Heat Pipe Degradation & Failure

Easily controlled reasons for heat pipe degradation and failure include the purity /compatibility of working fluid and enclosure metal as well as flaws in the manufacturing process such as improper pre-production cleaning, fluid charge, and tube evacuation.

On the customer side, reasons include using the heat pipe outside its design parameters such as high-power loading, unplanned temperature or environmental exposure. Fortunately, most producers and consumers of heat pipes diligently strive to prevent these mistakes.

Anecdotally, it’s not uncommon to hear of two-phases devices operating well for 20-30 years, with other parts of the device failing beforehand. Empirically, the de facto test method is accelerated life testing where the device is required to operate at elevated temperatures for extended periods.

In a 200 Deg C elevated temperature test that’s been running continuously for seven years, heat pipes are subject to temperatures well above normal operating conditions and hot enough to cause impurities and incompatible materials to outgas and collect in the condenser, causing an increase in delta-T.

Thermocouples at each end measure the heat pipe delta-T and heat pipes are tested for full performance (Qmax vs delta-T) at the beginning of the test and at regular intervals thereafter.

Over the seven-year duration, delta-t remains stable and within the expected range. To estimate lifespan, researchers have suggested using the Arrhenius model which predicts a doubling of failure rate for every 10 oC increase in temperature. Unfortunately for the calculation, we haven’t yet had a failure of any heat pipes. Celsia’s guidance: heat pipes that are properly designed, manufactured, and used will outlast the useful life of electronic applications.