While purchasing and producing a printing circuit board (PCB), several criteria, including profile, weight, and components, are considered. Yet, one of the crucial elements is PCB thickness. Why? Its conductivity and resistance depend on its thickness, essential factors that satisfy your PCB needs. Usually, the PCB’s functionality depends on its thickness.
Usually, a buyer or a designer must give only a few details regarding where and how it will work. It aids in choosing a specific thickness suitable for its usage and the area where it is employed.
The Fr-4 material is a popular PCB base material due to its low cost and versatility. It comprises fiberglass weaved to a thin, cloth-looking sheet enclosed with a fire-resistant epoxy resin. We measure the Fr-4 standard PCB thickness in millimeters or inches, usually from 3″ to 10″. However, its subsequent board thickness range can be extreme, so designers must consider component compatibility and space when designing PCBs.
The thickness tolerance of a PCB
We refer to the permissible range of variation in the thickness of a printed circuit board (PCB) during manufacture as the thickness tolerance. The performance, functionality, and compatibility of the board with electronic components are all impacted by the PCB thickness tolerance.
A PCB manufacturer will generally specify the thickness tolerance, which depends on the type of PCB, the material used, and the manufacturing process.
For instance, the allowed range of thickness variation would be between 1.44mm and 1.76mm if the nominal thickness of a PCB is 1.6mm and the manufacturer stipulates a thickness tolerance of +/-10%. Therefore, the actual PCB thickness can vary within this range while meeting the manufacturer’s requirements.
A PCB’s thickness tolerance guarantees the board’s mechanical stability and compatibility with electronic components. The PCB’s thickness tolerance might influence the pad-to-pad distance and solder paste thickness needed for electronic components like surface-mount devices (SMDs). A PCB may impact the performance and dependability of the electronic device with a thickness that is outside the permissible range if it cannot offer the required pad-to-pad separation or solder paste thickness.
A PCB’s thickness tolerance might also impact the board’s mechanical stability. A thickness outside the allowed range can affect the board’s rigidity and mechanical strength. The thickness of the PCB is a crucial component in determining the board’s stiffness. This may cause the board to bend, twist, or warp, resulting in problems including solder joint failure, component damage, or other mechanical failures.
Mechanical Strength
Another crucial aspect that can be affected by the FR4 substrate’s thickness is the mechanical strength of the PCB. More mechanical strength in thicker substrates makes them more resistant to bending and twisting forces. This can be useful when the PCB encounters mechanical stress, such as in portable electronic devices.
However, the higher stiffness of thicker substrates can render them more prone to cracking or breaking under stress. Moreover, thicker substrates may add to the PCB’s weight, which is a drawback for portable devices where it is essential.
Thermal Performance
The FR4 substrate’s thickness also impacts the thermal performance of the PCB. This is because the PCB substrate acts as a heat sink to remove the heat produced when electronic components operate. Therefore, thermal conductivity is a crucial factor affecting the FR4 substrate’s capacity to dissipate heat.
Thicker substrates are less effective in dissipating heat because they have lesser thermal conductivity. This may impact the electronic components’ performance, and reliability as their temperatures may rise. In addition, the temperature gradient across the PCB might widen, and thermal stress can result from thicker substrates’ higher thermal resistance.
The thickness of the FR4 substrate should change for the unique heat dissipation needs of the electronic components to provide effective heat dissipation and avoid thermal stress. To guarantee proper heat dissipation, several substrate thicknesses may occasionally be utilized in various locations on the PCB.
Power Distribution
The thickness of the FR4 material has an impact on power distribution, another crucial component of PCB design. The power distribution network (PDN) in a PCB is in charge of providing electricity to each component in the circuit. A network of copper traces and planes comprise the PDN, which serve as low-resistance routes for current flow.
The PCB’s power distribution efficiency depends on the resistance of the copper traces and planes, which relies on the FR4 material’s thickness. A thinner FR4 material, on the other hand, has a lower resistance, which lowers the voltage drop across the traces and planes and boosts the power transferred to the components.
Impedance Control
Impedance management is crucial to the creation of high-speed PCBs. A circuit’s resistance to the flow of alternating current is known as impedance. Impedance control in a PCB refers to keeping the impedance constant throughout the circuit to avoid reflections and signal distortion.
The FR4 thickness impacts the impedance of the traces in the PCB. The impedance is inversely proportional to the FR4 material’s thickness. As a result, thicker FR4 materials have higher impedances, while thinner ones have lower impedances. With a high-speed PCB design, maintaining impedance control requires careful consideration of the FR4 material’s thickness.
Signal Integrity
Signal integrity is one of the essential effects of FR4 thickness (fr4 standard thickness) on PCB effectiveness. The capacity of the electronic signals to maintain their quality as they pass across the PCB is signal integrity. However, the FR4 substrate’s thickness impacts the signal transmission speed, which can result in signal loss or distortion.
Due to the characteristics of the material, a signal experiences dielectric loss as it passes through the FR4 substrate. The dielectric loss is a function of the material’s dielectric constant (Dk), and the thickness of the FR4 substrate influences the Dk value. Therefore, the Dk value of the FR4 substrate similarly rises as the thickness does, leading to more pronounced signal attenuation and distortion. This effect is more evident when the signal wavelengths are shorter at higher frequencies.
The thickness of the FR4 substrate should be adjusted for the particular frequency range of the signals to reduce signal distortion and loss. For instance, a thinner substrate is necessary for high-frequency transmissions to prevent signal attenuation and maintain signal integrity.
Considerations for selecting the thickness
Component compatibility: Although FR-4 is used to create many different kinds of printed circuits, the components we can use depend on the material’s thickness. THT components, for instance, differ from other components and demand a thin PCB.
Space-saving: While designing a PCB, space-saving is crucial, especially for USB connectors and Bluetooth devices. The thinnest boards are helpful in arrangements where saving space is essential.
Design and adaptability: Thicker boards are ideal for most manufacturers over thinner ones. When using FR-4, expanding the board’s dimensions could break it if the substrate is too thin. Yet, thicker boards are flexible and allow for the creation of V-grooves.
The PCB’s usage environment is essential. Reduced stress is a given with thin PCBs for an electronic control unit in the medical industry. Too thin and hence too flexible, boards are more susceptible to heat. They may flex and assume an unfavorable angle during the component soldering processes.
Impedance control: Impedance control is made possible by the thickness of the board, which is implied by the dielectric environment thickness, in this case, FR-4. The board thickness is a deciding factor when the impedance is significant.
Connections: The FR-4 thickness also depends on the connections used for a printed circuit.
Summary
The thickness of the material used in printed circuit boards (PCBs), called FR4, can significantly affect their performance and properties. From 0.8mm to 1.6mm, the thinner the substrate, the stronger the mechanical strength and stiffness and the less signal loss. But other factors like the number of copper layers, trace width and spacing, solder mask, and surface polish can all impact the board’s performance, so designers must ensure they meet the application’s needs. The thickness tolerance of the PCB is essential, too, as it affects the mechanical stability and compatibility with electronic components.
