Thermal interface materials sit between a hot chip and a cold heatsink. Their job is to fill the microscopic air gaps between two imperfectly flat surfaces. Under compression, the material deforms, filling more gaps, improving thermal contact. The assumption is that this process is reversible — release the pressure, the material springs back, the thermal resistance returns to its original value.
Square-pulsed thermoreflectance measurements under mechanical load show that gels and thermal pads exhibit pronounced hysteresis. Compress them and the thermal conductivity increases as the material densifies and the interface improves. Release the pressure and the conductivity doesn't fully return to its pre-compression value. The bulk densification and interfacial contact changes persist — the material remembers being compressed.
Vacuum grease behaves differently. Its bulk thermal properties are nearly pressure-independent — squeezing it doesn't change the material's intrinsic conductivity. But its interfacial thermal resistance is strongly pressure-dependent. The grease itself doesn't change under load; what changes is how well it contacts the surfaces.
The distinction reveals two different mechanisms operating in the same measurement. For gels and pads, compression changes the bulk material. For grease, compression changes the interface. The same external variable — pressure — drives different internal processes depending on the material's mechanics. A thermal interface material's performance under load depends on which mechanism dominates, and you can't know which dominates without measuring bulk and interfacial contributions separately.
The simultaneous measurement of three thermal parameters under load makes this separation possible for the first time. Previous methods assumed one mechanism or the other. The reality is both, in material-dependent proportions.