In sealed enclosures, near optics, in vacuum and inside precision instruments, the wrong thermal interface material does not fail thermally - it contaminates. Silicone oil bleeding and volatile outgassing deposit films on lenses, relays, connectors and cold surfaces. This page explains where the film comes from, how it is measured, and how to choose a TIM that stays clean.
Where the contamination comes from
- Silicone oil bleed. Conventional pads are silicone loaded with thermally conductive filler; unbound low-molecular-weight siloxanes migrate out over time, especially with heat and pressure. The film is a few molecular layers thick but enough to defeat adhesives, fog optics and increase contact resistance of relays.
- Volatiles from grease and phase-change materials. Greases rely on oil vehicles; above their service temperature the oil evaporates or migrates, the joint dries, and the residue condenses on the nearest cold surface.
- Outgassing in vacuum. With no air to carry vapour away, volatiles re-condense on optical and cold surfaces - the classic satellite and vacuum-system contamination path.
How outgassing is measured
The standard test is ASTM E595 (also used as ECSS-Q-ST-70-02 in Europe): a 24 h exposure at 125 °C in vacuum, with the condensable fraction collected on a cooled plate. Two numbers result:
| Metric | Meaning | Common requirement |
|---|---|---|
| TML - Total Mass Loss | % of sample mass lost in the test | < 1.0% for space-grade materials |
| CVCM - Collected Volatile Condensable Materials | % that condensed on the cold plate | < 0.10% for space-grade; stricter near optics |
| WVR - Water Vapour Recovered | % recovered water | Informational |
For silicone bleed (as opposed to vacuum outgassing), the practical test is simply storing the pad against a clean glass slide at elevated temperature and looking for the interference film - a check any receiving inspection can run.
Choosing a clean TIM: the four options
| Option | Contamination risk | Best fit |
|---|---|---|
| Conventional silicone pad | Bleed possible; outgassing depends on cure and formulation | Cost-driven electronics with open enclosures |
| Low-volatile silicone pad (post-cured) | Low; ask for E595 data per lot family | Telecom, automotive, sealed consumer devices |
| Silicone-free pad | No siloxanes at all; watch other volatiles | Optics assembly, relay cabinets, paint and adhesive shops |
| Solid graphite / graphene | No oil vehicle, nothing to bleed; only adsorbed moisture to desorb | Vacuum, space, optics, precision instruments |
A solid, fully dense material such as our oriented-graphene pad has no liquid phase: there is nothing to bleed, and volatiles are limited to adsorbed moisture, which desorbs early in life. That is why graphene spreaders keep getting specified for vacuum and optical assemblies. For programme documentation we can arrange ASTM E595 reporting on the specific material - ask us rather than assuming a stock number.
Design practices that reduce the risk
- Pre-bake pads before assembly when the BOM allows it - drives off moisture and short-chain volatiles.
- Do not let silicone TIMs touch or share a compartment with uncured adhesives, optics or relay banks; isolate or go silicone-free.
- Specify post-cured or low-volatile grades explicitly in the PO - "silicone pad" alone does not bound the bleed behaviour.
- In vacuum systems, require E595 data for every polymer in the enclosure, not only the TIM - adhesives and cables contaminate too.
Where our materials sit
The 180 W/m·K graphene pad is a cured, solid elastomer bonded to oriented graphite - no grease vehicle, no pump-out, and its 1500 h / 150 °C ageing verification includes contact-stability checks. For optical benches, satellite payloads and laboratory instruments we also supply the 1200 W/m·K plate alone, which is a solid laminate with no elastomer at all. Tell us the contamination budget for your assembly and we will propose the stack that fits it.