Designing Your PCB
Designing a high-frequency PCB can be challenging; as a PCB designer, you need to understand the physics and implications of high-frequency PCB designs.
Various elements are required to create a high-frequency PCB design. These include routing, component placement, and grounding. The proper spacing between traces is also an important aspect of PCB design.
Although there is much debate about the routing angle, most designers agree on a 45-degree angle. This angle reduces capacitance. The best high-frequency PCB layout designs include the correct spacing and component placement. These two elements work together to make the circuitry on the board operate efficiently.
PCB Substrate Dimensional Properties
High-frequency printed circuit boards require materials with low dimensional properties: low moisture absorption, dimensional stability and chemical corrosion resistance. These properties enable the high-frequency PCB to withstand harsh conditions.
The right choice of PCB materials reduces inductance, crosstalk, and improves the frequency transmission speed.
The low dielectric constant ensures that the frequency is transmitted without any delay. The inverse relationship between the square root of the dielectric constant and the signal transmission rate means that higher frequencies will result in a larger loss.
High-frequency laminate materials stabilize the dielectric constant and reduce the dissipation factor. These laminates also offer excellent thermal performance and are particularly well-suited for circuits that can experience significant temperature variations.
Routing of Signals on Different PCB Layers
It can be challenging to keep analog signals from interfering. You can reduce interference by routing high-frequency signals on different layers of the PCB.
The separation of layers by fiberglass epoxy provides a shielding effect for the signal.
High-frequency signal lines require a smooth path. This is important because of the correlation between EMI and trace length. Radiation intensity decreases when a signal line runs parallel to another line.
Generally, to reduce crosstalk, a high-speed signal line should be separated by three times the routing width to minimize or eliminate its impact on devices.
Thermal Expansion
PCBs provide a rigid surface on which to attach components and a means to transfer electrical signals. These boards are susceptible to thermal expansion, which can cause problems or even complete failure.
When exposed to high temperatures, different materials expand at different rates. It is crucial to select a PCB material with a coefficient of thermal expansion (CTE), like copper. This will ensure reliability at higher temperatures.
Crosstalk and The Skin Effect
Crosstalk is the disturbance caused by unintended coupling between an EMI and an electrical signal. Crosstalk mainly results from magnetic and conductive coupling, which can be reduced by increasing the spacing between adjacent signal traces. Understanding crosstalk’s effects on the functionality of a PCB’s design is essential.
Common impedance coupling results from multiple signals sharing a common return path. Fringe electric and magnetic fields produce this type of coupling.
Common mode crosstalk causes more negative effects than differential mode crosstalk. Both types of crosstalk depend on the frequency of transmission, which affects the signal’s rise time. The rise time of a signal increases with increasing data rates.
The frequency of a signal affects the amplitude of its EM fields. There is increasing interference with higher frequencies. Increasing the bandwidth of the signal also increases the rate of response. The dielectric medium of the substrate material contributes to loss that is positively proportional to the frequency. You can control it by using a substrate material with a lower dielectric constant and considering the substrate’s dielectric thickness, which can be as thin as four mils (0.1mm).