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Gap Pad, Graphite, Polyimide, or EMI Gasket: Choosing Medical-Electronics Sealing, Thermal, and Shielding Materials
A medical-device controller board carries four different die-cut parts that get conflated because they all sit between two surfaces: a seal that keeps the enclosure clean, a thermal interface that moves heat, a dielectric that holds off a voltage, and an EMI gasket that closes a shielded seam. Each is chosen for a different property, so the first task is to name the job. The families on the H-O medical-electronics page sort by exactly that, and this guide walks each by the decision it wins.
Four jobs, four properties, one board
The four parts share a form factor but need opposite properties. The seal is chosen for closure force and cleanability; the thermal interface for heat transfer and conformability; the dielectric for dielectric strength and thickness; the EMI gasket for conductivity and compression. Asking one pad to seal and conduct heat and insulate is the classic conflation, because a good thermal pad is not a good dielectric if it is conductive, and a good seal is not a good thermal path.
So the families sort by job, with thermal behavior cited per the maker thermal data, dielectric per ASTM D149, and the biocompatibility basis per the ISO 10993 series where a part is patient-facing.
The thermal interface: gap pad or graphite
Where heat has to move from a component to a heatsink or chassis across a gap, two families cover the range. A boron-nitride silicone thermal gap pad is the conformable choice: it is soft enough to fill an uneven gap at low mounting pressure and it is electrically insulating, so it moves heat without shorting the component, which is why it suits a board where the thermal path must also be a dielectric.
A graphite thermal interface is the high-conductivity choice for a flatter, well-controlled gap where the path can be thin and the higher in-plane conductivity helps spread heat, noting that graphite is electrically conductive so it is kept away from anything it could short. The decision is gap control and whether the path must insulate.
The dielectric and the seal: polyimide and medical silicone
Where a conductor must be held off from its neighbor, polyimide film is the thin, tough, heat-stable dielectric, with the general-purpose polyimide HN grade as the workhorse barrier and slot liner sized to the working voltage with margin per ASTM D149. Where a polyimide foam is wanted, it adds a compressible, heat-stable insulating layer.
The enclosure seal that keeps the board clean is a BISCO MS-80 / MS-96 medical cellular medical silicone for a wipe-down lid or a BISCO MS-1600 solid medical silicone for a toleranced groove, chosen to the closure force as on the diagnostics enclosure page.
The shield: conductive elastomer and foil
Where a shielded seam has to close electrically, a conductive EMI shielding elastomer seals the seam both environmentally and electrically for a lid or panel, and a conductive foil tape bridges a seam or wraps a cable where a thin conductive path is wanted. As on the imaging page, a shield only works if its mating surfaces are conductive where they touch and it has a path to ground, and the failure mode of a gasket grounding against paint is the same here.
The seam continuity and the thermal pad's long-term contact are the subjects of the companion design article on this estate.
Sealing, Thermal, Dielectric, and Shielding Families by Job
| Family | Job | Property that matters | Electrically | Where it lives |
|---|---|---|---|---|
| Boron-nitride gap pad | Thermal, insulating | Heat transfer + conformability | Insulating | Component to heatsink, uneven gap |
| Graphite interface | Thermal, high-conductivity | In-plane spreading | Conductive | Flat, controlled gaps |
| Polyimide film | Dielectric barrier | Dielectric strength / thickness | Insulating | Slot liners, barriers |
| Medical silicone | Enclosure seal | Closure force / cleanability | Insulating | Wipe-down lids, grooves |
| Conductive EMI elastomer | Shielded seam | Conductivity + compression | Conductive | Lids, removable panels |
Illustrative and relative only; confirm each grade on its current TDS. Thermal behavior per the maker data; dielectric strength per ASTM D149; biocompatibility basis per the ISO 10993 series where patient-facing; shielding effectiveness is a system property verified by the device builder.
