Author: jiangwei68 Source: Photo News blog column
1. Flip chip
In 1998, Lumileds packaged the world's first high-power LED (1W LUXOEN device), which changed the LED device from the previous indicator application to a new solid-state light source that could replace traditional lighting, which has triggered the invention of incandescent lamps in human history. Another lighting revolution. The 1W LUXOEN device allows the power of LEDs to jump from tens of milliwatts to more than 1000 milliwatts, and the luminous flux of a single device has leap from less than one lm to more than a dozen lm. High Power LEDs Due to the high power density of the chip, the designer and manufacturer of the device must optimize the thermal system of the device in terms of structure and materials. At present, there are two types of GaN-based epitaxial substrate materials: one is sapphire represented by Nichia, Japan; the other is SiC substrate represented by CREE Corporation of the United States. A conventional sapphire substrate GaN chip structure, the electrode is located just on the light exit surface of the chip. In this configuration, a small portion of the p-GaN layer and the "light emitting" layer are etched to make electrical contact with the underlying n-GaN layer. Light is taken from the uppermost p-GaN layer. The limited conductivity of the p-GaN layer requires the reprecipitation of a current spreading metal layer on the surface of the p-GaN layer. This current diffusion layer is composed of Ni and Au, and absorbs part of the light, thereby reducing the light extraction efficiency of the chip. In order to reduce the absorption of the emitted light, the thickness of the current spreading layer should be reduced to several hundred nanometers. The reduction in thickness in turn limits the ability of the current spreading layer to uniformly and reliably diffuse large currents across the surface of the p-GaN layer. Therefore, this p-type contact structure restricts the operating power of the LED chip . At the same time, the heat of the pn junction of the structure is derived through the sapphire substrate, and the heat conduction path is long. Since the thermal conductivity of the sapphire is lower than that of the metal (35 W/mK), the thermal resistance of the LED chip of this structure is large. In addition, the p-electrode and the lead of this structure also block part of the light, so that the device power, light-emitting efficiency and thermal performance of the front-mounted LED chip are not optimal. In order to overcome these shortcomings of the packaged chip, Lumileds invented the Flip chip structure. In this configuration, light is taken from the sapphire substrate and does not have to be removed from the current spreading layer. Since the light is not emitted from the current diffusion layer, the opaque current diffusion layer can be thickened to increase the current density of the Flip chip. At the same time, the structure can also transfer the heat of the pn junction directly through the metal bump to the silicon substrate with high thermal conductivity (145W/mK), and the heat dissipation effect is better; and a new one is added between the pn junction and the p electrode. The reflective layer eliminates the light blocking of the electrodes and the leads, so the structure has superior characteristics in terms of electricity, light, heat, and the like.
2. High-power LED thermal analysis based on Flip chip
We know that one of the main parameters characterizing the thermal performance of a system is the thermal resistance of the system. Thermal resistance is defined as the ratio of the temperature difference between two specified points (or regions) to the heat dissipation power at which the temperature difference between the two points is generated under thermal equilibrium conditions. Thermal resistance symbol: Rθ or Rth; Thermal resistance unit: K/W or °C/W Generally, the flip-chip type high-power LED surface is mounted on a metal circuit board, and an external heat sink can be installed to increase the heat dissipation effect. A plurality of BUMPs (gold balls) soldered on the electrodes of the high-power LED chip are bonded to the corresponding BUMP on the Si substrate by eutectic bonding, and the Si substrate is bonded to the internal heat sink of the device through the bonding material. In order to have a better light-emitting effect, a heat collecting cup is formed on the heat sink, and the chip is placed in the center of the cup, and the heat sink is made of a metal material having a high thermal conductivity such as copper or aluminum.
3. Influence of substrate bonding materials on thermal characteristics of high power LEDs
The LED flip chip is stuck in the tube holder (the internal heat sink of the device), and can be applied in three ways: thermal adhesive paste, conductive silver paste paste and solder paste paste. The hardening temperature of the thermal conductive adhesive is generally lower than 150 ° C, and can even be cured at room temperature, but the thermal conductivity of the thermal conductive adhesive is small, and the thermal conductivity is poor. The curing temperature of the conductive silver paste is generally lower than 200 ° C, which has good thermal conductivity and good adhesion strength. The thermal conductivity of solder paste is the best of the three methods. It is generally used for soldering between metals and has excellent electrical conductivity. In the packaging of high-power LED devices, manufacturers easily ignore the influence of the substrate bonding material on the thermal conductivity of the device. In fact, the substrate bonding material is an important factor influencing the thermal conductivity characteristics of the device. If the processing is not good, the thermal resistance of the LED will be too large, resulting in the junction temperature of the device being too high under the rated working conditions, resulting in the device. The light output efficiency is lowered and the reliability is lowered. When lead-tin solder 63Sn/37Pb is selected, λ=39W/mK, and its thickness is equal to 20μm, RθAttach is equal to 0.026(K/W), even if its thickness is 100μm, RΘAttach is only equal to 0.131(K/W); Heat sink adhesive Ablefilm 5020K, λ = 0.7W / mK, while its thickness is equal to 20μm, RΘAttach is equal to 1.457 (K / W), when its thickness is 100μm, RΘAttach is equal to 7.286 (K / W); when we choose Conductive chip bonding adhesive Ablebond 84-1LMISR4, λ=2.5W/m•K, and its thickness is equal to 20μm, RΘAttach is equal to 0.408(K/W), when its thickness is 100μm, RΘAttach is equal to 2.041 (K/W) ). Therefore, the use of different bonding materials has a great influence on the thermal resistance. At the same time, it is also important to reduce the thickness of the material when printing or coating the die bonding material.
4, summary
LED chip junction temperature allows up to 125 ° C, if its worst working environment temperature is 65 ° C, for a 1 W high power LED, considering the thermal resistance of the external heat sink from the high power device is generally 40 (K / W The thermal resistance of the device pn junction to the device should be less than 20 (K/W). For a 5W high-power LED, if its worst working temperature is 65 °C, the thermal resistance from the pn junction to the environment should be less than 12 K/W to ensure that the junction temperature of the chip does not exceed 125 °C. Ablefilm 5020K heat sink adhesive, λ = 0.7W / m • K while its thickness is 100μm, only the thermal resistance R Θ Attach of the chip bonding material is equal to 7.286 (K / W). Therefore, in the package of Flip chip high-power LED device, it is very important to ensure the reliability and light-emitting characteristics of the device by selecting the appropriate chip substrate bonding material and ensuring the bonding thickness as small as possible in the mass production process.
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