Key takeaways
- A heat spreader moves heat sideways into a larger area. It does not replace a heat sink with fins.
- Thermal contact at the interface often matters more than the thickness of the spreader.
- EMI shielding depends on a continuous conductive path, so seams, apertures and grounding tabs decide performance.
- A stamped shield can do both jobs, but the thermal and electrical requirements must be designed together.
Two jobs that are often given to one part
A stamped heat spreader takes heat from a small source, such as a power device or a module, and spreads it across a larger area so that the heat can leave through convection, a chassis or a heat sink. A stamped EMI shield is a conductive enclosure that reduces the electromagnetic field escaping from or entering a circuit. Both are thin sheet metal parts, and both are frequently combined into one component to save space and assembly steps.
The combination is sensible, but it sets a conflict. Thermal performance wants a short, wide, well contacted path to a heat sink. Shielding wants a closed conductive surface with as few openings as possible. The design has to decide which requirement leads, then check that the other still works. A part that is excellent at one job and marginal at the other is a common outcome when the two are treated as separate drawings.
Material and thickness
Aluminium is the usual choice for a spreader because it is light, conducts heat well and is easy to stamp. Copper conducts heat better and is used where the thermal budget is tight, but it costs more, weighs more and needs a finish that controls oxidation. Steel is a poor thermal conductor, so it is normally chosen for a shield rather than a spreader.
Thickness is often over-specified. A thicker spreader carries heat along its own plane a little better, but the gain is modest compared with improving the contact area and the interface. A thin spreader that is pressed flat against a clean surface can outperform a thick one that touches only at a few high points. Thickness should be chosen for stiffness, forming and the shield's mechanical duty, then checked against the thermal model.
For shielding, the material needs enough conductivity and enough mechanical stability to keep seams closed. Aluminium and steel both work. A plated steel shield is common because the plating gives a stable conductive surface at the seams and the steel provides stiffness at a lower thickness.
Geometry that moves heat
The first geometric decision is the footprint. Heat leaves a spreader from its surface, so area matters. A spreader that is only slightly larger than the device adds little. A spreader that reaches a mounting foot, a chassis wall or a finned area gives the heat a path out of the local hotspot.
The second decision is the path. Heat has to travel from the device into the spreader and then from the spreader into the next body. Every joint in that path adds resistance. A formed foot that is bolted to a chassis with a controlled contact area is usually better than a lip that touches only at the edge of a hole. Formed ribs and shallow domes can add stiffness without adding a separate part, but they must not lift the contact face away from the device.
The third decision is flatness at the interface. A spreader that is bowed will contact at the centre or at the edges, not across the whole face. The drawing should call out flatness over the contact area, not over the whole blank, so the supplier knows where to control the part.
| Feature | Thermal effect | Shielding effect | What to specify |
|---|---|---|---|
| Base thickness | Small gain in spreading | Stiffness and seam stability | Gauge with a flatness callout |
| Contact face | Dominant interface resistance | Grounding path | Flatness and surface finish |
| Formed foot | Adds a path to chassis | Bond to chassis | Contact area and fixing method |
| Seam or clinch | Neutral | Continuity across the joint | Overlap, plating and inspection |
| Apertures | Ventilation if required | Leakage above a frequency | Size, pitch and slot orientation |
| Grounding tab | Neutral | Low impedance return | Tab length, plating and fixing |
The same feature rarely optimises both functions. Record which requirement leads, then verify the other one after the design is fixed.
Thermal contact is the part most designs get wrong
Heat crosses an interface through the small areas where two surfaces actually touch. Air fills the rest. The real contact area depends on flatness, roughness, pressure and the hardness of the materials. This is why a spreader with a large nominal area can still run hot.
The practical fixes are simple. Machine or coin the contact face if flatness is critical. Use a thermal interface material that fills the gaps and is specified for the expected pressure and temperature. Apply a controlled clamping force through screws, clips or the housing, and make sure the force is spread across the face rather than concentrated at one edge. Keep the interface clean and free of plating nodules, burrs and handling marks.
Do not assume that a thermal pad solves everything. A thick pad compensates for poor flatness but adds its own resistance. A thin pad needs flatter surfaces. The right combination is a design decision, not a purchasing preference, and it should be tested on the actual assembly rather than on a bare spreader.
