Hsiang-Chen Hsu

Numerical simulation of high power LED heat-dissipating system

In this paper, thermal analysis of the heat dissipation under different heat sink for high-power white Light Emitting Diode (LED) is presented. Junction temperature of LED is elevated as the power of LED increases, which brings up deterioration of light efficiency and other side effects. Heat dissipation is another design concern other than material and illumination efficiency. The purpose of this paper is to investigate the cooling of high-power LED chips and modules for design of heat sinks. Three types of heat sinks are designed for a tandem 12-chip module and an extensive numerical investigation of the heat sink design performance is conducted by Computational Fluid Dynamics software Fluent. The effects of heat sink geometry and adhesive material are also investigated. Design variables are the thickness of sink base, number, thickness and length of fins. The total wetted area is the dominant factor to the junction temperature. The objective of design regarding the junction temperatures around 50°C is easily achieved. However, its effect is limited at high values of these parameters, furthermore an excessive number of fins incurs reverse consequence due to problem of ventilation also waste of material.

Characteristic of Heat Affected Zone for Ultra Thin Gold Wire/Copper Wire and Advanced Finite Element Wirebonding Model

This paper demonstrates two major works, experimentally determines the material properties and accurately predicts the dynamic response of stressed region on the bond pad and beneath the contact area. The characteristic of heat affected zone (HAZ) on both Au wire and Cu wire have been carefully experimental evaluated. In addition, the dynamic response on Al-Cu pad and beneath the pad of Cu/low-k wafer during wire bonding process has been predicted by using finite element method. Material property for Cu wire from mechanical tensile test has shown much more rigid than Au wire. This would result in bond pad shoveling around the bottom of mashed ball during impact stage and the consequent thermosonic vibration stage. All the measured data serves as material inputs for the explicit commercial finite element software ANSYS/LS-DYNA. It is also demonstrated that the pad material for Cu/low K wafer can be replaced by Al-Cu pad or Cu pad to avoid large deformation on pad and cracking beneath the surface. A secondary electric flame-off (EFO) method has been conducted to reduce the strength of Cu wire and increase bondability and reliability. A series of comprehensive experimental works and FEA predictions have been performed in this study.

Advanced finite element model on Copper wire ball bonding

The mechanism of wirebonding bondability for ultra thin Copper wire is described in this research. Two major analyses are conducted in the present paper. In the first, the characteristic of heat affected zone (HAZ) and free air ball (FAB) on thin Copper (Cu) wire have been carefully experimental measured. Thin film interfacial micro-tribology behavior between Cu FAB and Al pad is examined by Atomic Force Microscopy (AFM). Secondary, the dynamic response on Aluminum (Al) bond pad and beneath the pad during wirebonding process has been successfully predicted by finite element analysis (FEA). Tensile mechanical properties of ultra thin Cu wire before/after electric flame-off (EFO) process have been investigated by self-design pull test fixture. Experimental obtained hardening constant in Hell-Petch equation has significantly influence on the localize stressed area on Al pad. This would result in Al pad squeezing (large plastic deformation) around the smashed FAB during impact stage and the consequent thermosonic vibration stage. Microstructure of FAB is also carefully investigated by nano indentation instruments. A real-time secondary EFO scheme has been conducted to reduce the strength of Cu wire and increase the bondability. All the measured data serves as material inputs for the finite element model based on explicit software ANSYS/LS-DYNA. A series of comprehensive FEA parametric predictions have been performed in this paper.

Novel design and fabrication of a geometrical obstacle-embedded micromixer with notched wall

A microfluidic embedded MEMS mixer with a Y-junction type channel and cylindrical obstructions was designed and fabricated for improving the fluid mixing mechanism under low Reynolds number () condition. The flow field was simulated numerically by software (COMSOL multiphysics®) first. The design was then realized through casting the device in PDMS by lithographed SU-8 photo-resistive mold on silicon wafer. Parametric experimental studies were conducted for optimal design. Two different fluids were pumped into the two legs of the Y-junction channel, and the fluids were broken-up by an embedded cylindrical obstacle in the middle of the tapered micro-channel. The chaotic convection took place in the mixing channel behind the embedded cylindrical obstacles. The flow motion was observed under CCD camera and analyzed by grey level. The developed micromixer in this study can enhance the fluid mixing by the interaction of diffusion and convection for wide range of Reynolds numbers (0.01 < < 100). Experimental results showed that the mixing index reached the required value at 0.1 within 0.024 seconds when the inlet fluid velocity is 0.499 m/s (i.e., at 1200 µl/min flow rate) for merely four cylindrical obstacles. A shorter mixing distance can be accomplished compared to the current devices reported due to faster mixing and shorter mixing time.

Advanced moisture diffusion model and hygro-thermo-mechanical design for flip chip BGA package

In the present paper, a comprehensive moisture diffusion model and characterization for encapsulated plastic Flip Chip (FC) Ball Grid Array (BGA) package are investigated. The transient moisture diffusion analysis described by Ficks second law is performed to evaluate the overall moisture distribution. Diffusivities in the moisture desorption model are determined under Arrhenius behaviors. Hygroscopic swelling properties of polymeric materials are characterized by using an existing TMA/TGA extraction method. With the so-called “thermal-wetness” analogous technique, finite element analysis (FEA) is developed to evaluate the entire moisture distribution on FC BGA package. The analytical expression for total expansion strain due to hygro-thermo-mechanical coupled effect is implemented using finite element software ANSYS. Finite element predictions reveal the significance of contribution of hygroswelling induced strain. Reliability analysis for FC BGA is performed in accordance with JEDEC standard JESD22-A120. A series of comprehensive experimental works and parametric studies are conducted in this research.

