❄️ Thermal Engineering & Interface Materials•15 Min Read

Thermal Paste vs PTM7950 vs Liquid Metal: The Physics of Pump-Out & Interface Degradation

You repaste your graphics card or gaming laptop with premium paste, enjoy 65°C core temperatures for two months, and suddenly observe GPU Hotspot temperatures spiking to 105°C with fans screaming at 100%. This is not cooler failure; it is the physical reality of the pump-out effect. This guide analyzes surface asperities, Honeywell PTM7950 phase-change physics, and liquid metal gallium chemistry.

Published: November 2026•By NickCraft Thermal R&D•Tested on RTX 4090 & Delidded i9

01. Microscopic Surface Roughness: Why Thermal Paste Exists

Even when a copper heatsink base and a silicon die appear mirror-polished, electron microscopes reveal microscopic peaks and valleys called surface asperities. Under dry contact, only about 1% to 2% of the surfaces actually touch:

The Thermal Conductivity Comparison:

Trapped Air Pockets: ~0.026 W/mK (Extreme thermal insulator)
Standard Silicone Paste: 4.0 – 8.5 W/mK (Fills air voids)
Honeywell PTM7950 Phase-Change: 8.5 W/mK (Zero void phase transition)
Gallium-Indium Liquid Metal: 73.0 W/mK (True metallic molecular bonding)
Pure Electrolytic Copper: 398.0 W/mK (Heatsink base)

The goal of a Thermal Interface Material (TIM) is not to replace copper, but to displace air. Therefore, the thinnest possible bond line thickness (BLT) yields the lowest thermal resistance: R_th = BLT / (k × A).

02. The Pump-Out Effect: Why Pastes Fail on GPUs and Laptops

Desktop CPUs have a thick, rigid nickel-plated copper lid (IHS) that dampens rapid thermal flexing. In contrast, modern graphics cards (NVIDIA AD102, AMD Navi 31) and gaming laptops are direct-die: the heatsink cold plate clamps directly onto bare silicon.

THE PUMP-OUT CYCLE (MONTHS 1 TO 4)

Silicon expands at 2.6 × 10⁻⁶ /K, while copper expands at 16.5 × 10⁻⁶ /K. When you launch a game, the center of the die heats up in milliseconds. The differential expansion bends the heatsink base slightly convex. When cooling down, it flattens. This continuous pulsing pumps liquid silicone oils out of the center, leaving behind dry, chalky fillers and bare silicon hotspots.

THE PTM7950 SOLID-STATE SOLUTION

Honeywell PTM7950 contains a proprietary phase-change polymer matrix. When cold (<45°C), it re-solidifies into an elastic pad that cannot be pumped out. When hot (>45°C), surface tension keeps it locked between the microscopic microscopic crevices. It survives thousands of thermal cycles with zero degradation.

03. Liquid Metal Chemistry: 73 W/mK Performance vs Gallium Corrosion

Liquid Metal (Thermal Grizzly Conductonaut, Coollaboratory Liquid Pro) delivers thermal conductivity ten times higher than conventional paste. However, working with eutectic gallium alloys requires strict adherence to metallurgical rules:

FATAL HAZARD: Aluminum AmalgamationNever allow liquid metal to touch aluminum heatsinks, mounting brackets, or laptop heatpipe fin stacks. Gallium diffuses into aluminum grain boundaries, destroying interatomic bonds. Within 30 minutes, solid aluminum loses all structural integrity and turns into brittle grey powder.

Copper Staining & Intermetallic DiffusionWhen applied to raw copper, gallium slowly reacts to form CuGa₂ (copper gallide), creating a silver-grey crust that absorbs the liquid layer. On bare copper, you must clean and apply a second coat after 4 to 6 months. Nickel plating completely stops this reaction.

Electrical Conductivity & Short Circuit RiskLiquid metal is electrically conductive. When applying on a bare GPU die or delidded CPU, you must insulate all surrounding Surface Mount Devices (SMD capacitors) using high-temperature silicone conformal coating, MG Chemicals 422B, or Kapton tape with a foam barrier dam.

04. Thermal Interface Material Comparison Matrix

Material Type	Thermal Conductivity	Pump-Out Resistance	Lifespan	Recommended Application
Standard Paste (MX-6, NT-H2)	4.5 – 8.5 W/mK	Low (Pumps out on direct-die)	1 – 2 Years	Desktop CPUs with IHS
Honeywell PTM7950 (0.2mm Pad)	8.5 W/mK	Immune (Phase Change)	5+ Years Maintenance-Free	GPUs, Gaming Laptops, Steam Deck
Liquid Metal (Conductonaut)	73.0 W/mK	High (If sealed in dam)	Requires top-up on copper	Delidded CPUs & Extreme Overclocking

Frequently Asked Questions

Technical insights on thermal pastes, phase-change pads, and liquid metal.

❓ Frequently Asked Questions (FAQ)

Everything you need to know about Unicode font generation, game rules, and device compatibility.

01.What causes the 'pump-out effect' in high-end GPUs and gaming laptops?▼

The pump-out effect occurs on direct-die silicon (GPUs and laptop CPUs) where there is no integrated heat spreader (IHS). As the silicon die cycles rapidly between 35°C idle and 85°C load, the heatsink copper and silicon expand and contract at different rates. This microscopic flexing acts as a hydraulic pump, physically squeezing traditional viscous thermal paste out toward the edges within 3 to 6 months, causing GPU hotspot temperatures to skyrocket past 105°C.

02.Why is Honeywell PTM7950 considered the gold standard for direct-die cooling?▼

Honeywell PTM7950 is a phase-change polymer material. Below 45°C, it remains a dry solid pad that is easy to handle. Once the GPU heats up past 45°C, it melts into a low-viscosity liquid that flows into microscopic surface asperities with zero air voids. When cooling down, it re-solidifies, completely preventing pump-out and delivering consistent thermal conductivity (8.5 W/mK) for over 5 years without maintenance.

03.What happens if you use Liquid Metal on an aluminum heatsink?▼

Liquid Metal (a eutectic alloy of Gallium, Indium, and Tin) causes catastrophic chemical destruction when exposed to aluminum. Gallium penetrates the aluminum crystal lattice, forming an amalgam that destroys its structural integrity within minutes. The aluminum becomes brittle and crumbles to dust under light pressure. Liquid Metal must ONLY be applied to nickel-plated copper or pure copper cold plates.

04.Why does Liquid Metal require re-application on bare copper after 6 months?▼

While Gallium does not destroy copper like it does aluminum, it undergoes slow solid-state diffusion into bare copper, forming an intermetallic copper-gallium alloy layer. This 'stains' the copper base and reduces the liquid volume, necessitating a cleaning and top-up application after 4 to 6 months. Nickel-plated copper provides an impervious barrier against gallium diffusion.

05.Which thermal interface material should I use for a desktop CPU with an IHS?▼

For desktop processors with an Integrated Heat Spreader (like AMD AM5 or Intel LGA1700), high-viscosity traditional paste (such as Arctic MX-6, Noctua NT-H2, or Thermal Grizzly Kryonaut) is ideal. Because the thick copper IHS prevents localized flexure, pump-out does not occur, providing safe, non-conductive, low-risk thermal transfer.