What to Specify on a High Speed PCB Order
What to Specify on a High Speed PCB Order
PCB Substrates and Their Dielectric Constant at High Frequencies
Figure 1: Dielectric constant (Er) vs frequency.
Normally, as the frequency increases the dielectric constant will drop and will flatten out at about 2 GHz. Nevertheless, it is important to tell the manufacturer what your operational frequency is and the desired value of the dielectric material as there could be differences depending on the selected material, the manufacturer of the substrate, or other possible variants.
PCB applications are increasing in frequency and complexity. As this trend continues, PCB designs and materials need to take further considerations in order to guarantee their functionality. Selecting the correct PCB substrate is important to ensure your circuit board will work properly. We explain below why this is important.
When it comes to traces (microstrips or striplines), at high frequencies they need to be modeled as transmission lines. This model implies a point to point connection between a source and a load, where a wave is transmitted. The characteristic impedance (Z0) is one of the fundamental parameters that describe a transmission line; this value is determined by the constructive characteristics of the trace and the PCB. The impedance that the load will see will depend on this parameter and the length of the transmission line (d).
Figure 2: Graphical representation of a transmission line. Vg and Zg are the source, Z0 the characteristic impedance of the transmission line and ZL the load.
As there is an impedance discontinuity between Z0 and ZL, the wave that travels through the transmission line part will be transmitted to the load and part will be reflected. This gives value to the reflection coefficient (π€) and the standing wave ratio (SWR), where SWR = (1+βπ€β)/(1-βπ€β). From the point of view of a designer, the objective is to adapt the transmission line with the load, achieving π€= 0 (or SWR=1). There are different techniques to do this, for example, changing the length of the transmission line and adding a quarter-wave impedance transformer or a stub, defining their values following a Smith chart.
How will the selection of a PCB substrate affect a transmission line? The characteristic impedance of, for example, a microstrip will be determined by the trace width (W) and thickness (T) and by the dielectric height (H) and the dielectric constant (Er). Also, we should consider, if correspond, the solder mask thickness (Tsm). If we are working with a differential pair, two parallel transmission lines with complementary signals with opposite polarity, we should consider the differential pair impedance, which also depends on the trace separation (S).
Figure 3 : Microstrip (left) and differential pair (right), with all the parameters needed to obtain the characteristic impedance and the differential pair impedance.
Additionally, it is essential to know the operational frequency of the signal that will travel through the traces. The dielectric material which the substrate of the PCB is made of will vary with frequency. This means that if the value of the dielectric constant doesnβt correspond to the operational frequency, the characteristic impedance will not be the desired according to the design requirements and the circuit will not function properly.
To sum up, when a PCB design is finished and should specify the operational frequency and the dielectric constant so your manufacturer will make sure that your desired characteristics impedance will be achieved.