Comparison of PCB Materials for High Speed and HDI Boards

Zachariah Peterson
|  Created: November 9, 2020
Comparison of PCB Materials for High Speed and HDI Boards

Any PCB uses a variety of materials to create a layer stackup, etch copper, apply solder mask, and print silkscreen. Each dielectric material you use to create your multilayer PCB stackup has different material properties, such as dielectric constant and thermal conductivity. When you’re designing for specialized applications, a thorough comparison of PCB materials can help you select the right base material for use in your next circuit board. Altium Designer gives you the features you need to select a base material for your multilayer PCB and prepare your new design for full-scale manufacturing.

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Any PCB stackup you create for your circuit board will include multiple materials. Many different materials will be used during the PCB manufacturing. The materials you choose will determine power/signal loss, interconnect impedance, temperature rise, copper surface roughness, and temperature rise in your PCB. Not every base material is ideal for every application, so you’ll need to pick just the right base material for your PCB stackup to balance performance in these different areas.

With a thorough comparison of PCB materials, you can determine the best base material to use for your next system. Once you determine the best material to use for your circuit boards, the PCB stackup design and analysis tools in Altium Designer help you create a PCB layout for high speed circuit boards or HDI designs. Here is a comparison of PCB materials to help you choose the right base material for your circuit boards.

Important Base Material Properties

When selecting a base material for your PCB, you need to consider the various material properties of the material and how they fit into your board’s application. Here are the important PCB material properties you need to consider in your PCB:

  • Relative Dielectric Constant: This is a complex number consisting of the relative permittivity (Dk) and dissipation factor (Df). The Df value is related to the loss tangent in the material.
  • Conductor Loss: Conductor losses come in AC and DC varieties, both of which are related to the electrical conductivity of your conductors. The electrical conductivity determines the skin depth, which determines AC losses. Note that conductor loss in copper on a PCB substrate are related to copper surface roughness, which will increase loss in the system and change the impedance of the interconnect.
  • Thermal Conductivity: This determines the rate at which heat is dissipated away from the substrate during operation, and it determines the temperature rise with respect to ambient temperature. This will then determine your thermal management strategy, such as the use of a heat sink or fan to keep components cool.
  • Coefficient of Thermal Expansion (CTE): This tells you how the board expands as its temperature increases. CTE is anisotropic, meaning the board will expand at different rates along different directions. Normally, we only care about expansion along the z-axis, i.e., perpendicular to the surface layer of a circuit board.
  • Glass Transition Temperature (Tg): The glass transition temperature tells you when the CTE value suddenly increases as temperature continues increasing. The CTE value above Tg is larger than the CTE value below Tg.
  • Dispersion: The relative dielectric constant is a function of the signal’s frequency. Therefore, dispersion is not a specific number but is rather a property that defines how signals with different frequencies move with different propagation delay values in an interconnect.

Fabricators continue to discover materials better suited to high speed circuit boards, HDI circuit boards, and high temperature environments. The various material properties above affect how high-speed signals propagate in a substrate due to dispersion, as well as how they disperse heat and withstand mechanical shock. Some of these materials are suitable for microwave and mmWave devices, or for use in high temperature environments, but they are more expensive to produce. Your best bet in choosing a material is to start with FR4 and evaluate whether this material will be suitable for your application.

Standard PCB Stackup Materials

The industry primarily uses grade FR4 nonconductive material between layers of copper to build printed circuit boards. FR4 is a NEMA grade designation for glass-reinforced epoxy laminate material. The designation represents the ratio of fiber to resin and indicates characteristics such as flame retardant, dielectric constant, loss factor, tensile strength, shear strength, glass transition temperature, and z-axis expansion coefficient. FR4 is flame retardant making it suitable for safety requirements, and it is robust in varying temperature and humidity environments, increasing the quality of performance.

Fiberglass-impregnated resin base material laminates for PCB stackup design

Standard fiberglass-impregnated resin laminates.

The major material components of PCBs are the polymer resin (dielectric material) with or without fillers, reinforcement, and metal foil. To form a PCB, alternate layers of dielectric, with or without reinforcement, are stacked in between the copper foil layers. The majority of the materials are epoxy, but some are BT, PPE, cyanate ester, and modified acrylates. The structure of a typical epoxy PCB laminate is shown above.

Fiberglass-impregnated resin base material laminate cross-section for PCB stackup design

Cross-section of a fiberglass-impregnated resin laminate.

These laminates are placed between layers of copper and become substrates with dielectric properties relevant to the circuits assembled onto the boards. Designers specify substrate thickness to meet dielectric requirements for the circuits. IPC-2221 contains tables specifying dielectric constant for FR4 and other laminate materials in PCB substrate layers. The best set of PCB design and layout tools will use this data to model impedance and loss in your PCB stackup for optimal routing and trace design.

