I'm making my first 4 layer PCB (1 signal, 2 GND plane, 3 PWR plane, 4 signal) with impedance control. I was checking the layer stack up for Altium to make sure the thickness and dielectric constant (dk) were correct and I realized they were not. Checking a couple of manufacturers I quickly realized I don't know what I'm doing.
My experience is low speed PCBs that work fine with FR4 on 2 layers and no impedance control. It looks like the dk varies by material and thickness. Asking the manufacturers what dk value their PCB material has just leads to "for what PCB material" question. I don't know, FR4 ok? LOL.
The design I'm working on has an Infineon psoc62, an eMMC memory and a GPS module. The only controlled impedance traces are for the eMMC, GPS antenna (both 50 Ω impedance) and USB 2.0 ports (one on the MCU and another from the GPS module, both 90 Ω impedance). The eMMC will have the highest clock source (from the MCU) at 45 MHz. I see different types of PCB material and their properties but I'm baffled.
How do you know what type of material to use? Is FR4 material ok for the 50 Ω and 90 Ω impedance? Is there a range for the value of the dielectric constant (dk) for a specific material and if so how do I know what to set it up in the stack up?
Best Answer
FR4 material is a composite material, not originally designed for controlled impedance, which is why the dk specifications are somewhat loose. However, it can be used for GHz designs (it's the staple for 2.45GHz WiFi), if the application is tolerant enough. This means almost all applications, so RF or high speed logic signals between modules and ICs. Do not use it for microwave filters, or metrology, for which you would use a proper RF material like RO4350.
A good rule of thumb for 50 ohms microstrip (trace above ground) on FR4 is for the track to be twice as wide as the substrate is thick.
Avoid using a pre-preg layer as the substrate, as if FR4 is poorly specified, pre-preg is even worse. Most board fabs will try to sell you a central core with two outer foils, as it's often a slightly cheaper build. Insist on two cores stuck together, this gives you core FR4 between the top signal layer and the ground reference plane below.