Two rules that sort the page
First, do not ask one part to do two electrical jobs at once: a thermal pad that must also insulate has to be the insulating boron-nitride type, not graphite, and a seal is not a shield unless it is the conductive grade. Second, the thermal path and the EMI seam are both about maintaining contact over the life, because a pad that dries out or a seam that opens fails even when the material was right, which the companion design article on this estate walks in detail.
Pick the property to the job, then the grade to the gap, closure force, or voltage.
How H-O Products helps
H-O Products is a precision die-cut converter, not a raw-material producer and not the device maker. We take boron-nitride silicone thermal gap pads, graphite thermal interfaces, polyimide film and foam, closed-cell and solid medical silicone, conductive EMI elastomer, and conductive foil tape from the material manufacturers and die-cut, kiss-cut on liner, laminate, and kit them to your drawing, with material traceability and lot-code TDS records under our ISO 9001:2015 certified quality management system in Winsted, Connecticut.
We recommend a family from which of the four jobs the part does, then size it to the gap, the closure force, or the working voltage, with grade-level values confirmed against the maker's current data. Thermal qualification, emissions qualification, dielectric verification, and device clearance stay with the device builder.
Frequently asked questions
Which thermal interface material should I use, a gap pad or a graphite sheet?
By the gap control and whether the path must insulate. A boron-nitride silicone gap pad is soft and conformable, filling an uneven gap at low mounting pressure, and it is electrically insulating, so it moves heat without shorting the component, which suits a path that must also be a dielectric. A graphite interface gives higher in-plane conductivity for a flat, well-controlled gap, but it is electrically conductive so it is kept away from anything it could short. The gap and the insulation requirement decide it.
How thick should a polyimide dielectric barrier be?
Thick enough to hold off the working voltage with margin, sized against the dielectric strength on the maker TDS per ASTM D149. A barrier sized to the nominal voltage with no margin is a service breakdown waiting to happen, because contamination, humidity, and edge effects lower the real breakdown below the bench value. Tell us the working voltage and any creepage and clearance constraint and we recommend a polyimide thickness and grade with headroom built in.
Can one part both shield EMI and seal the enclosure environmentally?
Yes, a conductive EMI shielding elastomer seals a seam both environmentally and electrically, so one gasket can do both jobs on a lid or panel. But it only shields if its mating surfaces are conductive where they touch and it has a path to ground, so the contact area cannot be painted or anodized in the seam. If the enclosure seal does not need to be conductive, a medical silicone is the cheaper, cleaner seal; the conductive grade is for where the seam must also shield.
Do the thermal and EMI parts have to survive sterilization too, or just the seal?
It depends where they sit. An internal thermal pad or EMI gasket inside a sealed enclosure may see only the internal environment, while the external seal sees the wipe-down and sterilization directly, so the duty differs by location. Tell us which parts are exposed to the sterilization route and which are internal, and we grade each to its actual environment. The sterilization validation across the assembly stays with the device builder.
Why is my board still hot when I already added a thermal pad?
Common reasons are a pad too thick or too firm for the gap, so it does not fully contact both surfaces at the available mounting pressure, or a pad that has dried out or pumped out over thermal cycling and lost contact, or simply the wrong material for the heat load. The thermal path depends on continuous contact across the whole interface, and the companion design article on this estate walks pump-out and contact loss, which are the usual reasons a pad underperforms over time.
How does H-O document the biocompatibility basis for a medical-electronics part?
Where a part is patient-facing, we convert materials whose makers publish a biocompatibility basis per the ISO 10993 series, such as cytotoxicity, sensitization, and irritation endpoints, and USP Class VI where applicable, and we pass it through with traceability. Many internal thermal, dielectric, and EMI parts are not patient-facing and may not carry that requirement, which the device owner determines. Tell us which parts are patient-facing and we point you at materials whose makers publish the basis.
Is a boron-nitride silicone thermal pad electrically insulating?
