A thermal interface material can have excellent datasheet conductivity and still perform poorly on a production assembly. The problem may be in the cold surface it is coupled to.
A machined cold plate or heat-sink base is rarely perfectly flat, at the scale relevant to heat transfer. Tool marks leave microscopic peaks and valleys. Larger-scale waviness varies the gap across the contact area. When two apparently flat surfaces are clamped together, contact initially occurs at a fraction of their nominal area. Air remains to insulate between the high points.
The thermal interface material fills those voids with something much more conductive than air. Its final performance then depends on the surface geometry, bondline thickness and pressure developed by the assembly.
Those are production characteristics. They can be annotated on a drawing, measured and controlled. When the surface is never verified, a datasheet-perfect TIM can still fail in the field, and no one owns the gap. A supplier of record owns the verified, audit-ready surface.
What a thermal interface material actually does
A machined Aluminum or Copper cold plate or heat sink contacting a processor’s top surface is a typical context for heat dissipation. Under magnification, neither surface looks flat. Contact occurs first at asperity peaks, with air occupying the remaining microscopic volumes.
Air has very poor thermal conductivity. Unfilled polymers are also poor conductors, at around 0.1 W/mK, but thermally conductive fillers can raise polymer-based TIM conductivity to roughly 7 W/mK, around 100 times better than the air it displaces. Common fillers include alumina, Aluminum nitride, Boron nitride and other thermally conductive particles.
The TIM conforms to the surface and displaces air. Experimental work on thermal contact has shown reductions in interface resistance of roughly five times when conductive grease replaces the air at an interface (Yovanovich et al., Electronics Cooling)..
A thicker TIM layer is not automatically better. Once the voids have been filled and adequate contact established, additional material creates a longer conduction path through a material that is still much less conductive than either Aluminum or Copper.
The surface and TIM should be treated as one thermal interface.
Thermal resistance, thermal impedance and conductivity are not the same thing
TIM specifications often mix three quantities that describe different aspects of thermal behavior.
- Thermal conductivity, k, is a material property expressed in W/mK.
For a uniform layer:
R = t / (k × A)
where:
t = layer thickness
k = thermal conductivity
A = heat-transfer area
A material with high conductivity can still produce an undesirable interface if the bondline is unnecessarily thick.
- Thermal resistance, normally expressed in K/W or °C/W, describes the temperature rise associated with a given heat flow through the tested interface.
Rth = ΔT / Q
where:
Rth = thermal resistance
ΔT = temperature difference across the interface
Q = heat flow
- Area-normalized thermal resistance or thermal impedance removes the test area from the result and is often reported in K·cm²/W or K·in²/W.
The unit matters when comparing datasheets. To convert Kin²/W to Kcm²/W, multiply by 6.45.
A conductivity value by itself does not specify the assembled interface. Bondline thickness and contact resistance remain in the heat path.
TIM classes and where they fit
The material form changes how the interface is assembled and how sensitive it is to the machined surface.
Thermal grease
Grease can form a very thin bondline and readily fills fine surface irregularities. Interface resistance around 1 Kcm²/W is achievable in representative applications, with optimized systems reaching approximately 0.2 Kcm²/W.
It requires controlled application. Too much grease increases bondline thickness. Too little can leave areas without complete coverage.
Thermal cycling presents another problem. Repeated expansion and contraction can gradually displace grease from the joint, generally described as pump-out. A grease that performs well in an initial bench test may therefore require cycling qualification for the intended assembly.
Gap pads and gap fillers
A compliant pad is useful when the interface has to accommodate a larger or less predictable gap. It can also provide electrical isolation.
Typical dielectric pad thicknesses range from 5 to 15 mil. Representative interface resistance for an elastomeric pad around 0.25 mm thick can lie in the 1 to 3 Kcm²/W range.
Compression matters. Pads commonly require contact pressures up to roughly 700 kPa to reach their intended compressed condition. The mounting hardware and the cold plate must tolerate that load on the device without excessive deflection.
Gap fillers cover interfaces where dimensional variation is too large for a very thin TIM bondline. Their thermal penalty is often accepted because maintaining physical contact across the tolerance stack is more important than achieving the minimum possible interface thickness.
Phase-change materials
A phase-change TIM is relatively firm during handling and softens when the assembly reaches its transition temperature. Common transition temperatures soften at 55 to 80°C.
After softening, the material can flow into surface irregularities and produce a thin interface. Representative thermal resistance values are around 0.3 to 0.7 Kcm²/W.
