Thermosyphons for Electronics Cooling

Thermosyphons for Electronics Cooling

This blog is written to give engineers a basic understanding of an often forgotten, but still relevant two-phase cooling solution – thermosyphons.

Recently, I visited the Henry Ford Museum in Michigan. A fabulous place that’s steeped in the kind of history that reminds one that the ‘tech revolution’ started well over one hundred years before our modern use of the term.  I’m talking about more than just the innovation that allowed the Model-T’s assembly time to be cut from 12 hours to 2.5 hours. It was an effort in materials innovation coupled with a desire to keep things as simple as possible, in some cases eschewing innovation for less costly, more dependable solutions.

The decision to rely on an atmospheric radiator cooling solution is one such example of simplicity in place of innovation. Although pumped liquid cooling was in use by Daimler at the time, Ford decided he didn’t need the added cost or point of failure.  The solution is known as a thermosyphon, which is a simple open or closed loop self-pumping mechanism that relies on the fact that hot water rises while cool water sinks. As the water in the engine block channels is heated, it’s forced up and into the radiator, where it is cooled and sinks to the bottom of the block where the cycle is repeated.

Thermosyphons are still used today in solar heaters and furnaces, but it’s their use in electronics cooling that I want to focus on today, albeit in a slightly different form. Certainly, the first step in the discussion should be to understand what’s generally known as a heat pipe thermosyphon. Let’s compare them to standard heat pipes in terms of design, limitations, and benefits.

Differences Between Thermosyphons & Heat Pipes

Thermosyphons can only be used when gravity is used to move liquid back to the evaporator

Thermosyphons used for electronics cooling are known as “Heat Pipe Thermosyphons” (HPTS) because they mimic their heat pipe counterparts in all but one way: the wick structure is fully or partially removed. But, why is this important and how do they work without this seemly critical component removed?

As you know, the wick structure inside a heat pipe is used to move liquid from the condenser to the evaporator after is has changed state from vapor to liquid at the condenser (fin stack). This capillary action allows efficient liquid transport even if the heat pipe is in an orientation that is neutral or against gravity. Without this built-in liquid transport mechanism, HPTS must rely on gravity to move the liquid back to the heat source (Fig 1 – Thermalfluidscentral.org). For this to occur, the angle should generally be greater than +5 degrees with the evaporator below the condenser.

For a given diameter, thermosyphons have a higher Qmax than heat pipes

The sintered wick lining the walls of a heat pipe reduces the available vapor space, a key component to determining a device’s Qmax. This isn’t a problem for heat pipes as it’s the capillary limit that determines its ability to transport heat. The chart below shows Qmax for various heat pipe sizes where the evaporator is directly below the condenser (+90 degree orientation).

Wickless thermosyphons of the same diameter as heat pipes have 100-200% higher Qmax than the heat pipe counterparts, all because of the additional vapor space. See Celsia’s heat pipe performance calculator.

Thermosyphons can carry heat farther distances than heat pipes

The simple reason thermosyphons operate more effectively over longer distances has to do with the ease with which liquid can travel from the condenser to the evaporator. With a heat pipe, the water is transported through the wick structure which is usually porous sintered metal.  In a thermosyphon, water easily flows back to the evaporator along the smooth or grooved inner walls. While a heat pipe’s practical heat transport limit is on the order to 1 to 2 meters, a thermosyphon can easily carry heat distances over 10 meters. This usually isn’t an issue with electronics cooling as most applications will use these devices in distances less than a couple of meters, but worth mentioning nonetheless.

 

Thermosyphons are susceptible to damage caused by freezing

The fluid level required for a thermosyphon generally covers between 20-80% of the length of the evaporator, assuming the vessel is in a vertical orientation. This is significantly more than traditional heat pipes and can be problematic if water is used in conditions where they are exposed to freezing temperatures. After repeated freeze/thaw cycles, the expansion of water will form bulges in the pipe, eventually fracturing the tube wall.  For HPTS designs using other fluids, freezing is generally not a concern.

Water (used in a copper system) is the highest performing fluid for most electronics temperatures followed by alcohols and then refrigerants. The advantage of non-water based working fluids is their ability to operate below freezing temperatures. Common combinations with proven life are copper/water, copper/alcohol, aluminum/refrigerant.

Thermosyphon Variations

There are several variations of this basic design that can improve thermal performance and/or reduce the possibility of structural damage due to freezing.

Add a wick to the evaporator section

A grooved or mesh wick will reduce thermal resistance, enabling higher power densities in the evaporator. However, this solution will do nothing to prevent damage caused by freezing as the same amount of liquid is required.

