Analyze LED heat sink substrate and technology development trend

In general, if the thermal energy generated by LED illumination cannot be exported, the junction temperature of the LED will be too high, which will affect the product life cycle, luminous efficiency and stability, and the junction temperature, luminous efficiency and lifetime of the LED. Relationships, which will be further explained below using the relationship diagram.

1, LED heat dissipation

According to different packaging technologies, the heat dissipation method is also different, and the various ways of cooling the LEDs can be roughly illustrated:

Description of the heat dissipation route:

(1). Dissipating heat from the air

(2). Thermal energy is directly exported by Systemcircuitboard

(3). Export thermal energy via gold wire

(4). For eutectic and Flipchip processes, thermal energy will be derived via vias to the system board.

Generally, an LED die (Die) is connected to a substrate (Substrate of LEDDie) by gold wire, eutectic or flip chip to form an LED chip, and then the LED chip is fixed on the circuit board of the system. (Systemcircuitboard). Therefore, the possible heat dissipation path of the LED is to dissipate heat directly from the air, or through the LED die substrate to the system board to the atmosphere. The rate at which heat is dissipated from the system board to the atmosphere depends on the design of the entire luminaire or system.

However, most of the thermal bottlenecks of the entire system at this stage occur mainly by transferring heat from the LED die to its substrate to the system board. The possible heat dissipation path of this part: one is to directly dissipate heat to the system circuit board by the die substrate. In this heat dissipation path, the heat dissipation capability of the LED die substrate material is a very important parameter. On the other hand, the heat generated by the LEDs will also pass through the electrode metal wires to the system board. Generally, the heat dissipation by the gold wire is limited by the elongated geometry of the wire itself. Recently, there is a Eutectic or Flipchip bonding method. This design greatly reduces the length of the wire and greatly increases the cross-sectional area of ​​the wire. As a result, the heat dissipation efficiency of the LED electrode wire to the system board is effective. Upgrade.

Through the above explanation of the heat dissipation path, it can be known that the selection of the heat-dissipating substrate material and the packaging method of the LED die occupy an extremely important part in the LED heat dissipation management, and the latter section will be outlined for the LED heat-dissipating substrate.

2, LED heat sink substrate

The LED heat-dissipating substrate mainly uses the heat-dissipating substrate material itself to have better thermal conductivity, and the heat source is derived from the LED die. Therefore, from the description of the LED heat dissipation method, the LED heat dissipation substrate can be subdivided into two categories, namely, the LED die substrate and the system circuit board. The two different heat dissipation substrates respectively carry the LED die and the LED chip. The thermal energy generated when the LED die emits light is radiated to the system board through the LED die, and then absorbed by the atmospheric environment to achieve the effect of heat dissipation.

2.1 system board

The system board is mainly used as an LED cooling system, and finally the thermal energy is led to the fins, the outer casing or the atmosphere.

In recent years, the production technology of printed circuit boards (PCBs) has been very mature. The system boards of early LED products are mostly PCB-based. However, with the increasing demand for high-power LEDs, the heat dissipation capability of PCB materials is limited, making it unusable. In order to improve the heat dissipation problem of high-power LEDs, high-power products have recently developed a high thermal conductivity aluminum substrate (MCPCB), which has achieved the purpose of heat dissipation of high-power products by utilizing the characteristics of better heat dissipation characteristics of metal materials. However, with the continuous development of LED brightness and performance requirements, although the system board can effectively dissipate the heat generated by the LED chip to the atmosphere, the heat generated by the LED die cannot be effectively transferred from the die to the system circuit. Board, in other words, when the LED power is more efficiently upgraded, the thermal bottleneck of the entire LED will appear on the LED die heat sink substrate.

2.2 LED die substrate

The LED die substrate is mainly used as a medium for deriving thermal energy between the LED die and the system board, and is combined with the LED die by a wire bonding, eutectic or flip chip process. Based on the consideration of heat dissipation, the current LED die substrates on the market are mainly ceramic substrates, and the circuit preparation methods can be roughly divided into three types: thick film ceramic substrate, low temperature co-fired multilayer ceramic, and thin film ceramic substrate. Conventional high-power LED components are mostly made of a thick film or a low-temperature co-fired ceramic substrate as a die-dissipating substrate, and the LED die is bonded to the ceramic substrate by a gold wire.

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 high heat dissipation coefficient instead of alumina, and includes a germanium substrate, a tantalum carbide substrate, an anodized aluminum substrate or an aluminum nitride substrate, and a material semiconductor of the tantalum and tantalum carbide substrate. The characteristics make it encounter the more severe test at the present stage, and the anodized aluminum substrate is easily turned on due to the insufficient strength of the anodized oxide layer, which makes it limited in practical application, and thus, at this stage The more mature and generally accepted is the use of aluminum nitride as the heat sink substrate; however, the current aluminum nitride substrate is not suitable for the traditional thick film process (the material must be subjected to atmospheric heat treatment at 850 ° C after silver paste printing). The material reliability problem arises. Therefore, the aluminum nitride substrate line needs to be prepared by a thin film process.