Vapor chambers have several performances and design advantages over heat pipes. First, they are more isothermal than solid metal or heat pipe-based solutions. This results in more uniform temperatures across the mold surface (fewer hot spots), as well as more uniform temperatures across the surface of the vapor chamber (lower delta-T).
Second, the use of a heat sink in the vapor chamber allows direct contact between the heat source and the device, reducing the interface thermal resistance. Heat pipe solutions typically require an additional base plate and TIM layer.
Third, height-constrained thermal solutions typically benefit from vapor chambers because they make the base thinner, while fin stacks allow more fin area because the heat pipe typically passes through the center of the fin stack.
When the ratio of vapor chamber to evaporator area is greater than 10:1
As with heat pipes, the thermal conductivity of the vapor chamber increases with length. This means that a vapor chamber of the same size as the heat source has little advantage over a solid piece of copper. A good rule of thumb is that the area of the vapor chamber should be equal to or greater than 10 times the area of the heat source. This may not be a problem with a large heat budget, or when a large airflow is driving a small fin stack. However, it is often the case that the bottom of the sink needs to be much larger than the heat source.
Using a vapor chamber when the primary goal is to spread heat
While vapor chambers can sometimes be used to transfer heat to remote radiators, we most often see vapor chambers used to disperse heat to local radiators. Heat pipes are ideal for connecting a heat source to a remote fin stack, especially since this often involves a series of twists and turns.
Thermal Conductivity and Performance of vapor chambers
When looking at the effective thermal conductivity of heat pipes and vapor chambers, it appears that vapor chambers have a lower thermal resistance than heat pipes. They do. This is because the steam chamber has a large cross-sectional area compared to a typical heat pipe. An average 6 mm heat pipe has a cross-section of 28 mm2, while even a small steam chamber, 3 mm x 40 mm, has a cross-section of 120 mm2 (dT = Q*L/(k*A)).
If the same power is transmitted, then the effective thermal conductivity decreases in proportion to the cross-section. A key point to remember is that despite the lower effective thermal conductivity of VCs, they offer performance advantages such as higher total capacity, better gravity-resistant operation, direct contact with the heat source, and lower delta-ts.
Integration of vapor chamber heat sinks
Vapor chambers can be attached to any type of heat sink (extruded, pried open, etc.), but are most commonly paired with zippered fins (also called fin packs) or machined heat sinks. There are two reasons for this. First, both types of heatsinks have very good thermal performance; zippered fins due to the ability to have very thin, closely spaced fins, and machined fins due to the almost infinite geometric design options. Sometimes we see them successfully paired with die-cast housings and in extreme environments with integrated fins.
Regardless of the heatsink type, the vapor chambers must be attached to the base/fins. They are either soldered (most often) or secured to the bottom of the fin stack with epoxy, the former offering better thermal conductivity. The solder used for these assemblies has a thermal conductivity of 20 to 50 W/mK, while the epoxy has 1/10 the thermal conductivity of the solder, making them suitable only for low power density applications at <10 W/cm2.