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Don’t allow for the debate between material selection dissuade you from proper FR4 dielectric selection.
TOPICS IN THIS SOLUTION
Enabling your board material selection process to go through without a hitch.
PCB designs start with the selection of an insulating substrate. Most general purpose, rigid boards rely on a material called FR-4 as the substrate core. Using the FR4 minimizes the production cost for PCBs and provides good structural stability and good reliability. FR-4 has excellent mechanical properties because of a high strength-to-weight ratio and high electrical insulating properties regardless of humidity. Because of these properties and low cost, FR-4 works well for many electronic products.
The designation “FR-4” refers to the flame-retardant properties of the dielectric material and type 4 woven glass reinforced epoxy laminate used for the substrate. The composite materials and the structure of FR-4 give the substrate-specific thermal, electrical, chemical, and mechanical properties that impact your PCB design and limit the use of the substrate for RF and microwave frequency applications. Altium Designer provides the tools needed for making the right decision about FR-4 within your design.
Every material has permittivity—a characteristic that describes the speed that an electrical signal travels through a material and the amount of electrical charge that material can store in a given volume. As you think about PCB design, you should note that permittivity uses Farads/meter as a unit measure. The use of that measure tells that permittivity controls the value of a capacitance beyond the physical dimensions of the capacitor.
In a vacuum absent of any materials, permittivity equals a base value:
8.854 x 10-12Farads/meter or Ɛ0.
Signal propagation speed occurs relative to the speed of light in a vacuum. When we compare the permittivity of a vacuum to the permittivity of other materials, the vacuum permittivity has a base value of 1.00.
In real-world applications, we rarely have the opportunities to work in a vacuum. Every material—including air—has a higher permittivity value than 1.00. With that, let’s examine another rule. While the speed of light slows down within a specific material, the wavelength of the electromagnetic wave also decreases within the same material.
The term “relative permittivity (ƐR)” also represents the dielectric constant (Dk) of a material. When working with PCBs, designers refer to the dielectric constant when determining how dielectrics behave at given frequencies. The dielectric constant varies with frequency and with differences in materials. FR-4 materials have a dielectric constant that ranges from 4.2 to 4.8.
Because dielectric materials are anisotropic, the properties of the materials also vary in different directions. The importance of this characteristic rests within dielectric values that extend along the x, y, and z axes of the material. Since greater electric fields could occur across the length (x) and width (y) of the dielectric than across the thickness (z) of the material, PCB designers must remain aware that measurements will show different Dk values at each axis.
In the last section, we discussed the relationship between permittivity and capacitance. As you design your multi-layered PCB, adjacent layers act as a capacitor. The thickness of the FR-4 material determines the thickness of the dielectric and the value of capacitance. In addition, high frequency PCB designs depend on impedance matching and impedance stability. If your multi-layer design includes high frequencies, the operation of the circuit also depends on establishing the correct capacitance for each layer.
Because the dielectric constant of FR-4 materials change with frequency, the capacitance of dielectric also changes. In addition, changes in the temperature also impact the stability of the FR-4 dielectric constant because FR-4 materials have a high thermal coefficient. All this decreases the predictability of how the dielectric material behaves and impedance stability. When we work with high frequency signals, our design must have a stable dielectric constant and matched impedances.
Achieving the correct dielectric constant also impacts the physical size of the PCB. Controlling the impedance affects the routing and the size of transmission lines. Reducing the size of the transmission lines in the circuit allows you to match physical design requirements by decreasing the board size. Dielectric materials with a higher dielectric constant result in smaller board sizes.
You can use Altium’s Design Rules along with these tools when considering basic material properties, circuit fabrication reliability, and end-use performance. Design Rules can establish dielectric requirements and establish the instructions for Altium Designer’s PCB Editor. Monitoring the rules with Design Rule Checker allows you to successfully implement your FR4 layers.
Altium Designer offers options a-plenty for your constraints and rules checking needs
The need for selecting the correct FR-4 material thickness begins with the design and continues with ordering the board. Thick FR-4 layers have greater thermal mass and—as a result—cause changes in the dielectric constant. Functions within the PCB Editor allow you to control the dielectric constant of the substrate material.
The PCB Editor supports up to 32 signal layers and 16 plane layers. As a result, you can define the insulating layers across the entire PCB for traditional rigid designs. Altium Designer also provides the flexibility to choose from a variety of materials or to use one high-frequency laminate with the combination of many layers of FR-4 laminate.
Each of these tools also enables multi-layer board designs. The Layer Stack Manager supports PCB designs with more than 30 layers. As you work with a multi-layer design, you can specify a name for the layer, show the type of layer, establish the layer thickness in either millimeters or mils, and set the dielectric constant. If your design changes, the Layer Stack Manager menu allows you to add and remove dielectric layers.
The information for each layer associated with attributes found in the Board Layer Stack. Table Commands and options allow you to show information about the FR-4 material. Another key functionality involves the use of the Layer Stack Manager dialog and the Layer Stack Legend within Draftsman to document complex layer stack structures. Draftsman’s Layer Stack Legend view shows detailed descriptions and information—such as Gerber files--for each layer in the stack. You can use the Layer Stack Legend command or select an icon from the Additional Views on the Altium Designer Active Bar.
The dielectric constant of FR-4 substrates and other materials affects the electrical performance and signal integrity of a PCB. You can use the PCB Editor and Layer Stack Manager in Altium Designer to select the appropriate dielectric materials, specify the dielectric constant, and establish the layer thickness.
Don’t let your software impede your dielectric capabilities
For multi-layer PCB designs, Altium Designer simplifies every part of the process and allows you to set the number and thickness of layers for achieving the correct capacitance. By providing an end-to-end design platform that includes integrated tools, the unified environment of Altium Designer moves the PCB design from schematic to documentation and to fabrication. With the use of the integrated tools found within Altium Designer, you can easily solve design issues with dielectric materials, the number of dielectric layers, and dielectric thickness.