A variable-height thermal-interface assembly for transferring heat from a heat source to a heat sink comprises a slidable interface between two contacting surfaces, the slidable interface inclined diagonally relative to the z-axis. The two contacting surfaces slide relative to one another parallel to the incline direction to provide z-axis expansion of the assembly. The assembly further comprises a spring clip, which when released applies a shear force across the slidable interface, causing the two contacting surfaces to slide relative to one another, coupling the sliding to provide z-axis expansion. The assembly further comprises a reversible locking device, which when locked prevents the two contacting surfaces from sliding relative to one another, such that the spring clip remains retracted, and when unlocked allows the two contacting surfaces to slide relative to one another, such that the spring clip is released.