The birth of conventional PCB thicknesses.
How building a board from base materials impacts the thickness and other parameters.
The impact of PCB thickness on the aspect ratio and how to mitigate this in HDI designs.
PCB thicknesses can vary depending on the requirements of the job, but may also fall into common variants.
The common PCB thickness of 63 mils/1.57mm is a convention, not a standard; there is no particular reason for its high rate of occurrence (although a particular stackup may achieve the best results from a board at this thickness). Designers and manufacturers simply settled upon it as the most agreeable touchpoint for PCB thickness.
This common PCB thickness traces its origin to a seemingly unexpected source: plywood. At the onset of the transistor era of electronics, engineers would design proto-PCBs using a thin synthetic plastic on top of single-ply plywood, with a measured thickness of 1/16”, or 62.5 mils to be exact. Since it is merely a convention, PCB thickness can and does vary significantly depending on the needs of the board.
Comparing Printed Circuit Thicknesses, Materials, and Stackup
Max layer count*
FR4 (glass fiber with expoxy resin)
FR4 /polyimide / polyester (PET)
*Will depend upon manufacturer capabilities
The Material Building Blocks of PCBs
Form factors for electronics are rapidly changing. On one hand, consumers demand thinner and lighter devices for portability. On the other hand, HDI designs for increased functionality and performance require additional layers for signal routing. In other words, the common 63 mils/1.57mm PCB thickness, while still ubiquitous, is becoming less common as devices have to become more specialized.
It’s important to understand that PCB thickness should foremost serve a purpose to the board and final design. A thinner board profile bumped up to 63 mils/1.57 mm requires extra material costs to do so, yet this may be a requirement for interfacing with a 63 mil edge connector. Absent this constraint or any other common edge connector variants (e.g., 93 mils/2.36 mm), board thickness should adhere to the minimum thickness that fulfills all its necessary electromechanical conditions.
PCB thickness is a direct function of the dielectric requirements and physical design of the board. After conversing with the engineer and manufacturer, designers will want to begin with the board layer stackup. It is here that designers will use a field solver to determine the physical parameters of the conductors for different target impedances. Depending on the stage of the product’s development, layout designers may have additional latitude in determining the board thickness. However, it is more likely that design documents have already called out the absolute thickness of the board. The task for layout then is to create a balanced layer stackup with given constraints.
Board thickness will depend on the core construction elements of the board:
- Copper-clad laminate - A prepreg sheet with copper foils bonded on both sides. Also known as cores when designing the stackup of a PCB, this material provides a dielectric sandwiched between two electric layers for signal routing or plane design.
- Prepreg - Prepreg alone can be used to increase the thickness of the board without adding electrical layers to the layout design process. This has several effects like decreasing the capacitance between layers of a board while increasing the inductance with a larger loop area (and vice versa). Additional factors, such as minimum thicknesses to avoid a dielectric breakdown in high-voltage electronics, can also be a consideration.
- Copper foil - Cores distribute two neighboring electrical layers, so certain layer counts may require only a single layer to be placed for the top and bottom. Copper foil can instead be bonded to the prepreg to make a single-sided laminate. The thickness of the foil can also vary depending on current and thermal properties: a thicker copper can more effectively carry large currents (especially at DC/low frequencies) as well as dissipate more heat.
How PCB Thickness Affects Via Reliability
The integrity of vias and other plated through-holes (PTHs) are intimately linked to the board thickness. As boards increase in the z-axis, the minimum diameter opening for through-holes has to increase as well. The aspect ratio relates the measurements of the hole depth to its diameter, and PTHs should exhibit an aspect ratio of 10:1 (and sometimes less for high-reliability purposes). The problem arises due to the mechanism of through-hole plating: holes with insufficient aspect ratios will have plated copper concentrated towards the hole openings with less material deposited at the vertical center of the hole. This imbalanced plating can result in thermal failure due to CTE mismatches between the via barrel and that of the surrounding material, which forms high-stress regions.
For designers, the aspect ratio means one thing: less space for routing. In HDI designs, this can be a non-starter. After conversing with the fabricator, designers may need to look into alternates to standard through-hole vias to satisfy the routing without sacrificing manufacturability and reliability. With a 1:1 aspect ratio and a 10 mil depth, microvias are an excellent solution to the issues caused by standard via fabrication in HDI boards. Instead of drilling through the entire width of the board, microvias are controlled-depth drilled to selectively drill between two layers at a time.
To span the width of the board, microvias can be placed consecutively:
- Stacked - Microvias are placed one on top of another (i.e., their x- and y-position don’t change). This is analogous to a standard through-hole via.
- Staggered - The x- and y-position of same-net microvias is not fixed. Designers can place microvias anywhere in the plane provided a trace or another copper feature is connecting them.
While staggered microvias allow for greater flexibility in routing, signal integrity can suffer with longer return paths. Alternatively, stacked microvias conserve more space, but are also more difficult to fabricate and less reliable. Any microvia structure will greatly complicate the fabrication process and are significant cost adders, yet they can offer unparalleled miniaturization for thick HDI boards.
Cadence Can See You Through the Thick of PCB Design
As rigid boards become more complex with greater functionality, PCB thickness as well as related DFM practices will similarly grow in importance. Design teams will need to utilize every tool at their disposal to optimize space, stackup performance, and cost. To prepare for manufacturing, Cadence’s PCB Design and Analysis suite offers a package of utilities for comprehensive simulation and modeling of boards and supporting design features before entering production. Results can then be seamlessly imported to OrCAD PCB Designer to realize designs with accelerated turnaround times.
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