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Insulating TIM, Gap Filler, Graphite, or Dielectric Film: Choosing Drive and Inverter Insulation
Every interface in a drive, inverter, or converter is either part of the insulation system or it is not, and that single question sorts the material families faster than any thermal number. Where a semiconductor must be electrically isolated from its heat sink, only the insulating constructions qualify: PROTECT pads, reinforced Sil‑Pad TSP grades, soft Gap Pad fillers, solid silicone, and the dielectric films.
Where the joint is already grounded, electrically conductive eGRAF graphite becomes available and usually wins the thermal comparison. This guide walks the families the H-O power electronics and drive insulation application page covers, with the governing methods named by designation: ASTM D5470 for thermal impedance, ASTM D149 for dielectric strength, ASTM D2240 for hardness, UL 94 for flammability classes, IEC 60085 for insulation thermal classes, and IEC 61800-5-1 as the drive-level insulation framework.
Per-grade values quoted here are the figures cited on the maker TDS for those grades; design margins belong to your insulation coordination.
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The first filter: isolating, or already grounded?
State the electrical role of every interface on the drawing before comparing a single W/m·K figure. The highest-conductivity material on the parent page is electrically conductive graphite; the best insulators give up thermal performance for dielectric strength. A TIM choice that starts with the conductivity column and discovers the isolation requirement later gets rebuilt, sometimes after a hipot failure, sometimes after a field event.
Dielectric strength for the insulating families is reported per ASTM D149 and volume and surface resistance per ASTM D257 on the TDS on file.
The families, by the decision each one wins
Insulating TIM pads: the isolated module-to-sink joint
PROTECT insulating TIM pads are the reference point for the classic power-electronics interface: the 1500FG grade reports 1.5 W/m·K, 500 V/mil dielectric strength per ASTM D149, UL 94 V-0, and a -100 to 204 °C service range on its TDS, with fiberglass reinforcement that resists cut-through at fastener edges and burrs.
The Sil‑Pad TSP family covers the same clamped, flat joints with fiberglass- and film-reinforced grades up to the 3.5 W/m·K class (TSP 3500), and the thin polyimide-reinforced TSP K1300 where the lowest bond line wins. Specify these where the interface is part of the insulation system and the mating faces are reasonably flat and clamped.
Gap fillers: the uneven stack
A stack of components at different heights, a stamped baseplate, or an honest tolerance chain wants a soft Gap Pad conformable gap filler instead of a thin pad that bridges the high spots. The TGP grades run 1.0 to 5.0 W/m·K in gauges from roughly 0.010 to 0.250 inch per the maker TDS, with softness classes down to the ultra-soft TGP 1000VOUS grade and a -60 to 200 °C service window.
The filler trades some bulk conductivity for full-footprint contact, which usually wins on uneven stacks; the parent page's failure-modes section covers both directions of getting this wrong.
Graphite TIM: grounded interfaces only
eGRAF graphite TIM is electrically conductive in every direction, which rules it out wherever the interface belongs to the insulation system and makes it the usual thermal winner everywhere else: baseplate-to-sink joints, grounded spreaders, and hot process interfaces. The HiTherm laminates come in 5, 10, and 20 mil classes with UL 94 V-0 on the laminate TDS, and the compressible high-temperature grade runs -40 to 400 °C per its TDS.
Graphite is also strongly anisotropic, conducting far more in-plane than through-plane, so read both directions from the TDS.
Adhesive films: the fastener-free spreader
Where a heat spreader, lid, or small sink mounts without clips or screws, a SECURE thermally conductive adhesive film carries the attachment and the thermal path in one layer; the 1500KT2 grade reports 53.1 kV/mm dielectric strength per ASTM D149 on its TDS in a thin-film bond line. The bond is permanent, so surface prep and rework policy are process decisions to settle before the line runs.
For bonding insulation layers adjacent to hot components, the silicone/acrylic hybrid family (SA1911 Polysil, 2378SL, 1003 Polysil) pairs a silicone face for the hot side with an acrylic face for the cooler substrate.
Solid silicone, films, and papers: the barriers and small jobs
Away from the main thermal path, converters are full of smaller insulation jobs. BISCO solid silicone pairs dielectric behavior with conformability for pads under magnetics, spacers between boards, and compliant seats, with hardness steps per ASTM D2240 and dielectric properties per ASTM D149 on the TDS, plus high-performance specialty grades for tight-tolerance, extra-soft, transparent, and fire-safe duties.
Polyimide film (Kapton HN and FN, Apical NP) is the thin-film dielectric standard for barriers under power modules and gate drivers, with the heat-sealable FN grade for laminated constructions. Aramid paper wraps harnesses and winding zones in 220 °C class insulation systems under the IEC 60085 framing.
PTFE constructions: the harness's hard miles
PTFE-coated fiberglass (the 6085 series) jackets wire bundles where abrasion is the killer, pairing a slick PTFE face with a woven glass core, and skived PTFE film (DW2000, AMS 3662 listed, with dielectric methods per ASTM D149 on its TDS) lines routing paths and serves as a non-stick release wrap. H-O slits both to width and die-cuts wrap segments, washers, and liner strips to the drawing.