Yes. Boron nitride is a thermally conductive but electrically insulating filler, so a boron-nitride silicone gap pad moves heat while holding off a voltage, which is exactly why it is chosen where the thermal path must also be a dielectric. This is the opposite of a graphite interface, which is electrically conductive. If your thermal path crosses a live component or trace, the insulating boron-nitride pad is the safe choice.
My EMI gasket passes on the bench but the enclosure fails emissions. Why?
Almost always because the gasket grounds against a painted or anodized surface in the real enclosure, so it never reaches bare metal and the interference current has no path, even though the gasket compresses. The contact area has to be masked to bare metal or a conductive finish, the gasket has to compress enough for continuous contact, and the seam has to stay continuous. The companion design article on this estate walks grounding paths and seam continuity in detail.
Can H-O laminate several layers, like a dielectric film with an adhesive, into one part?
Yes. We laminate a polyimide dielectric with an adhesive, a thermal pad with a carrier, or a stack of layers into a single die-cut part, kiss-cut on a liner for placement. A bondable FEP-coated polyimide can laminate without a separate adhesive where a thin stack matters. Send the layer stack and the geometry and we build and cut the multilayer part to your drawing with material traceability.
What is the difference between this page and the medical imaging EMI page?
Both cover insulation, shielding, and cushioning, but the medical imaging page centers on imaging boards, flex circuits, and detector cushioning, while this medical-electronics page centers on a device controller board where the enclosure seal, the thermal interface, the dielectric, and the EMI seam all meet. The material families overlap, and the companion imaging blogs on the diagnostics-optical-electrical-emi page go deeper on flex coverlay and detector shock; this page goes deeper on thermal interface and enclosure sealing together.
What do I send H-O to get a medical-electronics quote?
Name which job each part does, then send the specifics: for a thermal pad, the gap, the mounting pressure, and whether the path must insulate; for a dielectric, the working voltage and barrier geometry; for a seal, the closure force and gland; for an EMI gasket, the seam geometry and mating-surface finish. Add whether each part is patient-facing, any biocompatibility basis needed, and the quantities. With that, engineering recommends families and grades and quotes die-cut parts. The companion how-to-spec article on this estate is a full checklist.
Get a sealing, thermal, dielectric, or EMI recommendation
Tell us which job each part does, then the gap and mounting pressure, the working voltage, the closure force, or the seam geometry, and we will recommend families and grades and quote die-cut parts to your drawing.
Samples typically ship in 3 to 5 business days for common die-cut configurations on materials we keep on hand; standard production runs about two weeks after drawing approval; expedited service is available. MOQ varies by material and part.
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Related reading
Request a quote for your medical-electronics part
Send H-O the board or seam geometry and the duty for each part—engineering reviews every RFQ and converts samples on your geometry.
Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- ASTM D149 (dielectric breakdown voltage and dielectric strength of solid insulating materials), cited by designation for the polyimide dielectric basis.
- ISO 10993 series (biological evaluation endpoints) and USP Class VI, cited by designation where a part contacts a patient-facing surface.
- Manufacturer thermal data and technical data sheets for the boron-nitride silicone gap pads, graphite thermal interfaces, polyimide film and foam, closed-cell and solid medical silicone, conductive EMI elastomer, and conductive foil tape grades named on the parent application page.
This article summarizes material-selection and design principles for medical-electronics sealing, thermal management, and EMI protection using the families and published test methods cited on the parent application page. Comparisons are qualitative; per-grade values are maker technical-data-sheet figures and must be confirmed on the manufacturer's current TDS at your installed condition. Standards (ISO 10993, USP Class VI, ISO 11607, IEC, ASTM, UL) are cited by designation for context, not as a claim of qualification or registration.
H-O Products is a precision die-cut converter, not a raw-material producer and not the device maker; biocompatibility testing, sterilization validation, device clearance, and final-assembly certification belong to the device owner and its test houses.