In high-frequency PCB applications, the routing of signals across the various layers must be orthogonal. This means that the trace of the signal layer should be either horizontal or vertical. The signal layer must be free of any obstruction, so proper via placement is essential for smooth signal routing. Otherwise, the signal can be disrupted due to current spreading across the ground plan splits, resulting in loss of signal integrity.
Material properties
High frequency PCB materials have specific properties that can affect the performance of electronic circuits. For example, copper and aluminum have different thermal conductivities. Copper has a higher thermal conductivity and dissipates heat more quickly than aluminum. This property is measured in watts per square meter (W/m2), and it identifies how heat is absorbed and dissipated by a PCB material.
The material properties of a high frequency circuit board should be closely related to its frequency range. For instance, certain copper foil materials, such as rolled-annealed copper, are more suitable for high-frequency applications. The roughness of copper can vary, but a good datasheet will list approximate roughness in z-axis RMS. Copper that is smoother will have lower skin effect enhancement and a smaller impedance deviation. For short interconnects, it’s best to use copper that is closely matched to the frequency range of the circuit board.
High frequency PCB materials have special requirements for the fabrication process. Even the slightest alteration in r value in the base material can significantly impact the impedance of the PCB. Because of this, most designers opt for Rogers dielectric materials. These materials offer minimal dielectric loss and are ideal for rapid prototyping.
The dielectric constant (Dk) is an important property to look for in high-frequency PCB materials. This measure is often overlooked, but it is an important indicator of the performance of a circuit under uncontrolled conditions. Typically, the real Dk value of a material should be greater than that of a dielectric constant measured at room temperature.
High-frequency PCB materials are also less prone to dimensional instability than other materials. This makes them a better choice for mobile, high-speed applications. Furthermore, high-frequency PCBs have a low dissipation factor (DF) that reduces signal wastage. These materials are also more durable and resistant to chemical attacks than other types of PCBs.
Impedance matching
Impedance matching on high frequency PCBs is an important process for PCB design, as it ensures that signal lines transmit without reflections and deliver the required power to the receiving component. The amount of impedance difference between the signal trace and the substrate material is dependent on the signaling standards and supply voltage levels used in the circuit. Impedance matching becomes particularly complicated when the circuit has differential pairs.
Impedance matching prevents reflections at the interface, which can cause self-oscillation. The reflected signal has a short wavelength and will alter the shape of the original signal. For this reason, the characteristic impedance of the transmission line must match that of the load. If the impedances are not matched, reflections will occur at both the source and load, causing self-oscillation.
Impedance matching on high frequency PCBs is essential for obtaining high-speed performance. The impedance of the circuit must be matched to that of the transmission line, as otherwise the circuit will not perform well. Without proper impedance matching, high-speed digital circuits will not work.
The impedance of high-frequency circuits is affected by the type of materials used for the circuitry. For example, if the circuit contains a high-frequency signal, then the dielectric constant of the high-frequency components must be high. The dielectric constant of a high-frequency circuit should be at least 3.7 ohms.
High-frequency PCBs can be easily impedance-matched using the Pi-network or T-network. Using this technique can improve the EMI performance of a circuit. This technique is suitable for RF interfaces and CAN bus connections. A series/parallel resistor transformer can also be used. Another method is a simple jump line or a welded resistance.
Heat dissipation
When using high-frequency circuitry, heat dissipation is an important issue. The PCBs used in these circuits are typically composed of a copper-based material, which is a good thermal conductor. However, copper may not be a good choice in all cases. This is because copper circuitry is more likely to conduct heat than the PCB dielectric.
High-frequency PCBs can be made from a variety of materials. FR-4 is the most common high-frequency circuit material. Other common materials are pure polytetrafluoroethylene (PTFE), ceramic-filled PTFE, hydrocarbon ceramic, and high-temperature thermoplastic. However, the Dk stability of these materials varies, and the loss factor is dependent on surface contaminants and the laminate’s hygroscopicity. In addition, lead-free manufacturing temperatures are higher than standard soldering temperatures.
High-frequency PCBs should be designed to place components near the center, as this helps to distribute heat dissipation. However, high-power components should not be placed at the edges of the PCB because this would cause hot spots, and it would not be safe to place them there.
PCB temperatures are affected by the power consumed by the components and the size of the components. The smaller the components are, the more power they consume and the greater the risk of thermal failure. As a result, heat dissipation becomes an essential consideration for design. As an engineer, you must learn how to design PCBs that minimize thermal power consumption.
The thickness of copper pads plays a critical role in PCB thermal design. Copper traces must have low resistance to the flow of current. Resistance accounts for a significant portion of power loss and heat generation. It is important to design copper pads with sufficient area to maximize heat dissipation and reduce heat spots on the PCB circuit.
Surface roughness
Surface roughness is one of the most important parameters to consider when determining the performance of high frequency PCBs. Roughness can affect the adhesion and attenuation of signals, which can significantly reduce the overall PCB performance. This effect is even more severe when the PCBs contain conductors with high surface roughness.
Surface roughness can be measured in several ways. One method is by determining the root-mean-square height of bumps. This calculation provides a better measure of surface roughness than the roughness derived from a surface ridge. Although it is not possible to determine an exact profile for a working design, the roughness can be approximated to a maximum of 0.29mm.
The roughness of the conductor surface is an important issue. The lower the roughness, the better. Rough surfaces have little effect below a certain frequency, while high surface roughness causes attenuation at hundreds of MHz. So, it is important to choose a PCB material that has a low profile copper foil.
Another important issue to consider when manufacturing a high frequency PCB is the copper’s surface roughness. Rough copper surfaces can cause problems with signal integrity. Rough copper surfaces will also affect the overall thermal stability of a PCB. Copper layers are often treated with a chemical compound to control their roughness.
Surface roughness is not a simple thing to measure. There are a few different methods to measure the roughness of high frequency PCB. One method is to use a laser microscope. A laser microscope will provide you with more accurate results if you use the right lens.
Design layout
A good high-frequency PCB design layout should include good decisions regarding component placement, spacing and clearances, routing, stackup, and grounding. These decisions can affect the overall performance of the PCB. For example, improper spacing may cause unwanted coupling between components. A proper PCB layout should minimize this coupling.
A good PCB design should begin with a detailed schematic. This schematic should provide a complete description of the circuit. This will help minimize any EMI problems that might arise. A detailed schematic can also save time and effort as it provides the intended flow of the circuit. The schematic should include all information available.
When designing high-frequency PCBs, designers should pay close attention to the phenomenon of crosstalk between signal lines that are parallel. Crosstalk occurs when electromagnetic fields are coupled, resulting in unwanted noise signals. The distance between signal lines, PCB plate layer, spacing, electrical characteristics of the receiving and driving terminals, and signal line connection mode are all factors that can influence the amount of crosstalk.
High-frequency PCBs require a very high level of integration and wiring density. Multi-layer boards are essential for these designs, as they minimize signal reflections. The right size of the printed circuit board can help improve reliability and reduce parasitic inductance. It also allows you to make optimal use of the middle tier.
If you’re designing a high-speed PCB, you will want to know the best materials to use for the board. PCBONLINE carries the most popular high-frequency PCB materials including Rogers, Taconic, and Arlon. For your convenience, you can also use PCBONLINE’s online coupons and get $100 off your next purchase!