My 5-year-old son has grown increasingly fond of doughnuts. During those rare moments when he isn’t asking for another one to eat, he sometimes inquires about how they’re made and why they’re shaped with a hole in the center. I’ve tried my best to satisfy his curious mind, but I had no idea why doughnuts have holes. Thankfully, I could turn to Google for an answer.
Unless you have a curious child of your own, knowing why a doughnut has a hole probably has little use to you. On the other hand, PCB holes, as mundane as they may seem, are an important element in an electronics design. It’s possible that you too have been Googling for knowledge on these types of holes, and you’ve come to the right place. Understanding the basic design of PCB holes and when you should connect mounting holes to ground could save you some unnecessary trouble after installation.
How PCB Holes Can Be Used For EMI Reduction
As the name implies, PCB mounting holes helps to secure your PCB to an enclosure. Besides that, PCB mounting holes can be utilized for Electromagnetic Interference (EMI) reduction. PCBs that are sensitive to EMI are usually placed in a metal enclosure. For EMI reduction to be effective, the plated PCB mounting holes need to be connected to ground. Any electrical interference will be redirected to ground from the metal enclosure.
The common question that new designers ask is which ground do you connect to? In typical electronics devices, there are signal, chassis and earth grounds. As a rule of thumb, you do not want to connect your mounting holes to the signal ground. Signal ground is the reference point of your electronics components and channeling electrical interference into it can be a bad idea.
The ground that you’ll want to connect to is the chassis ground. This is a meeting point of all the ground connections of enclosures. Chassis ground should be connected at a single point, preferably by a star connection. This is so you can avoid causing a ground loop and multiple ground connections. Multiple ground connections can result in a slight voltage difference and result in current flowing between chassis grounds. The chassis ground is then connected to earth ground for safety measures.
Grounded PCB mounting holes can reduce EMI.
Why It’s Important To Have a Proper Earth Ground Connection
Connecting plated mounting holes to chassis ground is a best practice, but it isn’t the only best practice to adhere to. To ensure that your device is protected, your chassis ground must be connected to a proper earth ground. For example, if you construct an automated parking payment machine that isn’t properly grounded, you might have customers complaining of having experienced an “electric shock” when making their payment. This could happen when the customer touches the non-insulated metal part of the enclosure.
You can also experience minor electric shock when your computer power supply’s chassis is not properly grounded. The same could happen when the ground cable that connects the supply outlet to the building’s ground is broken. This can result in a floating ground at the respective machine.
The principle of EMI shielding relies on a proper earth ground connection. Having a floating ground connection will not only upset your customers if they experience minor electric shocks, but it can compromise their safety if your device short circuits.
Proper earthing is important for safety and EMI shielding.
Basic Tips For Designing PCB Mounting Holes
PCB mounting hole is a non-electrical feature that you may take for granted in your design. There are a few simple rules that guide me when I’m placing mounting holes. You should pay close attention to the coordinates of the mounting holes. Making a mistake here means that your PCB won’t be mounted properly in its enclosure. You’ll also need to ensure that the hole size is adequate for your choice of screws.
I make it a habit not to place mounting hole too far along the edge of the PCB. Having too little dielectric material at the edge can lead to cracks on the PCB during installation or removal. You should also allow sufficient clearance between the mounting holes and other components.
Have questions about EMI shielding and mounting holes? Talk to an expert at Altium.