PCB – Why 370HR is Used for Controlled Impedance Instead of FR4

If you specify controlled impedance/controlled dielectric, they will test your board to ensure that the traces are at the specified impedance. In your fabrication notes on your PCB printout, specify the nets and their targeted impedance (with tolerance, e.g., 50 ohms +/- 2 ohms).

They will either test a small test strip that is manufactured on the same panel as your boards; or they will test all nets as needed. This will help catch boards that do not meet spec, before they end up being stuffed with components.

BTW, the "weave" of the board may affect the actual impedance of any particular trace, even when the traces are built to spec (see PCB Dielectric Material Selection and Fiber Weave Effect on High-Speed Channel Routing - Altera Application Note).

Your board fabrication note should specify what the target impedance is, and on which traces those impedance values apply to. (Example: Trace width of 8 mils shall be at 50 ohms +/- 10%.) The fabricator may adjust your trace width slightly to meet the target impedance.

Controlled impedance implies that the the PCB is manufactured so that the impedance of traces are within a certain percentage of a specified value. Controlled Dielectric merely specifies that the dielectric constant of the PCB substrate is within a certain percentage of a specific value.

A controlled impedance board generally also has a controlled dielectric substrate, but the inverse is not necessarily true.

Advanced Circuits has a nice short overview here, and reproduced below.

Impedance is the sum of the resistance and reactance of an electrical circuit expressed in Ohms. The resistance being the opposition to current flow present in all materials. The reactance is the opposition to current flow resulting from the effect of the inherent capacitance and inductance of the conductor interacting with changes in voltage and current. In DC circuits there is no reactance and the resistance of copper conductors is typically insignificant. However in high speed AC circuits (those with sharp changes in voltage and/or current) the reactance and thus the impedance can become very significant. This can become critical to a design's functionality because of the effects that changes in the impedance along the signals path from transmitter to receiver will have on the efficiency of power transfer as well as signal integrity. While a circuit’s speed is often expressed as the frequency of the wave form: the critical concern is the speed at which the voltage and/or current is required to change.

Controlled Delectric
The typical design considerations involved in the determination of the requirement for controlling the impedance are the strength of the signals involved, the susceptibility of the circuit to noise and signal distortion, the criticality of signal timing and the speed at which the signal’s source is attempting to force a change in voltage and/or current.

The design considerations for setting the value of the impedance for the conductor is typically the output impedance of the transmitter and the input impedance of the receiver. The impedance of other conductors (i.e. coaxial cables) in the circuit path will also need to be considered. The acceptable range (tolerance) for the impedance will need to be determined and taken into account during the design phase as well as when specifying the PCB parameters. In many cases merely by using software models to determine the anticipated impedance with particular dielectric materials and spacing, followed by requesting these parameters be followed when the PCB’s are fabricated will suffice. This is what we call “controlled dielectric”. For more critical applications you will need to specify “controlled impedance” and supply the actual impedance requirements for the conductors and we will fine tune the dielectrics and conductors to meet these requirements. The impedance requirements will have to be specified based on the layer and the conductor widths.

TDR is Time Domain Reflectometry. Basically it's a measure of the reflections and spurious signals in a conductor arising from the source coupling into non-ideal impedances (e.g. the connecting wiring and termination). It's a common tool used for very high speed digital and analog work.

Therefore, a TDR report is generally either a simulation of a circuit board which tries to account from all of the coupling of a trace into nearby traces and grounds, together with the input and output buffers and terminations of two (or more) chips connected by PCB traces, or an actual text of the circuit-board after it has been manufactured.