Thermal Pad

A Thermal Pad (also known as a thermal interface pad or thermal gap pad) is a pre‑formed, soft, thermally conductive material placed between two surfaces to improve heat transfer. In AGL applications, thermal pads are used between heat‑generating components (such as LED PCBs, LED drivers, or power transistors) and heat sinks or the fixture housing. They fill microscopic air gaps and surface irregularities that would otherwise trap air and act as thermal insulators, significantly reducing thermal resistance and ensuring efficient cooling.

Key functions of a thermal pad in AGL applications include:

• Thermal Conduction: Providing a low‑resistance path for heat to flow from the component to the heat sink or housing.
• Electrical Insulation: Offering high dielectric strength to prevent electrical short circuits between the component and the heat sink, which is often metallic.
• Gap Filling: Complying to fill uneven gaps between surfaces, accommodating tolerances and ensuring full contact.
• Vibration Damping: Absorbing mechanical vibrations and shocks, protecting the component from stress.
• Pressure Distribution: Evenly distributing mounting pressure to avoid damage to fragile components (e.g., LED PCBs).

Thermal pads are preferred over thermal grease in many AGL applications because they are clean, easy to apply, pre‑formed to the required shape, and maintain consistent performance over time without drying out or migrating.

The thermal pad's purpose is to make sure the heat a component actually generates reaches the heat sink or housing meant to dissipate it, rather than being trapped by the microscopic air gaps that exist at any real-world mechanical interface — air being a poor conductor of heat, even a thin, seemingly negligible gap between an LED PCB and its heat sink can create a surprisingly large thermal bottleneck. By compressing to fill those gaps while still providing electrical insulation between a live component and a metal housing, the pad lets the fixture's heat-dissipation design actually deliver the thermal performance it was engineered for, and its vibration-damping properties protect delicate components from the mechanical stress an airfield's constant vibration would otherwise transmit directly into a rigid joint.

Thermal pads are used in virtually all thermally critical areas of AGL equipment:

• LED PCBs to Heat Sinks: Placed between the metal‑core PCB (MCPCB) and the fixture's aluminium heat sink to transfer heat from the LEDs to the fixture for dissipation.
• LED Drivers to Housing: Between the driver's power components (e.g., MOSFETs, transformers) and the metal housing to cool the driver.
• CCR Power Components: In Constant Current Regulators, thermal pads are used between high‑power transistors or rectifiers and the heatsink.
• Power Supplies and Rectifiers: In various power conversion stages within AGL systems.
• Control Electronics: For cooling of voltage regulators and other heat‑generating ICs.
• ALCMS Server and Networking Equipment: Where processors and power supplies require cooling.
• PAPI and Approach Light Fixtures: In high‑intensity LED assemblies.

Thermal pads are also used in:

• Retrofit Upgrades: When replacing older components with LED modules, thermal pads ensure proper thermal interface.
• Maintenance and Repair: Replacing degraded pads during servicing to restore thermal performance.
• Prototype and R&D: For testing thermal performance of new designs.

The pad is cut or supplied to size and placed directly between the two surfaces it thermally couples, typically the LED PCB and the heat sink, or a driver's power component and the housing wall, with its protective release film peeled away immediately before assembly to keep the contact faces clean. The parts are then clamped or bolted together to the specified compression, since a thermal pad performs correctly only within its designed compression range; too little clamping leaves air gaps, while excessive compression can squeeze the pad thinner than intended and reduce its insulating or gap-filling properties. It's fitted as a one-time consumable and replaced, not reused, whenever the joint is opened for servicing, since a pad that has already taken a compression set won't reseat evenly a second time.

A thermal pad functions by providing a highly conductive interface between two surfaces, minimising thermal resistance:

• Material Composition: Thermal pads are typically made from a silicone, acrylic, or urethane matrix filled with thermally conductive particles (e.g., aluminium oxide, boron nitride, or graphite). The matrix provides compliance and gap‑filling capability, while the fillers provide the thermal conductivity.
• Thermal Conductivity: The pad has a specified thermal conductivity (typically 1 – 6 W/m·K), which determines how efficiently it transfers heat. Higher conductivity means better heat transfer for a given thickness and area.
• Compression and Gap Filling: When compressed between the two surfaces (by mounting screws or clamping force), the pad deforms to fill microscopic air gaps and surface irregularities. This ensures that air (which has very low thermal conductivity) is displaced, maximising the contact area for heat transfer.
• Pressure and Thermal Resistance: The thermal resistance of the pad depends on the applied pressure. Higher pressure reduces the pad's thickness and improves contact, lowering thermal resistance. However, excessive pressure can damage components; the pad is designed to provide optimal performance at the recommended mounting pressure.
• Electrical Insulation: The matrix material is an electrical insulator, providing dielectric strength typically > 3 kV/mm, preventing electrical shorting between the component and the metallic heat sink.
• Long‑Term Stability: Thermal pads are designed to maintain their thermal and mechanical properties over a wide temperature range (-40°C to +150°C) and over many thermal cycles, without drying out, pumping out, or degrading.
• Adhesive or Non‑Adhesive: Some pads have a light adhesive on one or both sides to aid in assembly, while others rely solely on mechanical compression for retention.

In AGL fixtures, the thermal pad is typically placed between the LED PCB and the fixture's heat sink base. When the PCB is screwed down, the pad compresses, ensuring efficient heat flow from the LEDs to the heat sink, which then dissipates heat to the ambient air.

A thermal pad has no electronics, so its "control logic" is entirely mechanical and set once at assembly — by mounting screw torque and pad thickness — rather than adjusted in service. The heat path runs in one fixed direction: LED die → PCB copper → thermal pad → heat sink base → ambient air, and the pad's only function is to remove the air gap at the PCB-to-heat-sink joint, since air's thermal conductivity is orders of magnitude below any filled polymer. The governing variable is compressive pressure, set by the mounting screw pattern and torque spec: increasing pressure thins the pad and improves contact down to a design-recommended minimum thickness, below which further compression risks damaging the PCB or extruding the pad material rather than improving heat transfer. Because there is no active regulation, the "system" is self-limiting by geometry — the mounting hardware compresses the pad only until the PCB seats against its standoffs, which fixes the final gap and therefore the steady-state thermal resistance for the life of the fixture, unless the pad degrades (drying out, pumping out) or the joint is disturbed during later maintenance.

A thermal pad is manufactured as a pre-formed, soft, thermally conductive sheet, typically a silicone or polymer matrix loaded with thermally conductive filler particles (such as ceramic, aluminium oxide, or boron nitride) to achieve its rated thermal conductivity while remaining electrically insulating. The pad is calendared or moulded to a controlled thickness and supplied with a low-tack adhesive or liner on one or both faces to hold it in position during assembly between a heat-generating component, such as an LED PCB or driver, and the heat sink or fixture housing it is bonded to. Its compliant, conformable structure is designed to deform under the clamping pressure applied when the fixture is assembled, filling microscopic surface irregularities and air gaps on both mating surfaces so heat transfers by direct contact rather than through an insulating air layer. The material formulation is selected for a stable, self-extinguishing flammability rating and low outgassing, since the pad operates sealed within a fixture housing where trapped gases or degradation products could affect the fixture's optical or electrical components over its service life.