Fabrication and evaluation of a copper flat micro heat pipe working under adverse-gravity orientation

The fabrication of a prototype flat micro heat pipe (FMHP) of a size appropriate for mobile electronics and its performance test results are reported. To ensure reliable operation under repeated thermal loads and to enhance the heat transport capacity, copper is selected as the packaging material considering its high thermal conductivity and good strength. The wick structure of the FMHP consists of fan-shaped microgrooves with a width and depth of about 100 and 200 mu m, respectively. The fabrication of microgrooves was done using a laser micromachining technique and water was used as the working fluid. Fan-shaped microgrooves were found to induce a greater capillary pressure than triangular microgrooves of a similar size. Subsequent test results confirmed that despite its small size, 56 mm (L) x 8 mm (W) x 1.5 mm (H), the FMHP had a high heat transport capacity; the maximum heat transfer rate was 8 W under stable operation conditions and 13 W at the dryout point. In addition, the FMHP worked under adverse-gravity conditions with little change in cooling capacity, a key advantage for application in modern mobile electronics.