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Choosing a Thermal Interface Material: Gap Pads, TIM Films, and Graphite Spreaders by Impedance, Not Bulk Conductivity
The metric that decides a thermal interface material is its thermal impedance at the actual gap and clamp pressure, measured per ASTM D5470, not the headline bulk conductivity in W/m·K. A thin, conformable pad that wets both faces at low pressure can move more heat across a real joint than a thicker pad with a higher catalog conductivity.
This guide explains why that is, separates the three families an engineer actually chooses between, gap pads, thin TIM films, and flexible-graphite spreaders, and gives a starting direction for matching a family to your joint.
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The application problem
A thermal interface material does one job: it carries heat across the joint between a hot component and the surface that removes that heat, usually a heat sink, cold plate, or chassis. The problem is that no two machined or molded surfaces meet perfectly. Press a power-module baseplate against a heat sink and the real contact is a scatter of high points separated by microscopic air gaps, and air conducts heat at roughly 0.026 W/m·K, which is to say almost not at all.
The interface material exists to displace that air with something conformable that touches both faces.
The failure that follows from getting this wrong is a device that runs hotter than the thermal model predicted. The bulk material in the gap might have an impressive conductivity number, yet the part throttles, derates, or shortens its service life because the heat is not actually crossing the joint. The number on the data sheet described the material; the device feels the joint.
This shows up across the heat paths H-O Products converts parts for: an IGBT, SiC, or GaN power module or inverter dumping heat into a sink across a joint that must also keep the device electrically isolated; a high-power CPU or GPU die to a heat sink or cold plate; an EV cell or module to a liquid cooling plate across a gap that varies unit to unit; and LED boards shedding junction heat into a housing. Each is a contact-resistance problem first and a bulk-conductivity problem second.
Why impedance, not conductivity, is the deciding number
The in-application metric is thermal impedance, and it has two parts that add together: bulk conduction through the thickness of the material, plus the contact resistance at each of the two faces where the material meets metal. ASTM D5470 is the reference test method that captures both, by measuring impedance across a material at a stated thickness and contact pressure, and it is the number a thermal engineer should compare between candidates.
Three things follow from that definition, and together they overturn the instinct to chase the biggest W/m·K:
- Thinner usually wins on bulk resistance. Bulk resistance rises with thickness, so a material that fills the gap in a thinner bondline carries heat better through its body, even at a modest conductivity. A thick high-conductivity pad can lose to a thin one.
- Contact pressure sets the contact resistance. More pressure flattens the conformable material into the surface roughness and squeezes out trapped air, lowering the resistance at each face. The same material reports a lower impedance at higher pressure on a D5470 pressure series. The catalog conductivity does not move; the joint does.
- Conformability can beat conductivity. A soft pad that wets both faces and fills the real gap can present a lower total impedance than a stiffer, higher-conductivity material that bridges the high points and traps air in between.
That is why a thin, conformable pad at good contact pressure can outperform a thicker pad of higher bulk conductivity. The right question is not which material has the highest k, it is which form fills the actual gap at the available clamp force with the lowest impedance.
The three families, and the wrong-but-tempting choice
Almost every TIM decision reduces to choosing among three families, and to recognizing when a family is being used for a job it cannot do. Below, families are named as materials H-O Products converts; grade-level conductivity, impedance, and temperature limits live on the manufacturer’s data sheet and should be confirmed there.
- Boron-nitride silicone gap pads. Soft, conformable, and electrically isolating, these fill a large or uneven gap at low clamp force, which is what protects a fragile board or cell from over-stress. Commonly specified for power-module and inverter interfaces where the joint must also isolate the device, and for EV cell-to-plate gaps that vary unit to unit and favor a high-deflection, low-stress material. Available as soft conformable pad stock (soft conformable BN silicone gap pads) and as reinforced sheet for more robust handling under clamp load (reinforced BN silicone thermal pads).
- Thin TIM films. On a flat, tight, well-clamped joint, a thin dielectric film gives a lower through-plane impedance than a soft pad because the bondline is so thin that bulk resistance nearly disappears. Often specified for board-level chip cooling on a flat lid or cold plate, and for dielectric-critical power interfaces (TIM pads and films, and the dielectric PROTECT-series TIM pads where electrical isolation is the controlling requirement). A film cannot fill a large or uneven gap, so it is the wrong choice where the gap is variable.
