Impedance is a special requirement in HDI PCB circuit boards.
The factors that affect impedance are line width, dielectric constant, copper thickness, dielectric thickness and solder mask thickness. HDI PCB circuit boards need to be laser processed because of blind holes. The conventional dielectric thickness is generally 2.5-4mil (generally a 1080PP is used as an interlayer dielectric, which is most suitable for laser drilling). When the adjacent layer is used as the reference layer, the calculated resistance is too small. According to the relationship between dielectric thickness and impedance, the thicker the dielectric thickness, the greater the resistance value. Therefore, it is necessary to design an interlayer as a reference layer and increase the dielectric thickness to achieve the purpose of impedance matching. However, when the dielectric thickness reaches a certain level, it will no longer affect the impedance.
When the HDI PCB circuit board specifies that the adjacent layer must be used as the reference layer, when the resistance value is small, the impedance value cannot be increased by increasing the dielectric thickness. The impedance can only be matched by adjusting the line width and copper thickness. According to the inversely proportional relationship between the line width and the impedance value, the line width needs to be reduced to achieve the purpose of increasing the resistance value, but the line width of the impedance line is preferably designed to be above 3mil (the impedance line is above 3mil to ensure that the impedance is not distorted and the yield of the product is ensured). According to the inversely proportional relationship between the copper thickness and the impedance value, the thinner the copper thickness, the greater the impedance value, the copper thickness of the finished product is appropriately reduced to achieve matching impedance. However, because the HDI PCB board needs to be electroplated with blind holes and buried holes, the copper thickness is generally above 1OZ. If the impedance value cannot be reached, the copper thickness can be reduced to the minimum HOZ through special processes to achieve impedance matching.
The dielectric constant is a characteristic parameter of the board. Combined with the factory inventory and customer requirements, it is generally difficult to change the dielectric constant by changing the board, that is, it is generally rare to change the dielectric constant parameters to match the impedance.