Sintered wicks will lower the thermal resistance to the greatest level. It also allows optimization of the fluid charge, effectively reducing the required liquid. This all but eliminates the possibility of damage caused by freezing. In the bottom portion of the above figure, note that the wick is only used at the evaporator section of the thermosyphon.

Create separate loops for the vapor and liquid

Lastly, a looped thermosyphon can be used to separate the vapor flow from the liquid flow, further increasing Qmax. The vapor flow leads directly to the condenser where it cools and returns to liquid which travels back down to the evaporator. In the configuration to the right, the hollow evaporator at the bottom of the device uses a mechanism to prevent vapor from traveling up the liquid flow section, but still allowing liquid to flow in. The addition of a sintered wick to the evaporator allows less water to be used, decreasing the risk of damage caused by freezing.

 

Using Thermosyphons to Cool Electronics

Taking into account both the advantages and drawbacks of thermosyphons – when should they be used in electronics environments? Historically the number one use for these are in power electronics applications and recently some use in data centers. Power electronics would include stationary motor controllers in places like steel mills, mining etc. Also moving applications such as light rail and subway systems. Recently there has been renewed interest for data center applications as they move towards higher ambient temperatures and a reduced number of fans. Cooling in data centers accounts for about 33% of the operating costs, with cooling fans accounting for almost half that figure.

In summary, thermosyphons are still a relevant technology for cooling high power applications where the evaporator is below the condenser. Applications include power electronics such as IGBTs as well as radar systems, transmitters, and alternative energy generation.

 

Design Considerations When Using Heat Pipes (Pt. 2)

Design Considerations When Using Heat Pipes (Pt. 2)

Now that we’ve determined which heat pipes can effectively carry the required heat load (Pt 1), we need to ensure we select and size the heat exchanger to dissipate the heat into the surrounding air -based on available air flow. As mentioned in the prior post, this article is not intended to provide detailed training/equations on the proper design of heat pipes and heat sinks, but rather to offer guidance on the number and size of heat pipes used as well as to provide some tips for estimating heat sink size and determining attachment methods of the sink to the PCB. Types of Heat Sinks There are numerous choices from zipper pack fins to extruded fin stacks, each with their own cost and performance characteristics.  While the heat sink choice can markedly affect heat dissipation performance, the biggest performance boost for any type of heat exchanger comes with forced convection.

Types of Heat Sinks

Table 1: Types of Heat Sinks

Estimating Required Heat Sink Volume By using the simple equation below, we can estimate the overall volume of the heat sink required to cool a heat source. V= (Q*Rv)/Delta T Volume is the outer dimensions of the heat sink. Q is power in watts and Rv is the volumetric thermal resistance (cm3-C/W). Table X provided some well established guidelines for the latter under different air flow rates.  The first thing to notice is the enormous positive affect forced air has on thermal performance. Even with a gentle air flow, volumetric thermal resistance of the heat sink is reduced by two-thirds.

Air Flow vs Volumetric Thermal Resistance

Table 2: Air Flow vs Volumetric Thermal Resistance

Let’s say we have a 50W heat source (Q) who’s specified maximum operating junction temperature (Tjunction) is 100o C. Further, let’s assume we’re designing this device to operate in a max ambient temperature of 70o C – a rugged environment. By subtracting the ambient temperature from the maximum allowable electronics temperature, you arrive at the delta t: in this example it’s 100 – 70 = 30o C delta t. If we are designing a rugged computer, for instance, we’ll probably have a semi or fully sealed enclosure meaning that our heat sink needs to be designed for natural convection. The resulting heat sink volume would be: V = (50*500)/30= 833 cm3 = a really large heat sink of 9.4 x 9.4 x 9.4 cm. Obviously, we wouldn’t have a heat sink with 9.4 cm high fins in a natural convection environment so the length and width dimensions would have to grow substantially.  But, we might surmise that this application would probably need to use the chassis lid as the heat sink. There’s an example of just such an application that I talked about in a prior post on ruggedized electronics and whose heat load, ambient operating temperature, and fanless design match the above example.  Let’s look at just the lower half of the design. The bottom side of the PCB contains the processor and chipset at roughly 50W. A copper plate in conjunction with two heat pipes are used to spread and move heat to the chassis case, whose published dimensions are 25cm (L) X 15cm (W).  We know the overall thickness of the assembled case is 7.5 cm and it looks to me the top and bottom lids are each roughly 2.5 cm thick. This would result in an overall volume of 938 cm3 which is within 15% of the required 833cm3 we calculated earlier.

