This blog post offers some intermediate level vapor chamber heat sink design guidance for the most prevalent types of electronics applications: CPU/ASIC to amplifier applications with power ranging from around 20-250 watts, power densities greater than 20 W/cm2, and heat source sizes of between 10-30mm square. I’ll also focus on the most ubiquitous type of vapor chamber – a copper envelope with sintered copper wick using water as the working fluid.
Vapor Chambers Can Have Advantages Over Heat Pipe or Solid Metal Base Heat Sink Designs
Although vapor chambers are basically planar heat pipes, they have some key advantages. They are more isothermal than either solid metal or heat pipe based solutions. This allows a more uniform temperature across the die face (reduced hot spots) as well a more uniform temperature across the entire face of the vapor chamber (lower delta-T). Secondly, heat sinks using a vapor chamber allow engineers to have direct contact between the heat source and the device, reducing solution thermal resistance; heat pipe solutions usually require an additional base plate and TIM layer.
Use Vapor Chambers to Spread Heat and Heat Pipes to Move Heat
Heat pipes are ideal for connecting the heat source to a remote fin stack especially as this often involves a series of twists and turns. While vapor chambers can sometimes be used if the path is linear, we most often see vapor chambers used when the fin stack is attached or at the least in close proximity to the heat source.
Some possible exceptions to this rule of thumb: when two or more heat sources need to be held close in temperature and when you’ve got a tight thermal budget.
Use Vapor Chambers When Thermal Budgets are Tight
Thermal budget is simply the maximum ambient temperature at which the end product will operate minus the maximum temperature of the die case. For many outdoor or rugged applications, thermal budget can be below 40oC.
That means that the sum of all individual delta-Ts (from TIM to Air) must be lower than the calculated thermal budget. For typical applications in this category, we generally need the delta-T of the heat sink base to be 10oC or less. Visit our online calculator to see the difference in heat sink delta-Ts for your application.
The Area of the Vapor Chamber Should be at Least 10X the Area of the Heat Source
Like heat pipes, vapor chamber thermal conductivity increases with length. This means that a vapor chamber the same size as the heat source will offer little advantage over a solid piece of copper. A good rule of thumb says that the area of the vapor chamber should be equal to or greater than ten-times the area of the heat source. In situations where the thermal budget is large or when a lot of airflow drives a small fin stack this may not be an issue. However, it’s often the case that the base of the sink needs to be considerably larger than the heat source.
If you haven’t yet calculated the required heat sink size, we’ve got another online calculator that will give you an estimate of the total heat sink volume. From there you can make some basic assumptions about the required XYZ dimensions.
Match the Type of Vapor Chamber to the Application
While everyone is familiar with a traditional vapor chamber that’s made from two stamped pieces of metal (2-piece design), there’s another method for producing these devices that offer some unique advantages.
For shapes other than a rectangle, a 2-piece vapor chamber is needed because the stamped plates can be created in virtually any shape along the XY planes. Additionally, they’re able to have a higher embossment should the heat source be recessed.
A handful of manufacturers are now producing a 1-piece vapor chamber – called so because it begins life as a very large single copper tube. While its shape is limited to a rectangle, it can be bend in the Z-direction forming steps or U-shapes.
Other Vapor Chamber Design Considerations
Dimensions – While there are very few limits, vapor chambers used for cooling electronics rarely exceed 300-400 mm in both X and Y directions. Thickness is a function of both wick type and power to be dissipated. Sintered metal wicks, the most common type, drive a vapor chamber thickness of between 2.5-4.0mm.
Power Density – Ideal applications for vapor chambers are those where the heat source power density is greater than 20 W/cm2 but these devices can handle in excess of 300 W/cm2.
Protection – Nickel plating is the most common coating used for both heat pipes and vapor chambers. It’s used to protect against corrosion if the device might be exposed harsh environments or simply for aesthetic reasons.
Operating Temperatures – While vapor chambers can withstand numerous freeze/thaw cycles, their typical operating temperature range is between 1-100 oC.
Clamping Pressure – Vapor chambers are usually designed to withstand 60psi of pressure before becoming deformed. However, they can be altered to support up to 90psi.