heat pipe introductions to the inner workings, configurations, usage conditions and heat sink options for heat pipes and vapor chambers

The Importance of Heat Pipes & Vapor Chambers

Heat pipes and vapor chambers are used to increase the thermal performance of heat sinks that would otherwise use only a solid metal base and fins. This is possible because their effective thermal conductivity is, in most cases, dramatically higher than that of solid aluminum or copper. For reference, the thermal conductivity of aluminum is ~235 W/(mK), copper is ~400 W/(mK), and two-phase devices are typically upwards of 6,000 W/(mK) – often dramatically higher. Unlike solid metal, the effective thermal conductivity of two-phase devices changes depending on a host of variables, but mainly with the distance the heat is transferred. The longer the distance, within reason, the higher the thermal conductivity – all else being held constant. See online heat pipe performance calculator.

As you know, CPUs, GPUs, FETs, and a lot of ASICs, among others have increased in thermal design power (Watts of dissipated heat) and, perhaps more importantly, power density (W/cm2). This thermal problem is compounded if the required max operating temperature (Max Ambient) of the final device increases and/or if available airflow across the heat sink fins is limited (lower airflow in CFM). For clarification, increases to Max Ambient decreases the thermal budget (more on this later), potentially requiring a more efficient heat sink. Decreases to airflow, increase the required size of the heat sink, making it more difficult for solid metal to effectively move/spread heat to all corners of the sink. These are the broad indicators that a two-phase device may be beneficial.

Heat Pipe Design & Working Principles

Heat pipes have three elements: a vacuum sealed enclosure, a wicking structure, and a working fluid. By a large margin, the most common type is a copper enclosure, a sintered copper wick structure that bonds to the interior surface, and de-ionized water as the working fluid. This configuration generally translates to non-space environments with required Max Ambient of less than 80 oC and will be the config presented throughout these posts.

The graphic below illustrates the inner working of a heat pipe. The liquid turns to vapor at well below 100 oC because it’s in a vacuum and the vapor moves to the condenser because it’s an area of lower pressure. After the vapor condenses and becomes liquid, the sintered wick is responsible for returning it back to the evaporator. This process is called capillary action – the same principle that will soak the entirety of a paper towel if only one corner is exposed to water.

Heat pipes are typically available in sizes from 2-12mm diameter and can be flattened and bent. Moreover, wick properties such as thickness and porosity can be changed to tune thermal performance (Qmax or maximum power carrying capacity in watts). Click here to use an online heat pipe performance calculator to understand Qmax by pipe size and angle of orientation. A few points:

  • Larger diameter heat pipes have higher Qmax.
  • Qmax is additive. If one pipe can carry 20W, two can carry 40W and so on.
  • Qmax is reduced when the pipe is bent, the capillary action is against gravity, required operating altitude above sea level increases, and often when the pipe is flattened (a small amount of flattening typically won’t affect it).

The working principles of vapor chambers are identical to heat pipes. In fact, vapor chambers are often referred to as planar heat pipes. The distinction really comes down to the width to height aspect ratio. A flattened heat pipe typically won’t exceed 4:1, whereas a vapor chamber can go up to around 60:1.

Typical Configuration & Uses

Rules of Thumb

  • Use vapor chambers to spread heat across the base of a local fin array (condenser).
  • Use heat pipes to move heat to a remote fin array or enclosure wall.

 

There are always exceptions, but here are the reasons. Heat pipes can be bent in any direction making them ideal for snaking around PCB components. This makes them well suited for moving heat to a remote condenser which most often requires some maneuvering. Conversely, vapor chambers have a continuous internal vapor space. This allows heat to be distributed in every direction to the remote corners and edges of the fin array, maximizing total fin efficiency.

Telltale Signs You Might Need a Heat Pipe or Vapor Chamber Device

Here’s a list of conditions where two-phase devices might be considered:

  • Having to move heat more than 25-50mm from the heat source to a remote condenser. Below this, solid copper bar or rods will be almost as effective.
  • When the bottom area (base) of a local fin array is greater than 10X the area of the heat source. Remember, lower air flow means larger fin area for a given heat source. This often translates to a larger base footprint as you might not have the vertical space (Z-height) and you certainly won’t have the fin efficiency to increase fin height indefinitely. See online heat sink size calculator.
  • If a solid copper heat sink (fins and base) meets thermal requirements, but not weight/shock and vibe requirements. A solid copper base is considerably heavier than a comparable vapor chamber base. Additionally, using a two-phase base may allow the use of aluminum fins, further reducing weight.
  • When thermal budgets are below 40 oC, especially when that’s coupled with low/no airflow. To calculate thermal budget, subtract the max operating temperature at which the finished device is designed to operate (Max Ambient) from the max case temperature (Tcase) of the IC – or junction temperature for bare die ICs (Tjunction). This second figure will be provided by the IC manufacturer. You can use our online heat sink calculator to determine the total delta-T of your heat sink and compare that to your thermal budget.

Types of Heat Sinks Most Used with Two-Phase Devices

The vast majority of heat sinks are solid aluminum (sometimes copper), off-the-shelf, and manufactured using extrusion or die casting techniques. They’re typically rather small and cool low power and low power density electronics. These are the least expensive and least efficient in terms of thermal conductivity (performance).

As more demands are placed on the thermal solution, bonded, skived or forged fin heat sinks take center stage, albeit at increased prices as the manufacturing process is more involved – as well the fact they are often custom designs.

What you’ll see most often paired with heat pipe or vapor chamber designs are zipper fins (also called fin packs). These are literally just fins with no base. They are low weight and can achieve a very high fin density. Vapor chambers can be soldered to the bottom of these or heat pipes can be run through the center. However, the king of thermal conductivity are generally machined heat sinks. They offer the additional benefit of allowing for very complicated designs. As you’ve guessed, two-phase devices can often boost the already impressive performance of these sinks.

Attaching the Heat Sink to the Heat Source

The primary reason for considering a heat pipe or vapor chamber solution is improved performance. As such, I’m not going to talk about the use of thermal tape or epoxy as the primary means of attaching the heat sink to the die. We generally stick with one of several mechanical attachment methods where we can meet Mil or NEBS shock and vibe requirements. Additionally, the thermal resistance of thermal interface material (TIM) improves as the pressure between the die and the heat sink is increased. An inexpensive attachment method for small (low mass) heat sinks are stamped mounting plates. Although it requires two PCB holes, this method offers better shock and vibe protection thermal tape or epoxy and some TIM compression – although still only 5 PSI.

 

Figure 6 shows spring loaded plastic or steel push pins further increase TIM compression up to around 10 PSI. Installation is fast and simple but removal requires access to the back of the PCB. Push pins should not be considered for anything more than light duty shock and vibe requirements. Spring-loaded screws offer the highest degree of shock and vibe protection (Figure 7) as they are the most secure method of attaching a heat sink to the die and PCB. They offer the highest TIM preload at roughly 75 PSI and work very well with heavy heat sinks.