Figure 3: PerfecTron MIL-810 Computer

Figure 1: PerfecTron MIL-810 Computer

Had this same heat source been used in conjunction with forced air, the required heat sink volume would have been reduced by up to ten fold. Although beyond the scope of this article, the next steps in heat sink design would be some detailed excel modeling followed by CFD analysis. This will enable engineers to fully understand optimal base thickness, fin pitch, fin height, and base/fin material among other things. Mating Heat Pipes to the Condenser Whether we’re dealing with a heat exchanger that is local or remote to the heat source, the options for mating a heat pipe(s) to them are identical. Grooved Base It should go without saying that simply soldering a round pipe to a flat surface is far from optimal.  Circular or semi-circular grooves should be extruded or machined into the heat sink. It’s advisable to size the grooves about 0.1mm larger than the diameter of the heat pipe in order to allow enough room for the solder. The heat sink in Figure 2 uses both a local and remote heat sink.  The extruded heat exchanger is design to accommodate slightly flattened heat pipes, helping to maximize the contact between the copper mounting plate and the heat source. A remote stamped fin pack is used to further increase thermal performance. These types of heat exchanger are particularly useful because the pipes can run directly through the center of the stack, decreasing conduction loss across the fin length.  Because no base plate is required with this fin type, weight and cost can be reduced. Again the holes through which the heat pipes are mounted should be 0.1 mm larger than the pipe diameter. Had the pipe been completely round at the heat source, a thicker grooved mounting plate would have been required as seen in Figure 3.

Heat Pipe Mounting Options to Heat Sink

Figure 2 (L): Heat Pipes in Grooved Base     Figure 3 (C): Heat Pipes in Grooved Mounting Block     Figure 4 (R): Direct Contact Heat Pipes

Direct Contact If conduction losses due to the base plate and extra TIM layer are still unacceptable, further flatting and machining of the heat pipes allows direct contact with the heat source as seen in Figure 4 above. Performance gains from this configuration usually lead to between a 2-8o C reduction in temperature rise.  In cases where direct contact of the heat source to the heat pipes is required a vapor chamber, which can also be mounted directly, should be considered due to its improved heat spreading capacity. Attaching the Heat Sink to the Heat Source The primary reason for considering a heat pipe 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 pressure between the die and the heat sink is increased. An inexpensive attachment method for small (low mass) heat sinks are stamped mounting plate. 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.

Attaching Heat Sink to PCB

Figure 5 (L): Stamped Mounting Plate           Figure 6 (C): Spring Loaded Push Pins                Figure 7 (R): Spring loaded Metal Screws

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. About Celsia Celsia is a custom heat sink supplier specializing in heat pipe and vapor chamber thermal assemblies.  We are a U.S. based company, with wholly-owned Taiwan manufacturing facilities, that prides itself on solving difficult cooling challenges under tight deadlines.  Please contact us to schedule a thermal review of your next application.

Design Considerations When Using Heat Pipes (Pt. 1)

Design Considerations When Using Heat Pipes (Pt. 1)

This blog post offers some intermediate level heat pipe design guidance for the most prevalent types of electronics applications: mobile to embedded computing and server type applications with power ranging from 15-150 watts using processor die sizes of between 10x10mm to 30x30mm.  I’m using these constraints because throughout the article I’ll mention some simple tips or guidelines that don’t necessarily apply when talking about power electronics applications.  I’ll also be focusing on the most ubiquitous type of heat pipe – copper tube with sintered copper wick using water as the working fluid.

As you might have read in past blogs, heat pipes should be considered when the thermal design is conduction limited or when non-thermal goals such as weight can’t be achieved with other materials such as solid aluminum and/or copper.  So, if you’re too the point where heat pipes may be an option, keep the following things in mind.

Thermal Conductivity of Heat Pipes Rarely Reach Published Limits

Regularly published heat pipe thermal conductivity range from 10,000 to 100,000 W/mK. That’s up to about 250 times that of solid copper and 500 times that of solid aluminum.  But don’t count on those figures for typical electronics applications.  Unlike solid metal, the effective thermal conductivity of copper heat pipes varies tremendously with heat pipe length, and to a lesser degree with other factors.

Figure 1 illustrates the effect of length on heat pipe thermal conductivity. In this example, we used three heat pipes to transport heat from a 75 watt power source. While thermal conductivity of 10,000 W/mK is achieved at just under 100mm heat pipe length, a 200mm length has less than one-third the typically published maximum thermal conductivity of 100,000 w/mK.

