Heat Sink Basics

Copper‐HeatsinkBefore liquid two phase technologies became commercially available in the 1960’s, thermal engineers relied largely on the combination of solid metal blocks, usually aluminum or copper, attached to a fin stack of the same material choices. Fans were added to improve airflow through the device, further removing heat.

As devices became smaller and thermal loads increased, a more efficient approach was needed. Two phase devices substantially increased thermal conductivity and heat spreading, removing heat at a much faster rate than their metal only counterparts.

Sintered-Heat-PipeThe process begins with a metal tube or stamped sheet to which copper powder (pictured) or mesh is sintered to the inside wall as part of a baking process. This material forms a wicking structure though which liquid can move once the device is complete. In the case of vapor chambers vertical posts or specialty designed vapor spacers are then added for improved vapor flow and rigidity.

The device is then closed and vacuum sealed, after which water or another working liquid is added. Both the wicking structure (sintered powder, mesh, grooved) and the liquid (water, ammonia, nitrogen) can be changed to alter the thermal transport characteristics of the device.

 

Inner Working of Vapor Chambers and Heat Pipes

A completed two phase cooling module includes one or more heat pipes and/or vapor chambers, a fin stack to dissipate heat into the surroundings, and a mechanical method off connecting the heat sink to the heat source.

Heatpipe Inner Workings

 

When heat is applied to the vacuum sealed tube (evaporator) a portion of the liquid turns to vapor which travels to lower pressure areas. As it does, it begins to cool. Upon reaching the fin stack (condenser) it turns back to liquid where it is absorbed by the wick. Capillary action then moves the water back to the evaporator. This process can be likened to the full absorption of water into a sponge when only its corner is dipped in water. While gravity plays some role in this cycle, the natural capillary action of the wick (sintered metal, mesh, or grooves) is responsible for most of the liquid movement.

Different Types of Two-Phase Devices

Heat Pipe Hybrid 1-Piece Vapor Chamber Traditional 2-Piece Vapor Chamber
How does a heat pipe work One-piece-vapor-chamber-internal-view one-piece-vapor-chamber-2
Initial Form Factor Small diameter tube 3-8mn Very large diameter tube 20-75mn Upper and lower stamped plates
Shapes Round, flattened and/or bent in any direction Flattened rectangular, surface embosing & z-direction bendable Complex shapes in x and y direction, surface embosing
Internal Structure Hollow vapor space Single spacer added for structural & vapor flow integrity Numerous columns added for structural & vapor flow integrity
Sealing Method Swaged and welded on each end Pinched and welded on each end 4 sided diffusion bonding or welding
Typical Dimension Limits 3-8mm diameter or flattened to ~1.5-2mm. Length 500mm+ 2-4mm thick, up to 100mm wide by 300mm long 2.5-4mm thick, up to 100mm wide by 400mm long
Heat Source Contact Indirect contact. Baseplate needed unless flattened & machined Direct contact. Mounting pressure up to 90 PSI Direct contact. Mounting pressure up to 90 PSI
Typical Application Move heat using 1-2 pipes, but 3-5 pipes not uncommon for higher heat applciations Spread heat using a single vaporchamber with high surface area to cool high power/flux application Spread heat using a single vaporchamber with high surface area to cool high power/flux application
Relative Cost Lowest for a single unit, but increases quickly with custom wick structure, flattening and/or machining Comparable to 3-4 heat pipes in higher power and/or high heat flux applications More expensive than 1-piece design due to additional tooling cost and labor time

Heat Pipe and Vapor Chamber Heat Sink Examples

Stand Alone Two Phase Device Heat Sink using Two Phase Device
One Piece Vapor Chamber One-Piece-Vapor-Chamber Heatsink‐using-one-piece-vapor-chamber
Heat Pipe Heatpipe Heatsink-using-heatpipe
Two Piece Vapor Chamber Two‐piece‐vapor-chamber Heatsink‐using‐two‐piece-vapor-chamber