High speed Digital PCB Material Considerations

Due to the development of 5G cellular communication technology and systems, the construction of high-speed digital printed circuit boards (PCBs) has become increasingly important. A PCB design is considered high speed when designed to operate at frequencies high enough to degrade circuit performance significantly. One of the essential factors to consider when determining what constitutes high speed is the PCB material.

High-speed PCBs must handle mechanical and electrical characteristics without impeding their operational capabilities. They must provide excellent frequency and signal transmission performance, depending mainly on the materials used.

The main concern of the high-speed PCB is maintaining the integrity of digital signals over a wide range of high-speed frequencies. And since the high-frequency performance depends on the materials used in a high-speed PCB, designers use several parameters to determine the suitability of these materials.

In addition, inappropriate materials for high-speed PCB material will result in poor plated through holes and even impedance discontinuities in multilayer PCBs' transmission lines. Therefore, material considerations for high-speed digital PCBs are essential.

Here’s an explanation of the different factors engineers consider in the materials used in high-speed digital PCBs!

PCB Material Selection for High-Speed Digital Design

The choice of the dielectric material to be used in high-speed digital PCB is essentially made considering certain factors and properties of the material in question. These materials form the basis of PCBs and must have certain characteristics, as they form an insulating layer between the PCB's conductive layers and must conduct very little electricity.

High-speed signal integrity primarily depends on impedance consistency, transmission line loss, and delay. Therefore, the main parameters of PCB materials selection for high-speed digital PCBs are DK, DF, Dispersion, and Total Losses.

Here are some of the factors to consider in PCB material selection for high-speed digital design:

Dissipation Factor

The first factor to consider in a high-speed PCB material is the dissipation factor or the dielectric loss factor (DF or tan δ). It is a measure of how easily a PCB material exhibits power losses. This electrical property is essential to consider for signal integrity and distortion minimization.

A material with a lower loss tangent causes less power loss. Dielectric materials commonly used in PCBs have a loss tangent between 0.02 and 0.001, where the latter value is for high-end and low-loss materials. For example, a PCB material with a dissipation factor of 0.005 or less is better suited for a high-speed digital PCB than a medium-loss material with a dissipation factor of 0.010.

Dielectric Constant

Another factor determining the suitability of a high-speed digital PCB is the dielectric constant or relative permittivity (Dk or ε′) that changes the performance of the high-speed digital circuits by affecting the transmission lines' impedance on the substrate.

It is a fundamental factor that measures the electrical insulation level of a PCB material and ensures impedance matching and signal integrity. Like Df, a material with a low Dk value is best suited for high-speed PCBs as it acts as a good insulator for power planes and copper traces.

Dielectric materials commonly used in PCBs have typical values between 3.5 and 5.5. However, these values are strictly dependent on the signal frequency and decrease as the frequency increases.

As the name indicates, the best material is the one that can have a DK value as constant as possible with the variation in signal frequency and throughout the circuit lifetime.

Dispersion

Another essential factor to consider in a PCB material is dispersion, which refers to a change in the Dk value with frequency. Thus, low dispersion is a good aspect in high-speed digital PCBs since it exhibits minimal changes in Dk with frequency and can be due to different characteristics of the PCB material, such as dielectric material polarity, material loss, and the copper conductor's surface roughness.

Total Losses

Engineers also consider the total losses in PCB materials that increase with increasing frequencies. Since signal losses rise with the signal travel length, their integrity is also affected by this length carrying high-speed digital signal on the PCB.

Thus, losses are more of a frequency function. For example, for the higher-order harmonic frequencies, the total losses for the travel length of high-speed digital signals are significantly higher. For example, a 5 GHz will face significant signal losses for the fifth-harmonic signal components, although it may face low losses for the third-harmonic signal component.

High-speed digital signals can also be distorted by physical details, apart from the losses of dielectric constant, dissipation, and dispersion of a PCB material.

For example, the effective width of the transmission line changes due to right-angled bends in transmission lines, causing a discontinuity of impedance. When selecting materials for PCB, engineers look for mitered 45-degree bends to minimize the reflections facing high-speed signals and thus avoid such discontinuities in the signal path.

PCB Stackups with High-Speed Digital PCB Materials

PCB stacking means arranging the insulating layers and copper layers of a PCB before completing its layout design. It deals with PCB Layers, Cores, prepregs, laminates, vias, etc. The Right PCB Stack depends on all of these and can give you the integrity of the circuit and signals without noise, particularly a high-quality printed circuit.

Proper PCB stacking ensures the excellence of the electrical circuit. While a stack of layers allows you to get more circuits on a single board through the different layers of PCBs, the design structure of PCB stacking confers many other benefits:

• A stack of PCB layers can help you minimize the vulnerability of your circuit to external noise, minimize radiation and reduce issues of impedance and crosstalk on circuit configurations printed at high speed.
• A good PCB stack can help you balance your need for efficient and inexpensive manufacturing methods and signal integrity problems. It can also improve the electromagnetic compatibility of your design.
• A well-designed multi-layer PCB stacking maintains a higher quality than single-layer PCBs.

