Electronic Design - High-speed PCB design: the correct choice of PCB board

With the rapid development of digital systems, transmission line loss, which was previously considered insignificant, is now becoming the primary concern of PCB design. When the clock frequency is higher than 1GHz, the influence of frequency-dependent transmission loss has actually occurred, especially the high-speed SerDes interface, the signal has a very fast rise time, and the digital signal can carry more high-frequency energy than its own repetition frequency. These higher high-frequency energy components are used to construct ideal fast-converting digital signals. Today's high-speed serial buses often have a large amount of energy concentration on the 5th harmonic of the clock rate.

There are many high-speed digital applications, with speeds of 10 Gbit/s or higher. These applications use a fundamental frequency of 5 GHz and harmonics of 15 GHz, 25 GHz, etc. In this frequency range, most common PCB materials have very significant differences in dielectric loss (Df), and cause serious signal integrity problems. This is one of the reasons why high-speed digital PCBs use special plates designed for high-frequency applications. The formulation of these materials has a low loss factor and has minimal change over a wide frequency range. These plates were often used in high-frequency RF applications in the past and are even now used in 77 GHz and higher applications. In addition to the improvement of dielectric loss factors, these plates are also equipped with strict thickness control and Dk control, which is better to ensure signal integrity.

At the 2019 Taipei Computer Show, AMD released the third-generation Ryzen Ryzen processor. In addition to the performance of AMD’s 7-nanometer CPU, it began to suppress Intel. Its supporting X570 chipset also introduced support for PCIe 4.0. PCIe 4.0 NVMe SSDs have also begun to be introduced to the market, and it is expected that the PCIe 5.0 specification will be released two years later.

The data rate of PCIe 5.0 will reach a terrifying 32GT/s, which will aggravate the frequency-related insertion loss. The selected PCB material will have a huge impact on the insertion loss of each area.

If the impact of the board on the high-speed signal is not considered when designing the PCB, the old driver will also overturn the car!

When choosing a PCB board, it is necessary to strike a balance between meeting PCB design requirements, mass production, and cost. Simply put, design requirements include electrical and structural reliability. Usually the board problem is more important when designing very high-speed PCB boards (frequency greater than GHz). For example, the commonly used FR-4 material has a large dielectric loss Df (Dielectricloss) at a frequency of several GHz, which may not be applicable.

The operating speed of high-speed digital circuits is the main factor considered in PCB selection. The higher the circuit speed, the smaller the Df value of the selected PCB. Circuit boards with medium and low loss will be suitable for 10Gb/s digital circuits; boards with lower loss are suitable for 25Gb/s digital circuits; boards with ultra-low loss will be suitable for faster high-speed digital circuits, and the rate can be 50Gb /s or higher.

From the material Df:

Df between 0.01～0.005 circuit board suitable for upper limit of 10Gb/S digital circuit;

Df is between 0.005～0.003, the suitable upper limit of circuit board is 25Gb/S digital circuit;

The circuit board with Df not exceeding 0.0015 is suitable for 50Gb/S or even higher-speed digital circuits.

For high-speed PCBs, it is necessary to consider whether the material selection and design meet the signal integrity requirements when designing, which requires minimizing signal transmission loss.

PCB transmission loss is mainly composed of dielectric loss, conductor loss and radiation loss.

When the high frequency signal is transmitted from the driver to the receiver on the PCB along a long transmission line, the loss factor of the dielectric material has a great influence on the signal. Larger dissipation factor means higher dielectric absorption. Materials with larger loss factors will affect high-frequency signals on long transmission lines. Dielectric absorption increases high-frequency attenuation.

The most commonly used dielectric material for PCB is FR-4, which uses an epoxy resin glass laminate, which can meet the requirements of a variety of process conditions. The εr of FR-4 is between 4.1 and 4.5. GETEK is another material that can be used for high-speed circuit boards. GETEK is composed of epoxy resin (polyphenylene ether) with εr between 3.6 and 4.2.

Conductor loss

The flow of charge through the material causes energy loss. The conductor loss of the outer microstrip line and the inner strip line can be subdivided into two parts: DC and AC loss. The direct current mentioned here is a circuit below 1MHz. Although DC loss is generally not suitable for high-speed circuit design, the drop in resistance will encroach on the logic level and noise tolerance of multi-point systems (such as SODIMM DDR3/4 address and command control bus wiring). However, the on-board memory usually has a signal cable length of less than 3 inches. For this reason, this problem is not highlighted.

For a typical 5 mil wide, 1.4 mil thick (1oz copper), 1 inch long circuit, the resistance of the signal path is usually 0.1 ohm/inch when DC power is applied. The bulk resistivity of copper and most other metals is constant until the frequency approaches 100 GHz. In any case, it is the skin effect that triggers the frequency dependence of the conductor.

Alternating current has resistive or inductive conductor loss due to its frequency dependence. At low frequencies, some PCB designers think that resistance and inductance are the same as direct current, but as the frequency increases, the cross-sectional current distribution on the transmission line and the reference surface becomes uneven and moves to the outside of the conductor. Due to the skin effect, the current is forced to enter the outer surface of the copper, which greatly increases the loss. The redistribution of the current increases the resistance and decreases the coil inductance per unit length. As the frequency increases to more than 1GHz, the resistance continues to increase, and the coil inductance reaches a limit value and becomes an external inductance. The higher the frequency, the greater the tendency for current to flow on the outer surface of the conductor. The AC resistance will remain approximately equal to the DC resistance until the frequency rises to a certain point, that is, when the skin depth is less than the thickness of the conductor.