The operating temperature has to reach the transition region for the material to behave as intended. This can apply during qualification of equipment that otherwise spends much of its service life below the phase-change temperature.
Gels, adhesives and metallic interfaces
Thermal gels can accommodate irregular geometry without requiring the handling characteristics of a precut pad. Representative interface resistance may fall around 0.4 to 0.8 Kcm²/W.
Thermally conductive adhesives provide a structural function as well as a heat path. Some systems can achieve interface resistance below 1 Kcm²/W, with optimized products approaching 0.15 Kcm²/W. Once the adhesive cures, serviceability changes substantially because the interface is no longer intended to separate easily.
Metallic TIMs and solder can reach much lower interface resistance, sometimes below 0.05 K·cm²/W. Their use introduces metallurgical compatibility and process-temperature requirements that do not exist for a conventional grease application.
Thermal pad or thermal paste?
The decision usually starts with the gap the assembly has, under typical assembly conditions.
A well-controlled interface between two sufficiently flat surfaces can support a thin grease bondline. Grease conforms readily to small machining irregularities and imposes negligible dimensional stack of its own.
A 0.5 mm variation between a device and cold plate is a different problem. A thin grease layer is not intended to bridge that distance. A compliant pad or gap filler can absorb the geometric variation.
Assembly requirements impose additional constraints. A precut pad provides a defined material quantity and position. Grease application needs its own process control, particularly where dispensing volume affects the finished bondline.
For assemblies exposed to repeated thermal cycling, pump-out may become the deciding concern even when grease gives the lower initial resistance.
Specify the machined surface on the drawing
A drawing that specifies the TIM but leaves the mating surface under a general machining tolerance has left part of the thermal design under-controlled.
The functional contact area should be identifiable. Flatness and surface texture then apply to the surface that actually transfers heat.
Flatness
Flatness controls the larger-scale departure of the mating face from an ideal plane.
ISO 1101 provides the geometrical tolerancing framework for specifying flatness. The tolerance should apply to the finished functional surface, not merely the cold-plate blank before subsequent processing.
Despite a 100 x 100 mm cold plate being nominally flat after machining, later brazing may bow the base by a variable degree. A thin conductive grease layer cannot be expected to compensate uniformly for the resulting gap. Increasing clamp force may pull some of the distortion out, but it also changes loads on the device and cold plate, potentially straining the package or the solder joints.
For a thermal interface, the appropriate flatness limit comes from the permissible bondline, and the mechanical compliance available in the assembly.
A drawing might therefore identify a specific TIM contact zone and apply:
FLATNESS 0.05 mm OVER TIM CONTACT AREA, FINISHED CONDITION
The words finished condition are critical, when the component subsequently passes through joining, coating or other process capable of distorting the surface.
Surface roughness
Roughness describes the microscopic textures. This differs from flatness, which is a macroscopic measure.
Surface roughness can be specified using parameters defined through ASME B46.1 or ISO 21920. Ra is widely used, although it does not describe every aspect of a surface that can influence contact.
A rough surface creates deeper local valleys for the TIM to fill. Making the surface progressively smoother can reduce the volume of interface material required, but there is little value in specifying an expensive optical finish simply because the part carries a thermal interface.
The required Ra should be developed with the selected TIM and its intended bondline. A grease applied at high contact pressure behaves differently on the same surface from a relatively thick elastomeric pad.
The inspection method belongs with the requirement. A profilometer can verify the specified roughness over a defined evaluation length and direction.
Co-planarity and parallelism
One cold plate may contact several devices at once.
If three processor packages sit beneath the same machined plate, local surface finish can be acceptable while one contact area sits higher than another. The first device takes clamp load before the others have developed the required interface pressure.
The drawing needs geometric control between those mounting areas that share a common interfaced heat transfer part.
Depending on the datum structure and assembly function, this may be handled through profile, parallelism, dimensional control or a defined co-planarity requirement. The chosen control should correspond to the way the assembled devices locate against the plate.
Bondline thickness is a design output
Bondline thickness, or BLT, is the final thickness of TIM between the two mating surfaces under the assembled condition.
For a simple uniform layer:
RTIM = BLT / (k × A)
where:
RTIM = thermal resistance of the TIM
BLT = bondline thickness
k = thermal conductivity
A = heat-transfer area
Reducing BLT shortens the conduction path. In an actual joint, however, BLT is produced by the interaction of surface geometry, material rheology or compressibility and assembly pressure.
It should not be treated as an arbitrary minimum.
Contact pressure changes BLT
Greases and phase-change materials can seat at pressures around 300 kPa in representative applications. Elastomeric pads may require around 700 kPa to achieve their intended compression.
