Design and Manufacture of Controlled Impedance PCBs
Xekera Electronics designs and manufactures controlled impedance PCBs of all types. We offer controlled impedance PCBs with desired impedance of specific tracks relative to a ground plane. To achieve this, we consider the dielectric constant of the insulating material of the board, the vertical spacing between the specific tracks and the ground plane, and the width of the track.
According to ASTM D-150 “Dielectric constant is the ratio of the capacitance formed by two plates with a material between them to the capacitance of the same plates with air as the dielectric”.
The dielectric constant (Dk) of the insulating material of the PCB is the ratio of the capacitance of a capacitor when using the insulating material as dielectric to that of the capacitance of an identical capacitor with no dielectric material between its plates. The insertion of a dielectric always raises the capacitance of a parallel plate capacitor.
The ratio C/C0 expresses the dielectric constant (Dk), where C is the capacitance of the capacitor with the insulating material as the dielectric and C0 is its capacitance without the dielectric being present. As Dk is a ratio, it is a number without dimensions.
As high-frequency electrical signals travel through insulated copper, eddy-currents force a greater concentration of current density towards the outer edges of the copper conductor—a phenomenon known as skin-effect. At the same time, the dielectric material forming the insulation around the copper slows down the speed of the signals by a factor of square root of the Dk of the insulating material.
With vacuum or air forming the insulation around the copper conductor, the speed of the signals remains unaltered to the speed of light, as the Dk is 1. However, for an insulating material with a Dk of 4, the speed of the signals reduces by 2. Therefore, a material with a low Dk is preferable when selecting the dielectric material for a PCB.
In PCBs, a copper track, separated from a ground plane with the insulating dielectric, behaves as a distributed capacitor. In high-speed and high-frequency circuits, it is important for the designer to be able to predict the change in speed of the signal as it travels from the transmitter to the receiver through the copper traces on the PCB. The designer must also select a dielectric material whose Dk will remain constant over time and over different batches, to enable consistent performance of the PCB.
To achieve the desired impedance for a specific dielectric material, the designer must choose appropriate geometric configurations of the ground planes and adjacent tracks. The designer achieves the desired impedance by varying the gap between the ground plane and the track, and the width of the track.
Manufacturing Controlled Impedance PCBs
At Xekera, we advise our customers on their requirement of PCBs with a specific controlled impedance. We guide them on the use of base materials, copper weights, layer spacing, and track width and spacing for achieving their desired impedance. Our engineers use industry standard software for assisting customers with impedance calculations, and guide them in appropriate PCB design and fabrication.
We verify our calculations of impedance in actual production by specifically designing test coupons and fabricating them. We use high-resolution digital microscopes for physically measuring all relevant internal features of the PCB in all axes to verify the construction. To verify the performance, we test the coupons by using TDR or Time Domain Reflectometry instruments. The outcome is in the form of a graph, like the one shown above. Our calculations provide an optimum construction for all types of transmission lines and impedances.