EMI Shielding on Assembled Boards

Why Shielding Is Added at Assembly

Electromagnetic interference is a system problem, but the first line of defence is often at the board. A shield placed over a noisy circuit keeps its emissions inside, and a shield over a sensitive circuit keeps external fields out. Board level shielding is used when layout, grounding, and filtering alone cannot meet the emission or immunity requirement, or when the enclosure design leaves a circuit exposed.

The decision to shield is usually made late in a project, after an emission test has failed. That timing is unfortunate, because the shield has to be grounded to the board, and the ground structure should have been planned in the layout. A shield added to a board without a proper ground perimeter will not perform as expected regardless of how good the metal is.

Shielding also adds cost and assembly steps. The shield itself, the placement, the soldering or the mechanical attachment, and the rework difficulty all add to the price of the board, so it should be a deliberate design choice rather than an afterthought.

Shield Cans and Frames

The simplest shield is a metal can, stamped or drawn from a sheet, that is soldered or clipped over the circuit. A one-piece can is economical and provides good attenuation, but it is difficult to rework because removing it damages the solder joints and the can.

Two-piece shields separate the frame from the lid. The frame is soldered to the board and stays in place, while the lid clips on and can be removed for inspection or rework. That is the common choice for production boards where access is needed, and it costs slightly more and takes slightly more board area.

The metal and its thickness determine the attenuation, but the openings matter more than most people expect. A ventilation hole or a slot in the can will leak at frequencies whose wavelength is comparable to the opening, so the number, size, and orientation of openings should be reviewed against the frequencies the circuit emits. Where ventilation is required for thermal reasons, a pattern of small holes performs much better than a few large ones.

Conductive Gaskets and Fabric Over Foam

A shield that does not make continuous electrical contact with the ground plane leaks around its edges. Gaskets, made from conductive fabric over a foam core or from a conductive elastomer, fill the gap between the shield and the board or between the board and the enclosure.

The gasket must be compressed to work. Its performance depends on the compression force and the contact area, so the mechanical design has to hold the gasket in compression without over-compressing it. A gasket that is only lightly touching provides little shielding, and one that is crushed beyond its range loses elasticity and stops working.

Gasket placement should follow the ground structure on the board rather than the outline of the enclosure. The gap between the gasket and the nearest ground via should be short, because a long path from the gasket to the ground plane acts as an antenna and reduces the effectiveness of the shield.

Grounding the Shield

A shield is only as good as its ground connection. The frame or can should be grounded at many points around the perimeter, with vias to the ground plane placed close to the attachment points. A few widely spaced grounds leave long return paths and create resonances that can make the shield worse than no shield at some frequencies.

The ground plane beneath the shield should be solid, without slots that cross the return path. A split in the plane under a shield forces the return current around the split, which increases the loop area and the emissions. Where the plane must be split for other reasons, the shield should be grounded on the side that carries the return current.

The attachment method also matters. A soldered frame gives a continuous low-impedance connection; a clipped or spring-loaded shield gives a mechanical contact that can degrade with oxidation or vibration. Where the shield will be assembled in the field or removed for service, the contact design should be tested for stability over the product’s life.

Coating and Shielding Interaction

Conformal coating and shielding interact in ways that are easy to overlook. A coating on the ground pads prevents the shield frame from making electrical contact, so those areas must be masked before coating. The mask should follow the ground perimeter exactly, because a coating that creeps onto the contact surface creates an intermittent ground connection.

Where a shield covers a coated area, the coating is trapped under the can and its cure is affected by the restricted airflow. The cure schedule should be developed with the shield in place, or the coating should be fully cured before the shield is attached. A coating that outgasses under a shield can deposit volatiles on the components it was meant to protect.

Coating can also be used deliberately to improve shielding in some designs, where a conductive coating is applied to the inside of an enclosure. That is a different technique from board level shielding, but it shares the same grounding requirement: the conductive coating must be bonded to the ground structure to be effective.

Verification and Testing

Board level shielding is verified by measuring the emission or the immunity of the assembly with and without the shield, and by confirming that the shield is properly grounded. Near-field probes can locate the frequencies that dominate, and a spectrum analyser can show whether the shield reduces them.

Production verification is usually simpler. The shield’s presence, orientation, and seating are confirmed by visual inspection, and the ground contact can be checked by a resistance measurement between the shield and a known ground point. That measurement catches an oxidized or missing contact, which is a common cause of a shield that worked in the lab and does not work in the field.

The assembly process also needs to protect the shield. A shield that is dented during handling, or a frame that is bent before soldering, will not seat correctly and will leave a gap. Handling and packaging should be defined so that the shields reach the board in the condition the process assumes.

FAQ

When should board level shielding be used? When layout, grounding, and filtering cannot meet the emission or immunity requirement, or when a circuit is exposed by the enclosure design. It is most effective when the ground structure is planned in the layout.

One-piece or two-piece shields? One-piece cans are cheaper and provide good attenuation but are difficult to rework. Two-piece frames with removable lids cost more but allow access for inspection and repair.

Why does my shield not reduce emissions? Common causes are an incomplete ground perimeter, a coating on the contact surfaces, and openings in the can that leak at the frequency of interest. The ground connection is the first thing to check.

Do gaskets matter if the shield is soldered? They matter where the shield meets the enclosure or where a removable lid is used. A gasket that is not compressed or that is placed far from ground provides little benefit.

How is shielding verified in production? By visual inspection of the shield and a resistance check between the shield and a ground point, supported by emission measurements during qualification.

Conclusion

Board level shielding works when the metal, the grounding, and the assembly process are designed together. A continuous ground perimeter, a shield that seats properly, gaskets that are compressed against the right structure, and masking that keeps the contact surfaces clean together determine whether the shield performs in production as it did in the test chamber. For related topics, see our notes on PCB design and layout, PCB assembly, conformal coating, and quality management for how shielding is integrated in 2026.