Hygroscopic Swelling Effect on Polymeric Materials and Thermo-hygro-mechanical Design on Finger Printer Package

The objective of this paper is focused on the hygroscopic swelling effect on polymeric material used in electronic package and thermo-hygro-structure coupled design and reliability analysis for finger printer package. For moisture absorption/desorption analysis, the ambient environment for temperature and humidity are set to be 60degC60%RH, 85degC60%RH and 85degC85%RH, respectively. The transient moisture diffusion analysis described by Ficks equation is performed to evaluate the overall moisture distribution. Hygroscopic swelling properties such as coefficient of saturation (Csat), coefficient of moisture expansion (beta) and activation energy (Q) can be extracted through TMA (Thermal Mechanical Analysis) and TGA (Thermal Gravitational Analysis). A three-dimensional solid model of finger printer package based on finite element ANSYS software is developed to predict the thermal-induced strain, hygroscopic swelling and the residual stress distributions. The predicted thermal-induced displacements were found to be very good agreement with the Moire interferometer experimental in-plane deformation. The developed finite element 3D model, therefore, is applied to predict the mechanism of thermo-hygro-mechanical induced stress in accordance with JEDEC pre-condition standard JESD22-A120. An analytical expression for the total expansion strain due to thermo-hygro-mechanical coupled effect was proposed and the implementation procedures using software ANSYS were described in details. A series of comprehensive parametric studies were conducted in this paper.

Dynamic finite element analysis on underlay microstructure of Cu/low-K wafer during bonding process

In the present paper, the tensile mechanical properties of thin gold wire before/after electric flame-off (EFO) process have been investigated by self-design pull test fixture. Microstructure characteristics of free air ball (FAB) and heat affected zone (HAZ) are also carefully investigated. The accurate experimental material data should be reflected as input for precise finite element analysis. Numerical model based on explicit time integration scheme software ANSYS/LS-DYNA is developed to simulate the ball bond of wire bonding process. Because the crack of low-K layer and delamination of copper via are observed, dynamic transient analysis is performed to inspect the overall stress/strain distributions on the microstructure of Cu/Low-K wafer. Special emphasizes are focused on the copper via layout and optimal design of Cu/Low-K microstructure. A series of comprehensive parametric studies were conducted in this research.

Nanoscale bondability study on copper-aluminum intermetallic compound using molecular dynamics simulation

In this paper, the growth mechanism of intermetallic compound (IMC) layer between Copper (Cu) free air ball (FAB) and Aluminum (Al) bond pad is carefully examined. The test vehicle is pd-coated Cu wirebonds on Al pad in plastic ball grid array (PBGA) package. Palladium (Pd), the anti-oxide material coated on Cu wire will be blended in the Cu FAB when the ball is formed by an electrical flame-off (EFO). Preliminary results demonstrated that IMC cracks from the edge of bonding interface and spreads into the center area. This is the cause of open fail. The IMC between Cu and Al was initially generated in the form of CuAl2, and gradually increased the content of Cu and turned into CuAl when the working temperature was increased. The final stage of IMC growth is Cu9Al4 and the aluminum pad will be vanished as the result of Cu diffusivity. Bondability on nanoscale IMC of CuAl2, CuAl and Cu9Al4 are also cautiously investigated by using molecular dynamics (MD) simulations. Atomic-level tensile stress and tensile strain are predicted to examine the bonding strength of two IMCs along the bonding interface. Interfacial fracture is different in different tensile speed as well as the working temperature. A series of experimental works and MD simulations are conducted in this research.

Cutting PCB with a 532nm DPSS green laser

The use of lasers in the field of microelectronics is growing and diversifying as designers look to innovative technologies to enable the ever-increasing sophistication of their products. Lasers are employed in manufacturing processes as varied as semiconductor lithography, wafer dicing, micro-welding and via drilling. The primary driver for the use of laser technology is the relentless progression towards miniaturization - lasers offering a highly accurate, precise and non-contact alternative to conventional manufacturing processes - ideal for processing the delicate micro-scale components employed in modern electronic devices [1].

An experimental study on dicing 28 nm low-k water using laser grooving technique

For a nanoscale low-k dielectric wafer, Inter-Layer Dielectric and metal layers peelings, cracks, chipping, and delamination are the most common dicing defects by traditional blade sawing process. This paper demonstrates an investigation on uv laser grooving on low-k dielectric 65-, 45-, and 28-nm wafers. A series of parametric study on input laser power, frequency, grooving feed speed, defocus amount and street index has been conducted to improve dicing quality. The effects of the laser kerf geometry, grooving edge quality and defects are evaluated by using scanning electron microscopy (SEM) and focused ion beam (FIB). Experimental results have shown that the laser grooving technique is capable to improve the quality and yield issues in low-k wafer dicing process.