Figure 1: Heat Pipe Thermal Conductivity Changes with Length

Figure 1: Heat Pipe Thermal Conductivity Changes with Length

 

Heat Pipe Designs Are Like Automobiles

Performance data for heat pipes are usually of the Honda Accord variety: adequate for most things, but horrible if you have a specific purpose in mind – like track racing or off-roading.  Even when limiting our conversation to copper/water/sintered wick versions, heat pipe customization can markedly affect operational and performance characteristics.  And just like cars, design changes to a heat pipe range from the simple to the exotic.

As we’ll see later, flattening and bending will negatively affect heat pipe performance but are an easy customization that allow engineers to navigate the often crowded route from the evaporator the condenser. If the application requires lower thermal resistance than a typical heat pipe assembly provides, the heat pipes can be machined to create a flat surface capable of making direct contact with the heat source, eliminating the solid metal base plate and extra TIM layer.  A heat pipe whose diameter is different from one end to the other may be needed so that the evaporator end completely covers the heat source while the rest of it remain small enough for tight bends. These changes allow engineers to physically connect the heat pipe to mating components from the heat source to the condenser.  But what if the heat pipe still can’t meet power handling or orientation requirements?

Changes to the internal structure of the heat pipe design, most notably wick porosity and wick thickness, allow heat pipes to be tuned to meet specific operating parameters and performance characteristics.  For instance, when a given diameter heat pipe is required to operate at higher power loads or against gravity, the capillary pressure in the wick needs to increase.  For higher Qmax, this means a larger pore radius. For effectively working against gravity (condenser below evaporator), this means a smaller pore radius and/or increased wick thickness.  Additionally, it is possible to vary both wick thickness and porosity along the length of a single tube. Suppliers who specialize in heat pipe customization will regularly use custom formulated copper powders and/or unique mandrels to ensure the final product meets applications requirements.

Mechanical Engineering Choices Affect Heat Pipe Performance

Once the internal structure of the heat pipe design has been determined, it’s good to know how outward physical characteristics affect performance.  With heat pipes, size generally matters most. However, changes to outward design will degrade performance for any given heat pipe.

Flattening

Figure 2 shows power handing capacities (Qmax) for the most common heat pipe sizes. As noted earlier, Qmax may change amongst vendors for standard (Honda Accord type) heat pipes, but I’m taking some data from recent projects we’ve done – without getting too exotic.

Heat Pipe Heat Transport Capability

Figure 2: Heat Pipe Qmax is Customizable

Typically, sintered copper heat pipes can be flattened to a maximum of between 30-60% of their original diameter.  I know, some will tell you it’s the lower figure before the centerline starts to collapse but it’s really a function of technique – we regularly flatten our one-piece vapor chambers, which begin life as a very large heat pipe, down to 90%+. I wish I could provide a rule of thumb for how much performance will degrade for every 10% decrease in thickness, but it would be irresponsible.  Why?   The answer comes down to how much excess vapor space is available before the heat pipe is flattened.

Simply put, there are two performance limits important for terrestrial heat pipe applications: the wick limit and the vapor limit. The wick limit is the ability of the wick to transport water from the condenser back to the evaporator. As mentioned, we can tune the porosity and thickness of the wick to specific applications, allowing for changes to Qmax and/or ability to work against gravity.  The vapor limit for a particular application is driven by how much space is available for the vapor to move from the evaporator to the condenser.

The orange and blue lines in Figure 3 plot the respective limits for the various heat pipe sizes shown in Figure 2.  It’s the lesser of these two limits that determine Qmax and as shown the vapor limit is above the wick limit, albeit only slightly for the 3mm heat pipe.  As heat pipes are flattened, the cross sectional area available for vapor to move is gradually reduced, effectively moving the vapor limit down. So long as the vapor limit is above the wick limit, Qmax remains unchanged.

Heat Pipe Wick and Vapor Limits

Figure 3: Heat Pipe Wick & Vapor Performance Limits

In this example, we’ve chosen to flatten the heat pipes to the specifications in Figure 2. As seen by the gray line, the vapor limit is below the wick limit, reducing the Qmax.  Flattening the 3mm by a mere 33% causes the vapor limit to become the determining factor whereas we had to flatten the 8mm by over 60% for this to happen.

Bending

Bending the heat pipe will also affect the maximum power handling capacity.  Here again I’ll offer a couple rules of thumb. First, minimum bend radius is 3x the diameter of the heat pipe. Second, every 45-degree bend will reduce Qmax by about 2.5%. From Figure 2 above, an 8mm heat pipe, when flattened to 2.5mm, has a Qmax of 52 watts. Bending it 90 degrees would result in a further 5% reduction. The new Qmax would be 52-2.55 = 49.45 watts

For more technical information on bending both heat pipes and vapor chambers, please visit a past blog on how bending affects heat pipe performance.

