Impedance is a special requirement in HDI PCB circuit boards.
There are five factors that affect impedance: cable width, dielectric permittivity, copper thickness, dielectric thickness and solder mask thickness. HDI PCB circuit boards require laser processing due to blind holes. The thickness of conventional dielectrics is generally 2.5-4mil (generally A piece of 1080PP is used as the interlayer medium, which is most suitable for laser drilling). When the adjacent layer is used as the reference layer, the calculated resistance value is too small. According to the relationship between the thickness of the medium and the resistance, the thicker the medium, the greater the resistance In order to achieve the purpose of impedance matching, it is necessary to design the interlayer as a reference layer and increase the thickness of the medium. However, when the thickness of the medium reaches a certain level, it will no longer affect the impedance.
When the HDI PCB circuit board, it is specified that the adjacent layer must be the reference layer, and the resistance value is too small. You cannot increase the impedance value by increasing the dielectric thickness. You can only achieve matching impedance by adjusting the line width and copper thickness. The impedance value is inversely proportional, 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 more than 3mil (the impedance line is more than 3mil, in order to ensure that the impedance is not distorted and the yield of the product ). According to the inverse relationship between copper thickness and 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, due to the HDI PCB board, blind holes and buried holes need to be plated. The copper thickness is generally at least 1OZ or more. If the impedance value is really not reached, the copper thickness can be made above the minimum HOZ through a special process to achieve impedance matching.
The dielectric constant is the characteristic parameter of the plate. In combination with the inventory in the factory and customer requirements, it is generally difficult to change the dielectric constant by changing the plate, which means that it is generally seldom used to change the dielectric constant parameter to match the impedance.