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Sep 25, 2020

Difference Between Thickness of Standard and Customized PCBs

Procuring a printed circuit board (PCB) involves considering several factors such as its profile, weight, and components. Thickness of a PCB is one of the main criteria, as the thickness decides its performance and functionality. Users typically determine the best suitable thickness level depending on the application requirements.

Although several variables come into play when fabricating a PCB, the thickness of the board is an essential one. The required thickness depends not only on the functionality of the PCB, but also on its profile, weight, components assembly and the specifications of those components. Moreover, the above factors contribute not only to the overall performance of the PCB, but to its thickness level as well.

The evolvement of electronic equipment over the years has led to an overall shrinking in all dimensions, causing the PCB to follow suit. Therefore, most modern boards are thin and lightweight, making the thickness of the PCB an essential factor.

Thickness of Standard PCBs

PCBs come in different structures such as single sided, two sided, or multi-layered. The two sided and multi-layered PCBs may have a substrate layer in between. Each layer will also have one or two copper layers and all the above add up to the thickness of the PCB. Earlier, the typical PCB thickness was 1.57 mm or 0.063 inches, derived from plywood sheets that equipment manufacturers used for electronic devices that included PCBs.

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However, when developing multi-layer boards, manufacturers had to match the thickness of connectors between the boards. This led to the level of thickness of the boards becoming a significant variable. Moreover, the copper layer on the plate edges also had to be of a standard level.

The above allows the general thickness level to range from 0.2 mm or 0.008 inches to 6.1 mm or 0.240 inches, depending on the application or usage. Manufacturers use different types of cores or substrates when fabricating PCBs, including paper reinforced phenolic resins between layers of copper foil. Boards are thinner when they do not have edge connectors, and fabricators use lighter material for the substrate.

PCB Stackup

The PCB stackup may also define its thickness. For instance, a prepreg layer may separate two copper layers in a multi-layered board. The prepreg may be epoxy or paper reinforced phenolic resin. As the number of copper layers goes up with the increase in overall number of layers of a PCB, the copper thickness the manufacturer uses also contributes to the overall thickness of the board.

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PCB Copper Thickness

Copper traces on a PCB decide its overall functionality and its area of application. The thickness of copper the fabricator uses plays an essential role in deciding the standard PCB thickness. The standard measurement for copper on a PCB is in ounces. One ounce of copper spread evenly over an area of one square foot results in a thickness of 1.37 Mils or 0.035 mm.

Manufacturers typically use a thickness equivalent to 1 oz of copper for PCBs, meaning, the copper thickness is typically 0.035 mm. However, this thickness of copper may not be adequate if the current passing through the traces is high. In such cases, the manufacturer follows one of two choices—increasing the weight of copper or increasing the width of the copper trace. The general preference is to increase the width of the copper trace, as increasing the copper weight is not only more challenging, but is also expensive and requires more fabrication time.

PCB Copper Width

The designer primarily decides the width of the copper trace depending on the maximum amount of current the trace must carry. This is an important factor for PCB design, as it prevents overheating or damaging the PCB. As the resistance of a specific copper track does not change, current passing through increases the trace temperature. Increasing the current raises the temperature further, until it crosses the limit of the PCB leading to PCB damage. Therefore, the designer must ensure the trace is wide enough to allow passage of higher currents without causing damage.

Designers typically use a PCB track width calculator to calculate the appropriate width for allowing a specific current flow without damage. Consequently, tracks in the internal layers will have to be wider as they cannot easily dissipate the heat they generate. On the other hand, tracks on the external layers of the PCB can transfer their heat through convection.

Customized PCB Thickness

A fiberglass reinforced PCB with epoxy laminated sheets, also known as FR-4, is a fire-retardant material that manufacturers popularly use for manufacturing rigid circuit boards, and as a PCB base material. Although the fiberglass is a thin and cloth-type sheet, a fire-resistant epoxy resin encloses it, giving it the customary rigidity.

The popularity of FR-4 material stems from its low cost as well as its high dielectric strength and electrical insulation. Moreover, the material is lightweight, heat and water resistant. All these make FR-4 material suitable for a wide range of environmental conditions.

Designers must consider several factors for deciding the thickness of their PCB:

Components Used: The type of component the designer will use affects the PCB thickness. SMT components are lighter and a thin PCB is suitable for them. On the other hand, through hole components need thicker PCBs as they are larger and heavier in general.

Space Available: A thicker board may increase the height of the overall assembly, which may not fit in the space available—a thinner board may be necessary.

Design Requirements: Unless necessary, a thicker board is preferable as they are mechanically sturdier than thin boards. Manufacturing thicker boards is also easier on the same account. Cutting grooves to separate PCBs from panels is much easier in thicker boards. However, designers must contend with the extra weight of the thicker PCB.

Flexibility: Although one does not expect rigid boards to be highly flexible, some amount of flexibility is necessary for PCBs and depends on its use and application. In extreme cases, the designer has the option to use Polyimide in place of FR-4 for achieving more flexibility.

Assemblers may face problems with flexible boards, as these are likely to bend during soldering, displacing components on the surface.

Impedance Control: Maintaining proper thickness during manufacturing is essential for multilayered boards with controlled impedance features. Layers surrounding the track create the desired impedance, hence the importance of their thickness.

Edge Connection: If the PCB must match a connector at its edge, then the designer must specify its thickness, and the fabricator must keep it under control during manufacturing.

Weight of PCB: As the thickness of a PCB also defines its weight, a thinner and hence lighter board can save the customer on shipping costs.

Conclusion

Thickness of a PCB is of immense importance to its functionality. Therefore, for the best performance, a designer must decide whether they will use a PCB with standard thickness or customize its thickness.

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