Working Against Gravity

Let me start this section by saying that we’re using the same fairly ‘standard’ heat pipes that we’ve shown in prior examples so they haven’t been optimized to work against gravity (-90o).  Figure 4 illustrates how the relative position of the evaporator to the condenser can affect both Qmax and heat pipe selection. In each case, Qmax is reduced by roughly 95% from one orientation extreme to the next.

Heat Pipe Orientation

Figure 4: Affect of Orientation on Heat Pipe Performance

In situations where the condenser must be place below the evaporator, we would use a sintered material that allowed for smaller pore radius and/or increase the wick thickness.  For instance, if we optimized the 8mm heat pipe for use against gravity (-90o), we could raise its Qmax from 6W to 25W.

Heat Pipe Design Calculations: A Practical Example

Let’s use an example to illustrate how different heat pipe designs might be used to solve a thermal challenge.  Assume the following as know constraints: 70W heat source with dimensions 20 x 20mm and a single 90 degree heat pipe bend required to transport heat from evaporator to condenser. Further, the heat pipes will operate in a horizontal position.

  • To be at their most effective, heat pipes need to fully cover the heat source, which in this case is 20mm wide. From Figure 2, it appears we have two choices: three round 6mm pipes or two flattened 8mm pipes. Remember the three 6mm configuration will be placed in a mounting block with 1-2mm between the heat pipes.
  • Heat pipes can be used in conjunction to share the heat load. The 6mm configuration has a Qmax of 114W (3x38w), while the flattened 8mm configuration has a Qmax of 104W (2x52w)
Figure 5: Heat Pipe Configuration Choices

Figure 5: Heat Pipe Configuration Choices

 

  • It’s just good heat pipe design practice to build in a safety margin. You wouldn’t go bungee jumping with a rig only capable of supporting exactly your weight, would you? Same for heat pipes. I typically use 75% of their maximum capacity. So we’ve got 85.5W for the 6mm (75% x 104W) and 78W for the 8mm (75% x 104w)
  • Now we need to factor in bending. A 90 degree bend will cause Qmax of each configuration to be reduced by another 5%. The resulting Qmax for the 6mm configuration is now just over 81W and for the 8mm configuration it’s now 74W, both of which are higher than the 70W heat source we’re trying to cool.

As can be seen, both heat pipe design configurations are adequate to move heat from the evaporator to the condenser. So why choose one over the other? From a mechanical perspective it may simply come down to heat sink stack height at the evaporator. As you can see, the 8mm configuration has a much lower profile than does the 6mm configuration.  Conversely, condenser efficiency may be improved by having heat input in three locations versus two locations, necessitating the use of the 6mm configuration.

Please check back for Part 2 of this blog where I’ll be discussing condenser types and sizing as well as methods to attach the heat pipe assembly to the heat source.

Rugged Electronics: Thermal Challenges and Heat Pipe Solutions

Rugged Electronics: Thermal Challenges and Heat Pipe Solutions

The challenges faced by thermal engineers tasked with cooling rugged electronics are often extensive. This blog post is meant to provide an overview of ‘rugged’ standards as well as address some thermal management techniques being used.

Electronics for both military, marine and industrial applications are exposed to a variety of harsh conditions from extreme temperatures, wet or dusty environments, and situations where components are subject to repeated shock or exposure to corrosive substances.

Thermal engineers designing rugged systems must content with enclosures/chassis that offer little to no airflow and reliability requirements that limit design options. Here are a few of the US standards used to define whether ruggedized devices can stand up to harsh environments.

One the oldest and most widely used US standard is a military specification called MIL-810. It’s been used by all Department of Defense agencies since with 1960’s (with numerous revisions) and is regularly adopted by the private sector for use in commercial/industrial applications.  One of the advantages of MIL-810 is that test requirements, duration and sequences are clearly defined.  There are around thirty test categories ranging from the common like temperature and water resistance to the niche like pyro-shock and acceleration, but equipment is rarely, if ever, designed to pass all of them.

The two most often mentioned ‘rugged’ electronics standards for the industrial and commercial markets are NEMA-250 and IP. Issued by the National Electronics Manufacturers Association (NEMA), specification 250 details a protection rating for electronic enclosures which ranges from limiting access to hazardous system components to degrees of protection from dust, water and corrosives.   The Ingress Protection Ranking (IP Code) rates devices on the degree of solid particle and liquid protection as well as impact protection. Additionally, several ANSI accredited VITA standards specify environmental requirements for different classes of COTS plug-in modules.

