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FOLYSKY（WuHan） LTD.
Manufacturing of ceramic substrates:
It is difficult to manufacture high-purity ceramic substrates, as most ceramics have high melting points and hardness, which limits the possibility of ceramic machining. Therefore, glass with lower melting points is often doped in ceramic substrates for melting or bonding, making the final product easy to machine. The preparation process of Al2O3, BeO, and AlN substrates is very similar. The substrate material is ground into powder with a diameter of about a few micrometers, mixed with different glass fluxes and adhesives (including MgO and CaO of the powder). In addition, some organic adhesives and different plasticizers are added to the mixture, and then ball milled to prevent agglomeration and achieve uniform composition, forming green ceramic chips, and finally sintered at high temperature. At present, there are several main methods for ceramic molding:
Roller rolling: Spray the slurry onto a flat surface, partially dry to form a thin film with a viscosity like putty, and then roll the film into a pair of large parallel rollers to obtain a uniform thickness of raw porcelain.
● Casting forming: The slurry is coated onto a moving strip with a sharp blade to form a thin sheet. Compared to other processes, this is a low-pressure process.
Powder pressing: The powder is sintered in the hard mold cavity under high pressure (about 138MPa). Although uneven pressure may cause excessive warping, the sintered parts produced by this process are very dense and have small tolerances.
Isostatic pressing powder pressing: This process uses molds surrounded by water or glycerol and uses a pressure of up to 69MPa, which is more uniform and produces components with less warpage.
Extrusion molding: The process of extruding the slurry through a mold uses a lower viscosity of the slurry, making it difficult to obtain smaller tolerances. However, this process is very economical and can produce thinner components than other methods.
Characteristics of LED ceramic substrate:
After understanding the manufacturing methods of ceramic heat dissipation substrates, the next step is to explore the differences in the characteristics of each heat dissipation substrate, and what meanings each characteristic represents, and why it affects the key considerations that must be taken into account when applying heat dissipation substrates. In the comparison of the characteristics of ceramic heat dissipation substrates, this article further discusses the four characteristics of heat dissipation substrates: (1) thermal conductivity, (2) process temperature, (3) circuit manufacturing method, and (4) wire diameter width:
1. Thermal conductivity
The thermal conductivity, also known as the thermal conductivity coefficient, represents the ability of the substrate material itself to directly transmit heat energy. The higher the value, the better its heat dissipation ability. The main function of LED heat dissipation substrate is to effectively transfer heat energy from the LED chip to the system for heat dissipation, in order to reduce the temperature of the LED chip, increase luminous efficiency, and extend the lifespan of the LED. Therefore, the quality of the heat conduction effect of the heat dissipation substrate has become one of the important evaluation items when selecting the heat dissipation substrate in LED engineering. From the comparison of four ceramic heat dissipation substrates, it can be seen that although the thermal conductivity of Al2O3 material is about 20-24, LTCC added 30% -50% glass material to reduce its sintering temperature, reducing its thermal conductivity to around 2-5W/m · K; Due to its generally lower co firing temperature than the sintering temperature of pure Al2O3 substrate, HTCC has a low thermal conductivity coefficient of about 16-20W/mK for Al2O3 substrate due to its low material density. Generally speaking, the heat dissipation effect of LTCC and HTCC is not as ideal as that of DBC and DPC heat dissipation substrates.
2. Operating environment temperature
It mainly refers to the highest process temperature used in the production process of the product. In terms of a production process, the higher the temperature used, the higher the relative manufacturing cost, and the yield is difficult to control. The HTCC process itself, due to the different compositions of ceramic powder materials, has a process temperature of approximately 1300~1600 ℃, while the LTCC process temperature is also around 850~900 ℃. In addition, HTCC and LTCC must be laminated before sintering after the process, resulting in shrinkage ratio issues for each layer. Manufacturing is also striving to find solutions to address this issue. On the other hand, DBC has very strict requirements for process temperature accuracy. It is necessary to melt the copper layer into eutectic melt within the extremely stable temperature range of 1065~1085 ℃ in order to tightly bond with the ceramic substrate. If the temperature of the production process is not stable enough, it will inevitably lead to low yield. In terms of process temperature and margin, the DPC process temperature only requires a temperature of around 250-350 ℃ to complete the production of the heat dissipation substrate, completely avoiding the phenomenon of material damage or size variation caused by high temperature, and also eliminating the problem of high manufacturing costs.
3. Process capability
The process capability mainly refers to the process technology used to complete the metal circuits of various heat dissipation substrates. As the manufacturing/forming methods of the circuits directly affect the characteristics of circuit accuracy, surface roughness plating, and alignment accuracy, process resolution has become one of the important items that must be considered in the demand for fine circuits with small dimensions of high-power LEDs. Both LTCC and HTCC use thick film printing technology to complete circuit production. Thick film printing itself is limited by the tension problem of the screen. Generally speaking, the surface of the circuit is relatively rough, and it is easy to cause inaccurate alignment and excessive progressive tolerance. In addition, the multi-layer ceramic lamination sintering process also requires consideration of the shrinkage ratio, which limits its process resolution. Although DBC uses micro lithography technology to prepare metal circuits, due to its limited processing capacity, the lower limit of metal copper thickness is about 150-300um, which makes the upper resolution limit of its metal circuit only between 150-300um (based on a depth to width ratio of 1:1). DPC, on the other hand, is produced using a thin film process that utilizes vacuum coating and yellow light micro imaging processes to produce circuits, making the circuits on the substrate more precise and with high surface smoothness. Additionally, electroplating/electrochemical deposition methods are used to increase the thickness of the circuits. The thickness of DPC metal circuits can be designed according to the actual needs of the product (metal thickness and circuit resolution). Generally speaking, the resolution of DPC metal circuits is around 10-50um under the principle of a metal circuit aspect ratio of 1:1. Therefore, DPC eliminates the sintering shrinkage ratio of LTCC/HTCC and the problem of screen tension in the thick film process.
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