PCB material selection is the first step in your design process. Selecting the dielectric material for your design is very important, as it can impact the overall performance of the board. To help board designers determine what sort of material best suits their design needs, we have developed a circuit board material selector.
Sierra Circuits’ PCB Material Selector allows you to search and compare the properties and specifications of board materials that fit your application. There are 50+ rigid materials and 10+ flex materials available in this tool. For each material, there are various parameters listed, such as dielectric constant, dissipation factor, glass transition temperature, decomposition temperature, moisture absorption, etc.
Our circuit board material tool allows you to search and compare PCB materials that fit your application and meet your requirements.
Sierra Circuits PCB material selector
All the criteria for comparative selection are listed below:
In the Board Properties section, you can choose the following options:
If you select rigid as the material type and click on the “Go/Submit” button, our Material Selector lists out all the available rigid materials (even if no criteria are selected).
After selecting the required criteria, click on the “View” button to view the properties of that particular material. To compare the properties, select two or more different materials and hit the “Compare” button. A new window pops up comparing the properties of the chosen materials.
If you select flex as the material type and click the “GO/SUBMIT” button, our circuit board material tool lists out all the available flex materials. You can now have a look at all the materials along with their properties as shown here.
By clicking the “View” button, you can view the properties of that particular material. To compare the properties, select two different materials by clicking the checkbox and hitting the Compare button. A new window pops up comparing the properties of the chosen materials.
Select the option Yes to list out halogen-free materials. If the option All is selected, the tool considers all the available materials present. This filter only applies to rigid materials.
Thermal conductivity is the ability of a material to transfer heat by conduction. It is usually represented by the symbol K, and its units are W/mK (watts per meter per degree Kelvin)
Select the Very High Thermal Conductivity option if you require thermal conductivity greater than 1W/mK.
The key material properties that are used as a criterion for selection/filtering are categorized in the following table:
In this section, fill in the fields to narrow down the required material.
Based on the operating frequency (speed) and associated loss, PCB materials are categorized as:
Depending on your application, you can choose one of these 4 categories. Select option “All” to view all the materials available, irrespective of these categories.
The dissipation factor (Df) is the most important in determining the extent of signal loss in a PCB material; a higher value of Df leads to a higher loss. Furthermore, the signal loss also increases with the frequency or speed of operation. Therefore, for controlling the loss at high frequencies or high speeds, choose a PCB material with a low Df value. Thus, the range of Df values is the criteria for the above speed/loss categorization.
The dielectric constant (Dk) of a PCB material changes with frequency. Large variations of Dk with frequency cause larger signal distortion/rise time degradation, which becomes less acceptable for high-frequency or high-speed signals. Therefore, as the signal frequency or speed increases, it is desirable to have less variation in Dk with frequency. The need for less variation of Dk with frequency for low-loss PCB materials has been recognized and implemented by all board material manufacturers.
You can also choose the materials based on maximum signal frequency content (GHz)/fastest signal rise or fall time (ns)/highest data transfer rate (Gbps).
These 3 parameters are related to each other; if any one of the aforementioned values is specified, the material selection tool automatically calculates the remaining two values. For example, if you input the maximum signal frequency content, it automatically displays the corresponding values of the fastest signal rise/fall time (ns) and the highest data transfer rate (Gbps).
The Dk of most PCB materials ranges from 2 to 10. To choose the appropriate range out of this, a slider button has been provided in the Material Selector.
The Dk of a PCB material largely determines the trace width of controlled impedance transmission lines on the circuit board; the greater the Dk value, the lesser the trace width will be for the same target controlled impedance value. A higher Dk value also leads to higher dielectric loss. In general, high-speed/low-loss materials have lower Dk values. Thus, Dk becomes an important electrical performance criterion for material selection. For normal speed/normal loss materials, the Dk varies considerably with frequency; for very high speed/very low loss materials, the Dk remains effectively constant with frequency. The Dk of the most commonly used board material- FR4- for example FR370HR- is 3.92 @ 10Ghz.
The values of Dk specified in our Material Selector are with a material’s resin content at 50% (as per IPC guidelines for the listing of properties in the material’s datasheet). Since Dk varies with frequency as well, we have used the values at 10 GHz in this tool.
The range of the above-mentioned properties can be set using the respective sliding button.
A material’s dissipation factor is the most important factor in determining the signal attenuation or signal loss, as signals travel along a PCB conductor or trace. The lower a material’s dissipation factor is, the lesser will be the signal loss. Thus, Df is the most important selection criterion from the signal loss perspective. The Df of FR4 material FR370HR is 0.025 @ 10Ghz.