While we’ve touched on a few US standards/specifications for harsh environments, it should be noted that that there are many others from the extensive German DIN standards to STANAG standards used by NATO defense forces.

Irrespective of the specific ‘rugged’ standard, thermal engineers need to be creative with how heat is managed in electronic devices designed for harsh conditions. Depending on the required range of operating temperature and level of protection against vibration, shock, dust, sand, and or water rugged electronics chassis and thermal management design can take on many forms.  Let’s use a familiar example: a general purpose computer with Intel Core i7 processor and some level of internal expandability.

The first is a Comark Enduranode PC certified under IEC-60945 for marine use in a protected environment like the bridge of a ship. Test requirements/ranges are beyond that of a consumer PC and include Environmental, Safety and Electromagnetic: dry and damp heat, low temperature, vibration to simulate propeller and wave slamming, corrosion resistance, noise levels, limited access to high voltages within the chassis, and improved protection against electromagnetic interference.

Figure 1: Rugged Marine Computer

Figure 1: Rugged Marine Computer from Comark

Figure 1 shows the outside of this unit. It uses the small form factor chassis as a heat sink incorporating an external extruded aluminum fin stack with a corrosion resistant coating.  A solid state hard drive was chosen for reliability and resistance to vibration reasons. Additionally, all electronics are conformal coated to protection against moisture, dust, chemicals, and temperature extremes – in this case -20 – +50oC without fan and up to +60oC when the internal system fan is active.

The unit is vented on both sides and incorporates an on-demand fan but because I couldn’t find any internal pictures of this unit, I’ll make some assumptions about how heat is being managed. In addition to the already mentioned conformal coating on all electronics, heat generated from the 50 watt combined CPU and chipset could be spread using an aluminum baseplate, possibly with a low profile fin stack directly above it. Copper heat pipes are almost certainly used to transport heat to the finned chassis lid.

Figure 2: Heat Pipe Heat Sink

Figure 2: Heat Pipe Heat Sink

Figure 2 is a rendering of a heat sink for a different product (Logic GL-L05) with similar dimensions and thermal design, but could be representative of what’s in the Comark computer. Additionally, the power source may have its own fan and/or be a high efficiency version which generates less heat.

Let’s compare this to a more rugged MIL-810G computer designed to operate in environments from -40oC – +70oC (without CPU throttling) and withstand mechanical shock, temperature shock, and vibration as set forth in the MIL specification. The chassis is fanless and non-vented although not completely sealed due to the I/O ports, however it is tested for dust resistance. The 50 watt combined CPU and chipset are soldered on-board as is the solid state drive for improved shock and vibration capability.  While the electronics and power of this system are comparable to the Comark, the PerfecTron SR100 has an external volume roughly half that of the Comark unit.

Figure 3: PerfecTron MIL-810 Computer

Figure 3: PerfecTron MIL-810 Computer

For this example, I won’t have to speculate on the thermal solution as an exploded diagram was available from the PerfecTron website (Figure 3). Residing on the top side of the PCB are I/O functionality and the power source to which a copper heat spreader is attached. The spreader mates to two copper heat pipes embedded into the base of the upper heat sink lid. The CPU and chipset, located on the opposite side, are similarly attached to a copper spreader with heat pipes embedded in a lower heat sink lid.

While I’ve chosen rugged designs that both use heat pipes, other options are available to thermal engineers depending on the ambient temperature as well as the size, weight and power goals (SWaP) of the rugged electronic device. Here’s a concise article from Military Embedded Systems Magazine on Airflow Through, Liquid Flow Through & Spray Cooling.

Heat Pipe Reliability – What You Know is Wrong!

Heat Pipe Reliability – What You Know is Wrong!

Last week I gave an IMAPS presentation on heat pipe reliability with Ross Wilcoxon from Rockwell Collins. Thought I’d give you the highlights in graphical form along with some comments.

Product reliability is determined by understanding the reliability metrics for each of the components and or sub-assemblies within the final product. But, does the current thinking about heat pipe reliability make sense?

Calculating System Reliability

 

How to Test Heat Pipe Reliability

 

Neither Ross nor I have never seen or heard about heat pipes failing… for reasons other than stupidity or planned obsolescence.

Reasons for Heat Pipe Failure

 

There’s a lot of conflicting market and research data about the mean time between failure (MTBF) of heat pipes.

Heat Pipe Reliability Conflicting Data

Here’s the conventional reliability assurance approach with heat pipes.

Conventional Heat Pipe Reliability Approach

Temperature and failure are correlated; chemists and semiconductor folks believe it, but does a much adhered to rule of thumb apply to heat pipes. Namely, every 10 degree Celsius temperature rise will double the failure rate.