However, for proper stacking, you need premium-quality materials. In addition, you should choose the layer materials by comparing the standard values.

Today's increasingly compact trend of electronic products requires the three-dimensional design of multi-layer printed circuit boards. However, PCB stacking brought forward new problems related to this design perspective.

One of the issues is getting high-quality laminate buildings for the project. Good PCB laminate design is essential to reduce circuit radiation of PCB and associated circuits.

When selecting the high-speed PCB laminate material, several considerations are made regarding each substrate's thermal, mechanical, and electrical characteristics and how they interact.

Materials to be used in High-Speed Digital PCB stackups must:

• maintain a stable dielectric constant (Dk) during high-speed operations
• have a low dielectric loss factor (Df) to ensure no losses
• ensure that layers do not delaminate, break down or begin to peel when the frequency increases
• offer impeccable mechanical stability and excellent dissipation

Via Fill Types

Vias are often an integral part of PCB design as they ensure the proper functioning of signal transfer between the PCB layers, which means pathways can be considered conduits.

Via filling is a method that ensures that via holes are completely closed wither with resin or Soldermask. A Via hole is a conductive hole full of copper that connects the lines. Although designers can use other materials, copper is the most used material for this purpose. You can now use via fill to connect two layers.

Vias play an important role in the interconnection between PCB layers with the increase in the development of finer-pitch device applications and electronic products.

There are three main types of PCB vias:

• Through-hole vias
• Blind vias
• Buried vias

Each type has different functions and attributes contributing to the overall PCB performance. The most used is the through-hole vias, followed by blind roads. Buried pathways connect the inner layers; hence the name buried vias.

The technique where the vias are filled or closed with resin or a solder mask is via plugging. However, via tenting is a different technique where the resin or a solder mask only provides coverage and does not fill the via hole.

Via plugging protects the vias from the damaging flux of welding material during the welding process. If a via is not plugged or tented during the soldering process, solder can create unnecessary solder joints by flowing down the via from the pads.

Conductive and Non-Conductive Via Fill

The solder mask layer can be applied to a PCB after plugging via capping through a conductive or non-conductive material.

Conductive Via Fill

Conductive via fill has silver-coated copper particles filled with epoxy to provide maximum thermal and electrical conductivity. It helps carry a large amount of current throughout a PCB.

However, the major setback in a conductive via fill is the difference between its CTE (coefficient of thermal expansion) and the CTE of the surrounding laminate. This difference causes the conductive material to expand faster than the surrounding laminate during the PCB operation, causing a fracture between the via and the associated contact pad.

Non-Conductive Via Fill

A non-conductive via fill will still function as normal vias as it also has copper-plated vias that have good heat and power conductivity. However, they cannot withstand higher current loads than those filled with conductive materials.

High-Speed Digital PCB Stackup Selection Guideline

Now you know that high-speed PCBs are generated through designs designed to be less susceptible to signal integrity, power, and EMI/EMC problems.

High-speed designs use high-speed digital signals for data transfer between the various components. One of the main differences between a high-speed PCB and a normal PCB is the system's rise and fall time of the Edge rate. However, growth is indeed noticeable in this type of segment due to technologies such as the Internet of Things (IoT) and embedded computing.

These types of designs focus on interconnect design, layer stack design, and routing. If you do the first two steps properly, possibly the third is the same. Before designing your PCB layer stackup, it is important to consider the number of layers needed to house all the digital signals.

Material Selection Guidelines

Selecting the PCB laminating materials (resin, copper foil, fiberglass) for a PCB composition is the first step a designer takes, depending on the type of application. Here comes the issue of the availability of high-speed material when designing a high-speed digital PCB stackup.

A high-speed design PCB depends on the materials you select for your board and how the layer stacking is structured. As discussed above, certain factors are considered when selecting materials for designing high-speed digital PCBs.

Here are the guidelines for high-speed PCB stackup selection to achieve high-speed signals in your PCB design:

A Material with Low Loss Tangent and Dielectric Constant

The best PCB material for a high-speed design is the one that has a smaller dielectric constant value and the lowest loss tangent. For example, a material with Df less than 0.002 is a suitable choice.

Check if materials such as Rogers 4350B or Megtron 6N/6G (discussed below) are available after vendor characterization. These are good choices for a high-speed PCB design.

When designing high-speed PCBs, special attention is paid to the details of materials, including Dielectric Matrix, Fiberglass, and Copper. The change of pattern in these three materials is considered carefully since a higher data rate signal has a higher frequency element with a shorter signal wavelength creating more discontinuities.

