How Compression Affects Shielding Effectiveness in Conductive Foam

2026-10-16　|　Dr.H Materials Laboratory　|　8 min read

A datasheet that promises 90 dB of shielding effectiveness looks reassuring until the assembled unit measures something closer to sixty. In most cases the material is not at fault. Shielding effectiveness is not a fixed property of a foam sample; it is the result of the material, the joint structure, and the compressive state working together.

1. Where shielding effectiveness comes from

Shielding effectiveness (SE) describes how much an electromagnetic field is attenuated when it tries to pass through a shield, expressed in decibels. The standard explanatory framework divides it into three contributions. Reflection loss occurs because the impedance mismatch at the interface between air and the conductive layer bounces part of the wave back. Absorption loss accounts for energy dissipated inside the material as the wave propagates through it. A multiple-reflection correction term accounts for waves bouncing between internal interfaces, which reduces overall attenuation in thin or low-loss shields.

That model assumes a homogeneous, continuous shield in full contact. A real assembly does not look like that. Sheet-metal panels deviate from flatness, mating surfaces leave gaps, and conductive layers touch each other only at a finite set of contact points. Once contact becomes discontinuous, the shield stops behaving like a shield and starts behaving like a group of conductor fragments separated by slots.

Hence the significance of test fixtures. The nominal SE on a datasheet is measured with a defined fixture, defined pressure, defined frequency range and defined specimen size. The fixture supplies two things the assembly must supply on its own: a stable compressive force and a controlled contact interface.

Conclusion: Reflection loss, absorption loss and the multiple-reflection correction together define shielding effectiveness, but after assembly the dominant variables are the gaps and the contact resistance between interfaces.

2. Why compression governs shielding effectiveness

Compression is defined as (free thickness − installed thickness) ÷ free thickness, expressed as a percentage.

In the uncompressed state the conductive covering touches the mating surface only at a few high points. Contact area is small, contact points are sparse, and the interface is full of microscopic air gaps. As compression increases, three things change:

• Contact area grows, giving shielding currents a wider path;
• Contact point density rises, so the failure of any single point no longer dominates the total impedance;
• Gap continuity improves, as residual air gaps are flattened and broken up and the shielding path closes.

All three converge on the same quantity: contact resistance drops. When contact resistance stays high, the shield layer and the mating housing are no longer at the same potential, shielding currents cannot drain properly, and leakage grows — most visibly at higher frequencies. In conductive foam selection discussions, Dr.H New Material generally recommends agreeing on the available compression first and only then discussing the target dB figure.

Conclusion: Compression determines contact resistance through contact area, contact point density and gap continuity, and those three variables set the shielding you actually get.

3. What happens when compression is insufficient

The visible symptom is residual gaps after closing the joint. At low frequencies that may cause only mild degradation, but the picture changes at higher frequencies. Once a gap's linear dimension approaches a meaningful fraction of the wavelength, the gap behaves like a slot antenna: energy that should have been contained radiates or couples out efficiently.

Stability is the harder problem. Without a stop or hard limit, screw torque variation, moulded-housing dimensional tolerance and batch-to-batch variation in foam thickness all push the actual compression around within a band. Near the threshold, a small tolerance shift is enough to make measurements jump: units from the same build pass on some machines and fail on others.

Vibration amplifies this. Start-up shock, transport and fan vibration produce small relative movements at the joint, contact points open and close repeatedly, and shielding effectiveness shows up as a fluctuating reading rather than a stable value.

Conclusion: Insufficient compression leaves residual gaps and raises contact resistance; high frequencies are particularly sensitive, and tolerance drift or vibration tends to show up as instability rather than a constant loss.

4. Why over-compression is not insurance

The opposite habit — pressing harder for safety — has two failure paths of its own.

The first is loss of resilience. Once the cell structure of the core is crushed beyond its limit, it can no longer push back. When thermal cycling, vibration loosening or assembly movement follows, the covering cannot follow the surface and contact degrades immediately.

The second is compression set. Under sustained high compression the installed thickness decreases irreversibly over time. Nominal compression may still look adequate while the real contact pressure falls, so shielding decays gradually. Factory testing will not catch this; it tends to surface after the product has been running.

There is also a mechanical cost. Every step up in compression demands markedly more closing force, which in most cases shows up as housing warpage, snap-fit cracking, or screws that are difficult to seat and prone to stripping.

Conclusion: Over-compression causes resilience loss and compression set, so shielding decays over time, while higher assembly stress risks housing deformation and fastening problems.

5. Sizing thickness with a usable formula

Thickness selection separates two questions: how much gap must be filled, and what fraction should be compressed. The working formula is:

For example, if the measured maximum gap is 2.0 mm and the target compression rate is 30%:

The next standard grade up is 3.0 mm. Installed thickness then lands near 2.0 mm, giving roughly 33% compression, comfortably inside the workable band.

