COB package substrate thickness is reduced, heat conduction path is smooth

The purpose of thermal management is to reduce thermal resistance. The main components of the thermal resistance of the LED package are shown in Figure 1. It is necessary to start from various stages. For example, Hsueh-Han Wu et al. used IR infrared thermal imager and Computational Fluid Dynamics (CFD) software to analyze the junction temperature of each chip under the condition of chip spacing of 0.5, 1.0, 1.5, 2.0, and 2.5 mm, respectively. The distribution state shows that the junction temperature is the lowest when the chip pitch is 2.5 mm. Bum-Sik Seo et al. used thermal software simulation to design four heat sinks of different materials, sizes and structures for the 30 W COB downlight, and finally determined the lowest temperature. Minseok Ha et al. simulated the influence of different chip and solid crystal glue thickness on the thermal resistance of the light source through Finite Element Analysis (FEA) software. This paper will also explain the importance of thermal management from different aspects through thermal software, combined with practical application products and measured data, which has important guiding significance for practical operation.

Figure 1 shows the thermal resistance of a COB package

1. Influence of chip carrying current on output parameters of COB light source

Figure 2 shows a schematic diagram of the structure of a multi-chip COB source. The chip layout varies depending on the power. When the power is the same, the smaller the number of chips, the larger the current carrying by a single chip, and the higher the heat flux density, the higher the junction temperature and the lower the lifetime.

Figure 2 The structure of the COB power supply

Taking a 7 W, 9 W COB source as an example, Tables 1 and 2 describe the changes in current, voltage, luminous flux, and correlated color temperature (CCT) of the product at different chip counts. It can be seen that the current of a single chip increases, the voltage increases, and the luminous flux gradually decreases. When it drops to 10% of the initial value, the CCT change value changes from negative to positive. Figure 3-5 visually shows the above relationship.

Table 1 7W COB source light output parameters vary with chip current

Table 2 9W COB source light output parameters vary with chip current

Figure 3 7W COB source luminous flux and voltage with current curve

Fig. 4 Curve of luminous flux and voltage with current of 9W COB source

Figure 5 7W, 9W COB light source CCT with current curve

The reason is that the current increases, the heat generated by the product increases, the heat generated cannot be effectively derived, and the light radiation energy is reduced. Therefore, at the beginning of the chip layout design, it is necessary to carry out the necessary effective thermal management to ensure that the product output parameters meet the requirements.

2, EFD software simulation analysis

2.1 Influence of different chip carrying current on heat distribution

Take a 7 W COB light source (substrate thickness 0.7 mm) as an example. We assume that all light sources use the profile heatsink shown in Figure 6. All thermal materials have the same thermal conductivity. They only change the number of chips and change the current carrying current of a single chip. The maximum temperature of the light source is simulated by EFD software to determine the current carrying capacity. The impact of distribution.

Figure 6 profile radiator

Table 3 shows the maximum temperature of the light source for different chip currents. From the data, we can get that when If is greater than 116.7 mA, the heat accumulation ability is getting stronger and stronger. On the contrary, the export ability is getting worse. It can be seen from the simulation diagram given in Fig. 7 that when the current is too large, the temperature is concentrated inside the chip, cannot be effectively deduced, and the heat path is blocked, so the temperature difference between the light source and the heat sink is sharply increased. In this case, the light source is very Dead lights can appear in a short time!

Table 3 7W COB light source different single chip carrying current maximum temperature of the light source

Figure 7 Heat distribution under different chip currents

2.2 Influence of different substrate thickness on heat distribution

Taking the 7 W COB light source as an example, the maximum temperature of the light source under different substrate thicknesses when the current of the single chip is the same is simulated, as shown in Table 4. It can be seen that as the thickness of the substrate increases, the thermal path lengthens and the maximum temperature of the light source increases.

3, summary

In this paper, several factors affecting the heat distribution of COB light source are analyzed through data comparison and software simulation. The results show that under the same power conditions, the smaller the number of chips, the larger the carrying current of a single chip, the larger the thickness of the substrate, the more heat accumulated by the light source, the higher the temperature of the light source, and the lower the reliability!