Ultimate PCB Material Selection and Properties Guide

Ultimate PCB Material Selection and Properties Guide

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With all PCB material types, the purpose is to conduct electricity and provide insulation between conducting layers of copper. FR-4 is the most widely used material in this category. However, the needs of your board will invariably rely on various PCB material properties. Read the following PCB material selection guide to learn what to look for when it comes to the thermal, electrical, chemical, and mechanical properties of different PCB material types.

PCB Material Properties

Thermal Properties

• Glass Transition Temperature (Tg): A PCB substrate softens when exposed to temperatures above a certain threshold. The substrate hardens back to its natural state once removed from heat. The range at which this transformation can be applied to a substrate is known as glass transition temperature (Tg) — a unit noted in degrees Celsius.
• Decomposition Temperature (Td): When exposed to temperatures past a certain threshold, a PCB substrate will decompose, losing five percent or more of its overall mass. This temperature range is known as decomposition temperature (Td), noted in degrees Celsius. Unlike Tg transformations, the effects of decomposition are permanent. Hence, choosing a PCB material that can handle the temperature range of your working environment is crucial. The temperature required to solder a PCB substrate is generally between 200°C and 250°C. Ideally, the Tg should be below that range and the Td should be higher.
• Coefficient of Thermal Expansion (CTE): A PCB’s expansion rate is known as its CTE. When a substrate is exposed to temperatures exceeding its Tg, the material will also undergo a rise in CTE, measured in parts per million (ppm). Since a substrate generally has a higher CTE than a copper layer, this difference can cause interconnection problems when heat is applied. Typically, due to the constraints of woven glass, the CTE is in the range of 10 to 20 ppm across the X and Y axes. Even when the temperature surpasses the Tg threshold, the CTE remains the same. To be on the safe side, a CTE of 70 ppm or under is recommended.
• Thermal Conductivity (k): The heat-conductive properties of a PCB are referred to as thermal conductivity (k). A material’s thermal conductivity directly correlates to the ability of the PCB to transfer heat. If the k-level is low, the level of heat transfer will also be low, and vice versa. Thermal conductivity rates are measured in watts per meter (W/M) at Kelvin (K). The thermal conductivity range for many dielectric materials is between 0.3 and 6 W/m-K, whereas copper has a k-level of 386 W/M-°C. Therefore, the copper layer carries heat faster than the dielectric in a PCB.

Electrical Properties

• Dielectric Constant or Relative Permittivity (Er or Dk): Two crucial factors that determine the electrical performance of a dielectric constant are impedance and signal integrity. The dielectric constant (Er) or relative permittivity (Dk) of a PCB material is typically between 3.5 and 5.5. A material’s Er level depends on frequency and usually drops as the frequency rises. The Dk level changes less on certain PCB materials than on others. For applications involving high frequencies, a material must maintain a stable dielectric constant over a broad range of frequencies.
• Dielectric Loss Tangent or Dissipation Factor (Tan δ or Df): A material loses less power if its loss tangent is low. The dielectric loss tangent (Tan δ) of materials used in printed circuit boards is generally between 0.02 to 0.001. The former figure applies to more widely used materials, while the latter usually applies to high-end materials. Tan δ also increases along with frequency. In digital circuitry, loss tangent is generally a factor of minor importance, except for applications where the frequency level exceeds 1 GHz. Loss tangent is more consequential for analog signals.
• Volume Resistivity (ρ): The resistance of a dielectric material to insulation or electricity is known as its volume resistance (ρ). PCB material with high resistivity will be less likely to facilitate electric charges. Ohm-meters (Ω-m) and ohm-centimeters (Ω-cm) are used to measure the resistivity of a given dielectric. For a printed circuit board, high resistivity is essential, preferably in the range of 10³ to 10¹⁰ Megaohm-centimeters. External factors like heat, cold, and moisture can impact a material’s resistivity.
• Surface Resistivity (ρS): The surface resistance of a dielectric material to insulation and electricity is referred to as surface resistivity (ρS). Like the volume resistivity, the ρS should be high, preferably in the range of 10³ to 10⁹ Megaohms per square. As with volume resistivity, factors like temperature extremes and moisture can impact surface resistivity.
• Electrical Strength: Across the Z direction of a printed circuit board, the ability of a dielectric material to resist electrical breakdown is referred to as electrical strength, measured in Volts/mil. Most PCB materials have an electrical strength between 800 V/mil and 1500 V/mil.

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Power and Heat Impact on PCB Material Selection

Chemical Properties

• Flammability Specs (UL94): The flame-retardant properties of plastics, also known as plastics flammability, are ranked from highest to lowest in the Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances (UL94). The requirements for PCB materials in UL94 state that specimens cannot burn for longer than 10 seconds with flaming combustion. For a set of five specimens, the combined specimens cannot burn for longer than 50 seconds with flaming combustion.
• Moisture Absorption: When submerged in liquid, the ability of a dielectric material to withstand this kind of exposure is referred to as moisture absorption. Most PCB materials have a moisture absorption value between 0.01 percent and 0.20 percent. The electrical and thermal properties of a dielectric are both influenced by the material’s moisture absorption.
• Methylene Chloride Resistance: The chemical resistance of a PCB material is referred to as methylene chloride resistance (MCR), which most directly measures the resistance of a dielectric to methylene chloride absorption. Dielectrics often have an MCR between 0.01 percent and 0.20 percent.

