So you’ve just completed your PCB design, and you’re about ready to send those CAD files over to the fabrication house. But there’s just one problem—you’ve designed a new form factor with unusual specs, and you’ve got a nagging feeling that you should probably check those dimensions and tolerances before you pick a manufacturer.
But where do you start? Which numbers are most important to consider when designing for manufacturing? In this post, we’ll walk you through the different dimensions and tolerances to consider for the manufacturability of your PCB design.
Board size will have a direct impact on the cost of manufacturing your PCB. Here’s a quick list of the bare board dimensions that are critical to the fabrication house:
Board size (LxW): At the end of the day, your PCB will have to be cut from a larger slab of material. For single board processing, the minimum size of the longest edge of a single board acceptable by most manufacturer conveyors is 2.0”. For smaller boards, panelization is typically required.
Number of layers: The more layers you have the more your board will cost. 1-2 layers is pretty standard, with some manufacturers going as high as 20 layers or more.
Thickness: Generally, there are thickness requirements associated with the board itself and its individual inner layers. 0.020” of inner layer clearance is pretty standard, with premiums available for tighter tolerances.
If your PCB is towards the extremes (large or small) of the form factor spectrum, it might be a good idea to check the minimum and maximum board size requirements of the manufacturer you want to use. The minimum and maximum dimensions of the board itself may also be affected by increasing the number of layers.
Panelization helps manufacturers keep costs down by allowing you to organize multiple boards on a single panel for processing through a manufacturing line. Here are the important dimensions and tolerances you need to know to incorporate panelization into your design:
Panel size: 18 x 24 inch panels are standard, with ½ inch perimeter of clearance for double sided boards. Such a panel might fit a maximum single board size of 16 x 22 inches.
Routing/Scoring clearances: There’s a ± 0.010 inch tolerance on board outlines and internal cutouts, with 0.100 inch spacing between individual PCBs for tab rout spacing.
Both you and your manufacturing house benefit from eliminating the amount of wasted material on the panel. You can choose the panel for your PCB design after picking one of the standard formats available at the fabrication house.
Drilling holes and vias in PCBs
There are a number of dimensions and tolerances to consider when drilling any holes in your PCB:
Drill diameters: Drills are often manufactured in increments of 0.05 mm (or 2 mils). You’ll want to be familiar with both imperial and metric units, as well as the drill gauge system.
Production hole oversize: If you’ll be plating your througholes, it’s a good idea to oversize the drill a little to account for the thickness of the plating (e.g. 4 mil or 0.10 mm for PTH).
Common hole tolerances: You’ll generally be fine going with the minimum hole size recommended by your fabrication house for vias, typically 0.3-0.4 mm
Conductive pads and traces
The conductive parts of your board usually have minimum clearance requirements for adhering to EMI/EMC considerations. Here are some important dimensions to consider when placing pads and traces.
Copper trace width/spacing: The minimum air gap between copper features and the minimum trace width are both on the order of 0.003 to 0.010 inches. You can expect to pay a premium for spacings or widths less than 0.007 inches.
Pads and annular rings: Pad sizes are often driven by industry requirements. Depending on the tolerance your manufacturer needs, you can optimize drill and pad sizes for an ideal amount of annular ring. Annular ring is the amount of copper that’s measured from the outside of the pad to the inside of the drill hole. Simply, subtract the drill size from the pad size and divide by two. Be sure to factor in some margin, as vias can be drilled off center due to wandering drills and cause issues like breakout.
Manufacturability starts with CAD
From adhering to EMI/EMC clearances on electrical components, to the careful placement of vias and traces across a multi-layer board, PCB design can get quite complex. If you want to be sure your designs will make it on the manufacturing floor, PCB design software can help.