The values of Df specified in our Material Selector are with a material’s resin content at 50% (as per IPC guidelines for the listing of properties in the material’s datasheet). Since Df varies with frequency as well, we have used the values at 10 GHz in this tool.
It is a measure of the voltage threshold above which electric breakdown can occur between 2 electrical conductors on the surface of a material. The smaller the CTI class value, the higher the threshold voltage. The CTI of the most commonly used PCB material- FR4- for example FR370HR- is CTI Class 3 with the electric breakdown voltage range being 175V-249V.
Electrical strength, given in volts per unit thickness, measures the maximum electrical field a board material can withstand in the Z direction before its electrical breakdown. In this tool, the unit used is volts/mil. The dielectric electrical strength of the most commonly used PCB material- FR4- for example, FR370HR- is 1350 V/mil. Most of the IPC-certified materials have a dielectric electrical strength >= 750 V/mil.
It is the electrical resistance of any material of unit length and unit cross-sectional area (A) when measured across the length (l). Volume resistivity is expressed in units of Ω-cm (ohm-cm). For a dielectric material, since resistivity is very high, it is usually expressed in units of MΩ-cm. The volume resistivity of FR4 material, for example, FR370HR, is 3.0 x 108 MΩ-cm.
Resistance R = ρ⋅ (L/A); where ρ is the resistivity
The inverse of volume resistivity is called electrical conductivity (σ), which is expressed in Mhos/cm or Siemens/cm.
It is the electrical resistance of any material surface of a square shape of any dimension when measured across one of the dimensions. Surface resistivity is expressed in units of ohms or ohms per square. For a dielectric material, since resistivity is very high, it is usually expressed in units of Mohms or Mohms per square. The surface resistivity of the most commonly used PCB material FR4- for example, FR370HR- is 3.0 x 106 MΩ-cm.
Resistance R = ρₛ(L/W); where ρ is the resistivity
The ability of an insulating material (measured in terms of time) to withstand a high-voltage electric arc and resist the formation of a conducting path along its surface. The conducting path is due to the chemical and thermal decomposition of the material. For example, the arc resistance of FR4 material FR370HR is 115 seconds.
It is the voltage at which breakdown of a dielectric material occurs, usually expressed in kV. The dielectric breakdown voltage of FR4 – FR370HR is 50kV.
In this section, you can set the range of various thermal properties like glass transition temperature (Tg), decomposition temperature (Td), coefficient of thermal expansion (CTE) along X, Y, and Z axes, and thermal conductivity (K).
Tg is the temperature around which a circuit board substrate transitions from a glassy, rigid state to a softened, deformable state. Above Tg, the material has a substantially higher coefficient of thermal expansion than that below Tg. However, the deformation with temperature around Tg is completely elastic; when the temperature drops from a temperature higher than Tg to below Tg, the material returns to its original state. For all lead-free (ROHS compatible) materials, the Tg requirement is >= 170 °C. The Tg of FR4 material FR370HR is 180 °C.
Td is the temperature at which a PCB material chemically decomposes, and the change is completely irreversible; once the material crosses Td, it cannot return to its original state after cooling down and therefore becomes totally non-functional. The Td of FR4 material FR370HR is 340 °C.
It is the rate of expansion of a board material in the X, Y, and Z directions as the temperature increases. These values refer to the rate of expansion below Tg. The X, Y CTE of FR4 material FR370HR is (13,14) ppm/°C. The Z-axis CTE of FR370HR is 45 ppm/°C.
The unit of thermal conductivity (K) is W/mK (Watt per meter per degree Kelvin). The K of most PCB materials is in the range of 0.2 to 0.5 W/m⋅K; for FR4 material FR370HR, it is 0.4 W/mK.
Based on the values of thermal conductivity, we have categorized them into 3 categories, as given in the drop-down list:
The amount of time, in minutes, a dielectric material can withstand before failure or delamination at 260°C. The T260 time of FR4- for instance, FR370HR- is 60 minutes.
The amount of time, in minutes, a dielectric material can withstand before failure or delamination at 288°C. The T288 time of FR370HR is 30 minutes.
Here, you can set the range of tolerable moisture absorption of the required material. If a CAF-resistant material is required, you can select the option “Yes” else select the option “All”.
It is a measure of the ability of a board material to resist water absorption when immersed in water. It is given by the percentage increase in weight of a dielectric material due to water absorption under controlled conditions as specified in the IPC test method standard IPC-TM-650- 2.6.2.1A. The moisture absorption of FR4 material FR370HR is 0.15%.
CAF (conductive anodic filament) is a metallic filament that is formed due to electrochemical migration. Formation of CAF might lead to electrical failure. This can be avoid