Flex and Rigid-Flex Boards

Mechanical Properties

• Peel Strength: The bonding ability between dielectric and copper layers in a printed circuit board is known as peel strength. During the PCB manufacturing stage, peel strength is tested on one-ounce-thick copper traces under the following three conditions: after thermal stress, at high temperatures, and after chemical exposure.
• Flexural Strength: The ability of a dielectric material to bear physical stress without breaking is referred to as flexural strength, measured in kilograms per square meter or pounds per square inch. To test the flexural strength of a printed circuit board, force is applied at the center while only the ends are supported. Dielectric strength is also measured by the tensile modulus, which determines the stress/strain ratio of a PCB material and how well it holds in each direction. Tensile modulus is often referred to as Young’s modulus and is used by some manufacturers instead of flexural strength as the measure of a PCB’s ability to withstand stress.
• Density: A dielectric material’s density is measured in grams per cubic centimeter (g/cc). Alternatively, PCB density can also be measured in pounds per cubic inch (lb/in³).
• Time to Delamination: The duration of a dielectric material’s resistance to delaminate-causing effects is referred to as “time to delamination,” which determines how long it will take for a PCB’s layers to come undone when exposed to temperatures beyond a certain threshold. A PCB material, be it laminate or fiberglass, can also delaminate if exposed to thermal shock or moisture.

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Material for HDI Considerations

Factors such as low-quality copper foil and dielectrics can impact the mechanisms of a PCB. With the proper selection of dielectric materials, you can prevent your circuit board from sending out faulty signals.

The molecular nature of dielectrics can render your system vulnerable to loss. Each time a signal is generated, the magnetic fields within the molecules vibrate. The effects of this are determined by the height of the signal frequency. When vibrations occur, energy becomes heat and is consequently lost in the system.

Copper conductors can also cause loss within a system. When electrons stray from the center of a conductor, the frequency increases. For example, a nickel-finished copper conductor will experience loss as significant amounts of current flow through the nickel instead of the copper. A conductor comprised entirely of copper can also see loss if micro ridges are present, as these can send the current up and down and cause resistance.

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What Features Can You Choose?

To prevent loss in your system, check your options in both of the following categories to ensure that you select materials that are best suited for high frequencies:

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• Substrate: Various materials are contained within the substrate of a printed circuit board, such as epoxy and glass fibers. A substrate must have a low dielectric constant to produce high-frequency circuits.
• Foil: Different types of foil can be applied to copper. Select a copper that can most reliably minimize resistance.

Low-quality and mismatched substrates and foils can lead to costly loss, but the right choices can help you get long-lasting PCB performance.

Best Practices and Further Considerations

To select the best substrate and foil for high-frequency applications, keep the following practices in mind:

• Match the Dielectric Constants: On a printed circuit board, the Dk’s of the different dielectric constants need to match one another. If the Dk’s are mismatched, problems will likely occur. For example, a resin paired with a woven layer can create issues if their Dk values differ.
• Match Coefficient of Thermal Expansion (CTE): Of all the substrate qualities related to temperature, CTE is the most important. As with Dk, the CTE between two substrates must match. Otherwise, the two substrates might expand at different speeds and consistencies, which can cause defects during the fabrication process. Differing CTE’s can impact the Dk’s when the PCB is in use.
• Tight Substrate Weave: The substrate must be woven tightly for the material and mesh to interact properly. Loose weaving can negatively impact the Dk’s.
• Avoid Using FR-4: Because of its low cost, FR-4 is a common choice. However, for circuits that generate high frequencies, FR-4 is an ill-suited material.
• Use Only Smooth Foils: To mitigate losses at high frequencies, use smooth copper foil.
• Use a Conductive Foil: Poor conductors can dampen the circuit. To prevent this, use only the right conductive foil.

When it comes to PCBs, performance hinges on quality, which can only be ensured with high-quality, well-matched parts. As you look through catalogs and consider which PCB might best suit your needs, keep the following considerations in mind:

• Cost: High-quality substrates can be a significant investment. The adage “you get what you pay for” holds true for printed circuit boards. Investing in quality parts upfront can save costs associated with part failures down the line.
• Manufacturing Defects: With materials designed to operate at high frequencies, checking for potential defects is crucial. This is especially true for PTFE and certain other materials.

By selecting the proper materials, making the right investment, and checking for manufacturing defects, you can ensure many years of performance without loss from your printed circuit board.

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At Millennium Circuits Limited (MCL), we provide top-of-the-line printed circuit boards. Each PCB we offer is priced competitively and engineered to perfection. As Pennsylvania’s leading supplier of PCBs, it is our mission to provide the finest service on order volumes of all sizes.

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After three straight years of being named among the “Top 50 Fastest Growing Companies in Central PA,” MCL was ranked among the state’s Top 250 companies in 2015—an amazing milestone for a company that started just ten years earlier with the intention to become a leading supplier of printed circuit boards. With a 98 percent “Excellent” quality rating and a 99 percent on-time delivery record, the team at MCL can provide you with the very best in customer service. To learn more about our PCBs, visit our products and services pages and contact us today to connect with our representatives.

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