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Graphite, Boron Nitride Silicone, or Gap Pad: Choosing a TIM for EV Power Modules
Thermal-interface selection for an inverter, onboard charger, or BMS is mostly a geometry-and-electricity question wearing a materials costume. The geometry question: is the joint a flat, high-flux bondline under real clamp pressure, or a large, uneven gap at low pressure? The electricity question: may the interface conduct, or must it isolate a live part from a grounded heat sink?
Answer both and the family follows almost automatically: graphite for the flat clamped bondline where electrical conductivity is acceptable, boron nitride silicone where the heat path must also isolate, a soft conformable gap pad where the gap is the problem, and the broader TIM pad-and-film families in between.
This guide walks the decision the way the H-O power-electronics application page structures it, with thermal impedance cited the only honest way: per ASTM D5470 at a stated pressure and thickness, from the manufacturer's data sheet.
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The first fork: thin TIM or gap filler
A thin TIM (graphite or a thin film) is for a flat, high-flux bondline under clamp pressure, where the goal is the smallest possible thickness so contact resistance is squeezed out. A gap filler is a soft, thick, conformable pad for a large or uneven air gap at low pressure, where the goal is to fill a space a thin film cannot close.
The geometry decides which one you need before any conductivity number is compared, and most selection mistakes in this category are geometry mistakes: a thin film specified across a warped gap it cannot bridge, or a thick soft pad squeezed into a flat clamped joint that wanted the thinnest interface available.
The second fork: may the interface conduct?
The electrical question splits the thin-TIM side. Flexible graphite is electrically conductive, so it is the wrong choice wherever the heat path crosses from a live part to a grounded sink. That job belongs to boron nitride silicone: the boron nitride filler raises thermal conductivity without making the pad electrically conductive, so one die-cut pad cools and isolates at once. Asking the dielectric question early costs nothing; discovering it late costs a requalification.
The families, by the joint that picks them
Graphite TIMs: thin, high-conductivity, clamped
Graphite TIMs excel at the flat, high-flux module bondline: they conform into surface roughness under clamp pressure, give very low impedance at small thickness, and are clean and dry to handle. The catches are the two forks above: graphite is electrically conductive, and it needs real, even clamp pressure to seat, so it suits a bolted module-to-cold-plate joint more than a loose, low-pressure gap.
Where the joint is flat, the flux is high, the pressure is controlled, and conductivity at the interface is acceptable, graphite is the lead, with the grade-level impedance confirmed per ASTM D5470 for your conditions.
Boron nitride silicone pads: the dielectric TIM
Boron nitride silicone thermal pads take every joint where cooling and isolation meet: a live busbar or terminal shedding heat into a grounded plate, a BMS power stage isolated from its chassis, a module whose baseplate may not be grounded through the interface. One die-cut pad does both jobs, which usually beats a two-layer improvisation of film plus filler both thermally and in assembly time.
Soft conformable gap pads: when the gap is the enemy
Soft conformable gap pads close large, uneven, low-pressure gaps: board-to-housing paths, component clusters of varying height, castings whose tolerance stack made the gap a range rather than a number. Their job is conformity at low stress, spreading gentle contact across components that cannot take bolt-level pressure.
TIM pads and films: the in-between library
The broader TIM pads and films family covers the constructions between the poles: films for thin bondlines, pads of intermediate softness and thickness, and carriers that survive rework. It is the place engineering shops when the joint does not sit cleanly at either extreme.
EV Power-Module TIM Families by Joint Type
| Family | Best joint | Pressure need | Electrical isolation | Typical EV location |
|---|---|---|---|---|
| Graphite TIM | Flat, high-flux, thin | Real, even clamp | Conductive: no | Bolted module-to-cold-plate joints |
| Boron nitride silicone pad | Cool + isolate | Moderate | Dielectric filler | Live-to-grounded heat paths, BMS stages |
| Soft conformable gap pad | Large, uneven gaps | Low | Grade-dependent | Board-to-housing, mixed-height clusters |
| TIM pads & films | Intermediate joints | Construction-dependent | Grade-dependent | Chargers, DC-DC converters, rework-exposed joints |
Illustrative and relative only; confirm each grade's impedance curve per ASTM D5470 at your pressure, thickness, and surface finish on the manufacturer's current data sheet, then verify on the bench.
Why the data-sheet number is an impedance, not a conductivity
The parent page's drawing-callout guidance is the part most RFQs miss: specify the impedance at your real bondline conditions, not a headline conductivity. Thermal impedance is reported per ASTM D5470 at a stated pressure and thickness, so a meaningful callout names the target impedance together with the assembly pressure, the bondline thickness, and the surface finish it applies to.