As clamp pressure rises, a grease layer becomes thinner and excess material moves outward. A pad compresses and displaces outwards according to its stress-strain behavior.
The pressure seen by the TIM also depends on the stiffness of the hardware. A nominal screw torque does not guarantee uniform interface pressure across a large cold plate. Local plate bending can produce high pressure near fasteners and a thicker bondline toward the center.
If BLT is a critical thermal characteristic, the assembly and cold-plate stiffness belong in the same tolerance analysis.
Why the thinnest possible bondline can fail
Very low BLT leaves little capacity to absorb flatness variation. Local regions may lose material coverage when the mating surfaces move under thermal cycling.
Grease is also susceptible to pump-out. Repeated changes in package and cold-plate dimensions can drive material away from the active region.
Pads behave differently under prolonged compression. Compression set can reduce their ability to follow later movement of the interface.
A practical BLT therefore has to remain compatible with the actual surface and service environment.
Alloy matters
6061, 7075, 2xxx alloys and high-Silicon Aluminum castings should not be assumed to respond identically to anodic coating.
There is therefore no universal rule that hardcoat grow every surface by 0.001 in.
Anodizing type, alloy, pretreatment, process conditions and geometry all influence the end result.
For repeat production, the actual processor’s demonstrated results on the specified alloy and geometry are more valuable than a generic guide allowance.
Verifying thermal interface performance
There are distinct areas of inspection required in production. The materials, machined surfaces and geometry of parts can be measured directly or source-validated. The thermal behavior of the completed interface requires a controlled thermal test.
ASTM D5470
ASTM D5470 is the principal test method used for measuring thermal transmission properties of thermally conductive electrical insulation and related interface materials.
The test places the specimen between controlled meter bars and applies heat through the stack. Temperature measurements and heat flow are used to determine the thermal behavior of the interface.
Results can be evaluated over different bondline thicknesses, to separate bulk material resistance from contact interface influences.
A D5470 result is useful only when the test conditions are understood. Pressure, thickness and temperature affect many TIMs. Comparing two headline conductivity values while ignoring those conditions will likely produce the wrong material selection.
For production hardware, D5470 data establishes the TIM side of the interface. It does not verify that the delivered cold plate has the flatness or surface condition assumed when the TIM was selected.
CMM inspection of the mating face
Flatness should be inspected after processes capable of distorting the part are complete.
A CMM can measure the functional surface against the drawing’s geometric tolerance. The measurement strategy needs enough points to reveal the form errors that matter over the contact area.
Sparse sampling can miss a local high or low region. A large cold plate with a broad TIM interface may justify a denser measurement pattern than an ordinary machined mounting pad.
If several device locations share the same plate, the CMM program can also inspect their relative position.
Profilometer inspection
Surface roughness requires a separate measurement.
The profilometer trace should correspond to the specified standard and evaluation parameters. Measurement direction matters on a strongly directional machined surface because a trace parallel to milling marks can produce a different result from one crossing them.
The drawing can state the required Ra and any measurement direction needed for the functional surface.
A generic SURFACE FINISH 1.6 µm Ra ALL OVER often spends machining effort on irrelevant faces, while saying too little about the thermal interface itself.
Upstream manufacturing can move the surface
A cold plate may meet its machining inspection and lose flatness during the next process.
Vacuum brazing exposes the assembly to elevated temperature and can leave residual distortion after cooling. Friction stir welding introduces localized heat and plastic deformation. Either process can move a broad mating face enough to affect a thin TIM bondline.
Flatness inspection therefore belongs after the last step capable of materially changing the surface.
If final machining follows brazing, the process plan needs enough stock to recover the required contact face. Machining may also have to preserve wall thickness above internal coolant passages.
Anodizing changes the surface condition again. Coating thickness affects dimensions, while pretreatment and handling can alter the condition of a carefully prepared mating face. Some thermal interfaces are masked from anodizing; others use the finished anodized surface. The drawing should make that distinction explicit.
Cleaning before TIM application
A surface can meet its Ra and flatness requirements and still be unsuitable for TIM application.
Machining coolant and cutting oil can remain in fine surface texture after ordinary handling. Oxide films develop on exposed metals. Particles introduced during storage or assembly can locally increase bondline thickness.
Cleaning requirements depend on the TIM chemistry and the product environment. A bonded interface may require more stringent surface preparation than a removable gap pad.
The useful acceptance condition is the surface immediately before TIM application. Packaging and handling after machining are therefore part of maintaining that condition.
