Board Level Shielding and RF Cans

When a Shield Is Needed

A board level shield, often called a can, is a metal cover over a section of the circuit that confines the field of an emitter or protects a sensitive receiver. It is used when the layout, the filtering and the grounding have not achieved the required isolation, or when a radio has to coexist with a switching supply on a small board. A shield is effective and it is expensive: it adds a component, a placement or a soldering operation, it occupies height and it complicates the rework. The decision to use one should follow from a measurement or a calculation rather than from a habit of covering anything that looks like an analogue circuit.

What a Shield Does and Does Not Do

A metal can reflects and absorbs the field, so it reduces the coupling between what is inside and what is outside. It does not, by itself, stop a signal that leaves on a trace or a via that passes through it, and it does not help if the interference is conducted rather than radiated. Its effectiveness depends on the can’s material, its thickness and, most importantly, on its bonding to the ground. A can that is held by a clip with a poor contact behaves as an antenna rather than as a shield, which is a common reason a shield fails to solve the problem it was added for.

Fences, Clips and Frames

The common construction is a fence, a metal frame soldered to the board along a path that surrounds the circuit, with a clip on top to hold the cover. The fence defines the shield’s boundary on the board, and it has to be continuous except where the traces must pass. The clip holds the cover and provides the electrical contact, and its force and its material decide the bonding’s quality over time. A one piece can with a soldered flange gives the best bond and is the hardest to rework; a clip on frame is easier to service and depends on the clip’s contact. The cover may also have a mesh or a vent for thermal reasons.

The Layout Inside the Shield

What is inside the shield matters as much as the shield. The ground plane under the circuit should be continuous and should connect to the fence along its whole length, not only at the corners. The traces that cross the boundary should do so at a point where their return also crosses, and each crossing is a potential leakage path that should be minimised; a filtered feedthrough is often used. The components inside should be placed so that the noisy one is not adjacent to the sensitive one, since the shield reduces the external coupling and not the internal. The can’s height should clear the tallest component with a margin.

Material and Plating

The can is usually a stamped steel or a plated metal. The material and the plating determine the shielding’s effectiveness, the solderability of the fence and the corrosion resistance. A tin plated steel is common; a material with a low conductivity needs a thicker section to achieve the same attenuation. The fence’s soldering to the board requires a solderable surface and a footprint that the process can handle, and the soldering of a long, thin fence is a thermal challenge that the profile has to accommodate. Where the can is intended to be removable, the contact surfaces should be designed for the number of cycles the product will see.

Thermal and Mechanical Considerations

A can traps heat, so a component inside one runs hotter than the same component in the open. The thermal design should account for that, and the can may need vents or a thermal path to the board or the enclosure. Mechanically, the can adds stiffness and mass, and its attachment has to survive the vibration and the handling; a large can with a few clips can lift at a corner and lose its bond. The can’s removal for rework has to be planned, since a soldered fence that is desoldered repeatedly damages the board’s pads and can loosen the surrounding components.

Verification

The shield’s effectiveness should be verified by measurement rather than assumed. A near field probe scan before and after the shield is fitted shows the reduction in the field, and an emission or immunity measurement on the finished product shows whether the requirement is met. The contact resistance of the fence and the clip should be checked on a sample, since the bond is the shield’s weak point. Where a shield is added to solve a problem that was not diagnosed, the measurement may show that it did not help, which is the outcome that a diagnostic approach avoids.

Shielding Without a Can

A can is not the only way to reduce the coupling. Increasing the separation between the aggressor and the victim, keeping the victim’s trace over a continuous plane, slowing the aggressor’s edge, filtering the power at the source and improving the grounding all reduce the field before a shield is considered. Where a small reduction is enough, one of those measures achieves it at a fraction of the cost of a can and without the height, the weight and the rework penalty. The sequence should be to diagnose the coupling path, correct the layout if the correction is available, and use a can only where the geometry leaves no alternative.

FAQ

When is a board level shield needed? When layout, filtering and grounding have not achieved the required isolation, or when a radio and a noise source must coexist closely.

What does a shield not fix? A conducted path, a trace crossing the boundary, or a coupling between two circuits inside the same can.

Why is the bonding so important? A poor contact makes the can behave as an antenna, so the shield can make the problem worse.

What is a fence and clip? A soldered frame that defines the boundary, with a clip that holds the removable cover and provides the contact.

How is a shield verified? By a near field scan before and after, and by the emission and immunity measurements on the product.

Conclusion

A board level shield is effective when it is bonded properly and diagnosed from a measurement, so treat the fence’s contact as the critical feature. Confirm the result with a scan. Shielding practice belongs to PCB assembly, the boundary design is part of PCB design and layout, and the verification is part of PCBA testing. Shielded designs are first built during prototype PCB assembly in 2026.