Thermal Shock testing is meant to accelerate failure modes due to very rapid changes of temperature (<10 seconds liquid-to-liquid).  Examples of these conditions include hand or wave soldering, self-heating of power semiconductors or turn on of optical devices.  Liquid-to-liquid thermal shock uses heating/cooling by conduction; whereas air-to-air temperature cycle testing is by convection and therefore a slower process.  
    
Test conditions for thermal shock normally specify 15 cycles and a minimum dwell of 2 minutes.  MIL-STD-883 specifies three temperature ranges for Thermal Shock: 0°C to 100°C, -55°C to 125°C, or –65°C to 150°C.  Telcordia GR 468 specifies 0°C to 100°C, which can be performed in water. The two higher ranges require the use of a perfluorocarbon fluid with low freezing point and high boiling point.  Silicon Cert Laboratories uses Galden D02-DS, which has a useful range of -97°C to +175°C, in both hot and cold tanks.  The basket, holding the parts under test, switches from one tank to the other in eight seconds.  It returns to the load/unload position automatically after the specified number of cycles.