Drive & Inverter Insulation Families Side by Side
| Family | Electrical role | Thermal duty | Conforms / absorbs tolerance | Typical drive role |
|---|---|---|---|---|
| PROTECT pads / Sil‑Pad TSP | Insulating | 1.5 to 3.5 W/m·K classes per TDS | Thin, reinforced | Flat, clamped module-to-sink joints |
| Gap Pad fillers | Insulating | 1.0 to 5.0 W/m·K classes per TDS | Soft, thick | Uneven stacks, tolerance chains |
| eGRAF graphite TIM | Conductive, all directions | Usually wins when allowed | Compressible grades | Grounded joints, hot interfaces |
| SECURE adhesive films | Insulating per grade TDS | Thin-film bond line | Needs flat, clean faces | Fastener-free bonded spreaders |
| BISCO solid silicone | Insulating | Barrier, not TIM | Compliant seat | Pads, spacers, compliant barriers |
| Polyimide film / aramid paper | Insulating | Thin dielectric layer | Film folds; paper creases | Barriers, gate-drive zones, harness wrap |
Illustrative and relative only; confirm each grade on its current technical data sheet. Thermal impedance per ASTM D5470 at your clamping pressure; dielectric strength per ASTM D149 at your gauge; UL 94 classifications are per grade, not per family.
Thickness is the most expensive dimension
The junction does not care about W/m·K alone; it cares about the whole stack's resistance, which grows with every layer and every extra mil. Compare candidates per ASTM D5470 thermal-impedance data at your pressure on the TDS, not by conductivity alone: a 5 mil pad at the 1.5 W/m·K class can beat a 40 mil pad at the 3.0 W/m·K class.
Measure the worst-case gap and flatness, then pick thickness to fill it under the available clamping force, and no more. A dielectric film added under a module for isolation is also a thermal layer the budget must absorb.
How H-O Products helps
H-O Products is a precision die-cut converter, not a raw-material producer.
We do not manufacture the pads, films, or papers; we buy roll and sheet stock from the material manufacturers and convert it to your drawing: die-cut pads to the module footprint with fastener clearances, kiss-cut on liner for peel-and-place assembly, CNC knife-cut, slit to width, and laminated multi-layer constructions such as a dielectric film bonded to a thermal pad in one part, 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, gap, thermal budget, temperature, and assembly method, and the grade-level values are confirmed against the manufacturer's current TDS.
Frequently asked questions
Which insulating TIM goes between an IGBT or SiC module and its heat sink?
Start from the gap and the clamping force, not the conductivity number. A flat, well-clamped module wants a thin reinforced insulator: a PROTECT pad (1500FG: 1.5 W/m·K, 500 V/mil per ASTM D149, UL 94 V-0 per TDS) or a Sil‑Pad TSP grade (to the 3.5 W/m·K class). A stack with height variation wants a soft Gap Pad filler that conforms instead of bridging. Compare candidates by ASTM D5470 thermal-impedance data at your pressure on the TDS, and verify the dielectric class against your insulation coordination.
Is graphite TIM electrically conductive, and when can I use it?
Yes, graphite is electrically conductive in every direction, and that rules it out wherever the interface is part of the insulation system. Use eGRAF graphite where the joint is already at ground or isolation is carried by another layer: baseplate-to-sink interfaces, grounded spreaders, and hot process joints, where the compressible high-temperature grade runs -40 to 400 °C per its TDS. On those interfaces graphite usually wins the thermal comparison. State the electrical role of the interface on the drawing so the choice is deliberate.
Sil-Pad versus Gap Pad: what is the actual difference?
Construction and job. A Sil‑Pad TSP grade is a thin, reinforced insulator pad built for clamped, flat interfaces where it takes screw force without cutting through; thickness is in the few-mil class. A Gap Pad TGP grade is a soft, thick, conformable filler in gauges to roughly 0.250 inch per TDS, built to absorb stack-height variation under modest pressure. Specifying a gap filler on a flat clamped joint wastes thermal budget; specifying a thin reinforced pad over an uneven stack leaves air. Both families are electrically insulating per their TDS.
What goes under the power module to isolate it from the chassis?
A die-cut dielectric layer: polyimide film where thin gauge and dimensional stability rule, or a BISCO solid silicone pad where the barrier also needs compliance. Polyimide carries high dielectric strength per ASTM D149 in thin gauges, but it is a clean-coupon number: protect the film at burrs, sheared edges, and fastener heads, and carry the working margin in your insulation coordination per the IEC 61800-5-1 framework. The heat-sealable FN grade adds a bondable face where the barrier becomes part of a lamination.
Get a drive-insulation material recommendation
Tell us the electrical role of the interface, the worst-case gap, the thermal budget, the service temperature, and how the part goes on, 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 voltage and dielectric strength); ASTM D257 (DC resistance of insulating materials); ASTM D2240 (rubber hardness, Shore).
- UL 94 (flammability classifications, per grade); IEC 60085 (thermal classes for electrical insulation); IEC 61800-5-1 (adjustable-speed drive safety framework); IEC 61439 / UL 508A (assembly standards for the enclosure).
- Manufacturer technical data sheets: insulating TIM pads, gap fillers, graphite TIM, adhesive films, solid silicone, polyimide film, aramid paper, PTFE constructions.
This article summarizes insulation and TIM material-selection principles for drives, inverters, and converters using the families and published methods named above. Comparisons are qualitative; per-grade values quoted are the maker-TDS figures cited on the parent application page and must be confirmed on the manufacturer's current technical data sheet for your gauge and pressure. Working dielectric margins belong to your insulation coordination per the IEC 61800-5-1 framework, and assembly-level outcomes belong to the finished equipment's own standards.
All parts are custom manufactured to order; minimum production run quantities vary by material and part. H-O is a die-cut converter, not a raw-material producer, and does not independently certify materials or finished assemblies.