Use the Right Off-The-Shelf Metal Cans and Clips to Shield Against EMI/RFI
Today’s circuits swim in a sea of electromagnetic (EM) energy of widely varying intensity and frequency. As a result, EM interference (EMI), radio frequency interference (RFI)—often grouped as electromagnetic compatibility (EMC) topics—are pervasive, related phenomena that affect circuit performance and formal product approval. Although these have been issues of concern since the early days of electronics, they are now presenting increasingly difficult challenges due to the widespread availability of wireless connectivity, use of higher frequencies, more-sensitive circuits, and lower voltage rails.
Interference that affects a circuit can be due to both intentional and unintentional nearby emitters of electromagnetic energy and may be caused by natural or man-made sources. The circuit itself may also emit undesirable or unacceptable EM energy that affects nearby electronics. Among the most common solutions to mitigate EMI/RFI energy problems is to add shielding around the critical parts of the circuit board or even an entire module. During the breadboard and prototype stages, this shielding can be improvised to understand, attenuate, and solve the problem. However, such improvised solutions are not compatible with a manufacturing environment, or with test, debug, and repair stations.
This article identifies the basic challenges of EMC on pc boards, assemblies, and products. It then looks at off-the-shelf shielding solutions from Harwin and how to use them for technical effectiveness and production compatibility.
EMC issues take two paths
Electrical interference energy can travel from a source to a “victim” circuit via conduction or radiation (Figure 1). In the conduction case, the energy travels through conductors such as wires or cables. Designers usually attenuate this energy using ferrite beads, filters, chokes, and other passive components. In the radiated case, the energy path is through air or vacuum going from source to victim, with no metallic conductors.
These unwanted effects can sometimes be reduced by repositioning components at the source or victim, but this is a time-consuming process that is usually impractical, impossible, or ineffective. Similarly, filtering is not a viable option since much of the offending EMI/RFI energy is within the operating radio frequency (RF) band of interest, and such filtering would also reduce the strength of the desired signal as well, compromising system performance.
For some radiated EMI cases, a technique called “spread spectrum” is sometimes used to reduce peak EMI emission at the operating frequency. In this approach, the circuit’s clock is randomly “dithered” around its nominal frequency, as a form of frequency hopping. This spreads the RF energy across the spectrum, but it does not reduce the overall energy emitted (Figure 2).
The spread spectrum approach is considered to be a “cheat” by some designers as it is done primarily to meet emission limits, while others consider it a simple and elegant solution. It is primarily applicable to DC-DC switching regulators where a fixed operating frequency is not critical; but spread spectrum frequency hopping is not suitable for the many situations where carrier and operating frequency stability are critical.
Passive shielding: often the answer
In most EMC cases the offending energy circuit is beyond the designer’s control, yet it must be reduced at the source or the victim. An effective and widely used solution for dealing with radiated EMI/RFI is to add grounded metal shielding around the offending energy source or the victim, depending on the circumstances. This presents two engineering problems:
- Which area(s) of the pc board needs shielding?
- How should this shielding be implemented for a production environment to minimize time to market, cost, and impact on production?
In many cases, the area or areas needing shielding are obvious, such as an RF transceiver section; in others, it will take multiple efforts to locate the part of the circuit that is either emitting too much EMI/RFI or is susceptible to it. To find these areas, designers often build a small, EMI-tight conducting box to enclose and shield the area being investigated. Depending on the product and design, this box may need to be as small as a fingernail or large enough to enclose an entire pc board.
For smaller RF enclosures, it is possible to use thin copper sheeting folded into a box, with the seams either soldered or covered with copper tape that has a conductive adhesive. For medium and larger enclosures, scraps of clad pc board can be cut to the size needed to build the box, with all seams taped or soldered (Figure 3). In some cases, the seams are first “tack soldered” in a few places for basic stability and then covered with the conductive tape.
The box is then placed over the area of the board being evaluated and the seam line between the open bottom and the pc board is soldered to a low impedance RF ground. In practice, this may actually be more challenging than it appears, since the pc board often does not yet have a ground trace corresponding to the perimeter of the constructed can. While a few points of connection may suffice, a more continuous grounded seam means that there is less path for RF leakage into or out of the can assembly.
There’s another concern with this soldered-on approach. Due to the thin tracks of many pc boards, soldering or unsoldering the test can from the boards will likely damage the delicate tracks and ruin the board. Therefore, it’s a good idea to do some measurements of the situation using RF probes and sniffers before building and attaching these shielding cans.
A better prototype shield approach
Fabricating a shielding can using copper foil or copper-clad pc boards does work, but it is a time-consuming process. Also, it requires dealing with the FR-4 substrate (if using pc boards), which is difficult to cut without the right set-up and leaves nasty fiberglass “splinters” in the user’s fingers unless gloves are worn. Even using a bare copper sheet has issues, as it can slice fingers if handled carelessly, and may require access to a small bending brake to achieve proper 90° folds of the edges and corners. What at first may seem like a simple DIY approach to building a shielding test box is not as quick and easy as it appears, although it is certainly doable.
Fortunately, there’s a better solution using the Harwin S01-806005KIT RFI Shield Can Kit. This kit comes with two shield-can sheets etched with a 5 millimeter (mm) square grid, 24 RFI shield clips, and easy-to-follow instructions. To make a basic folded box, just draw a simple diagram of the required box dimension, cut away the unneeded sheet material, and fold the remaining material on the etched lines using a metal ruler as a guide and informal bending brake (Figure 4).
