Other than the electronics industry, many other industries use technologies that make extensive use of PCBs and high frequency parts. PCBs place the circuit and offer a variety of applications, helping to create end-to-end products across numerous industries. Due to the numerous benefits we observe in them, high-frequency PCBs are widely employed in science and electronics in general. Some of them are:
- Moderately affordable; they can be made in large quantities;
- Reusable; they can be used repeatedly;
- Very tough, providing the circuit a long shelf life;
- A compact size reduces wire wastage.
The aforementioned well-founded components are what give the electronic/electric circuit execution certainty.
Frequency and its Relation to PCB’s
Low frequency ensures that the signal parameters always fall within data characterisation and that the circuit always operates as intended. But, if we speed up the system, the greater frequency will have an effect on the circuit’s characteristics, putting the system at risk. Data communication over transmission lines from the sender to the receiver may suffer as a result of I/O signaling. There is relatively little effect on the circuit when the speed is modest. But, as speed is increased, higher frequency takes over, causing issues like crosstalk, ringing, heating, etc. to disturb the signal’s responsiveness and harming the signal’s integrity.
Why do we need High-Frequency PCBs?
At high frequencies, electronic circuits exhibit radically diverse behavior. This is mostly caused by a shift in the way the passive components behave (resistors, inductors, and capacitors). It also affects the following in a parasitic manner: active elements, tracks on PCBs, Grounding techniques. Compared to typical circuit boards, signals have a significantly tighter impedance tolerance and are noise-prone. The noise produced by high frequency would always interfere with signals between two items. A higher frequency wave has more energy than a lower frequency wave of equal amplitude since this requires more energy.
Figure 1: Low and high frequency signals
Factors affecting high-frequency PCBs
It’s crucial to confirm that the fabricated PCBs can operate at high frequencies without breaking down. Many crucial elements that have an impact on PCBs that operate at high frequencies include:
Choice of Materials
Materials to support higher frequencies are used in the construction of PCBs operating at high frequencies. The requirement that the material used to make a board have a low dielectric constant (Dk) and low dielectric loss tangent is taken into account (Df). Dielectrics are insulating substances with a built-in capacity to hold electrical charge despite being poor electrical conductors. The insulating material’s ability to store electrical charge as a result of dielectric polarization is indicated by the dielectric constant (Dk). These accumulated charges subsequently prevent the signals from spreading and result in a propagation delay that is unexpected in high-frequency activities.
Because to its low cost and ease of use, FR-4, or Flame Retardant Type 4, has been the industry standard material for PCB production. It is made of braided epoxy with glass reinforcement. Although FR-4 is a reliable insulator and robust material for PCB construction, it has a significant flaw when used at high frequencies. Signal loss increases as a result of the material’s changing dielectric constant at high frequencies. The rate at which energy carried by an electromagnetic field when traveling through a dielectric is measured by the dielectric loss tangent (Df). Tan () serves to define it. The amount of energy absorbed by a dielectric increases as high frequencies pass through it; this causes the molecules to vibrate more quickly, which ultimately causes signal loss.
Figure 2: Flame Retardant Type 4 (FR-4) Source: https://www.protoexpress.com/blog/wp-content/uploads/2021/07/FR4_02.jpg
Thermal Management of PCB
Another crucial feature of PCB technology that designers must consider is thermal control. High-frequency PCBs will undergo loss when fed with high-frequency signals, and eventually the circuit material will begin to produce heat. More loss results from higher frequencies passing through the material, and as a result, the heat produced by the PCB gradually increases. The maximum rated value that a PCB is permitted to run at is known as the Maximum Operational Temperature, or MOT. The performance of the PCB is in danger once the temperature of the PCB surpasses the MOT value. One of the primary negative impacts of overheating is PCB expansion. As devices get smaller and smaller, space becomes less of an issue. In these circumstances, operating the PCB at a greater temperature will cause it to expand, potentially damaging the PCB.
- Absorption of moisture. Varying water absorption rates result in moisture, which has an impact on the circuit’s dielectric constant and dielectric loss. The electrical conductivity of the circuit is altered by even the smallest amount of moisture. Hence, it is important to choose materials that will minimize water absorption.
- Matching Impedance. The load impedance should match the impedance of the signal being transmitted during transmission. As there is currently no reflection in the transmission, it can be seen that the load is absorbing the energy. Impedance matching in high-frequency PCBs is related to the signal’s baseline. Impedance matching should therefore be made to ensure signal transmission without interruption.
- Prevent crosstalk. Crosstalk is the term for signals that are primarily caused by the coupling of electromagnetic fields and are not necessary for the operation of the circuit.
