The constitutive equation of wire conductors and the working principle of strain gages were used to derive the mathematical model. The experimental investigations involved three environmental temperatures (25, 50 and 80 °C) and two strain rates (1 and 3 mm/s) were employed to confirm the validity of the mathematical model. In the elastic range and the initiation of the plastic range, the variations in the electrical resistance of aluminum wire conductors are dominated by the thermal effect. The tensile strain replaces the thermal effect within the later half of the plastic range, however. We also proposed the electrical resistance charts of aluminum wire conductor that can be applied the variations into the electronic circuit and system simulation software (such as SPICE, PSPICE, etc.) to produce access simulations of the electrical circuits in package design.

Relationship between medial plica and medial femoral condyle—a three-dimensional dynamic finite element model

BACKGROUND Many researches reported that the pathologic medial plica impinges on the facing medial femoral condyle during knee motion and leads to erosive changes of the articular cartilage. The purpose of this study was to construct a simplified three-dimensional dynamic finite element human knee model to evaluate the dynamics behaviour between different types of medial plicae with the facing medial femoral condyles during knee motion. METHODS A three-dimensional dynamic finite element model composed of femur, tibia, covering cartilage and medial plica was developed. The kinematics of this simulation model was verified by previous findings during arthroscopic examination. The validated model was used to investigate and compare the magnitudes of the cyclic pressures acting on the cartilage of the medial femoral condyles by three different types of medial plicae with various Youngs moduli. FINDINGS All types of plicae remained in contact with the medial femoral condyles and shifted medially when the knees moved from extension to flexion. The contact pressures were positively correlated with the Youngs moduli of the medial plicae. During the whole range of motion, the maximum contact pressures of all simulation scenarios occurred when the knees moved beyond 50° of flexion. When the Youngs moduli of medial plicae were set greater than 60 MPa, all types of medial plicae would elicit contact pressures greater than 10 MPa on the medial femoral condyles. INTERPRETATION The close relationship and possible high contact pressure between fibrotic medial plica and medial femoral condyle during knee motion might be a cause of cartilage damage on the medial femoral condyle and warrants further investigation.

Numerical study on the bonding tool position, tip profile and planarity angle influences on TAB/ILB interconnection reliability

Abstract Tape automated bonding (TAB) is a widely used interconnection technology for high-pincount and fine-pitch IC packaging. In this study, a three-dimensional computational model was developed for analyzing TAB inner lead bonding (ILB) process. This experimental study on the thermomechanical properties of copper leads was achieved using high precision micro-force tensile tests. A stress–stain relation between the copper lead and different temperature ranges was successfully implemented into the finite element model to study large plastic deformation in ILB formation. The resulting ILB lead profile and bump sinking values obtained from the simulations agreed well with the experimental observations from actual manufacturing data with the same bonding parameters. The tool position and lead length effects are analyzed to study the residual stress distribution after ILB. A 10-lead model was developed to study how the tool tip profile and planarity ‘angle affect the co-planarity between the bonding tool and the stage. The numerical results show that the permissible tool profile variance should not exceed 1.25 μm and the acceptable planarity angle is 0.005° to achieve the minimum bump deformation requirement.