PCM's behaviour can be explained as a combination of two interfaces: As the power is turned on and the components are still cool, the system behaves like a dry joint and the temperature difference rapidly increases to about 13°C. As the system temperature increases, the PCM melts and the clamping pressure exerted by the spring clips forces the liquid to spread in the thermal joint. As the liquid spreads, the molten PCM displaces the interstitial air and the distance between the surfaces decreases. Both of these processes act to reduce the thermal resistance of the interface and the temperature difference is seen to decrease fairly rapidly, reaching the performance of thermal grease. In effect, the solid PCM film has turned into thermal grease and a grease-like joint has been formed. The next time the processor is turned on, the interface will not experience the large temperature difference because the thermal joint has already been established.