Characteristic Impedance PCB Design of Material Composition Structure

1) Stabilization of dielectric properties of substrate materials

In order to achieve a shortening of the wavelength under the high frequency circuit, the substrate material is required to have a stable dielectric constant value. That is, from a microscopic point of view, the dielectric constant values are uniform. For substrate materials with fiberglass reinforced materials, an important way to achieve this property is to use 100% of the fiberglass cloth that has been fiber-opened. Thereby, the uniformity of the distribution of the glass fiber cloth and the resin in the composition of the substrate material is achieved.

The fiberizing treatment of the glass fiber cloth is a treatment for reprocessing the glass fiber cloth by a high-pressure water jet method. At the end of the fiber-treated fiberglass, the warp and weft yarns are embossed, and the surrounding pores are obvious. The fiberglass cloth which is opened by the "spraying acupuncture method" by using the high-pressure water on the warp and weft of the glass fiber cloth is opened, and the fibers in the warp and weft yarn are loosened and evenly spread to be flat. . The bulging of the warp and weft interlacing points is slowed down. Its fibers are dispersed and filled with pores around the interlacing points, which serves to facilitate uniform distribution of the resin in various parts of the substrate material.

Most of the board materials have a resin weight ratio in the range of 35-65%. The relationship between the amount of different resins in the substrate material of FR-4 and the dielectric properties at different frequencies. When the amount of the resin is higher, the closer to the dielectric constant value of the resin itself, the lower the dielectric constant of the entire substrate material. As shown in Fig. 9, when the amount of the resin is smaller, the dielectric loss factor value of the entire substrate material is closer to the dielectric loss factor value of the glass fiber cloth, that is, the dielectric loss factor value is smaller. The dielectric constant value of the entire substrate material can be theoretically calculated at the ratio of the content of the different resin to the glass fiber cloth.

2) The effect of the amount of resin on the substrate material on other properties

The resin amount of the substrate material not only greatly affects the dielectric properties, but also the coefficient of thermal expansion (Tg) of the substrate material, the thickness direction (Z direction) of the board, and the plane direction (X, Y direction) (ie, Stable size also has an important impact. When the amount of the resin is small, the Tg of the sheet is high and the coefficient of thermal expansion is small. Under high temperature conditions, the resin used generally causes a water decomposition reaction, so that its insulation resistance is lowered, resulting in deterioration of insulation of the substrate material. The greater the amount of resin, the more pronounced this change characteristic. The thermal expansion coefficient of the board directly relates to the reliability of the through hole of the printed circuit board and the reliability of the soldering.

3) Relationship between thermal expansion coefficient of substrate material and glass fiber cloth and resin

Glass fiber cloth is used as the reinforced substrate material, and its thermal expansion coefficient in the X and Y directions is mainly related to the glass fiber cloth manufacturing method (monofilament diameter, linear density of glass yarn, fabric density, etc.). The magnitude of the thermal expansion coefficient in the thickness direction of the substrate material is mainly dependent on the tree expansion coefficient. The addition of a filler to the structure of such a substrate material suppresses the amount of thermal expansion of the substrate material by reducing the amount of the resin.

4) The effect of the filling material of the substrate material on the performance of the substrate material

Some non-halogenated substrate materials, substrate materials suitable for laser drilling, are generally mixed with an inorganic filler material in the substrate material resin. In the case where the specified thickness of the board is reached, the proportion of the resin used is reduced due to the entry of the filler material. This will improve on the board Tg. The amount of thermal expansion in the X, Y, and Z directions will be reduced. Since the filler material to be added has a high dielectric constant and a low dielectric loss factor characteristic, the electrical constant of the filler material sheet added to the resin is increased. And there is a decrease in the dielectric loss factor.