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TIM Pump-Out at Altitude: Why a Soft Interface Migrates Out of a Breathing Joint
A thermal interface that passes qualification can still fail at hour three thousand, because a soft material in a joint that flexes has been migrating outward the whole time. Each cycle works it further out of the gap until the center runs dry. This is pump-out, a mechanical failure of a thermal part, and altitude makes it worse by adding a pressure cycle to every climb. This article walks the mechanism and the construction choices that stop it, with methods named by designation.
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The interface that passed qualification and failed at hour three thousand
Avionics thermal failures rarely show at acceptance test. The box passes thermal survey, the interfaces read cool, qualification closes. Then field hours accumulate and a hot spot drifts, because a soft thermal interface in a joint that flexes has been slowly migrating outward the whole time. Each thermal cycle works material toward the edges until the center runs dry, and the interface that met its ASTM D5470 impedance on day one fails years later.
This is pump-out, and it is a mechanical failure of a thermal part driven by the one environment avionics cannot escape: a joint that breathes with temperature and altitude. This article walks the mechanism, why altitude cycling makes it worse, and the construction choices that stop it.
What pump-out actually is
A thermal interface exists to replace air in a joint, so it is necessarily soft enough to conform. That same compliance is what lets it move. When the two faces of a joint cycle apart and back, a soft, flowable interface is pumped a little further out of the gap on each excursion, the way working a tube of paste slowly empties it.
The drier the center gets, the worse the local impedance, and the worse the local impedance, the hotter and more aggressively the joint cycles, which pumps faster. The TDS impedance was real; what the data sheet cannot tell you is whether your joint will hold the material in place over thousands of cycles. That is a function of joint mechanics and material construction, not of the headline conductivity.
Why altitude makes a breathing joint breathe harder
Two cycles overlap in an airborne box. The thermal cycle expands and contracts the joint with every power and ambient swing. The altitude cycle adds a pressure swing on every climb and descent, and any joint that is not perfectly rigid flexes as the differential changes. A conduction-cooled module clamped to a chassis rail, a cover that is stiff at sea level and a little less so at altitude, a wedge-lock whose preload shifts with temperature, all of these turn a nominally static gap into one that opens and closes.
The qualitative point is not a number; it is that an avionics joint sees more cycles, and more kinds of cycle, than a benchtop test implies, and pump-out integrates every one of them. The environmental framing that captures this, RTCA DO-160 and MIL-STD-810, belongs in the material review, not just the box test plan.
The constructions that resist pump-out
The cure is to match construction to joint mechanics. A joint that breathes wants a material that does not flow.
Reinforced pads instead of soft fillers on breathing joints
A reinforced construction (Rogers PROTECT or Sil-Pad TSP) carries a carrier that holds the silicone in place, so the pad conforms under clamp but does not migrate out of a cycling gap the way an unreinforced soft filler will. Soft gap fillers remain the right answer where the gap is genuinely static, but on a joint the drawing describes as cover-compliant or cycling, the reinforced pad is the durable choice. Read both candidates per ASTM D5470 at the joint's real pressure, then let the joint mechanics break the tie.
Graphite sheet, which does not flow at all
Where the joint is grounded, compressible eGRAF HiTherm graphite (such as the HiTherm HT-1205 grade) carries the through-path without flowing, because a solid graphite laminate has nothing to pump out. It conforms by compressing, not by flowing, so a breathing grounded joint that would slowly empty a paste-like filler holds a graphite ply for the life of the box. As always, graphite is conductive, so the isolation scheme is verified first.
State the joint's mechanical behavior on the drawing
The single most useful thing a drawing can carry for pump-out is one word about the joint: bolted-stiff, cover-compliant, or cycling. That word changes the recommendation more than the wattage does, because it decides whether the interface needs to resist migration at all. A bolted-stiff joint can use the soft filler that gives the lowest impedance; a cycling joint needs a construction that stays put.
Match the Construction to How the Joint Moves
| Joint behavior | Pump-out risk | Construction that fits | Electrical note |
|---|---|---|---|
| Bolted-stiff, static gap | Low | Soft gap filler for lowest impedance | Insulating, per ASTM D149 |
| Cover-compliant | Moderate | Reinforced pad that holds position | Insulating |
| Cycling with temperature and altitude | High | Reinforced pad or graphite sheet | Graphite grounded only, isolation verified |
| Grounded and cycling | High | Compressible graphite, does not flow | Conductive, verify creepage and clearance |
Illustrative and relative only; confirm each grade on its current technical data sheet. The joint's mechanical behavior comes from the program's structural analysis; DO-160 / MIL-STD-810 qualification belongs to the box builder.
