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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 Options, Classifications and Substrate Property Considerations
- 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. In the process, the PCB loses five percent or more of its overall mass. The temperature range in which this process occurs is known as decomposition temperature (Td), which is noted in degrees Celsius. While the effects of a Tg transformation self-reverse once the material is removed from its glass transition temperature, the effects of decomposition temperature on a substrate are permanent. As such, it is best to use a PCB material that can handle the temperature range of your working environment. 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 that exceed its Tg, the material will also undergo a rise in CTE, which is measured in parts per million (ppm). A substrate will generally have a high CTE than a copper layer. This difference is sometimes the source of interconnection problems when heat is applied. Due to the constraints of woven glass around PCB material, CTE is typically 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. The CTE should be kept to a bare minimum across the axis because this is the direction in which the material will expand. 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, so too will be the level of heat transfer, 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/mk By comparison, copper has a k-level of 386 W/M-ºC. As such, the copper layer carries heat faster than the dielectric in a PCB.
- Dielectric Constant or Relative Permittivity (Er or Dk): Two of the most important 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 generally between 3.5 and 5.5. A material’s Er level depends on frequency and will usually drop as the frequency rises. The Dk level changes less on certain PCB materials than on others. For a material to be safe for applications that involve high frequencies, it 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 in the range of 0.02 to 0.001. The former figure applies to the more widely used materials. However, the latter figure generally applies to high-end materials. Tan δ also increases along with frequency. When it comes to digital circuitry, loss tangent is generally a factor of minor importance. Expectations to this rule would include any application where the frequency level exceeds 1Ghz. 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) — as well as ohm-centimeters (Ω-cm) — are used to measure the resistivity of a given dielectric. Like all dielectric insulators, the material on a printed circuit board must have high resistivity, preferably in the range of 103 to 1010 Megaohm-centimeters. External factors like heat, cold and moisture can have an impact on 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). As with a material’s ρ level, the ρS must be high, preferably in the range of 103 to 109 cMegaohms per square. As with ρ, a material’s ρS level can be impacted by temperature extremes and moisture.
- Electrical Strength:Across the Z direction of a printed circuit board, the ability of a dielectric material to resist an electrical breakdown is referred to as electrical strength, which is measured in Volts/mil. Most PCB materials have an electrical strength of between 800 V/mil and 1500 V/mil.
- 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. The majority of 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.
- 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, which is 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, which is used in lieu of flexural strength by some manufacturers 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). Alternately, PCB density can also be measured in pounds per cubic inch (lb/in^3).
- 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 the layers of a PCB to come undone from one another 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.
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 are vibrated. The effects of this are determined by the height of the signal frequency. When vibrations do occur, energy becomes heat and is consequently lost in the system.
Copper conductors can also be a cause of loss within a system. When electrons stray from the center of a conductor, the frequency goes upward. For example, a nickel-finished copper conductor will see 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.
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:
- 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 circuits of high frequency.
- Foil:There are several different types of foil that 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 applications of high frequency, you need to keep certain practices in mind:
- Match the Dielectric Constants:On a printed circuit board, the Dk’s of the different dielectric constants need to be matched to one another. If the Dk’s contradict, problems will likely occur. An example of mismatched DK’s could be a resin paired with a woven layer.
- Match Coefficient of Thermal Expansion (CTE):Of the substrate qualities that pertain 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. During the fabrication process, differing CTE’s can result in defects. When the PCB is in use, the clashing CTE’s could impact the Dk’s.
- Tight Substrate Weave:The substrate must be woven tightly for the material and mesh to properly interact. If not, the DK’s will be negatively impacted.
- Do Not Use FR-4:Because of its low cost, FR-4 is a common choice. For circuits that generate high frequencies, however, FR-4 is an ill-suited material.
- Use Only Smooth Foils:To mitigate losses at the highest of frequencies, use smooth copper foil.
- Use a Conductive Foil:Poor conductors can have a dampening effect on the circuit. To ensure this does not happen, only use the right conductive foil.
When it comes to PCBs, performance is all about quality, which you can only ensure 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 as well:
- Cost:Substrates of good quality can be a sizable investment. The fact is, you get what you pay for when it comes to printed circuit boards. If you are not willing to properly invest, you stand to lose far more down the line as parts fail.
- Manufacturing Defects:With any material designed to operate at high frequencies, it is always wise to double-check for possible defects, which are common in PTFE and certain other materials.
When you select the proper materials, invest the right amount of cash and check for manufacturing defects, you are much more likely to get many years of performance without loss from your printed circuit board.
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