Mounting holes seem simple enough—they let you mount your PCB to an enclosure or a surface. Simply pick a screw size that makes sense for the size of your board and the surface you wish to mount it to, and drill accordingly.
But as with everything in PCB design, things get a bit more complicated when you start adding high speed signals and smaller form factors to the mix. Suddenly every conductive surface has an impact on your EMI footprint, and that includes your mounting holes. Tolerances have to be respected to ensure that you leave enough clearance for all the components, vias, and traces that must fit onto your board.
If you had that gnawing feeling in your gut that there may be more to mounting hole design than meets the eye, you’ve come to the right place. Here’s everything you need to know about drilling PCB mounting holes.
Supported: Plated mounting holes which generally get tied to the ground plane.
Unsupported: Non-plated mounting holes which are never connected to a ground plane layer and require an outer keep-out zone to isolate it from other components and traces.
In addition to plating, mounting holes can also have an annular ring or copper pad that may or may not have its own ring of vias that also connect to the ground plane.
EMI shielding of course. Connecting ground to chassis via mounting helps turn the whole system into a Faraday cage.
As for that optional ring of vias, their purpose is to ensure that the connection to ground remains solid even if a screw thread happens to strip the copper plating from the main mounting hole.
The PCB aspect ratio is simply the ratio of the depth of a PCB hole over its diameter. It’s purpose is to quantify the difficulty of plating a hole in a PCB.
The larger the aspect ratio, the more difficult it is to plate. This is due to a tendency for capillary action to cause plating to be thicker at the terminating edges of the barrel leading to a thinner center. If the center of a plated hole is too thin, its structural integrity is compromised, and it can crack due to thermal or mechanical stress.
When designers normally talk about aspect ratios, they’re usually thinking of the conductive vias that link multiple layers in a board together. But if you’re planning on plating your mounting holes, it pays to pay attention to this number when selecting a fabrication house.
When it comes to the mounting hardware itself, your needs will largely depend on the dynamic environments that your PCB will be subjected to. That said, here’s a quick list of common hardware used to mount PCBs:
Philips head screws are the most common type of screw head you’ll encounter: the iconic plus impression on the head fits to a screwdriver.
Hex nut bolts have a hexagonal head that can be gripped by a wrench instead of a driver. This avoids a problem with phillips head screws which can be stripped by a driver.
Flat washers help evenly distribute the load of a threaded fastener such as a bolt or screw.
Lock washers prevent screws from loosening due to vibrations from the environment.
The requirements and standards will vary greatly with application and industry. There’s a big difference between the dynamics encountered by a programmable pressure cooker and the avionics box on a rocket. Refer to your industry mounting standards for best practices to incorporate in your design.
In this post we focused on PCB mounting holes because of the relative lack of coverage seen across the web. But when you add the countless vias and through holes that links your PCB’s components and multiple layers together in a circuit, the sheer number of holes that need to be drilled can be overwhelming.
Fortunately, Cadence’s suite of PCB design and analysis software has all the tools you need to map out holes across your board. From customizable symbols to drill charts and legends, Cadence has everything you need to prep your bare boards for drilling.