Contents
PCB Material Considerations for High-Performance Designs
First, it’s essential to understand what values can help determine whether a material is suitable for your high-performance design. A few properties to get familiar with include:
- Dielectric constant (Dk): Measures a material’s ability to store electrical energy in an electrical field. Mostly, it’s relevant to the non-conductive materials in PCBs. High-performance boards typically use substrates with a low Dk to ensure better signal integrity at high frequencies. On the other hand, conductive materials like copper have an infinite or near-infinite Dk.
- Loss tangent (Df): Refers to degradation of signal integrity due to a substrate or dielectric material. Essentially, the PCB’s substrate causes loss of electromagnetic energy, in turn affecting the signals passing through conductive pathways. We also refer to this as the dissipation factor or dielectric loss.
- Durability and mechanical strength: Your selection of materials needs to be functionally capable of handling the PCB’s operational demands and stresses of manufacturing. We use high-performance PCBs in high-demand electronics and applications requiring solid and reliable PCBs.
- Thermal conductivity: Denotes the rate at which a material conducts heat. Conductors like copper have high thermal conductivity and transfer heat effectively. However, the substrate (insulating layer) is also a factor in thermal management. In addition to electric insulation, it must help draw heat away from the traces, which tend to be thicker and generate more heat than standard PCBs.
- Coefficient of thermal expansion (CTE): Temperature fluctuations can impact a material’s shape and size. The CTE describes a material’s susceptibility to changes in size caused by heat. Materials with a low CTE are preferable as their dimensions are more stable, reducing the risk of warping or other issues.
- Cost and availability: It’s important to balance the price of materials with their performance. For instance, silver may be the most effective conductor of electricity and heat. But it’s far more expensive and harder to source than copper.
Via Fill Types for High-Performance PCBs
Vias are fundamental components of the trace design, as they connect the various conductive layers of PCBs. High-speed and high-performance PCB vias have unique design requirements, particularly in construction.
Via Types for High-Speed PCB Designs
Choose from the following via types for your high-performance design:
- Through-hole via: The most conventional and common via type; it connects all the layers of the multilayer PCB, from the top-most outer layer to the bottom. The biggest disadvantage of through-hole (or thru-hole) vias is that they take up a lot of space and route without prejudice. This means they connect all the conductive layers of the PCB, even the ones you may not want connected. Ultimately, this is a suboptimal PCB design, and it may be better to use an alternative for your high-speed PCB.
- Microvia: This is a miniaturized via we commonly use in high-density interconnect (HDI) PCBs. Because high-performance designs tend to be densely packed with components, they tend to have limited space. Microvias help us address these space restrictions. Additionally, their smaller diameters reduce parasitic impedance (inductance, capacitance and resistance), which negatively impact signal integrity and reduce signal transmission speed.
- Blind vias: Unlike through-hole vias, these connect the topmost layer of the PCB to only a few inner layers. Because they don’t go all the way through, they cut down the distance that signals have to travel. This, in turn, reduces the dissipation factor and helps to maintain signal integrity.
- Buried vias: Connect two or more inner conductive layers. Instead of continuous through-hole vias that connect all the layers, these allow you to connect select inner layers of the PCB. Again, this enables you to address the space constraints of high-performance PCBs while maintaining the best signal integrity. Typically, we use buried vias with a combination of other via types (such as blind vias).
- Via-in-pad: This places a via directly on a surface mount pad. You can then route and connect the pad (and its components) to the inner conductive layers of the PCB. Via-in-pads allow you to shorten signal paths, optimize your trace design, and deal with signal integrity loss issues.
- Stitching via: Connects conductive layers to ground planes, which provide return paths for signals. This helps to lower electromagnetic interference (EMI) and maintain optimal signal integrity.
The above via types offer various trace routing options. You can combine many of these options with other methods to optimize your PCB’s power distribution network and trace design.
For example, one way to optimize through-hole vias is through controlled depth drilling (CCD), also known as back drilling.
With this technique, we remove the bottom unused portion of a through-hole via, known as the stub.
Stubs can cause issues such as signal reflections, which can negatively impact the performance of your PCB.
[Image: Fragment Black Printed Circuit Computer Board With Vias]
Conductive vs Non-Conductive Via Fills
We fill vias to prevent air or fluid from entering them, which can lead to performance issues or failures during the assembly process. There are two via fill types:
- Conductive: This type of fill enhances the via’s electrical and thermal performance. Available in materials like copper or silver, conductive fills are best for designs that require high electrical performance and thermal management. Conductive fills can also add to the mechanical strength and reliability of the via.
- Non-conductive: We mainly use this type of fill to stop contaminants from entering the vias. Contaminants can be materials or substances like solder that manufacturers use in the assembly process. Non-conductive via filling traditionally consists of epoxy. While these fills don’t conduct electricity, they can absorb heat. Additionally, they’re less expensive than conductive vias.
Conductive fillings are best for high-performance and high-speed PCBs because they enhance the vias’ electrical and thermal performance. On the other hand, a good use of non-conductive filling materials would be in cost-sensitive applications.
A Checklist for High-Performance PCB Material Selection
We can separate PCB materials into two main categories: conductive and dielectric materials, known as the substrate. Both of these have considerations for material selection. As a recap:
- Conductive materials are the traces and other components of the PCB’s power distribution network.
- The dielectric/substrate materials are the laminate and insulating layers of the PCB.