A PCB Material with Smaller Dielectric Height

Opt for materials with smaller dielectric height to achieve high-speed signal routing. For high-speed designs, you need smaller trace width.

There's often a compromise between selecting shorter trace width and wider trace width. The latter takes more routing space but has lower insertion loss and less skin depth.

A PCB material with a smaller dielectric height also results in a smaller transition via and smaller PCB height that prevents impedance mismatching as much as possible.

Which Material is the Best for High-Speed Digital PCB

When it comes to designing high-speed PCBs, the standard FR4-based fiberglass isn't the best. PCB materials, including enhanced FR-4, polyamide, and PTFE, are for high-speed designs.

FR-4 is a favorite PCB material in many applications, but it is not acceptable as a PCB material for high-speed digital circuitry because it cannot introduce insertion loss and distortion satisfactorily. 

Since the dielectric constant (Dk) is the first aspect a designer starts with when selecting the proper PCB material for high-speed PCBsboards, suppliers describe their materials according to the following aspects: 

• Dielectric constant in the z-axis or the x-y plane and 
• Dielectric constant at a typical test frequency (1 or 10 GHz)

In addition, high-speed designs have a high requirement on the channels, such as a close match with the phase and amplitude. Finally, a PCB material can only handle signals rich in harmonic content if it can handle high-speed digital signals.

Enhanced epoxy is a material with better electrical properties, which is suitable for high-speed multi-layer PCBs. Applications using high-speed PCBs include routers, servers, power amplifiers, storage area networks, transceiver modules, and high-speed data channels.

Rogers 4350B and Megtron 6—Two Low-Loss Materials for High-Speed Digital PCBs

Two low-loss materials typically used in high-speed PCB materials are Rogers 4350B and Megtron 6, with low dielectric constant (D k) and low dissipation factor (Df) values.

These materials cost more than ordinary FR-4 laminates and are based on hydrocarbon resins. They cannot be covered with a quarter ounce of copper and come with low-profile slats to prevent high-frequency signal reflection.

Since prepregs used in Rogers 4350, core require higher pressure, it is much more costly than Megtron 6. In addition, its core material is ideally flat and repeatable for maximum impedance control.

Comparatively, Megtron 6 does not involve temperature, incompatible pressure, motion, or cure time and is more like conventional FR-4. For a high-speed PCB stackup, designers can build a single laminate with less expensive FR-4 material's inner layer and multiple layers or outer layers using Megtron 6 with a cap structure or foil.

In addition, impedance control and a simple buildup development also depend on a broader selection of prepreg thickness, resin content, and Megtron 6 core material.

Layers Selection

High-speed signal routing is achieved through enough stripline layers in all high-speed PCBs. The recommended stripline routing for critical high-speed signals is above 15 Gbps and below 15 Gbps for non-critical high-speed signals.

Dual stripline routing is only recommended when the signal routing is perpendicular on both stripline layers. While designing high-speed PCBs, designers must avoid longitudinal broadside coupling of differential pairs.

In addition, Stripline is recommended over microstrip as it requires smaller trace width, resulting in more space for signal routing. If you select microstrip routing, the solder mask must be removed.

Stackup Combination Selection

The concern here is to ensure that signal routing crossings are perpendicular on both stripline layers to make a ground/signal/ground stackup combination feasible. It is essential to minimize broadside coupling that causes cross-talk.

When designing a PCB, one of the main issues is the layers on the board to achieve the specific functions for the operation of the circuit.

During the design of a printed circuit, layers consideration is one significant issue; that is, choosing the layers of ground planes, wiring, and power planes in order to get the specific functions for the operation of the circuit. Of course, the number of layers will be linked to the signal integrity, requirements, manufacturing costs, etc.

Tips for Placement of Components in a High-Speed PCB Design

Regarding the placement of components, there are no specific rules for this type of high-speed PCB. Many designers believe that placing the largest central processing IC component near the center of the board will help your project.

The integrity of the signal is of great importance, which is why we have listed below a series of tips that will help you with a high-speed PCB design:

• Maintain short paths between components to ensure high-speed signals
• Minimize routing through vias
• Check with your manufacturer about materials and processes that can help prevent such entanglement effects.
• Use an electromagnetic coupling calculation to determine the proper spacing.

Final Words

We advise you to conduct an in-depth analysis to design a high-speed digital PCB suitable for your project. It is important to analyze in-depth aspects such as routing, power, and signal integrity discussed above. All you need is a partner who will facilitate your project through manufacturing and assembly processes, such as Hemeixin PCB, a reference name in designing and manufacturing a wide range of reliable and robust PCBs.

The specialists at the Hemeixin PCB have expertise in designing high-speed designs, ensuring the availability of high-quality materials in their manufacture.

Visit their website to place your order now!