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Graphite, Dielectric Pad, or Gap Filler: Choosing Avionics Thermal Materials
Avionics thermal management looks like a heat problem and is decided by an electrical one. Before any conductivity number matters, the joint declares whether it must isolate, because the most expensive failure in this zone is a conductive spreader sitting across an unverified isolation path. The families on the H-O avionics thermal page sort first by electrical role and then by joint mechanics, and this guide walks each one by the decision it wins, with methods named by designation.
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The gating question is electrical, not thermal
Avionics thermal management looks like a heat problem and is decided by an electrical one. Before any conductivity number matters, the joint declares whether it must isolate, because the most expensive TIM failure in this zone is not a degraded interface, it is a conductive spreader sitting across an unverified isolation path. So the families on the H-O avionics thermal page sort first by electrical role and only then by heat density and joint mechanics.
This guide walks them by the decision each one wins, with methods named by designation: ASTM D5470 for thermal impedance, ASTM D149 for dielectric breakdown where the joint must isolate, ASTM E595 for outgassing in contamination-sensitive bays, and ASTM D1056 for the compression behavior of the gasket set. The environmental framing, MIL-STD-810 and RTCA DO-160, belongs in the material review, not just the box test plan.
Per-grade values are the maker-TDS figures cited on the parent page.
Isolated joints: the dielectric path
Reinforced dielectric pads: the flat, clamped interface
A flat, well-clamped module-to-rail or baseplate joint that must isolate wants a thin reinforced construction: Rogers PROTECT pads (the PROTECT 1500FG grade with its UL 94 V-0 listing per TDS) or Sil-Pad TSP boron-nitride silicone grades. The reinforcement takes clamp force without cutting through, the silicone carries the heat, and dielectric strength per ASTM D149 on the TDS is the second gate after impedance. Boron-nitride-filled silicone thermal pads raise the conductivity where the isolated joint needs more path.
Gap fillers: the uneven, low-pressure stack
A toleranced multi-height stack, or a joint a compliant cover cannot load to a reinforced pad's window, wants a soft Gap Pad filler that conforms at low stress instead of bridging. The filler trades some bulk conductivity for full-footprint contact, which usually wins on uneven or lightly clamped stacks, and it fills where a reinforced pad would starve. The choice between the two is the joint's pressure window, read per ASTM D5470 at your clamp pressure, not the headline conductivity.
Grounded joints: the graphite path
Where the joint is grounded and isolation is handled elsewhere, graphite usually wins the thermal comparison, and it does two different jobs separated by direction. As a through-path TIM, compressible eGRAF HiTherm graphite (the polymer-enhanced HiTherm HT-1205 grade among them) carries grounded module and baseplate joints and resists the pump-out that defeats soft fillers.
As a spreader, SpreaderShield flexible graphite (the SS400 grade among the ladder) exploits the material's anisotropy: in-plane conductivity far exceeds through-plane, so a thin film behind a board, lid, or wall moves a hot spot sideways into structure and rail area the component cannot reach.
Both are electrically conductive, so the isolation scheme must be verified before either is specified, and a spreader with nowhere to send its heat just relocates the problem.
The film and gasket layers
Thin dielectric and wear films do the boundary and protection jobs around the TIM. Kapton and Apical polyimide film laminates a dielectric boundary onto a graphite ply so the isolation travels with the part, and APTIV PEEK film (the APTIV 2000 grade) serves as a wear ply under retention hardware where a softer film would powder. AeroZero thin aerogel film insulates electronics-adjacent layers where millimeters decide.
The gasket set, BISCO HT-12xx solid silicone and BISCO silicone foam, closes display bezels, window-heater interfaces, and chassis perimeters, with compression behavior per ASTM D1056 on the TDS.
Avionics Thermal Families by Electrical Role and Job
| Family | Job | Electrical role | Method position | Where it lives |
|---|---|---|---|---|
| PROTECT / Sil-Pad TSP | Path (through) | Insulating | ASTM D5470 + D149 | Flat, clamped isolated joints |
| Gap Pad filler | Path (through) | Insulating | ASTM D5470 low stress | Uneven, lightly clamped stacks |
| eGRAF HiTherm | Path (through) | Conductive | ASTM D5470, resists pump-out | Grounded joints, breathing interfaces |
| SpreaderShield | Path (sideways) | Conductive | In-plane per TDS | Lateral hot-spot spreading to rails |
| Kapton / APTIV PEEK | Boundary / wear | Insulating | ASTM D149 / wear | Dielectric boundary, retention wear plies |
| BISCO HT-12xx / silicone foam | Gasket | Insulating | ASTM D1056 | Bezels, window-heater, perimeters |
Illustrative and relative only; confirm each grade on its current technical data sheet. Path comparisons per ASTM D5470 at your pressure; isolation per ASTM D149; outgassing per ASTM E595 per grade; DO-160 / MIL-STD-810 qualification belongs to the box builder.
