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July 10, 2021

Why is Thickness So Crucial for Printed Circuit Boards in the USA?

The thickness of a printed circuit board in the USA depends primarily on its application. Maintaining the specified thickness during PCB fabrication in the USA depends on the manufacturing processes involved. Designers specify the required thickness depending on the application and functionality requirements, and the thickness may vary from design to design. PCB fabrication in the USA also involves different manufacturing processes, and their methods of achieving the final thickness of printed circuit boards USA may vary, along with the tolerance levels. The thickness of printed circuit boards in the USA is important because of three factors:

  • Cost
  • Performance
  • Quality

thickness of printed circuit boards

Cost

The cost of material forms a major part of the quotation of a printed circuit board in the USA. If the fabricator is consistently producing boards with thickness in excess of the specification and tolerance, they are using up more material than their customers are paying them for—it is a loss to the company. On the other hand, if the fabricator is consistently supplying thinner boards against the specification and tolerance agreed upon, they are charging the customer more than the quoted price. The cost of material beyond the specified tolerances may not seem much for a single board or for short runs of a PCB, but for large-scale production runs, its value may become significant.

Performance

Using a thinner printed circuit board in the USA where a thicker board is necessary, may be fraught with several performance issues. A thinner PCB may not be capable of withstanding the vibration levels in the application, and may develop cracks. On the other hand, a board that is thicker than necessary, may not fit very well in the equipment, leading to aesthetic and performance problems.

Quality

A printed circuit board in the USA that deviates from the specified thickness beyond the tolerance levels, may not pass the quality standards of the manufacturer. The incoming quality inspection of the customer may also reject such PCBs.

Analysis of Thickness of Printed Circuit Boards USA

Initially, printed circuit boards were simple with single- and double-layer boards built around a single core or copper clad. One could consider multi-layer boards as an arrangement of several layers of pre-preg and copper foil stacked around the core of a double-layer board. More complicated boards now have multiple cores.

Therefore, the overall thickness is the cumulative thickness of the cores and the individual thickness of all the layers making up the board. Although the standard thickness of an industrial PCB is 63 mil (1.57 mm), it is possible to have thicknesses ranging from 8 mil (0.2032 mm) to 240 mil (6.096 mm).

Apart from the core and layers of pre-preg and copper foils, the outermost surfaces of the printed circuit board may also have a layer of solder mask and a layer of silkscreen that add to the board’s thickness. For instance, a 63 mil (1.57 mm) thick double-layer PCB may have a core of 59 mil (1.5 mm), two copper layers of 1.38 mil (0.035 mm) each, two solder mask layers of 0.394 mil (0.01 mm) each, and two silkscreen layers of 0.226 mil (0.006 mm) each.

Modern printed circuit boards in the USA have standard thickness depending on the number of layers in them. For instance, if the PCB is a four-layer board, its typical standard thickness would be 31 mil ±10%; standard thicknesses for a six-layer board can be 31 mil ±10% or 62 mil ±10%. Similarly, for an eight-layer board the standard thickness could be 62 mil, 93 mil, or 125 mil ±10%.

Printed Circuit Board Manufacturers USA

Core Thickness of Printed Circuit Boards USA

The substrate or core of a printed circuit board is usually of epoxy fiberglass with copper foil bonded on its two sides. Cores of different thicknesses are available, along with various thicknesses of copper foils. The substrate gives the PCB its rigidity.

With increasing complexity of printed circuit boards in the USA, there was a need for multiple layers to allow for the larger number of interconnections between SMDs. Consequently, the standard PCB thickness jumped from 63 mil to 93 mil. To comply with RoHS and WEEE requirements, PCB fabrication in the USA uses high-temperature substrate materials like 370HR from Isola in thicknesses of 8, 10, 14, 22, and 39 mil in multiple layers.

Pre-Preg Thickness of Printed Circuit Boards USA

Pre-pregs consist of fiber-weaves that manufacturers have impregnated with a resin bonding agent. Fabricators use pre-pregs to bond etched cores together. They can vary the thickness of pre-preg layers to adjust the final thickness of the board.

Copper Layer Thickness of Printed Circuit Boards USA

The copper layer thickness may vary from ½ oz to 3 oz based on the current handling requirements of the board. Usually, the core layer has the same copper weight on both sides to prevent the board from bending with heat during assembly.

Why Choosing Thickness is Crucial?

While some designers prefer to go with a standard thickness, others want to customize the thickness of the printed circuit board in the USA. Whatever be the decision in the final stage of the design, the thickness affects several factors. Some major factors are:

  • Weight: Thick boards are heavy. If the device is lightweight, the designer may have to go for a thin PCB.
  • Flexibility: Some projects demand a flexible PCB, and a thick board cannot fulfil that requirement. Designers may have to specially design a thin, flexible board.
  • Space Constraint: If the project has a space constraint, a thick PCB may not fit inside. Designers may have to use a thin board with low-height components.
  • Connectors and Components: Type of components on the board also determines its thickness. A thin board will sag and bend if there are many heavy components on board. It is usually difficult to solder large, multi-pin components on very thin boards.
  • Impedance: Achieving a certain impedance requires a specific thickness of the dielectric or pre-preg. The designer must change the dielectric type to reduce the thickness but want to keep the impedance intact.

Conclusion

The thickness of printed circuit boards in the USA is crucial for their proper functioning in specific applications. This is because the thickness defines the mechanical rigidity, signal integrity, and current carrying capacity of the board. In most cases, designers and PCB fabrication in the USA follow IPC guidelines and standards for specifying the board thicknesses and tolerances for various applications.

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