high-frequency circuit board assembly
High-frequency circuit boards are crucial to the modern world, from allowing seamless wireless connectivity and accurate medical diagnoses to ensuring safe aerospace operations. These vital components help drive our technological progress forward by enabling faster, more efficient and higher-performing electronic devices.
However, unlike ordinary PCBs, high-frequency circuit boards require extensive design considerations in order to provide the highest possible performance. These include strict routing methods, impedance control, and layout optimization to prevent interference, ringing, cross-talk, and ground bounce. The physical layout of components, transmission lines, and ground planes also plays a critical role in achieving the optimal results.
When assembling a high-frequency circuit board assembly, it is necessary to use specialized materials that have low dielectric losses and dispersion. Conventional FR-4 substrates, which are often used in standard PCBs, typically have higher losses than needed for high-frequency applications, and can cause distortion and ringing in the signals that travel through them. Specialized materials like Isola I-speed, Rogers 4350B and Panasonic Megtron 6 offer low loss and can withstand high-frequency applications.
How is high-frequency circuit board assembly different?
In addition to selecting a proper material, high-frequency PCB assembly requires precise control over trace width and spacing. Impedance matching is important, and the ideal system impedance is 50 ohms or lower. This is accomplished using various routing techniques and impedance control tools such as differential signaling, impedance-controlled routing, rounded corners at turns, shorter routing lengths, transmission line structures with fewer layers and decoupling capacitors on power supply pins.
It is also important to limit the number of vias that are used in a high-frequency circuit board. This reduces the signal’s impedance and improves its speed. It is also advisable to keep the leads of each signal as short as possible. Longer leads are more susceptible to radiation, and the more they extend from the PCB surface, the more likely they are to interfere with other components in the device.
Another way to reduce interference is to employ a 45-degree bend or arc break in the leads of each signal on the circuit board. This can reduce the likelihood of electromagnetic emissions, such as radiated heat and coupling. High-frequency circuit boards also require a layered structure with a ground plane close to the outer layers. This helps reduce capacitive effects and enables a better view of the conductor widths.
In order to prevent parasitic capacitance, the PCB pad size must be as small as possible, and they should be matched to the component pins. This will minimize the effect of parasitic capacitance, and it will also increase mechanical strength.
High-frequency circuit boards can have several different signaling formats, but the most common is the stripline, which has a void on top of the outer layer and a ground plane below it. This type of signaling is more effective than microstrip, which requires the signal to be divided between two planes, causing EMI problems. The stripline, on the other hand, is able to contain EMI radiation within the two planes.