Two rules that sort the page
First, settle the electrical role before the thermal number: an isolated joint restricts the choice to insulating families with ASTM D149 data, while a grounded joint admits graphite, which usually wins the impedance comparison there, and any conductive material on a joint that must isolate is verified for creepage and clearance at every revision.
Second, match construction to joint mechanics: joints that breathe with thermal cycling favor reinforced pads or graphite sheet that does not flow, while soft fillers belong where gaps are static, which is the pump-out failure this estate's companion article walks in detail.
Outgassing, finally, is a per-grade property: screen the exact converted construction (base plus adhesive plus liner) against the program's contamination limits per ASTM E595.
How H-O Products helps
H-O Products is a precision die-cut converter, not a raw-material producer. We take dielectric pad, gap filler, graphite TIM and spreader, polyimide and PEEK film, thin aerogel, and silicone stock from the material manufacturers and die-cut, kiss-cut on liner, laminate, and slit it 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 and grade direction from your electrical role, heat density, joint mechanics, pressure window, and contamination limits, and grade-level values are confirmed against the manufacturer's current TDS. Box qualification to DO-160 or MIL-STD-810 stays with the box builder.
Frequently asked questions
What thermal interface materials are used in avionics?
A small set sorted first by electrical role. Isolated joints use insulating reinforced pads (Rogers PROTECT, Sil-Pad TSP boron-nitride silicone) or soft gap fillers, each carrying dielectric strength per ASTM D149. Grounded joints use electrically conductive graphite: compressible eGRAF HiTherm as a through-path TIM, and SpreaderShield flexible graphite as a lateral spreader. Around them sit a Kapton or PEEK film layer and a silicone gasket set. The choice within a role is made per ASTM D5470 impedance at the joint's pressure.
Is graphite TIM safe to use in an avionics box?
Yes, on grounded joints, where it usually wins the thermal comparison, provided it is treated as an electrical component. Graphite is electrically conductive, so the isolation scheme, chassis grounding, and creepage and clearance must be verified before it is specified and re-checked at every design revision. The most reliable practice is to bound the graphite ply with a laminated Kapton dielectric boundary so the isolation travels with the part, and to record the verification on the drawing.
How do I choose between a reinforced pad and a gap filler?
By the joint's pressure window, read per ASTM D5470 at your clamp pressure rather than by the headline conductivity. A flat, well-clamped joint that can reach a reinforced pad's working pressure favors the thin reinforced construction, which gives the lowest impedance there. A toleranced multi-height stack, or a joint a compliant cover cannot load, favors a soft gap filler that conforms at low stress and fills where a reinforced pad would starve. The structure's ability to deliver pressure decides the family.
When does outgassing matter for an avionics thermal material?
Whenever the part sits in or migrates toward an optics- or sensor-sensitive bay. Outgassing is a per-grade property, not a family trait: percent total mass loss and collected volatile condensable material per ASTM E595 are reported grade by grade, and the adhesive layers, liners, and laminations each add their own contribution. Screen the exact converted construction, base material plus adhesive plus any liner residue, against the program's contamination limits per the E595 data for each grade.
Does H-O qualify boxes to DO-160 or certify TIM performance?
No. H-O converts the documented materials to the qualified drawing, with material traceability and lot-code TDS records that support your qualification file. Box-level qualification to RTCA DO-160 or MIL-STD-810, and the verification that the thermal design holds in the field, belong to the box builder and its test houses. Our role is bounded: die-cut, kiss-cut, and laminate the pads, fillers, graphite, films, and gaskets to your drawing, repeatably.
Get an avionics thermal recommendation
Tell us the electrical role of the joint, the heat density, the joint mechanics and clamp pressure, the interface flatness, and any contamination limits, and we will recommend a family and grade direction 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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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- ASTM D5470 (thermal impedance of thin thermally conductive solids; compare at your pressure and thickness).
- ASTM D149 (dielectric breakdown strength where the joint must isolate).
- ASTM E595 (total mass loss and collected volatile condensable material; outgassing, reported per grade).
- ASTM D1056 (compression behavior of the silicone gasket set); MIL-STD-810 and RTCA DO-160 (environmental framing, owned by the box builder).
- Manufacturer technical data sheets for the dielectric pad, gap filler, graphite TIM and spreader, polyimide and PEEK film, thin aerogel, and silicone grades named on the parent application page.
This article summarizes thermal interface material-selection principles for avionics using the families and published test methods cited on the parent application page. Comparisons are qualitative; per-grade values quoted are maker-TDS figures and must be confirmed on the manufacturer's current technical data sheet at your pressure, thickness, and installed condition. Standards (ASTM, MIL-STD, RTCA DO-160) are cited by designation for context, not as a claim of qualification.
H-O is a precision die-cut converter, not a raw-material producer, and does not perform thermal design, certify materials, or qualify enclosures to DO-160 or MIL-STD-810; those belong to the box builder and its test houses.