The following techniques can be used to lessen crosstalks in a PCB.
Minimum width and trace separation: Horizontal crosstalk happens when traces are too close to one another. Making sure that the traces have the smallest widths feasible and that the distance between the centers of the two traces is at least three times that distance will solve this problem.
Figure 3: Trace and current
Parallel traces on the next levels of perpendicular layers create vertical crosstalk. By making sure that the traces in the next layers are perpendicular to one another while routing, this can be prevented. For instance, if a layer’s trace runs from North to South, the following layers’ traces may go from East to West, making the traces perpendicular to one another.
Figure 4: Alternate direction of traces
Using Solid Ground Plane: Inserting a ground plane in between two adjacent planes reduces the chances of broadside coupling. Problems like reducing bypass capacitor current loop, controlling trace impedance, etc are also solved. Crosstalk can be minimized by isolating high-frequency carrying traces from traces containing asynchronous signals. High-frequency signals like clocks should be kept apart from other traces carrying signals. As asynchronous signals like reset or interrupt lines are infrequently used, they are typically kept adjacent to the power lines or the switching lines. Crosstalk can be reduced by using differential signals, which combine signals with various polarizations but the same amplitude to create a single high-speed signal. The signal is unaffected by external noise because electromagnetic noise equally affects both lines since in reception the signal is acquired as the difference between the two voltage lines. The spacing between differential signal pairs and the other signal-carrying traces should be at least three times the width of the track. Utilizing a Material with a Low Dielectric Constant: Because mutual inductance and mutual capacitance between the traces are decreased, crosstalk is negligible on Boards made from materials with low dielectric constants.
Thermal Management Techniques for High-Frequency PCB
Thermal via arrays: By incorporating through arrays over the copper-filled sections, heat can be effectively managed. This allows heat to escape through the vias and into the surrounding air. Here, a via’s diameter should be at least 0.1 mm in order to effectively disperse heat. The more vias there are, the more heat is dissipated.
Figure 5: Thermal via arrays for a QFN component
Larger copper traces are used to increase heat dissipation. Furthermore, it aids in reducing hot spots on the PCB circuit. Wider traces should be made, but care must be given to ensure that they are not made excessively wide to avoid crosstalk. Heatsinks and cooling fans are employed if the heat generated by the PCBs is greater than what can be dissipated without causing any problems. The most heat-producing components, such as CPUs and MCUs, typically have heatsinks attached to them. They are then frequently either bolted onto the Board or left exposed to the air. When the PCB is in an enclosed space, a cooling fan is employed to help the hot air effectively dissipate into the surroundings. In addition to the methods already described, it is advisable to keep the elements that produce heat apart from those that are susceptible to it. This will prevent any harm from being done to the latter. Potential thermal hotspots in the PCBs can be found using PCB analysis tools. The components that produce heat must be positioned toward the top if the Boards are mounted vertically. Identifying the heat distribution of the PCB can be aided by simulation tools like Altium Designer, PADS Standards, ANSYS, and others.
Material Selection for High-Frequency PCB’s
For the production of High-frequency PCB boards, there are special materials required which give high-speed signals. Some of the materials are as follows:
- Rogers 4350B HF: Similar to the FR4, this material also has a low fabrication cost. It also offers excellent dimensional stability.
- Taconic TLX: This material consists of PTFE fiberglass, it is physically a stable material providing the best thermal, mechanical, and electrical properties. However, the only problem it has is, being difficult to fabricate.
- Taconic RF-35 Ceramic: This is a low-cost material that is made of ceramic-filled PTFE and glass. It is easy to fabricate but it has moderate peel strength, perfect electrical performance as well as low power dissipation.
- Rogers RO3001: This consists of a bonding film with a comparatively low dielectric constant. It is also highly resistant to chemicals and high temperatures.
- ARLON 85N: The ARLON 85N has a very high thermal resistance. It is made of pure polyamide resin.
Dielectric Constant of the Materials Selected
The material chosen should have the lowest feasible dielectric constant (Dk). The square root of the material’s dielectric constant has an inverse relationship with the signal transmission rate. When the chosen material has a high electric constant, there is a delay; the dielectric loss (DC) should also be minimal. Depending on how little the dielectric loss is, the signal loss will be at its lowest.
|Item||Material A||Material B||Material C||Material D|
The above table shows the comparison of substrate material dielectric constant, dielectric loss, Temperature(T) and the ion-migration resistance, moisture resistance, manufacturability, and cost. As per the result, Material C is used for this type of high-frequency and high-speed multilayer printed circuit board.