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.
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.
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. Last, do not use it for microwave filters, or metrology, for which you would use a proper RF material like RO4350.
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