A bulk conductivity figure alone is misleading because it ignores contact resistance, which can dominate a thin joint; most of the real interface resistance in a clamped joint is contact resistance, and it falls steeply as pressure rises before flattening toward a floor set by thickness and conductivity. The companion article in this trio walks that physics in detail; for selection purposes the rule is short: compare families on the D5470 impedance curve at your pressure, never on the conductivity headline.
How H-O Products helps
H-O Products is a precision die-cut converter, not a raw-material producer. We do not make the graphite, boron nitride silicone, gap-pad, or film stock; we buy sheet, roll, and slab stock from the material manufacturers and convert it to your drawing: die-cut and kiss-cut TIM pads with liner presentation, laminated constructions, EMI gaskets, and housing seals, slit and kitted for the line, with material traceability and lot-level data-sheet records under our ISO 9001:2015 certified quality management system.
We recommend a family and grade direction from your heat, gap, pressure, and dielectric requirements, and the grade impedance is confirmed per ASTM D5470 on the manufacturer's data sheet for your conditions, then verified on your bench.
Frequently asked questions
When is graphite the wrong TIM even though it is the most conductive option?
In two situations the parent page calls out plainly. First, wherever the heat path must isolate a live part from a grounded sink: flexible graphite is electrically conductive, and that job belongs to a boron nitride silicone pad. Second, in loose, low-pressure joints: graphite needs real, even clamp pressure to seat into surface roughness, so a low-pressure gap wants a soft conformable pad instead. Conductivity never overrides geometry and electricity.
What is the practical difference between a thin TIM and a gap filler?
The joint they are built for. A thin TIM (graphite or thin film) serves a flat, high-flux bondline under clamp pressure, where the goal is minimum thickness so contact resistance is squeezed out. A gap filler is soft, thick, and conformable for a large or uneven gap at low pressure, where the goal is closing a space a film cannot bridge. The geometry decides; comparing their conductivities across that divide is comparing answers to different questions.
What should the TIM callout on my drawing actually say?
The target thermal impedance at your conditions: impedance per ASTM D5470, together with the assembly pressure, the bondline thickness, and the surface finish it applies to. A bulk conductivity number alone ignores contact resistance, which can dominate a thin joint. The cleanest path is to send the heat to dissipate, the allowable rise, the gap and flatness, and the clamp pressure, and let the grade be matched to the data-sheet impedance curve, then verified on the bench.
Are these materials certified to CISPR 25 or IEC 60529?
No; those standards apply to an assembly or a measurement, not to a raw material. CISPR 25 and the IEC 61000 series evaluate emissions and immunity at the assembly level, and IEC 60529 rates the tested enclosure. A gasket or seal supports the assembly's compliance, and the materials are evaluated against the cited methods through the manufacturer's data sheet; H-O does not independently certify a material to them unless explicitly stated on the quote.
Get a TIM material recommendation
Tell us the heat to dissipate, the allowable rise, the gap and flatness, the clamp pressure, and any dielectric requirement, and we will recommend a family and grade direction and quote die-cut parts on your footprint.
Samples typically ship in 3 to 5 business days for common die-cut configurations; standard production runs about two weeks; special orders are quoted to the job. MOQ varies by material and part.
Family-owned in Winsted, Connecticut since 1971 · ISO 9001:2015 certified · OEMs worldwide send us a drawing — and get back their part.
Related reading
ApplicationEV Power Electronics, BMS & Inverter Thermal Management and EMI SealingRead →
GuideContact Resistance and the Slot-Antenna Seam: The Two Quiet Failure Modes in EV Power ElectronicsRead →
GuideHow to Spec a TIM, EMI Gasket, or Housing Seal: What to Send H-O for a QuoteRead →
IndustryEV & Battery hubRead →
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Send H-O your drawing or describe the 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 D5470 (thermal impedance of thermally conductive interface materials, at stated pressure and thickness).
- CISPR 25 and the IEC 61000 series (assembly-level emissions and immunity context); IEC 60529 (enclosure ingress rating); UL 94 (flammability listings).
- Manufacturer technical data sheets: graphite TIMs, boron nitride silicone thermal pads, soft conformable gap pads, TIM pads and films, conductive silicone and sponge EMI gaskets.
This article summarizes TIM selection principles for EV power electronics using the families and published methods named above. Comparisons are qualitative; no impedance, conductivity, or shielding value is stated for any grade, and grade-level data is confirmed per the cited methods on the manufacturer's current technical data sheet at your conditions, then verified on the bench.
Assembly-level outcomes (CISPR 25, IEC 61000, IEC 60529) belong to the tested assembly. 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.