A PCB has several layers and their individual thicknesses add up to the final thickness of a Printed Circuit Board. The major contribution comes to PCB thickness come from the core layer, prepreg layers, and copper layers. Although a PCB may have additional layers like two solder masks and one silkscreen layer, their contribution to the overall thickness is far lower. Among the thicker layers, the core layer makes the board rigid, the prepreg layers provide the insulation, while the copper layers provide the path for signal flow and various interconnections. Functionality of a board therefore depends to a large extent on the PCB thickness.
PCB Thickness — Constituents
The insulating material separating the two innermost copper layers forms the core of the PCB. This is a dielectric material and is also known as the substrate. The core is actually a copper clad and forms the main thickness of a double- or multi-layered board stack-up. During fabrication, the fabricator does not alter the thickness of the core.
Additional dielectric material in the internal layers is necessary to support multiple layers of copper on both sides of the core. Manufacturers achieve the specified overall board thickness by adjusting the thickness of these internal layers.
The overall board thickness also depends on the number of copper layers it has and their weight or thickness. Any plating on the copper layers also contributes to this thickness.
Some PCBs require two or more cores separated by prepreg layers between them. This is necessary for improving the stiffness of the PCB, but the process increases the board thickness.
PCB Thickness — Importance
In the industry, PCBs in standard thickness are in greater demand. Standard thicknesses not only simplify designing systems, but most manufacturing equipment can use these PCBs effectively, resulting in optimizing costs. The PCB industry provides boards in standard thicknesses of 0.0079 inches, 0.020 inches, 0.016 inches, 0.093 inches, and 0.125 inches. In metric terms, the standard thickness available are 0.3-0.8 mm in increments of 0.1 mm, and 1.0, 1.2, 1.6, 2.0, 2.4, and 3.3 mm thereafter However, PCB designers often resort to designing boards with non-standard thicknesses for complying with operational requirements.
The thickness of a PCB also depends on its application. Designers consider several factors to decide on their PCB thickness. Some major considerations are:
- Current Carrying Capability
- Materials used
- Number of Layers
- Signal Integrity
- Vias used
- Operational Environment
- Manufacturing Machinery
Thin and flexible boards are suitable for lightweight applications. Thick boards are sturdier and suitable for heavier components.
Current Carrying Capability
The thickness of copper is important in boards, as copper layers carry all the signal and power. Copper weight defines the thickness of the copper layer. Standard copper weight is 1 to 2 oz, signifying 35-70 microns thickness. Manufacturers use higher weight of copper according to the board requirements. For handling higher amounts of current, boards need higher copper weight, but this increases the overall PCB thickness.
The overall PCB thickness also depends on the thickness of its core and prepreg the manufacturer is using. Apart from providing the board with its rigidity and structure, its reliability and longevity also depends on the choice of the material for both the core and prepreg. Both the core and prepreg use laminates and substrates available in various options. Designers mix and match suitable materials for achieving thermal, mechanical, and electrical properties of the board. But this also determines the overall thickness of the board.
Number of Layers
The number of layers in the PCB also affects the board thickness. Sometimes manufacturers must keep board thickness to a specific value. They achieve this by adjusting the thickness of individual layers. However, as the number of layers in the board increases, this process may become more and more difficult for the manufacturer.
If the board must handle high-speed signals, the designer may have to define the impedance characteristics of several traces. To achieve this, they may have to introduce layers with a defined geometry and, in the process, impact the overall thickness of the board.
Manufacturers of printed circuit boards use different types of vias. These include blind vias, buried vias, and through vias. Thicker boards can use vias with higher diameter. Thinner boards must use vias with smaller diameters. For very thin boards, manufacturers use micro vias. With a laser beam drilling them, micro vias have very small diameters.
A rugged operating environment where the PCB may be subject to vibration may require a thick board to sustain long-term operations. Designers also consider the operating environment before deciding on the thickness of a PCB.
For circuit board fabrication, the manufacturing machinery must be suitable for handling the board thickness. This is especially true for PCB drilling machines and depanelizing machines for separating individual PCBs from a panel. For through holes, the drilling machine must be capable of drilling through the PCB thickness. There are two methods for depanelization—routing for thicker boards and scoring for thinner PCBs.
PCB Thickness — Power Handling
As explained above, boards with thicker copper layers can handle higher power. The cross-sectional area of the copper traces must present a low resistance to the flow of current. Lower track resistance is necessary to prevent undue temperature rise in the board. Designers can enhance the cross-sectional area of a copper trace by making the copper trace wider, adding more layers, or making the copper trace thicker.
Wider Copper Trace
If there is adequate space on the board, the designer can increase the width of a trace, allowing it to carry more current, while they continue to use the standard copper weight of 1 to 2 oz or 35-70 microns thickness. A wider copper trace effectively has a larger cross-sectional area, thereby presenting a lower resistance to the current flow.
If the designer cannot increase the width of the trace due to board size limitations, they can increase the number of layers in the board or increase the copper weight.
Higher Number of Layers
With additional layers, multiple traces on adjacent layers can share the current flow. Even with the standard copper weight of 1 to 2 oz or 35-70 microns thickness, paralleling multiple traces effectively increases their cross-sectional area. However, adding more layers increases the overall PCB thickness.
Higher Copper Weight
If the designer cannot use the two solutions above, they can increase the copper weight of the layer. A thicker copper layer provides a higher cross-sectional area for a trace of the same width. However, adding a layer with a higher copper weight not only makes the board thicker overall, but also increases manufacturing complexities.
All copper traces present an internal resistance opposing the flow of current through them. However, this raises the temperature of the tracks. By reducing the resistance of the track the designer can arrest the temperature rise within limits.
A track heats up as current passes through it. But it loses heat through convection to the surrounding air. The temperature stabilizes when the heat generated equals the amount lost. However, heat loss through convection to surrounding air is true only for the traces present on the outermost layers of a multi-layered board.
Traces in the inner layers can lose heat only through the PCB material surrounding it. Thermal conductivity of the dielectric material is far lower than that of air. This prevents traces in the inner layers from carrying high currents. For this reason, designers assign high current traces to only the outermost layers.
PCB Thickness — Design
Cost is one of the primary factors influencing PCB thickness at the beginning of the design process. Designers generally start with a standard thickness, and make the necessary changes as the design moves forward. However, the designer must focus on certain issues when making changes in PCB thickness:
For non-standard PCB thicknesses, designers may have to evaluate manufacturers early on for fabricating their special boards. Generally, manufacturing non-standard boards is comparatively more expensive.
Fabricating standard thickness boards requires making little or no changes to existing manufacturing processes. However, for accommodating non-standard boards, manufacturers may need to introduce a change in their processes and equipment settings. This increases the turnaround time, which the designer must discuss with the manufacturer and factor in their schedule.
As a non-standard board comes at an extra expense, designers must evaluate whether the business can absorb the extra cost. In comparison with standard-thickness boards, non-standard boards are not only more expensive, they also have a longer lead time. To mitigate a longer lead time, designers may explore boards with a standard thickness but made of special materials. Although more expensive than regular boards, they may not involve a longer lead time. Such options may work as a trade-off for the designer, rather than opting for a non-standard board.
PCB thickness has a direct relationship to its power handling capacity. PCBs that must carry high power, whether high current or high voltage, are generally thicker. They require higher amounts of insulation, lower track resistance, and higher mechanical sturdiness that result in an increase in board thickness.