Conductive Material Considerations
When selecting materials for the conductive pathways of your high-performance PCB designs, you must ensure that they support optimal signal integrity. Your main options in this regard are:
- Copper: Due to its cost-effectiveness, low resistivity, high thermal conductivity, exceptional electrical performance, and availability, copper is the most common trace material in PCB manufacturing.
- Gold: A substantially more expensive alternative to copper that doesn’t perform as well as copper does as a conductor. However, gold is superior in resisting corrosion and oxidation, making it more durable than copper. Generally, gold is better paired with copper than used as a substitute. For instance, we can use copper traces with an immersion gold (ENIG) surface finish, which acts as a protectant and a wetting agent.
- Aluminum: Although its electrical performance is inferior to copper’s, aluminum is more lightweight and better suited to lightweight PCBs. It also has good thermal conductivity, so it’s commonly used in metal-core PCBs.
- Silver: One of the few widely available base materials that outperforms copper in both thermal and conductivity. Additionally, it’s less susceptible to oxidation than copper. The only reason it isn’t as common as copper is because it’s far more expensive. However, just like gold, manufacturers use silver in immersion silver (IAg) plating.
Copper is usually the best material for traces, even in high-performance designs. It balances performance with cost.
It’s good to know that emerging materials and technologies could potentially enhance or replace copper shortly. As the industry continues experimenting with graphene, advanced alloys, and polymers, we may soon have more viable alternatives for copper.
Substrate Materials for High-Performance PCBs
Substrate materials do most of the work around a PCB’s structural and mechanical strength. Besides that, substrates need to be able to help facilitate the transmission of high-speed signals. Here are the best substrate materials for high-performance PCBs:
- High-performance FR-4: This modified version of FR-4 features a high insulation resistance constant (IRC) and glass transition temperature (Tg). It has a dielectric constant ranging between 3.8 and 4.8. The range can vary based on thickness, glass weave styles, and resin content. Because of its availability, performance, and price, it’s one of the most common dielectric materials for high-performance PCBs.
- Ceramic substrates: These metal-oxide-based substrates offer high mechanical strength and strong insulation. Although they’re typically more expensive than glass fiber-based substrates, their superior thermal conductivity and low Dk make them ideal for high-speed PCBs.
- Metal core: An extra metal plate improves thermal management as high-performance PCBs often generate tremendous heat. Adding a metal plate to the substrate can help dissipate this heat. Typically made from copper (C110) or aluminum, the latter is often preferred due to its lightweight properties.
- Polyimide: A synthetic material known for its mechanical strength and low glass transition temperature (Tg). Polyimides are flexible because manufacturers can alter or adjust many of their key properties through doping. Polyimide substrates are best for military, aerospace, and communications use cases.
- Polytetrafluoroethylene (PTFE) composites: Feature high-thermal conductivity and superior mechanical strength. We combine PTFE with other materials, such as FR4 and ceramics, to create substrates for microwave and radio-frequency (RF) PCBs.
Generally, it’s best to avoid wood-fiber or paper-based substrates (like CEM-1 and CEM-2) as they don’t have the mechanical strength nor the dielectric properties to support high-speed signals.
You’ll also notice that certain manufacturers and brands have their own proprietary materials. These materials are usually composites or specially built blends of glass-fiber and epoxy-based materials.
Where To Place Components in Your High-Speed PCB Design
[Image: Surface Mount Technology Component Placement]
As we previously alluded to, your selection of materials won’t be the only thing impacting your PCB’s performance.
You need to consider how and where you place your PCB’s components. As a result, it’s essential to execute proper floor planning, especially for a custom PCB layout.
Creating a floor plan allows you to visualize the layout of your components before finalizing their placement.
It also enables you to uncover any issues with routing or your circuit board pattern in the early stages of the design process.
- Placement for thermal management: If possible, place high-heat generating components near the edge of the PCB. This practice prevents the PCB from trapping heat in the center. Additionally, components should have ample space between them to encourage airflow and help with heat dissipation.
- Placement of high-speed components: Place high-speed components as closely together as possible. This reduces the length that signals have to travel between them.
- Central processor unit placement: Central processing units (for motherboards) should be placed near the center of the PCB. This makes it easier to establish and connect critical signal paths.
- Placement of sensitive components: You should not place components sensitive to EMI near the edge of the PCB. This only makes them more vulnerable to interference.
- Group by function: Place components with similar purposes or functions near or in relative proximity to each other. For instance, grouping all power-regulating components in a single section or area.
- Best routing placement: Aim for direct routing channels when possible. Avoiding vias is ideal, as this placement strategy facilitates more efficient signal pathways.
- Placement of differential pairs: Maintain optimal signal integrity by ensuring that differentials are as close to each other as possible. In addition to this, route them symmetrically to minimize signal distortion.
- Placement of decoupling capacitors: Place decoupling capacitors as close to the power pins of integrated circuits (such as the central processing unit) as possible. This enables them to filter out electric noise as efficiently as possible.
Design a PCB With High Performance in Mind
Good PCB design isn’t about using the best components – it’s about getting the best out of components.
By implementing good placement and design principles, along with a well-reasoned choice of materials, you can build effective high-performance PCBs.
The ultimate goal should be to limit EMI/EMC disturbances while ensuring good signal integrity and efficient power distribution for your boards.
Once you’re confident in your design, you must decide on a manufacturer with the skills, resources, and experience to produce it.
Contact WellPCB to learn how we can help make your high-performance PCB a reality.