The can is now ready to attach to the circuit board by simply snapping it into the supplied S1711-46R RFI shield clips, which can be reflow or even hand soldered to the board (Figure 5). This is a much better approach than attempting to solder the can directly to the board, and it also allows for easy removal of the can as needed for test, measurement, evaluation, and debug of the “canned” circuit.
Prototype is not production
While DIY cans or the Harwin Shield Can Kit can point to an EMC solution, they are not compatible with high-volume or even low-volume production. Clearly, building a quantity of enclosures from pc board “scraps” or folded sheet copper requires additional production steps and time, and is a non-standard item to put on the bill of materials (BOM). Even if that is acceptable, attaching these to the pc board via soldering along the joint between enclosure and board is a manual operation, unlike the standard reflow soldering of the other components; there’s also a good chance of damaging the board, and removal for test or repair is impractical.
Again, there’s a better approach to solving the problem by using prefabricated RF shielding cans and matching mounting clips from Harwin. These highly RF conductive, unplated nickel-silver rectangular cans are available in a wide range of footprint sizes and heights, from a diminutive 10 mm x 10 mm x 3 mm high (0.394 x 0.394 x 0.12 inches) with 0.15 mm material thickness for the S03-10100300R (Figure 6), to larger cans such as the S01-50250500 which measures 25 mm x 50 mm x 5 mm high (approximately 1 x 2 x 0.20 inch) with a thickness of 0.3 mm.
These cans alone solve only part of the production-friendly requirement. For this reason, Harwin offers a wide variety of clips that are ready for automated placement and can be reflow soldered to the pc board (Figure 7). Cans then snap and unsnap into the clips allowing for easy access to the component for cleaning or rework. The various clips accommodate different board situations in layout, orientation, access, and interference with adjacent pc board tracks and lands, as well as can material thickness.
Micro clips with a profile as low as 0.8 mm (0.031 inch) are available, as well as 90° corner clips designed to address localized eddy interference. Shield clips are compatible with off-the-shelf or custom cans as thin as 0.13 mm to as thick as 1.00 mm.
Factoring in RF attenuation, cooling
There’s a basic fact about solid-surface metal cans surrounding circuit components: they can impede cooling convection airflow from the surfaces of the components they enclose. This might seem to rule out shielding cans in many applications, but that’s not really the situation. The reason is that the metal of the can is quite thin, from 0.15 to 0.3 millimeters depending on specific can model and size. That thinness presents only a small barrier to heat flow via conduction from inside the can to its outside. Once the heat has been conducted to the outside surface, it can be carried away by free or forced-air convection or other means.
In this respect, a thin metal can is far better thermally than a shielded enclosure made of common FR-4 pc board material, which presents a much higher thermal-impedance barrier with conductivity of between 1 and 3 watts/meter-Kelvin (W/m-K) and a standard thickness of 1.6 millimeters. Compare this figure to the conductivity of the nickel-silver, which is about 1000 times higher, and is also much thinner (again, just 0.15 to 0.3 millimeters). Basic thermal modeling can quantify the impact of the thin metal can on cooling. Also, in almost all cases, it is good practice to follow the standard technique of using the underlying pc board copper with its high thermal conductivity to carry away a substantial amount of heat from the mounted components.
One apparent solution to improving thermal convection with shielding cans is to put holes in the can surface. However, this adds a new set of issues. The holes have to be small enough and spaced far enough apart that they do not allow RF leakage. As the maximum allowed diameter and spacing is a function of wavelength, a typical first-order guideline is that any openings should be no more than one-tenth of the shortest wavelength being shielded.
However, deciding on the critical wavelength and thus hole size is not always easy or obvious, as the offending RF energy may be at frequencies that are higher (and thus at a shorter wavelength) than the apparent operating or carrier frequency of the product. Consider that an offending gigahertz frequency signal can overload and saturate a nearby megahertz frequency front-end amplifier. Thus, the maximum hole size allowed would have to be much smaller than dictated by a simple first-pass analysis of the product operating frequency.
Keep in mind that in addition to ensuring circuit performance, another objective of the shielding can and clips may be to provide RF attenuation over a wide frequency range to meet regulatory requirements for the product. These EMC-related regulatory standards define the maximum RFI/EMI that a product can create within the various zones of the RF spectrum, as well as the product’s allowable susceptibility as an EMI/RFI victim, regardless of the nominal operating frequency.
Therefore, shielding must often do more than just assure performance at the obvious operating frequency, but instead may have to also provide attenuation across the broader EM spectrum. Using cooling holes dimensioned only for the nominal operating frequency can reduce the attenuation achieved at those shorter wavelengths and may affect regulatory approval.
Electromagnetic compatibility and issues of RFI/EMI affect nearly all electronic products and applications, and the increasing use of wireless links along with higher frequencies is making the design situation more challenging. The solution to many problems due to radiated EMI/RFI often involves basic RF shielding using a metal can to fully enclose the affected circuitry.
These cans are available as standard items in a wide variety of sizes, along with a selection of pc board clips in various configurations, allowing the cans to be easily attached or removed from the circuit board. These clips are also fully compatible with equipment used for insertion and soldering of SMT packaged components in a volume-production environment.
This article was originally featured at digikey.com on 04-Nov-2020 and is reproduced by kind permission of Digi-Key. Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical web-site manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN.
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