Standard PCB Board Thickness: A Brief History

Key Takeaways

• PCB thicknesses are variable but generally follow conventions devised before printed circuits existed.

• Designers may still want to adhere to standard thicknesses for compatibility with board edge connectors and manufacturer comfortability.

• However, board design with weight as a mission-critical factor can benefit from non-standard thicknesses.

After designing the stackup to ensure the correct impedance structures for the various board layers, some designers may pay no heed to the board thickness. After all, engineering design documents guide this aspect of the design, and the layout focuses on the electrical aspects of the board rather than the physical characteristics (so long as this doesn’t affect mating, enclosure mounting, etc.) Yet designers should be aware that standard PCB board thicknesses, though common for a reason, are more a convention than a standard, offering an additional angle of modularity and optimization that can realize significant cost savings on high-volume production lots.

Do I Need a Standard PCB Board Thickness?

The Origins of Standard PCB Board Thickness

In some aspects, PCB board thickness reflects the board's complexity – designers shouldn’t expect to fit a layer count well into the double digits within the standard 63 mil/1.57 mm thickness. However, most low- to moderate-layer count boards will adhere to this informal standard for manufacturing familiarity. The traditional thickness values enshrine some of the earliest DFM capabilities in PCB production: breadboarding that predates PCB manufacturing would use nonconductive 1/16th-inch plywood as the substrate for electronic assemblies. As system designers began to construct multi-board electronic assemblies, there was a push to reduce the number of connections between boards. Since a conductive surface was already present on both PCBs, it made more sense to simplify the connector and make one of the boards a plug (ergo, the birth of edge connectors).

Unsurprisingly, typical board thicknesses besides the 63 mil convention also trace back to plywood thickness, with 93 mils representing 3/32s of an inch, 125 mils representing 1/8 of an inch, etc. That’s not to say these are the only board thicknesses available; manufacturers can construct any thickness from the combination of available substrate thicknesses and well-defined process controls. Design intent and constraints will override any manufacturing conventions, and provided these requests don’t fall into the extremes of board thickness (these may incur extra costs due to yield issues), any manufacturer worth its salt will be happy to work alongside designers for a board that fulfills its stated parameters.

Board thicknesses can deviate from conventional manufacturing values due to design requirements or other production considerations. Foremost, unnecessary thickness in the design adds to material costs and can complicate manufacturing processes (there is a sweet spot here – as manufacturing methods evolved around the 63 mil value, extraordinarily thin rigid PCBs are likely to encounter production issues). It’s important to remember that substrate thickness contributes to dielectric factors and impedance calculations, so “unnecessary thickness” requires thoughtful consideration for signal integrity. However, surplus material can be more than a cost issue: mission-critical weight reductions are necessary for aerospace and similar applications, while boards in high-vibration environments can improve the assembly’s reliability with a less is more approach.

Determining An Appropriate Board Thickness

Designers need not remain beholden to the technology formats of yesteryear (although they remain suitable in many instances today). More than ever, board dimensions and features are customizable to meet the circuit's electrical needs and the enclosure's physical requirements. As form follows function, the best step is for designers to begin at the lowest level of design and work their way up:

1. General circuit density - The conflicting demands of product miniaturization and feature/functionality (and the power, EMI, and signal integrity constraints accompanying them) are the primary factors shaping the stackup. Initial design documents may not indicate a request for layer count/thickness and instead rely on the designer to propose the minimum layer count necessary to produce a layout that meets design intent without sacrificing performance. Designers should begin by assigning/creating footprints to the line items in the BOM before netlisting the design and importing it into the board viewing environment to roughly gauge the placement and routing density per the board dimensions.

2. Board-edge connectors - Mating boards to other boards or system connections requires a particular thickness for some specific edge connector model. The board edge connector will not significantly influence the layout, but adjustments must account for changes to the board itself. Speak with engineers, procurement, and other relevant teams if a change is necessary to ensure design consistency and per-board costs remain satisfactory.

3. Enclosure design - Most production-stage boards require system-level integration for protection and operational goals. The impact on the board design can be minimal in the cases of a simple box enclosure, or they could be a defining element, like in wearables and small, dense electronics (e.g., smartphones). For the former, the constraints of the board relative to the enclosure can affect the thickness of the board, and the latter will change everything from suitable stackup materials to production methods. At the earliest possible point, communication between engineering, design, and manufacturing will need to set the DFM design rules to reduce manufacturing downtime spent on excessive revisions.

Cadence Offers Solutions Through Thick and Thin of Board Design

Standard PCB board thickness is anything but: while manufacturers often fabricate to these familiar values, there is no requirement (short of connector or system integration) that requires them. Depending on the design criteria for the board, reducing the thickness can improve cost and performance, where the added weight of a thicker board is superfluous. Simulation models and prototype boards can offer product development teams some idea of the quantitative benefit to varying board thickness. Cadence’s PCB Design and Analysis Software suite offers a suitably robust design environment for all aspects of ECAD. With the fast and powerful (yet easy to use) OrCAD PCB Designer, the turnaround time for board layout is minimal while still adhering to the producibility of a DFM-led design.

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