The factors affecting impedance are trace width, dielectric constant, copper thickness, dielectric thickness, and solder mask thickness. HDI PCBs, due to the presence of blind vias requiring laser processing, typically have a dielectric thickness of 2.5-4 mil (usually a single 1080PP sheet is used as the interlayer dielectric, most suitable for laser drilling). When using an adjacent layer as a reference layer, if the calculated resistance is too low, according to the principle that dielectric thickness is directly proportional to impedance (the thicker the dielectric, the higher the resistance), a spacer layer needs to be designed as a reference layer to increase the dielectric thickness and achieve impedance matching. However, once the dielectric thickness reaches a certain level, it will no longer affect the impedance.
When an HDI PCB requires adjacent layers to be used as reference layers, and the resistance is too low, increasing the dielectric thickness cannot increase the impedance. The only way to achieve impedance matching is to adjust the trace width and copper thickness. Following the inverse relationship between trace width and impedance, the trace width needs to be reduced to increase the resistance. However, the trace width of the impedance trace should ideally be designed to be at least 3 mil (a minimum of 3 mil is necessary to ensure impedance integrity and product yield). Based on the inverse relationship between copper thickness and impedance (thinner copper results in higher impedance), the copper thickness of the finished product should be appropriately reduced to achieve impedance matching. However, because HDI PCBs require plated blind and buried vias, the copper thickness is generally at least 1 oz. If the required impedance cannot be achieved, a special process can be used to increase the copper thickness to at least 1 oz to achieve impedance matching.
The dielectric constant is a characteristic parameter of the board material. Considering factory inventory and customer requirements, it is generally difficult to change the dielectric constant by modifying the board material. In other words, changing the dielectric constant parameter is rarely used to match impedance.
