Reducing electromagnetic interference (EMI) is a key challenge when designing electronic devices. In this article, we will take a look into EMI challenges, the role of board-level shields (BLS) in reducing EMI, and the key criteria to keep in mind when selecting BLS.
EMI Issues
EMI is the crosstalk between different parts within a system that can interfere with each other. EMI affects operations, and proper EMI management provides FCC compliance for products to be sold in the marketplace. In a laptop or tablet for example, you have a CPU and a radio for wireless access, and these can interfere with each other. Memory devices also radiate noise that can interfere with a wireless device’s performance. When EMI is present, it can reduce the clarity of a Wi-Fi or Bluetooth radio. Another example is switching power supplies, which radiate EMI and affect the proper operation of memory devices.
EMI has always been a design issue, but it is an increasing problem due to the trends like the Internet of Things (IoT) demanding smaller, higher-functioning devices. Newer devices also tend to operate at higher frequencies with wireless components moving from 800 or 900 MHz a few years ago to 5 GHz today.
EMI Solutions
An old approach to minimizing EMI was to physically separate an EMI source from other components. But with end products getting smaller and smaller, this approach isn’t possible anymore.
Another approach is to use complex PCB designs or filtering to reduce noise. For example, in higher-frequency devices (10-20 GHz) you can use microwave absorbers. A microwave absorber is designed to absorb radio frequency and convert it to heat. There are both narrowband and broadband absorbers. Designers use the narrowband type to absorb one specific frequency while they use a broadband absorber to absorb a range of frequencies. The problem with using complex PCB designs or filtering, however, is that it can be very expensive.
The more common alternative is to deploy a shield around a noisy component. A BLS is basically a five-sided, stamped piece of metal that leverages the PCB ground plane to create a faraday cage around the component. Most BLS are custom designs around a specific IC or area of a customer’s system, but there are also standard designs for standard components or standard area sizes.
Custom designs are a little more expensive because of tooling costs, and they extend time to market because it takes four to six weeks for them to be manufactured, whereas the standard BLS designs are ready off the shelf and don’t include tooling costs. If designers plan for shielding when designing a PCB, they can define a board area early on or use components that are compatible with standard BLS products. One issue, however, is that standard BLS products may limit the design due to the height of the shield – many standard BLS parts are too tall for certain applications such as tablets or cell phones. TE Connectivity offers both standard designs and custom solutions for BLS.
Using a BLS
There are two basic approaches to using a BLS. You can either use it to contain a noisy component by putting a shield around it to contain the radiating frequency, or you use it to protect a component, putting a shield around a component you want to isolate from EMI.
For example, if a radio is next to a memory device and you want to eliminate EMI from the radio from impacting the memory device’s operation, you shield the memory device to protect it because you don’t want to reduce the radio’s performance by shielding it. Another approach is where you have a power supply, for example – you don’t want the power supply to radiate EMI and impact other components, so you contain the power supply with a BLS.
Criteria for choosing a BLS
When selecting a BLS for your application, there are four key criteria to consider. The first is accessibility. Should the component being shielded be accessible? If so, designers should use a two-piece shield with a four-sided frame and a cover that can be removed. If accessibility is not an issue, designers can use a one-piece, five-sided shield.
The second consideration is the operating frequency of the component you’re planning to shield. Higher frequencies require shields with small aperture sizes (slots or seams in the metal shield), while lower frequencies can work with shields with larger aperture sizes. For example, at a low frequency such as 800 MHz, you can have hole size 4 times larger than at a high frequency like 4 GHz and still achieve a similar range of shielding effectiveness.
The third thing to consider is weight. When designing a cell phone, tablet or laptop, for example, you often want to minimize weight, so you might choose an aluminum shield rather than one made of heavier cold rolled steel. There are also shields made of other materials, like nickel silver, but they cost more than the standard materials.
Heat dissipation is another concern. When you shield a component, you can create thermal issues for the component you’re covering. If you’re trying to shield a high-end CPU, for example, heat will become an issue and the component won’t operate correctly. The solution to this issue is to use aluminum rather than steel BLS products. Aluminum has very high thermal conductivity, so you could use it as a heat spreader to pull heat away from a component.
EMI is a fact of life with electronic components, and in an era of smaller and smaller products, BLS are a standard method of dealing with it. By considering the need for shielding as you design a PCB, you can use off-the-shelf BLS products that lower costs and reduce time to market.
Vu Lac is the Staff R&D/Product Development Engineer within TE Connectivity’s Data and Devices business unit. He earned a B.S./B.A. degree in Electrical Engineering with a minor in Mathematics from the University of San Diego. Vu has nearly ten years of experience developing EMI/RFI shielding solutions for the datacom, consumer, industrial and automotive markets. He has worked with companies such as Microsoft, Cisco, Itron, and Honeywell to design board level shields for their EMI/RFI reduction requirements.
