This article is intended as a follow-on to a couple of posts we made in the past about using vapor chambers rather than heat pipes. If you haven’t read those and are not familiar with designing with vapor chambers, we suggest you reference this links before continuing.
Quick Tools for Determining if Vapor Chambers Should be Considered
Most suspicions that vapor chambers may be the right choice can be validated using two online calculators. The first estimates heat sink size based on a few parameters like power and available airflow. This one is important because larger heat sinks will have more conduction loss in the base (a higher delta-T) which may put you over your thermal budget.
The second calculates total heat sink delta-T (from TIM to Fin-to-Air) and compares a sink with an aluminum or copper base to one with a vapor chamber base. While CFD modeling will refine these results and prototype testing will solidify them, these are two good first steps to take.
Vapor Chamber Manufacturing and Operational Limits
We should cover the design and use limits of the vapor chamber itself before moving on to how they’re integrated into a thermal solution
Power Handling Capability
Vapor chambers and heat pipes have roughly the same power handling capacity, from a few watts to a kilowatt. However, if one heat pipe can meet thermal and physical requirements, it’s probably cheaper to use them – depending on post-production operations like fly-cutting. That’s why a move to vapor chambers from heat pipes usually involves higher power and/or higher power densities. Anything less and heat pipes may suffice.
Shapes & Dimensions
The traditional method for producing vapor chambers begins with two stamped plates, mirror images of each other, that eventually get diffusion bonded together. This gives the designer enormous leeway in the X and Y dimensions. Length and width max dimensions are governed by press and furnace size as well as applications requirements. Consequently, you typically don’t find vapor chambers in excess of around 300 x 300mm.
A few manufacturers also have the capability to produce vapor chambers that start as a very large tube (25-70 mm diameter) which is sintered, flattened, and has an internal support structure added to it. The main advantages are cost and the ability to be shaped into L and U configuration. The drawback is they can only be produced in rectangular shapes. Dimensional limits due to manufacturing capability for these typically range in the 100mm wide to 300mm length.
Both types of vapor chambers, particularly when designed with a sintered wick structure, are between 2.5-4mm thick depending on the power to be moved or spread.
Two-piece vapor chambers made of two stamped plates are generally not bent post stamping. Any small ‘steps’ or bends are done as part of the stamping process. However, one-piece vapor chambers that start as a tube are bend post-production in the factory. While band radius changes somewhat depending on vapor chamber width, thickness and location of the bend, a typical bend radius is on the order of >/=12mm.
Vapor chamber surface flatness is particularly important because, unlike heat pipes, they are designed to make direct contact with the heat source. Flatness is controlled in the component contact areas to a nominal flatness is .002”/1” but post-machining, while adding cost, can bring this down to 0.001”/1”. This is typically only necessary when mating to higher power density components with similar flatness for very thin bond line thickness and low interface resistances.
Because heat pipes can handle up to 180-200 oC before structural damage to the vessel compromises performance (a cylinder is an inherently strong shape), doesn’t mean a vapor chamber can do the same. Without modifications, vapor chambers can withstand deformation to around 110 oC. For a copper water vapor chamber to handle higher temperatures, the wall thickness needs to be increased, additional internal support structures are added, and/or an exoskeleton (metal plate) is used on one side of the vapor chamber (the other side is supported by the base of the heat sink).
Vapor chambers are hollow and require internal support to withstand clamping pressures. Standard designs use supports for up to 60psi of pressure before becoming deformed. However, they can be altered to support up to 90psi.
All copper parts are passivated to protect against short term discoloration. Nickel plating is the most common coating used for both heat pipes and vapor chambers for corrosion protection or cosmetic reasons.
Vapor Chamber Heat Sink Options & Attachment
Vapor chambers can be attached to any kind of heat sink (extruded, skived, etc) but most often they are paired with zipper fins, also known as fin packs, or machined heat sinks. There are two reasons for this. First, both of these heat sinks have very good thermal performance; zipper fins due to the ability to have very thin, closely spaced fins, and machined due to virtually infinite geometrical design options. Sometimes we see them successfully paired with die-cast housings with integrated fins used in extreme environments.
Regardless of heat sink type, vapor chambers must be attached to the base/fins. They are soldered (most common) or epoxied to the base of the fin stack, the former having better thermal conductivity. Solders used for these assemblies have thermal conductivities on the order of 20 to 50 W/mK while epoxies are on the order of 1/10th of solder conductivities which makes them only useful for low power density applications <10 W/cm2.
Soldering takes place at temperatures generally above the max temp for vapor chambers so special care must be taken in designing solder fixtures. These fixtures must be able to withstand the internal pressures generated in the vapor chamber during the soldering process to prevent vapor chamber deformation. The pressure chart below indicates the internal vapor chamber pressures vs temperature.
The solder fixture (shown below in purple) is designed to conform to that of the heat sink assembly, preventing it from deforming during the soldering process. The upper and lower portions are clamped or bolted together to prevent the vapor chamber from expanding.