Finger contact on the mating face is easy to prevent and difficult to inspect quantitatively after it has occurred.
Aerospace, defense and controlled environments
Thermal hardware used in aerospace or vacuum service can introduce material restrictions beyond ordinary thermal performance.
Silicone-containing TIMs may require outgassing evaluation. ASTM E595 is used to assess mass loss, and net volatile condensable material from materials intended for vacuum environments.
Dissimilar metallic interfaces can introduce galvanic concerns. MIL-STD-889 provides guidance for controlling dissimilar-metal combinations in applicable defense hardware. A metallic TIM or conductive filler can change the material combination present at the joint.
These requirements should be established before the TIM and surface treatment are frozen. A material selected purely from its conductivity value can create a qualification problem elsewhere in the assembly.
IPC-A-610 may also apply to workmanship aspects of electronic assemblies containing the thermal interface, depending on the product and assembly configuration.
Fitting the interface into production quality control
AS9100 or another quality-management system controls how the supplier manages production. It does not determine the required cold-plate flatness or surface roughness.
Those characteristics belong on the engineering definition.
For aerospace hardware, AS9102 first article inspection can record the relevant surface characteristics. A ballooned drawing can tie the functional contact area’s flatness and roughness requirements to measured results.
PPAP can carry the same characteristics into repeat production where that framework applies. A control plan may call for periodic profilometer inspection while a critical flatness characteristic is checked by CMM at a different frequency.
Traceability becomes useful when the final surface depends on several manufacturing stages. If a brazed cold plate later fails flatness inspection, its records should identify the joining lot and the final machining process used to recover the contact face.
Surface callouts need revision control as well. Changing from a pad to grease can alter the acceptable roughness, flatness or bondline requirement without changing the cold plate’s basic geometry.
Approving a supplier for machined thermal hardware
Start with a drawing containing an actual thermal interface. Ask the shop how it would manufacture and inspect that face.
For a brazed cold plate, the important flatness result is the one obtained after brazing and any final machining. A supplier showing inspection data from a pre-braze plate has not demonstrated the finished condition. That is the black-box behavior to reject: accept the finished-condition result or nothing.
Review the measurement setup used for the mating surface. A CMM program should sample enough of the area to detect the expected distortion. For roughness, look at the profilometer method and where the trace is taken.
Then follow the surface through cleaning and packaging. A carefully machined interface can leave the inspection room within tolerance and arrive at assembly with oil residue or handling contamination.
The production documentation should preserve the drawing revision and inspection results associated with the shipped lot. Where AS9102 or PPAP applies, those records can sit inside the corresponding approval package.
Jiga’s role is on the machined-hardware side of the interface. As supplier of record, Jiga controls the surfaces and dimensions that decide how the specified TIM behaves once the assembly is clamped, and owns that result: on time, on spec, audit-ready.
Frequently Asked Questions
What is a thermal interface material and why does surface finish matter?
A thermal interface material fills microscopic and macro-scale gaps between mating thermal surfaces, replacing poorly conducting air. Surface finish affects the size and distribution of those gaps. Flatness controls larger form error, while roughness describes finer texture. Both influence the bondline the TIM has to fill under the available contact pressure.
Thermal pad vs. thermal paste: which should I specify?
Thermal paste suits closely controlled mating surfaces where a thin bondline can be maintained. Pads are useful when the interface has a larger gap, requires electrical isolation or needs a more controlled assembly material. Pressure, tolerance stack, service temperature and thermal cycling should be evaluated using the intended assembly, rather than from conductivity alone.
What flatness and surface roughness should I specify for a cold plate or heat sink?
There is no universal flatness or Ra value for every TIM interface. The limit should follow from the allowable bondline, TIM type, contact pressure and size of the mating area. Apply the flatness requirement to the finished functional surface and specify roughness using an appropriate standard such as ASME B46.1 or ISO 21920.
What is bondline thickness and how do I control it in production?
Bondline thickness is the compressed thickness of TIM remaining between the mating surfaces in the assembled condition. Surface flatness, TIM quantity or initial thickness, material compressibility and clamp pressure influence the result. Production control can use defined hardware, assembly torque or force, dimensional limits and direct BLT verification where the characteristic is critical.
How is thermal interface performance tested and verified?
ASTM D5470 is widely used to characterize thermal transmission through TIMs under controlled conditions. The machined hardware requires its own verification. CMM inspection can measure flatness and relative surface geometry, while a profilometer measures surface roughness. Assembly-level thermal testing can then confirm performance under the intended pressure, bondline and operating condition.