Temperature and Failure are Correlated

 

In examining past research, it appears that some erroneous assumptions were made.

Heat Pipes and Arrhenius Equation

 

Ross plotted the findings from a heat pipe failure study and showed that they did follow an Arrhenius curve. Then he estimated the activation energy; a key portion of the Arrhenius equation.

Heat Pipe Reliability Data Follows an Arrhenius Curve

 

He then used the estimated activation energy of -4507 to determine failure rate predictions at various temperatures. He found that our much believed rule of thumb (double the failure for every 10 deg. C rise) was only true for one point on the graph.

Reliability of Heat Pipes Doesnt follow a Rule of Thump

 

As you can see below, the observations made in the Mochizuki study (that heat pipes follow an Arrhenius curve) should not have lead to the assumption that it followed the half-life rule of thumb. While the blue curve, below, seems to be a better approximation of heat pipe reliability in the field, it’s based on an erroneous assumption.

Big Differences in Heat Pipe MTBF

 

To determine a more accurate predictive test, we need to find a suitable activation energy. The problem is we’ve yet to get the most common type of heat pipes (water, sintered copper, copper) to fail in the lab.

Celsia Heat Pipe Test Data

 

Celsia is continuing its elevated heat pipe testing until we can determine a new activation energy. In the mean time, companies should remember so long as heat pipes are properly manufactured and tested for the most common early types of failures heat pipes should last in excess of 30 years. Here are some simple guidelines.

Heat Pipe Reliability Test Requirements

Here’s a specific example of how we test at Celsia.

Heat Pipe Testing Process

To summarize..

Heat Pipe Reliability Summary

Guidelines for Circular Heat Pipe Thermal Testing

Guidelines for Circular Heat Pipe Thermal Testing

For today’s blog I’d like to present some work several of my colleagues and I have been working on for submittal to the fall session of the JEDEC Committee: Guidelines for Heat Pipe Thermal Test Set for Testing of Circular Heat Pipes.

As we’re always open to comments before submittal, please feel free to do so at the bottom of this blog or in one of the LinkedIn groups in which this is posted.

1.  Scope

This document provides guidelines to establish the functionality required of a circular heat pipe test set which can be used to consistently measure the performance of a straight, circular heat pipe. This document, coupled with JESD51-12-01 which defines the heat pipe thermal test procedure, JESD51-73 which defines terms and definitions and JESD51-xx which defines the test results reporting format, provide a family of guidance which allow for consistency of process.

The primary goal of this document is to establish consistent test set parameters for heat input, heat removal and power and temperature measurement such that presented heat pipe test results are consistent across the industry. Suppliers may choose to limit the extent of data extracted from a test but for that data which is presented, the results should conform to the test set criteria established by this document. In doing so, the output of the thermal test set defined by this document provides a common basis for discussion between heat pipe suppliers and users.

2.  Normative References

The following standards contain provisions that, through reference in this text, constitute provisions of this guideline. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on these standards are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below.

[1] JESD51-70, Overview

[2] JESD51-73, Terms and Definitions

[3] JESD51-12-01, Circular Heat Pipe Test Procedure

[4] JESD51-xx, Heat Pipe Thermal Test Data Sheet

3.  Terms, Definitions, Symbols and Abbreviations

Standard JESD51-70 [1] is an overview document addressing the performance testing and characterization of heat pipes (circular, flat and vapor chamber). Terms, definitions and symbols are defined in JESD51-73 [2].

4.  Thermal test set variables

The following are a list of the process variable outputs which the referenced thermal test set must be capable of controlling/outputting:

  •  Condenser temperature, Tc (°C)
  • Evaporator temperature, Te (°C)
  • Adiabatic temperature, Ta-e, Ta-c (°C)
  • Input power (W)
  • Pipe orientation with respect to gravity

5.  Components of the Thermal Test Set which Impact Consistency of Test Results

To allow for consistency and comparability of thermal test results, factors which impact the measured performance of a heat pipe must be kept consistent.

The items defined in [4] above are the test outputs required to effectively compare the thermal performance capabilities of a heat pipe. To facilitate consistency and comparability in generating those results, the test set elements must be consistent so that the results from the test set are comparable from test to test, be they multiple tests from a single vendor or comparative results from alternate vendors. The critical elements of the test set are identified below and the recommended configuration of this standard for each of these elements will be fully defined in subsequent sections of this standard.

