Due to limited space in consumer electronics, traditional heat sinks and fans become irrelevant anymore. Besides, thin film graphite sheets, from 20-100um thick, gradually phase out as well due to limited thermal dissipation capability and weak mechanic performances, particularly poor layer to layer cohesion. Meanwhile slim heat pipes and vapor chambers are taking the momentum.
The basic structure of a heat pipe includes copper tube (wall), wick in the inner wall, and phase changing fluid. The wall can be copper, aluminum and other metal or alloys. The wick can be sintered powder, groove, screen mesh. Water is typically adopted as the phase changing fluid for consumer electronics. In the applications, heat comes in from the evaporator end. liquid water evaporates and gas flows to the condenser end carrying heats. Then gas cools down and turns into liquid, flow back from wick. The wick structure is the one of key factors in determining the cooling performances. Heat pipes work very well as 1-D heat transfer; however it is limited for 2-D heat dissipation. Vapor chambers are quite useful instead.
As shown in above picture (cited from Prof. Garimella SV, Purdue University), the working principles of Vapor Chambers (VC) are quite similar as that of heat pipes. For simple understanding, it can be considered as a flattened heat pipes. The thickness of ultra slim VC has been reduced 0.35mm. The following link can be a good introductory video.
VC can be made starting from either metal foils or tubes (pictures from Delta website).
For super slim VCs, the thickness of VC is about 400um currently, and it is projected to be about 250um within a couple of years. Once it gets thinner, the following issues become increasingly important:
- Ultra thin copper foil may not be mechanically strong enough; other high performance metals, e.g. Ti, Stainless steel, or metal alloys are needed. However only a few companies can make these kind of foils.
- If different metals are used, galvanic corrosion between different metals may not be ignored.
- Water purity. De-ionized water is required to avoid metal corrosion
- Sealing. One important failure mode is the sealing fail, and water leaks out with time, leading to degrading performances.
The following are three videos about heat pipe manufacturing: part 1, part 2, part 3.