The pattern: a thermal part with a mechanical failure mode
Pump-out belongs to a family of avionics thermal failures that share a root: the part was specified by its day-one thermal number and installed into a mechanical environment the number does not describe. The companion failures on the parent page, a film powdered under a vibration-punished clamp, a pad specified outside its pressure window, are the same story in different mechanics.
The cure is the same too: name the mechanical environment on the drawing so the construction can be matched to it, and read the thermal comparison per ASTM D5470 within the constructions the mechanics allow.
How H-O Products helps
H-O Products is a precision die-cut converter, not a raw-material producer. We take reinforced dielectric pad, soft gap filler, and compressible graphite stock and die-cut, kiss-cut on liner, and laminate it to your drawing, including graphite laminated to a Kapton dielectric boundary as one part, with material traceability and lot-code TDS records under our ISO 9001:2015 certified quality management system in Winsted, Connecticut.
We help by recommending a construction matched to the joint mechanics you describe and flagging when a soft filler is headed for a breathing joint; the structural analysis that classifies the joint, and the DO-160 or MIL-STD-810 qualification, stay with the box builder.
Frequently asked questions
What is TIM pump-out, and why does it happen at altitude?
Pump-out is the slow migration of a soft thermal interface material out of a joint that flexes. The material is necessarily compliant so it can conform, and that compliance lets each cycle work it a little further out of the gap until the center runs dry and the local impedance climbs. Altitude makes it worse because an airborne joint sees two overlapping cycles: a thermal cycle on every power and ambient swing, and a pressure cycle on every climb and descent that flexes any joint that is not perfectly rigid.
Pump-out integrates every cycle the box ever sees.
How do I stop a thermal interface from pumping out?
Match the construction to how the joint moves. A joint that breathes wants a material that does not flow: a reinforced pad whose carrier holds the silicone in place, or a compressible graphite sheet that conforms by compressing rather than flowing and so has nothing to pump out. Soft gap fillers remain correct where the gap is genuinely static. The most useful step is to state the joint's mechanical behavior, bolted-stiff, cover-compliant, or cycling, on the drawing, because that selects the construction.
Does a higher conductivity pad resist pump-out better?
No, and assuming it does is the trap. Pump-out is a mechanical property of the construction and the joint, not a function of the headline conductivity. A high-conductivity soft filler can pump out of a breathing joint while a lower-number reinforced pad or graphite sheet stays put and outperforms it over service life. Compare candidates per ASTM D5470 at the joint's real pressure, but let the joint mechanics, not the conductivity column, choose the construction on a cycling joint.
Why does graphite resist pump-out when a soft pad does not?
Because a solid graphite laminate conforms by compressing, not by flowing, so there is nothing to migrate out of the gap. A paste-like or very soft filler is worked outward a little on each cycle; a compressible graphite ply simply compresses and springs back in place. On a grounded joint that breathes, that makes graphite a durable through-path choice, provided the isolation scheme is verified first because graphite is electrically conductive.
Does H-O test for pump-out or qualify the joint?
No. H-O converts the reinforced pad, laminated graphite, or soft filler that the joint mechanics call for, to your drawing, and recommends a construction matched to the behavior you describe. Classifying the joint as bolted-stiff, cover-compliant, or cycling comes from the program's structural analysis, and confirming that the interface survives the service life under RTCA DO-160 and MIL-STD-810 belongs to the box builder and its test houses. Our records support that file; they do not replace it.
Have a cycling thermal joint reviewed
Send the drawing with the joint mechanics named (bolted-stiff, cover-compliant, or cycling), the electrical role, the clamp pressure, and the heat density, and we will recommend a construction that stays put and quote die-cut parts.
Samples typically ship in 3 to 5 business days for common die-cut configurations on materials we keep on hand; standard production runs about two weeks after drawing approval; expedited service is available. 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
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- ASTM D5470 (thermal impedance at pressure; compare candidates at the joint's real clamp pressure).
- RTCA DO-160 and MIL-STD-810 (environmental qualification framing capturing thermal and altitude cycling; owned by the box builder).
- ASTM D149 (dielectric breakdown for the isolating pad families).
- Manufacturer technical data sheets for the reinforced dielectric pad, soft gap filler, and compressible graphite grades named on the parent application page.
This article summarizes thermal interface material-selection principles for avionics using the families and published test methods cited on the parent application page. Comparisons are qualitative; per-grade values quoted are maker-TDS figures and must be confirmed on the manufacturer's current technical data sheet at your pressure, thickness, and installed condition. Standards (ASTM, MIL-STD, RTCA DO-160) are cited by designation for context, not as a claim of qualification.
H-O is a precision die-cut converter, not a raw-material producer, and does not perform thermal design, certify materials, or qualify enclosures to DO-160 or MIL-STD-810; those belong to the box builder and its test houses.