- Flexible-graphite heat spreaders. These solve a different problem. Graphite is anisotropic: very high in-plane conductivity, commonly cited around 1350 to 1600 W/m·K per the maker’s data sheet, and low through-plane conductivity, on the order of a few W/m·K. That makes it excellent at spreading a concentrated hot spot sideways across a surface, in a phone, a display, an LED board, and similar, but a poor through-plane TIM. Available as natural-graphite (natural graphite spreaders), higher-conductivity synthetic (synthetic graphite TIM), and a broader spreader family (graphite thermal management). Graphite also conducts electricity, so near live parts it must be isolated with an adjacent dielectric layer.
The classic mistake is to treat all three as interchangeable because they all show up under the word TIM. A graphite spreader used as a through-plane interface starves the joint; a thin film stretched across a variable gap traps air; a thick gap pad on a flat, high-pressure joint adds bondline resistance the joint did not need. The first decision is the job: conduct across the gap, or spread along the surface.
Design and converting considerations
Once the family is right, the part becomes a converting and detailing problem, which is where a die-cut converter adds value over a sheet of raw stock.
- Match the form to the gap and pressure. Specify the nominal gap and its tolerance, and the clamp force or pressure the joint actually reaches. Those two numbers, not the conductivity headline, determine whether a pad or a film fills the joint, and they set the D5470 conditions a candidate should be compared at.
- Add the dielectric where graphite meets live parts. Because graphite is electrically conductive, a spreader near a live circuit is laminated to or paired with an adjacent dielectric film so the heat path does not become a short.
- Define adhesive, liner, and pull-tab. A pressure-sensitive adhesive and a release liner with a pull-tab speed placement on a production line, while a no-adhesive face is sometimes preferred where the part is clamped and serviceability matters. These are converting choices made on the drawing.
- Choose the converting process by feature and volume. Thin pads and films are commonly die-cut to print or kiss-cut on a liner; thicker blocks and complex profiles or prototypes can be laser- or waterjet-cut; multi-layer interfaces are built by adhesive lamination of TIM, dielectric, adhesive, and liner into one part. Thickness and outline tolerances are set on the drawing and confirmed at quote.
- Favor a die-cut solid pad where the assembly is serviced. A converted solid pad gives a repeatable, predictable compression and is removable for serviced or upgraded products, unlike a dispensed filler, which is a different process outside die-cut converting.
Common failure modes
| Failure mode | Root cause | Consequence |
|---|---|---|
| Chasing bulk conductivity | Selecting on the headline W/m·K instead of D5470 impedance at the real gap and pressure | A high-k material posts a higher in-application impedance than a thinner, more conformable one; the device runs hot |
| Wrong job entirely | A graphite spreader used as a through-plane TIM, or a TIM used where the need was to spread a hot spot | Heat does not cross or does not spread; the thermal budget is missed |
| Film on a variable gap | A thin film specified where the gap is large or uneven | The film cannot fill the gap, air is trapped, contact resistance climbs |
| Over-thick or under-thick pad | Thickness not matched to the gap; too thin leaves air, too thick adds bulk resistance or over-stresses the joint | Higher impedance, or a fragile board or cell over-compressed |
| Conductive graphite across a live gap | A graphite spreader placed near live parts without an adjacent dielectric | Electrical short or leakage path where isolation was required |
| Missed isolation requirement | A conductive family used where the heat sink had to stay electrically isolated from the device | Loss of the dielectric barrier the design depended on |
TIM Family Direction Selector
This tool points you toward a material family by the job and the joint. It does not compute an impedance from spec values; once you have a direction, compare candidate grades on ASTM D5470 impedance at your gap and pressure. The static matrix below shows the directions without JavaScript.
| Situation | Direction | Note |
|---|---|---|
| Conduct · flat joint · higher pressure | Thin TIM film | Thin bondline drives impedance down on a well-clamped flat joint. |
| Conduct · uneven gap · low pressure | Soft BN silicone gap pad | Conformable, isolating, low-stress; fills a variable gap. |
| Conduct · uneven gap · higher pressure | Gap pad (reinforced sheet) | Robust handling under clamp; verify deflection and impedance on the TDS. |
| Conduct · flat joint · low pressure | Soft gap pad | Film may not seat without pressure; a soft pad wets the faces. |
| Spread a hot spot along a surface | Flexible-graphite spreader | Very high in-plane k; poor through-plane; isolate near live parts. |
| Spread · near live parts | Graphite + adjacent dielectric | Graphite conducts electricity; pair with a dielectric film. |
Clear direction Verify on TDS Directions reflect the gap, pressure, and isolation logic above; they are a starting point, not a substitute for D5470 comparison and a drawing review.
Illustrative only; confirm against current material data and a drawing review. This selector points to a material family from the job and the joint. It does not assert any grade’s conductivity or impedance; compare candidate grades on ASTM D5470 data at your actual gap and contact pressure before specifying.
