Shield Can Assembly: Footprint and Solder

The Can as an Assembly

A shield can is a mechanical part that is also an electrical one, and getting it onto a board reliably is an assembly problem rather than a layout problem alone. The can is a large, thin, flat metal part that has to sit on a ring of solder along its full perimeter, be held flat while the solder is molten, and end up bonded continuously enough that the shield works. The two failure modes are a can that lifts at one corner, which leaves a gap that radiates, and a can that is pressed down so hard that it bridges two segments of the ring and shorts two nets together.

The Footprint Ring

The footprint is a ring of pads or a continuous strip, and its width determines how much solder can be deposited and how much the can can move without losing contact. A very narrow ring prints a thin deposit that may not form a continuous fillet, and a very wide one consumes board area and increases the risk of bridging to nearby traces. The ring should also be segmented where the shield needs to be a single electrical node, since a continuous ring of pads that are not all connected to the same reference produces multiple paths with different impedances. Where the can has fingers rather than a continuous wall, the pads have to align with the fingers and the pitch has to be small enough for the frequency, which is a layout requirement rather than an assembly one.

Stencil Apertures for the Fence

The stencil aperture for a can ring is long and narrow, and that shape has an area ratio that is often marginal. A long aperture also suffers from poor paste release at its ends, and the deposit tends to be thinner in the middle of a long line than at the ends, which produces an uneven fillet. The practical responses are to split a long aperture into a series of shorter ones with small gaps, to reduce the stencil thickness for that area where a step is available, and to accept a slightly lower deposit than a component pad would use. Where the aperture is split, the gaps should still be small compared with the wavelength of interest, since the gaps in the solder become gaps in the shield if the can does not bridge them.

Solder Volume and Fillet

The solder has to form a fillet along the full length of the can wall, which requires enough volume to fill the gap between the wall and the pad plus a fillet. Too little and the can sits on a thin layer that may not be continuous; too much and the solder wicks up the can wall, which is unsightly but usually harmless, or it spreads sideways and bridges to a neighbouring pad, which is not. The deposit is normally designed for a fillet that is visible on the outside of the can, since that is what the inspection checks, and the volume is adjusted from the appearance and from the electrical test rather than from a calculation.

Planarity and the Can

A can is stamped from sheet metal, and it is never perfectly flat. The lack of flatness means that the first point of contact between the can and the solder is one corner, and if the assembly is heated without support the can will rock and settle on that corner, leaving a gap on the opposite side. The remedies are to specify a flatness tolerance for the can, to use a can with a formed rim that adds stiffness, to hold the can with a fixture or a weight during reflow where the process allows, and to design the footprint so that the ring is as wide as the tolerance requires. The gap that results from poor flatness is often invisible after assembly, which is why the flatness is a specification rather than something to be inspected later.

Reflow Profile

The can is a large metal mass, and it takes heat from the board at the moment the solder melts, which can cool the joint below the melting point and produce a partial bond. This is the reason a shield can often needs a longer dwell at peak, or a slightly higher peak, than the rest of the assembly. The can also acts as a heat spreader, so the joints on the ring reach temperature more slowly than a small pad would, and the profile has to be written around the can rather than around the average component. Where the can is applied in a second reflow pass, the first pass components must survive the second excursion, which they usually do but which should be confirmed.

Fitting and Rework

Where the can is clipped rather than soldered, the fitting is a mechanical operation and the footprint is a contact surface rather than a solder joint. The clip has to be designed so that the contact force is maintained over the life of the product, and the pad has to be finished in a way that resists oxidation, since a contact surface without solder is exposed to the environment. Where a soldered can has to be removed for rework, the operation is difficult, because the can conducts heat away along the wall and the solder all around the perimeter has to be molten at the same time. The usual approach is a hot plate or a focused infrared preheat to bring the whole ring up together, followed by a controlled removal, and the operation is more likely to damage the board than a component rework.

Inspecting the Can Joint

The joint under a shield can is difficult to inspect, because most of it is hidden by the can itself. Optical inspection sees only the outside edge of the fillet, and X-ray through a metal can is largely blocked by the can itself, which is exactly what makes it a shield. This means the verification has to be electrical and mechanical rather than visual: a continuity measurement around the perimeter, a functional check of the shielded circuit, and where the design allows it, a pull or a peel test on a sample. Where the shield is critical to the product’s compliance, a sample should be cross sectioned periodically to confirm that the fillet is continuous, because a gap that is invisible from outside can be the difference between passing and failing an emissions test. The most useful production control is therefore the combination of the stencil check, the profile, and a continuity measurement, rather than an inspection of the joint itself.

FAQ

Why does a can lift at a corner? Because the stamped can is not perfectly flat, so it rocks during reflow and settles on the highest point.

How much solder is needed? Enough to form a continuous fillet along the whole perimeter, without spreading to a neighbouring pad.

Why split the stencil aperture? Because a long narrow aperture releases paste poorly; splitting it into shorter apertures gives a more even deposit.

Does the can change the profile? Yes. The can is a large metal mass that draws heat from the joint, so it usually needs a longer dwell or a slightly higher peak.

Can a soldered can be removed? With difficulty. The whole perimeter has to be molten at once, which usually needs a hot plate or an infrared preheat.

Conclusion

A shield can is a large mechanical part soldered around its perimeter, so the footprint width, the stencil apertures, the solder volume and the can’s flatness all decide whether the joint is continuous. Design the ring for the tolerance, split long apertures, adjust the profile for the metal mass, and specify the flatness. Shield design and grounding are described in PCB design and layout, the assembly process in SMT PCB assembly, and the continuity checks belong to PCBA testing. Radio assemblies are normally proven through prototype PCB assembly in 2026.