Take a brief look at many PCB manufacturer websites, and you’ll likely see a standard thickness advertised repeatedly. The 1.57 mm board thickness is something of a standard in the PCB industry, and it can cause one to wonder why many standard values, such as 50 Ohms impedance, are used in PCB design. The reason standard PCB sizes were chosen is largely historical, and the 1.57 mm board thickness has remained popular to this day.
If you work with a reputable and experienced manufacturer, you’ll have access to a broader range of standard PCB sizes than 1.57 mm. The panel size is also important to consider during design because this determines your throughput per panel. If you need a customized board thickness with precisely controlled laminate thickness for controlled impedance, a reputable manufacturer can give you access to a range of laminate options for your custom stackup.
The “Standard” PCB Sizes and Thicknesses
The story behind the standard PCB thickness is an interesting one that stretches back to the commercialization of integrated circuits. When electronic devices were transitioning away from discrete components and integrated circuits were being highly commercialized, PCBs were typically laid out by using a sheet of plywood as a breadboard.
The plywood functioned as a sturdy, insulating substrate that doubled as a workbench. The other advantage of plywood is that the top layer of plywood could be replaced with a material called Bakelite. It just so happens the standard thickness of a single ply in plywood is 1/16 in., or 1.57 mm.
Once board-to-board connectors became common for building multi-board systems and placing boards in rack-mount units, this thickness became a de facto standard within the industry, and subsequent standard PCB sizes evolved around this particular thickness. There is no specific reason for this particular board thickness to be used in place of any other board thickness other than the fact it has become a de facto standard. All that has changed since the early days is the materials used for PCB substrates.
Although many fabrication houses will likely advertise their ability to produce 1.57 mm boards, the only reason to use this specific board size is if you are designing to some standard (e.g., DDR, PCIe, M.2, or another computing interface). Other standardized interfaces, such as Mini PCIe, use 1 mm thick boards instead of 1.57 mm.
Other popular board sizes include 2.36 mm and 3.18 mm, which just happen to be 150% and 200% of the 1.57 mm board thickness, respectively. A standard four-layer PCB stackup can be easily extended to six layers with 2.36 mm thickness, or eight layers with 3.18 mm thickness, by simply adding new prepreg/core layers to the stackup.
Standard Panel Sizes
Most designers target standard board thickness values, but boards should also be designed to target standard panel sizes. The most common standard panel sizes are 18” × 24”, 18” × 12”, 9” × 24”, and 9” × 12”. A ½” clearance around the panel border is required for handling panels, although some manufacturers will extend this required clearance out to 1” for multilayer boards. There also needs to be some space between boards on a panel to allow for manual removal or automated routing.
The panel size will limit the number of boards that can be reliably produced per panel. This means standard PCB panel sizes should be considered when floorplanning your board to maximize your yield per panel. Something as simple as rotating your boards or slightly modifying the board dimensions can help you produce a few extra boards per panel and reduce your overall costs.
An example is shown in the image below. In this example, the board size was reduced by a small amount on each side, which provided enough area to produce two extra boards per panel. If you need to produce a mix of different board sizes with the same thickness, you could place multiple boards onto the same panel with different arrangements to try and maximize your throughput per panel. There are several tools you can find online to help with this, or your manufacturer can help you layout a panel before production.
A minor decrease in board size and rotating the boards allows two more boards to be produced per panel.
If you need something other than the standard PCB sizes for your stackup, or you are producing boards with odd shapes, you should consult with your manufacturer to obtain the laminate thickness you need and determine the panel size. A reliable manufacturer can obtain a variety of core and pre-preg laminate thicknesses for advanced custom stackups.
At VSE, we have experience working with the standard PCB sizes mentioned above for rigid boards, as well as other sizes based on laminate thicknesses for custom stackups. We also work regularly with rigid-flex and flex assemblies. We work with our customers to target common pre-preg and core layers available from our suppliers. Our quick turnaround PCB assembly services provide maximum flexibility and help minimize costs and cycle times. Contact us today to learn more about partnering with us for your next project.