Measure the interface, not just the part
A spreader can pass every dimensional check and still fail thermally because the mating surface is bowed or the clamping force is low. Include the assembled interface in the validation plan.
Making the shield work
A shield works by reflecting and absorbing electromagnetic energy. Its performance depends on the conductivity of the material, the completeness of the enclosure and the size of the openings. A perfect box is easy to model. A stamped box with a seam, a cable exit and a row of cooling slots is not.
Seams are the first weak point. Two surfaces that touch at a few points make a poor conductor at high frequency. Overlapping flanges, clinched joints, conductive gaskets or a continuous weld all improve the path. The finish matters too, because an insulating oxide or a painted surface at the joint breaks the connection. If the shield is painted, mask the joint faces.
Apertures are the second weak point. A slot behaves as an antenna when its longest dimension approaches a fraction of the wavelength of the noise. Many small holes are usually better than one long slot. If ventilation is needed, keep the openings short and arrange them so the longest dimension is not aligned with the dominant current path. Cable penetrations need a conductive gland or a shielded connector, otherwise the shield has a door in it.
Grounding is the third. The shield needs a low impedance connection to the reference plane. A single long tab can act as an inductor. Several short, wide bonds around the perimeter are better. The fixing method should bite through paint and oxide, and the contact should survive vibration and thermal cycling.
Inspection and validation
Dimensional inspection covers the contact face flatness, the footprint, the height of the shield wall, the position of the mounting holes and the aperture pattern. For a combined part, the flatness of the thermal face and the squareness of the shield wall are usually the critical calls. Add a profile tolerance if the wall has to seat against a gasket.
Functional checks are separate. Thermal validation should measure the device temperature at the expected load, with the production interface material and the production clamping method. Shielding validation is usually a radiated emissions or immunity test on the complete assembly, not on the bare shield. A shield that passes a bench continuity check can still fail in the chamber because the cable exit or the seam behaves differently at the test frequency.
For production, add a continuity check across the seams and the grounding points, a visual check for plating damage and a flatness check on a sample basis. If the part is supplied with a conductive gasket or a clip, inspect the assembly, not only the metal.
What to send for a useful quote
Send the drawing with the interface area, the flatness callout and the mating surface defined. State the device power and the expected ambient, the shield's frequency range and the test standard if one applies, and the assembly method. Add the finish, the masking requirements and the volume.
If the part is still in design, send the thermal model and the enclosure concept. A supplier can then flag whether the form, the seam and the aperture pattern are practical in a stamping tool, and whether the flatness call can be held without a secondary operation.
- Device power, ambient and allowable case temperature
- Contact area, flatness and surface finish
- Shield frequency range and test standard
- Seam, gasket and grounding method
- Finish and masked areas
- Annual volume and assembly sequence
Frequently asked questions
Can a stamped heat spreader replace a heat sink?
Only in low power designs where the heat can spread into a chassis or a larger surface. A spreader has no fins and little surface area of its own, so it cannot dissipate much heat to still air. It is usually part of a path to a heat sink or an enclosure.
What material is best for an EMI shield?
Aluminium and plated steel are both common. The choice depends on conductivity, stiffness, weight, corrosion and the plating needed at the seams. The finish at the joint faces matters as much as the base material.
Do slots in a shield ruin the shielding?
They reduce it, and the effect depends on the slot length relative to the frequency. Many short openings are better than one long slot. Keep the longest dimension short and test the complete assembly rather than assuming a rule of thumb.
How flat does a heat spreader need to be?
It depends on the interface material and the clamping force. A thin thermal pad needs a flatter face than a thick gap filler. Specify flatness over the contact area and validate the assembled interface, because the mating surface contributes as much as the spreader.
Sources and further reading
- IEC 62368-1, Audio/video, information and communication technology equipment, safety requirements
- CISPR 16 series, Specification for radio disturbance and immunity measuring apparatus and methods
- Qumek drawing review notes for stamped thermal and shielding parts
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Designing a part that has to spread heat and shield?
Send the drawing and the interface conditions. The engineering desk will review the contact face, seam and aperture pattern before tooling.
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