Three boundary conditions matter when applying the formula:

• Target compression rates generally fall in the 20%–70% range (subject to the actual test report). Below roughly 20%, contact resistance is often high and unstable; above roughly 70%, resilience and long-term reliability become a concern.
• Use the maximum gap, never the average. Shielding is set by the weakest section, so the widest gap defines the leakage ceiling. Selecting on an average leaves the wide spots under-compressed — exactly the condition batch sampling tends to miss. Tolerances also stack: when housing tolerance, foam thickness tolerance and assembly variation all reach their extremes, the real gap approaches the theoretical maximum. Where tolerance data is incomplete, add margin on top of the maximum gap and confirm it with the supplier.
• Check the minimum gap as well. Where envelope constraints limit wall thickness, verify that compression at the narrowest point stays below about 70%, to avoid locally crushing the foam.

Conclusion: Divide the measured maximum gap by (1 − target compression rate) to get free thickness, round up to the next standard grade, then check the minimum gap for over-compression.

6. Matching the core material and the covering layer

Conductive foam is not one material but a combination, and the division of labour is clear.

The core material, typically polyurethane sponge or a comparable elastomer, supplies the compression travel and the push-back force. It decides how much force is needed to reach a given compression and how much of that travel is recoverable, which is what resilience measures. For Dr.H New Material conductive foam, core elongation is ≥80% per JIS K6400-5 (subject to the actual test report).

The covering layer — conductive fabric or a metallised layer — forms the conductive path. It sets the surface resistance and determines how shielding currents travel toward the grounding point. A surface resistance of ≤0.05 Ω/in² (subject to the actual test report) indicates good lateral conduction across the surface.

Together they answer a practical question: how much compression is enough. A soft core paired with a ductile conductive fabric reaches high contact coverage at low closing force; a firmer core needs more assembly pressure to get the same contact area. Where the available closing force is fixed, select core hardness first, then let the covering layer make up the electrical performance. Because the core and covering can be combined in different ways, there is room to tune the design to a given force budget.

Conclusion: The core governs resilience and compression force while the covering layer governs surface resistance and the shielding path; together they set the minimum compression needed.

7. How the mounting structure changes the result

The same foam behaves differently in different joints. These factors belong in the design review:

• Compression direction — vertical compression through the thickness is the intended working condition. Lateral shear stretches the covering and tears at the joint interface; keep assembly instructions explicit about the load direction;
• Grooves and locating features — a groove or recess constrains the installed thickness and pulls actual compression closer to the design value;
• Stop height — a mechanical stop is the most direct control available: it makes compression a function of stop height rather than screw torque, which markedly improves batch consistency;
• Flange and mating-surface flatness — a warped flange distributes compression unevenly along its length, over-pressed at the peaks and under-pressed at the valleys. For long joints, consider stiffening the flange or tightening fastener spacing;
• Fastener spacing — if spacing is too wide, the region between two screws sees insufficient force. It is common practice to place extra fasteners near corners, joints and other points where the seam changes direction.

One point deserves emphasis: in a joint with no hard limit, compression is effectively uncontrolled and depends on how each operator feels about torque, so variation within a single batch can be significant. This is one of the more common sources of deviation between measured and published values. Where the design allows it, provide a stop or a locating step.

Conclusion: Structure determines the compression state; without a stop or limit, compression cannot be controlled, and that is a main source of deviation from nominal shielding values.

8. Test methods and why conditions differ

Before comparing numbers from different suppliers, establish what conditions produced them. The same foam can read differently with a different fixture, closing force, frequency range or specimen size.

The fixture determines contact interface condition; the force sets compression and therefore contact resistance; the frequency range decides whether gaps fall into a sensitive size range; and specimen size influences how much leakage comes from the edges. If any one of these differs, the comparison does not hold.

The typical shielding effectiveness range for Dr.H New Material conductive foam is ≥60–80 dB @ 10 MHz–3 GHz (subject to the actual test report), and that figure also depends on the conditions above. For real projects, write the agreed test method into the technical requirement — fixture type, closing force or compression rate, frequency range and specimen size — and request the matching report as part of delivery.

Conclusion: Shielding effectiveness must be compared together with fixture, pressure, frequency range and specimen size; without aligned conditions, data comparison has little reference value.

9. Common mistakes

10. Summary and recommended actions

The discussion reduces to four actions that can be checked item by item at design review:

1. Measure the maximum gap and the tolerances. Do not rely on nominal dimensions from the drawing. Measure the maximum gap and account for the stack-up of housing tolerance, foam thickness tolerance and assembly variation;
2. Fix a target compression rate. For reference, this generally falls in the 20%–70% range (subject to the actual test report). Choose a value consistent with the available closing force, then derive free thickness and round up to a standard grade;
3. Specify fastening and limit features. Prefer a stop or locating step, state the compression direction and fastener spacing, and avoid designs where compression drifts unchecked;
4. Define frequency range and test method. Record fixture type, closing force or compression rate, frequency range and specimen size, and accept delivery against the test method agreed by both parties.

Conclusion: Shielding performance comes from the material, the structure and the assembly process together; treating compression as a controlled design variable gets closer to real results than chasing a higher nominal dB number.

Dr.H New Material supports die-cutting of conductive foam into strips, frames and custom shapes to drawing, with adhesive backing and lamination among the secondary processes available, for EMI shielding and grounding across structural joints. All data above is for reference in selection only.

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