Ceramic substrate for resistor technology
As a ceramic substrate commonly used in passive components, it is not a traditional ceramic, but a fine ceramic. The ceramic substrate is made of high-purity inorganic material, and after precise control of chemical composition and uniformity, and then formed by a certain method, and finally sintered at a high temperature, it has high mechanical strength, low dielectric constant, low thermal expansion coefficient, high thermal conductivity, good chemical stability, and other advantages have been widely used.
For the most demanding chip and thick film resistors, the mechanical and electrical properties of ceramic substrate materials have a great influence on the resistive film layer. We will introduce the ceramic substrate to the resistor film layer from several aspects.
Surface roughness refers to the average deviation of peaks, valleys, and centerlines on a continuous surface, expressed in Ra, in um. The smaller the surface roughness, the smoother the surface. The surface roughness parameter has an important influence on the continuity of the resistive film layer. To give an extreme example: for a thin film resistor, if the thickness of the film is only 200 Å to 400 Å (0.02 um to 0.04 um), the film of this thickness is covered on a rough surface of 1 um (10000 Å), and the resistance element is along the mountain. And the deep valley spreads, rather than evenly distributed, the result is that the actual resistance and nominal resistance of the resistor are very different, and there is a greater possibility of forming an incomplete, fractured, cracked discontinuous film.
For substrate materials, there is another important feature – dielectric loss. It is characterized by the tangent of the phase angle of the polarization current hysteresis voltage in the dielectric material, and its relationship with the signal frequency and circuit distribution parameters C, R.
δ is called the loss angle. When the value of δ is large, the signal will transmit loss or even disappear in the form of heat. For high-frequency applications, dielectric loss is extremely important.
The signal transmission speed v is inversely proportional to the square root of the substrate dielectric constant. Therefore, for high-speed circuits, the substrate is required to have a lower dielectric constant.
In some high-power applications, ceramic substrates are required to have good heat dissipation performance, and high thermal conductivity substrates such as AlN substrates and BeO substrates are required. For resistors, a higher thermal conductivity substrate can achieve greater power ratings in the same size.
Thermal expansion coefficient
For different components, the coefficient of thermal expansion is different. For a semiconductor chip, it is required that the thermal expansion coefficient of the substrate is as close as possible to Si so that the stress generated in the operation-stop temperature cycle of the large-scale integrated circuit can be greatly reduced. This application generally employs a SiC substrate. For the metal foil resistor, the thermal expansion coefficient of the foil and the thermal expansion coefficient of the ceramic substrate must be strictly measured and matched to achieve a very low-temperature coefficient, so the thermal expansion coefficient of the substrate is a key factor.
Common ceramic substrate for the resistor.
1. Alumina ceramic substrate:
From a realistic perspective, the most widely used resistors are Al2O3 substrates, and their processing technology is also the most advanced compared to other materials. According to the percentage of alumina (Al2O3), it can be divided into: 75 porcelain, 96 porcelain, 99.5 porcelain. The alumina content is different, its electrical properties are almost unaffected, but its mechanical properties and thermal conductivity vary greatly. A substrate having a low purity has many glass phase components and large surface roughness. The higher the purity of the substrate, the smoother, denser, and the lower the dielectric loss, but the higher the price.
Therefore, the thin film resistors and thick film resistors, the alumina substrates they use are not exactly the same.