How H-O Products helps
H-O Products is a precision die-cut converter, made to order. We convert boron-nitride silicone gap pads and reinforced thermal-pad sheet, thin dielectric TIM films, and flexible-graphite spreaders into custom parts built to your drawing, die-cut or kiss-cut to print, laser- or waterjet-cut for thick blocks and complex profiles, and laminated where a TIM, a dielectric layer, an adhesive, and a liner need to become one part.
We do not produce the raw stock and we do not dispense liquid or putty interface materials; we convert solid sheet and roll stock to your part. Where the choice is between a conformable pad and a thin film, we help you reason from the gap and the clamp pressure to a family, point you to the ASTM D5470 data to confirm on the manufacturer’s current technical data sheet, and turn it into a repeatable die-cut part.
We work as an ISO 9001:2015 certified organization in Winsted, CT, with material data sheets, certificates of conformance, and lot traceability available on request.
Match the TIM to the joint, not the headline number
Send us the thermal job, the gap and tolerance, the available clamp pressure, and whether the joint must electrically isolate. We will recommend a material family direction, flag the ASTM D5470 data to confirm on the data sheet, and convert a die-cut interface part to your footprint. Samples typically ship in 3 to 5 business days for common configurations; standard production runs about two weeks.
Family-owned in Winsted, Connecticut since 1971 · ISO 9001:2015 certified · OEMs worldwide send us a drawing — and get back their part.
Frequently asked questions
Why does thermal impedance matter more than bulk conductivity for a TIM?
Because the device feels the whole joint, not just the material. Thermal impedance per ASTM D5470 adds bulk conduction through the thickness to the contact resistance at each face, and it is measured at a stated thickness and pressure. A thin, conformable material that wets both faces at the available clamp force can present a lower total impedance than a thicker material with a higher catalog conductivity, so impedance at your gap and pressure is the number to compare.
What is ASTM D5470?
ASTM D5470 is the reference test method for the thermal transmission properties of thermally conductive interface materials. It measures thermal impedance, and apparent thermal conductivity from a thickness series, across a material at a stated contact pressure, which is why it captures the contact-resistance behavior that governs real joints.
When should I use a gap pad instead of a thin TIM film?
Use a soft boron-nitride silicone gap pad when the gap is large, uneven, or varies unit to unit, or when the clamp force is low and a fragile board or cell must not be over-stressed; the pad conforms and isolates. Use a thin TIM film on a flat, tight, well-clamped joint, where the thin bondline drives impedance down. Confirm grade properties on the manufacturer’s current data sheet.
Is a graphite heat spreader a thermal interface material?
Not in the through-plane sense. Flexible graphite is anisotropic, with very high in-plane conductivity and low through-plane conductivity, so it spreads a concentrated hot spot sideways along a surface rather than carrying heat across a joint. It is the right family for spreading, not for a through-plane interface. Graphite also conducts electricity, so near live parts it needs an adjacent dielectric layer.
Can H-O dispense a liquid or putty TIM?
No. H-O Products is a precision die-cut converter and works with solid sheet and roll stock, converting gap pads, films, and graphite spreaders into custom die-cut parts. Dispensed liquid, putty, and phase-change interface materials are a different process and are outside die-cut converting.
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References and standards
- ASTM D5470 – Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials (thermal impedance and apparent conductivity of interface materials).
- ASTM C177 – Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus (insulation conductivity, for context on the opposite job).
- UL 94 – Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances (flammability classes HB, V-2, V-1, V-0, 5V; grade- and thickness-specific).
- IEC 60243 – Electric strength of insulating materials (dielectric strength, kV/mm), referenced where an interface must also isolate.
- Conduction and thermal-contact-resistance principles from standard heat-transfer references, the basis for the impedance-versus-pressure behavior discussed above.
- Manufacturer technical data sheets for boron-nitride silicone gap pads, dielectric silicone TIM films, and flexible-graphite spreaders, the source of grade-level conductivity, impedance, temperature, and flammability figures. Confirm current grade designations and properties on the manufacturer’s current technical data sheet.
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This article cites general engineering principles and published test standards (ASTM D5470 for interface-material thermal impedance, ASTM C177 for insulation conductivity, UL 94 for flammability, IEC 60243 for dielectric strength). Material names refer to thermal material families H-O Products converts, identified by material type.
Performance figures, including any conductivity or impedance values, are grade dependent and illustrative; confirm all values against the manufacturer’s current material data sheet and an engineering drawing review before specifying. H-O Products is a precision die-cut converter and does not produce raw material stock or dispense liquid, putty, or phase-change interface materials.