PCB Technical - PCB circuit board technology LED heat dissipation substrate

I. Introduction

With the rise of global environmental awareness, energy saving and power saving have become the current trend. The LED industry is one of the most watched PCB industries in recent years. So far, LED products have the advantages of energy saving, power saving, high efficiency, fast response time, long life cycle, no mercury, and environmental protection benefits...etc. However, usually about 20% of the input power of LED high-power products can be converted into light, and the remaining 80% of electric energy can be converted into heat.

The relationship between LED junction temperature and luminous efficiency. When the junction temperature rises from 25°C to 100°C, the luminous efficiency will decline by 20% to 75%, and the yellow light decline is the most serious by 75%. In addition, the higher the operating environment temperature of the LED, the lower the life of its PCB products. When the operating temperature rises from 63°C to 74°C, the average LED life will be reduced by 3/4. Therefore, in order to improve the luminous efficiency of LEDs, the heat dissipation management and design of the LED system has become an important topic. Before understanding the heat dissipation problems of LEDs, it is necessary to understand the heat dissipation pathways, and then improve the heat dissipation bottleneck.

2. LED heat dissipation method

According to different packaging technologies, the heat dissipation methods are also different, and the various heat dissipation methods of LEDs

1. Dissipate heat from the air

2. The thermal energy is directly derived from the Systemcircuitboard

3. Export heat energy via gold wire

4. If it is eutectic and Flipchip process, the heat will be exported to the system PCB circuit board through the through hole)

Three, LED heat dissipation substrate

The LED heat-dissipating substrate mainly utilizes the better thermal conductivity of the heat-dissipating substrate material itself to lead the heat source from the LED die. Therefore, from the description of the LED heat dissipation path, we can subdivide the LED heat dissipation substrate into two categories, namely (1) LED die substrate and (2) system circuit board. These two different heat dissipation substrates respectively carry the LED crystal. The heat energy generated when the LED chip emits light from the LED chip passes through the LED chip to dissipate the heat from the substrate to the system circuit board, and is then absorbed by the atmospheric environment to achieve the effect of heat dissipation.

1. System PCB

The system circuit board is mainly used as the LED heat dissipation system, and finally conducts the heat energy to the heat dissipation fins, the shell or the material in the atmosphere. In recent years, the production technology of printed circuit boards (PCB) has been very sophisticated. The system circuit boards of early LED products were mostly PCB. However, as the demand for high-power LEDs increases, the heat dissipation capacity of PCB materials is limited, making it impossible to apply them to them. For high-power products, in order to improve the heat dissipation problem of high-power LEDs, a high thermal conductivity aluminum substrate (MCPCB) has recently been developed, which uses the characteristics of better heat dissipation characteristics of metal materials to achieve the purpose of heat dissipation for high-power products. However, with the continuous development of LED brightness and performance requirements, although the system circuit board can effectively dissipate the heat generated by the LED chip to the atmosphere, the heat generated by the LED die cannot be effectively conducted from the die to the system circuit. In other words, when the LED power is increased more efficiently, the heat dissipation bottleneck of the entire LED will appear in the LED die heat dissipation substrate. The next article will do a more in-depth discussion on the LED die substrate.

2. LED die substrate

The LED die substrate is mainly used as a medium for deriving heat energy between the LED die and the system circuit board, and is combined with the LED die through the process of wire bonding, eutectic or flip chip. Based on heat dissipation considerations, the current LED die substrates on the market are mainly ceramic substrates, which can be roughly divided into three types: thick-film ceramic substrates, low-temperature co-fired multilayer ceramics, and thin-film ceramic substrates. Traditional high-power LED components usually use thick-film or low-temperature co-fired ceramic substrates as die heat dissipation substrates, and then combine the LED dies and the ceramic substrates with gold wires. As mentioned in the introduction, this gold wire connection limits the effectiveness of heat dissipation along the electrode contacts. Therefore, in recent years, major domestic and foreign manufacturers have all worked hard to solve this problem.

There are two solutions. One is to find a substrate material with a high heat dissipation coefficient to replace aluminum oxide, including silicon substrates, silicon carbide substrates, anodized aluminum substrates or aluminum nitride substrates. Among them, silicon and silicon carbide substrates are semiconductor materials. Due to its characteristics, it has encountered more severe tests at this stage, and the anodized aluminum substrate is likely to be conductive due to the insufficient strength of the anodized oxide layer, which makes its practical application limited. Therefore, at this stage The more mature and generally accepted is aluminum nitride as the heat dissipation substrate

In recent years, due to the development of aluminum substrates, the heat dissipation problem of system circuit boards has been gradually improved, and even flexible flexible PCBs have gradually been developed.

Four, LED ceramic heat dissipation substrate introduction

1. Thick film ceramic substrate

Thick film ceramic substrates are produced using screen printing technology. The material is printed on the substrate by a squeegee, and then dried, sintered, and lasered. At present, the main domestic thick film ceramic substrate manufacturers are He Shentang and Jiuhao Waiting for the company. Generally speaking, the lines made by the screen printing method are prone to rough lines and inaccurate alignment due to the problems of the screen. Therefore, for high-power LED products that require smaller and smaller sizes and more sophisticated circuits in the future, or LED products that require accurate alignment of eutectic or flip-chip processes, the accuracy of thick-film ceramic substrates Has gradually been insufficient for use.

2. Low-temperature co-fired multilayer ceramics

Low-temperature co-fired multilayer ceramic technology uses ceramics as the substrate material. The circuit is printed on the substrate by screen printing, and then the multilayer ceramic substrate is integrated, and finally formed by low-temperature sintering. The main domestic manufacturers include Jingde Electronics, Fengxin and other companies. The metal circuit layer of the low-temperature co-fired multilayer ceramic substrate is also made by the screen printing process, which may also cause alignment errors due to the mesh problem. In addition, after the multilayer ceramic is laminated and sintered, the shrinkage ratio will also be considered . Therefore, if low-temperature co-fired multilayer ceramics are used in eutectic/flip-chip LED products that require precise circuit alignment, it will be even more stringent.

Five, the development trend of LED products of international PCB manufacturers

The current development trend of LED products can be seen from the observation of the power and size of LED products recently published by major LED packaging manufacturers. Ways to dissipate heat from LED dies. Therefore, ceramic heat dissipation substrates have become a very important part of the structure of high-power, small-size LED products. The following table 2 is a simple summary of the development status and product categories of major LED products at home and abroad.