  • Evaporator – the heat input length and the definition of single sided or fully encased, uniformity of clamping pressure, uniformity of heat flux input to the heat pipe, the interface between the evaporator section of the heat pipe and the heat source
  • Condenser – the heat rejection length and the definition of single sided or fully encased, the uniformity and rate of heat rejection from the condenser attachment, the interface between the condenser section of the heat pipe and the heat sink, ability to control/maintain heat pipe condenser temperature as power level is adjusted
  • Temperature measurement – temperature measurement locations and temperature measurement method
  • Power input – ability to incrementally adjust power level
  • Orientation with respect to gravity – ability to maintain/adjust orientation of the test article with respect to gravity

6.  Thermal Test Set Description

A representative configuration of the circular heat pipe test fixture is shown in Figure 1.

Circular Heat Pipe Test Set

Figure 1 – Circular Heat Pipe Test Set

 

The evaporator block of the test set should fully encapsulate the heated length of the heat pipe. It should be split along its centerline and should be machined to assure that it exerts a uniform clamping force on the heat pipe such that when the evaporator block is clamped in place the heat pipe cannot be rotated or extracted by pulling on the pipe with manual force.

The evaporator block should be heated on only the lower clamping surface. The evaporator block material should be copper. The conduction length between the nearest heat pipe surface and the electrical heater surface should be a minimum of two heat pipe diameters separated. (See Figure 2)

The evaporator block should have a passage for a spring loaded thermocouple that will interface on the heat pipe surface, on the heated side, centered in the heated length.

The clamping force, when the heat pipe is inserted into the evaporator block should be a minimum of xxx N/m2 of force.

The interface between the heat pipe and the evaporator block should be coated with a light layer of thermal interface grease prior to installation of the test article.

Figure 2 - Evaporator Block Design

Figure 2 – Evaporator Block Design

 
The condenser block of the test set should fully encapsulate the cooled length of the heat pipe. It should be split along its centerline and should be machined to assure that it exerts a uniform clamping force on the heat pipe such that when the condenser block is clamped in place the heat pipe cannot be rotated or extracted by pulling on the pipe with manual force.

The condenser block should be cooled on only the lower clamping surface. The condenser block material should be copper. The conduction length between the nearest heat pipe surface and the liquid coolant passage in the condenser block should be a minimum of two heat pipe diameters separated. The method of cooling the condenser block should be liquid cooling circulated though coolant passages in the condenser block. (See Figure 3)

The condenser block should have a passage for a spring loaded thermocouple that will interface on the heat pipe surface, on the coolant side, centered in the heated length.

The clamping force, when the heat pipe is inserted into the condenser block should be a minimum of xxx N/m2 of force.

The interface between the heat pipe and the condenser block should be coated with a light layer of thermal interface grease prior to installation of the test article.

Figure 3 - Condenser Block Design

Figure 3 – Condenser Block Design

 

The evaporator and condenser blocks should be located on a ridged base (noted as adjustable length base in Figure 1) that keeps the heat pipe, when clamped in the evaporator block and condenser block oriented in the same plane in both the X and Y axis.

The rigid base should be designed such that it can be rotated, allowing the heat pipe test orientation to be changed from horizontal to gravity aided (evaporator down) or gravity inhibited (evaporator up).

7.  Thermocouple Measurements/Locations

The following temperatures shall be measured as outputs from the test set.

  • Theater
  • Tevaporator
  • Tadiabatic-evaporator
  • Tadiabatic-condenser
  • Tcondenser
  • Theat sink

The location of these temperature measurements is identified in Figure 4.

In the testing of the heat pipe, this data can be recorded manually or as an element in a data acquisition system.

Figure 4 - Temperature Measurement Locations

Figure 4 – Temperature Measurement Locations

 

8.  Power Input

Power input to the heat pipe is delivered via electric resistance heaters. The electrical power supplied to those resistance heaters should be delivered via a regulated power supply with power measurement capability. This can be a watt-meter or measurement of voltage/amperage draw. This input power will be recorded as the test power.

This data can be recorded manually or as an element in a data acquisition system.

9.  Test Set Orientation

A method of determining the heat pipe orientation should be included as a part of the test set.

This data can be recorded manually or as an element in a data acquisition system.

10.  Power Input

Power input to the heat pipe is delivered via electric resistance heaters. The electrical power supplied to those resistance heaters should be delivered via a regulated power supply with power measurement capability. This can be a watt-meter or measurement of voltage/amperage draw. This input power will be recorded as the test power.

This data can be recorded manually or as an element in a data acquisition system.


Contributors to this document include:

George Meyer – Celsia Inc.

Jim Petroski – Mentor Graphics

Bernie Siegal – TEA

Jerry Toth – Thermacore