Shih-Jeh Wu

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.

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.

Reliability Design and Optimization Process on Through Mold Via Using Ultrafast Laser

In this study, a through mold via (TMV) process using ultrafast Laser has been developed for the interconnection on 3D package-on-package (PoP) application. Epoxy molding compound (EMC) is a composite material consisted of epoxy resin, filler particles (fused silica) and other constituents. Among these chemical compositions, the size and spatial distribution of fillers on the EMC dominate the sidewall quality on drilled trench. However, random filler leakage on the sidewall leads to an irregular and uneven trench and these drop out fillers become a problem needed to be solved. Faster removal of debris can be achieved by controlling the laser drilling parameters. Parametric studies have been performed to determine the specification of trench and design rule of material. The optimal desmear process on TMV bottom PoP package developed in this study demonstrates the reliability improvement on follow-up sputtering or filling process.

Through Mold Via (TMV) by gas-aided laser

Cutting of packaged IC involves removal of different materials including semiconductor, metal and polymeric compound. Dicing saw has long been utilized to perform the task in regular (rectangular) shape. However, chipping and micro cracking at the edges and deteriorated strength of the packaged die are still problems to be solved. Laser has the advantages of being capable of cutting irregular shape and finer edge quality with less maintain cost. However, the laser beam has effects on different materials to be process and debris becomes a problem to be solved. Laser cutting is a standard industrial process for cutting sheet metals. The process relies on the removal of the melted material with the aid of a pressurized assist gas. In the same token we studied the effect of different aiding gases on cutting packaged IC epoxy molding compound (EMC) by a UV (355nm wavelength) laser. Among the main variables controlling the process, the assisting gas type is an essential factor. This study presents a study on the influence of different assisting gases (argon, nitrogen, oxygen and air) on the edge quality. In addition, the cutting speed can be improved due to faster removal of debris. Edge quality on though molding via (TMV) technology has been dramatically improved by gas-aided laser.

Internal patterning of the glass backlight module for LCD panel by laser

Backlight module is an essential part of Liquid Crystal Display panel. Its function is to deflect and scatter the light from LED source so that the whole display can be illuminated homogeneously through the light guide plate. The commonly design is applying different patterns of V-cut grooves on the surfaces of thin polymer material plates. The process may be by etching or injection molding. The parts are then bonded together with the light source. In this study we designed a one-step alternative method on thin non-alkaline glasses. The light deflection-diffusion pattern was engraved inside the glass between both sides by a picosecond infrared laser. The spot size of the laser is small enough to create dot and line microstructures less than 20μm size. The laser cuts are in round or oval shape and the light deflection is in the manner of Lambertian scattering, which is different from the deflection-reflection V-cut designs. This method can simplify the manufacturing in one step and suitable for different materials. The luminance existence pattern was then examined with led light source by a spectroradiometer.

Thermal design for high power arrayed LED heat-dissipating system

In this paper, an improved heat dissipating system for high power arrayed light emitted display (LED) was designed and validated. Heat sink was embedded to reduce the hot spots and improve the reliability of LED lighting system. As the power of LED increases lots of heat will be generated and, in turn raises the temperature. This will greatly deteriorate light efficiency and shift the wavelength. Heat dissipation under different heat sink has been carefully designed by finite element analysis. Various kinds of heat sinks are designed for LED chips and an extensive numerical investigation of the heat sink design performance is conducted by commercial finite element package ANSYS. Concerning a single chip LED and arrayed chips LED, Heat sinks with more fins lead to lower junction temperature because of larger surface. The effects of heat sink structure and material, adhesive and environment temperature are investigated. It can be seen that the higher the thermal conductivity of the adhesive, the thinner the thickness of the adhesive, the higher the thermal conductivity of the heat sink materials, the lower the junction temperature and junction-ambient thermal resistance are. As the ambient temperature increases, the junction temperature increases linearly while the junction ambient thermal resistance changes little.

Electrochemical migration failure on FR-4 PCB by hygro-thermo-vapor pressure coupled analysis

Corrosion reliability recently becomes an issue due to the trend of miniaturization of electronic products. The phenomenon of electronic corrosion is ionic migration which is also known as electrochemical migration (ECM) by surplus moisture remained inside the package. High temperature may also deteriorate the ECM failure. The hydrated metal ions (positively charged) will migrate towards the cathode and form a dendrite. In this paper, electrochemical migration failure caused by hygro-thermal swelling and residual moisture has been carefully investigated for FR-4 PCB board plated with Cu. An analytical moisture diffusion solution is proposed to determine the moisture distribution and consequent hygroscopic induced strain as well as stress. By applying Fickian diffusion law, the “thermal wetness” analogous technique is used to solve moisture absorption and desorption models. The analytical expression for total expansion strain due to hygro-thermo-vapor pressure coupled effect is implemented using finite element software ANSYS. Finite element predictions reveal the significance of contribution of hygroswelling induced effective stress/strain. Hygroscopic properties such as moisture diffusivity and coefficient of moisture expansion are characterized by an integrated TMA/TGA scheme. Solubility and vapor pressure effect are included to study popcorn failure during reflow process.