Materials for HDI and High-Speed Circuit Boards

The backbone resin of the industry has been the epoxy resin. Epoxy has been a staple due to its relatively low cost, strong adhesion (both to the metal foils and to itself), and desirable thermal, mechanical, and electrical properties. The basic epoxy chemistry has been dramatically changed over the years. Alternative resin-based PCB stackup materials are typically selected to address specific shortcomings of epoxy-resin systems. BT-Epoxy is common for organic chip packages due to its thermal stability, while polyimide and cyanate ester resins are used for their lower Dk and Df values.

Besides thermosetting resins, thermoplastic resins are utilized including polyimide and polytetrafluoroethylene (PTFE). Unlike the thermoplastic version of polyimide which is relatively brittle, the thermosetting version is flexible and is supplied in film form. It is typically used to make flexible circuits as well as the combination circuits called rigid-flex. It is also more expensive than epoxy and is only used as needed. Many companies have gone to a “Halogen-Free” requirement in anticipation of an eventual ban on bromine-based flame retardants used in advanced PCB dielectric materials.

Such properties are ideal for high speed and HDI PCBs. Lower dielectric constant allows signals in these boards to travel faster and lowers capacitive coupling between adjacent signal traces. This helps ensure signal integrity in these boards by reducing crosstalk between adjacent signal lines. This is especially important in HDI boards, where traces are packed into small spaces.

Alternative PCB Stackup Dielectric Options

If you’re designing for a high temperature or high power application, there are alternative substrate options available. These alternative substrates can also be used for HDI and high speed PCB designs, giving you more flexibility to choose the best base material for your circuit board.

Rigid-flex PCB with flexible base material in Altium Designer

Rigid-flex PCB design is easy in Altium Designer.

Create Your PCB Stackup in Altium Designer

The Layer Stack Manager in Altium Designer includes a library of standard fiberglass weave materials with well-defined dielectric constant, CTE value, and dispersion models. If you are working with a more specialized material, such as RT Duroid or an aluminum core PCB, you can define dielectric constant and loss from a datasheet. You can then design controlled impedance traces with an integrated field solver. Everything you need to ensure signal integrity and route HDI boards is present in Altium Designer.

Altium Designer’s Draftsman tool provides information on copper and PCB stackup dielectric material options required for PCB manufacturing. Materials for solder paste and hole plating are communicated as well. Use of RoHS materials is becoming the norm as removal of lead solder and other toxic materials become mandatory worldwide.

Assembly instructions and fabrication details are enumerated on the general purpose mechanical layers built into the layer stack. Special layers within the Layer Stack Manager are dedicated to specify silkscreen, solder and paste mask, drill information, keep-outs, and interconnect layers.

The Unified PCB Design Environment in Altium Designer

The unified environment in Altium Designer also includes layout and routing features for creating high speed PCBs, HDI circuit boards, and rigid-flex PCBs. Every function in Altium Designer is built on a rules-driven design engine, and all your PCB layout and routing tools are accessible within a single program. The simulation features in Altium Designer are also ideal for examining signal integrity in high speed and HDI circuit boards.

3D printed circuit board model and PCB stackup design in Altium Designer

Create your PCB stackup and specify the dielectric constant for your circuit boards in Altium Designer.

Materials for high speed and HDI circuit boards is a serious subject for PCB designers and electrical engineers. Several resources exist that provide a comparison of PCB materials, but you need design software that lets you use any material for PCB stackup design. The PCB design features in Altium Designer let you select materials for your advanced PCB stackup, and you’ll have access to a full suite of CAD features and simulation tools.

Altium Designer on Altium 365 delivers an unprecedented amount of integration to the electronics industry until now relegated to the world of software development, allowing designers to work from home and reach unprecedented levels of efficiency.

We have only scratched the surface of what is possible to do with Altium Designer on Altium 365. You can check the product page for a more in-depth feature description or one of the On-Demand Webinars.

 

About Author

About Author

Zachariah Peterson has an extensive technical background in academia and industry. He currently provides research, design, and marketing services to companies in the electronics industry. Prior to working in the PCB industry, he taught at Portland State University and conducted research on random laser theory, materials, and stability. His background in scientific research spans topics in nanoparticle lasers, electronic and optoelectronic semiconductor devices, environmental sensors, and stochastics. His work has been published in over a dozen peer-reviewed journals and conference proceedings, and he has written 1000+ technical blogs on PCB design for a number of companies. He is a member of IEEE Photonics Society, IEEE Electronics Packaging Society, and the American Physical Society, and he currently serves on the INCITS Quantum Computing Technical Advisory Committee.

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