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Why do Devices Fail EMC Testing?

Why do devices fail EMC testing? If you’re a product designer, you may have wondered this very thing—especially if your device has failed and has had to undergo a mitigation process to pass a second round of testing. There are various reasons that a device may fail EMC testing, but we’re going to discuss the most common reasons below.

It’s critical that product designers understand basic EMC principles, even if they don’t entirely understand EMC engineering, because it can save your company thousands of dollars and lots of wasted time. When a device undergoes EMC testing, engineers determine whether or not the device’s emissions will affect other devices (Radiated Testing- Emissions) and whether or not the device is susceptible to surrounding EMI (Radiated Testing- Immunity). Electromagnetic interference (EMI) is a disturbance that causes the malfunction of electronic equipment. While some EMI does not pose a significant threat—such as static noise on the radio—EMI that affects critical infrastructure, military assets, and medical equipment can pose economic and, sometimes, life-threatening risks.

PCB Design

So why might a device fail EMC testing?

The first and most common reason is poor PCB design. When designing a PC board with EMC in mind, a designer must consider current flow and high frequency signals. According to circuit theory, currents flow in loops from the source to the load and then back to the source. Sometimes, EMC testing failure can occur due to poorly defined or broken return paths. You must remember to define a return path for high frequency signals ( > 50 – 100 kHz) because undefined return paths can result in large current loops from the source to the load and back to the source. Minimizing the area of these loops will effectively minimize EMI.

Breaks in the return path often cause radiated emissions and susceptibility. A break in the return path forces the conduction current to find the nearest (low impedance) path back to the source, causing the electromagnetic field to leak out.    

Another important consideration is PCB stack-up. A six-layer board stack-up should include signal layers with adjacent return planes as well as adjacent power and power-return planes.  Without an adjacent return plane, the propagating wave return path will find the closest metal to take it back to the source.

Cables, Connectors, and Shielding  

Radiated emissions are often caused by cable radiation. Typically, designers will attach connectors to the circuit board and push them through holes in the shield, which causes EMI when the cable is plugged in. Pushing connectors through the shield defeats the purpose of the shield. Designers have trouble with shielded enclosures because it is difficult to penetrate them with power or I/O cables without causing leakage, but EMI gaskets and other bonding techniques can help. Another way shielding effectiveness is reduced is by using pigtail connectors, which introduce high impedance.


SSN (simultaneous switching noise) occurs when output devices are partially turned on at the same time, causing a large current pulse to shoot between the supply rail and power return pin of the IC. This happens when the output stage of a digital IC switches from low to high and vice versa. SSN is often accompanied by noise and propagates throughout the PCB, causing EMI. An efficiently designed PDN (power distribution network) that minimizes the time it takes to transfer energy from the power source to the IC will effectively minimize SSN. To do this, the PDN must first be as short and direct as possible. Use capacitors that can handle a large amount of energy—and to tackle the remaining energy, use decoupling capacitors with a minimal amount of series inductance. For the lowest series inductance, mount the decoupling capacitors near the IC.  

By considering the above issues, you can help prevent EMI and ensure your device passes EMC testing. Other important steps to take that can minimize your chances of failing EMC testing include EMC Design and Pre-Compliance Testing. EMC Design entails designing a product with EMC in mind. You can find an informative article on why EMC Design is so important here. After designing your product, the next step is to perform pre-compliance testing in your own facility. This will help illuminate EMI issues and allow you to mitigate them before you take your product to a testing lab. You can read about how to perform pre-compliance testing here.

If EMC is an all too foreign topic to you and EMC Design and Pre-Compliance Testing aren’t in your scope of skills, consider using Rhein Tech’s consulting services. Rhein Tech’s engineers will evaluate and then discuss your product’s design pitfalls with you before testing. And unlike other testing labs, if your product fails EMC testing, our engineers will illuminate and help mitigate EMI issues so you pass the second time. Let us save you money and time; request a quote from us today! Source:

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