Below are the questions asked during the event, along with their respective answers.
Q: Are vapor chambers currently used in space applications?
A: Yes, Boyd has vapor chambers for space applications.
Q: How about using vapor chamber for heat transportation as well as heat spreading?
A: Vapor chambers can be designed and built in many shapes. If you were to design a vapor chamber that is long and narrow it can be used to carry heat primarily in one direction similar to a heat pipe.
Q: What are the challenges for the cooling of microprocessor chips using vapor chamber?
A: Microprocessors can be effectively cooling using vapor chambers as part of a thermal solution. Each package and mechanical form factor has its own challenges.
Q: The vapor space inside the chamber enables the thermal conductivity? Or is it the other way around?
A: The vapor space enables a low dT over the vapor chamber surface. The effective thermal conductivity can then be calculated based on the distance between the points measured and the overall cross-section of the vapor chamber between those points.
Q: Is the performance of the vapor chamber affected by orientation?
A: Assuming that the vapor chamber is within its Qmax limit there is typically very little difference in it’s dT or effective thermal conductivity based on orientation. The maximum amount of power that can be moved between the evaporator and condenser (Qmax) is affected by external forces such as gravity.
Q: Are freezing temperatures an issue for vapor chambers?
A: Vapor chambers and heat pipes that use water as a working fluid are commonly used for applications in aerospace & defense as well as other harsh outdoor environments for telecommunications equipment. When using these devices outdoors we need to consider the maximum vertical dimension with gravity, fluid charge, and any parallel conduction paths to aid in the process to thaw out the vapor chamber prior to the component temperatures reaching their maximum.
Q: If multiple wicks are used within the same vapor chamber, with varying pore sizes and materials, how well does the liquid wick over the interface between these?
A: Wick to wick continuity is very critical in such vapor chamber designs and Boyd has a number of methods to accomplish this.
Q: Which is the theoretical width of a thin vs ultra-thin vapor chamber?
A: Vapor chambers can be made in narrow form factors but at some point, a flat heat pipe may be a better option if the shape is not complex.
Q: What is a typical high pressure in vapor chambers, and is it higher in ultra-thin vapor chambers?
A: Internal pressure depends on working fluid type and temperature, not the vapor chamber thickness or wall material. For water-based systems, there is a vacuum within the vapor chamber <100C and a positive pressure in the vapor chamber >100C.
Q: Does the condenser design affects the performance of the vapor chamber? I mean temperature difference remains the same as a spreader.
A: Although the evaporator tends to be more critical due to the location of the high heat flux from the component and highest dT, the condenser is important. The condenser needs to be a size large enough to allow for the transfer of the power to the fins. The condenser size and shape should take into account any remaining non-condensable gas (NCG) and fluid return to the evaporator.
Q: Do you supply to mobile phone manufacturers as well, like Apple/ Samsung?
A: We have customers for vapor chambers in many industries and for many products including use in mobile electronics
Q: What are the obstacles to the commercialization of the vapor chambers in the market?
A: Boyd has had vapor chambers in mass production for many years (early 1990’s).
Q: What are the manufacturing limits that determine the minimum thickness of SS, Cu, versus Ti vapor chambers?
A: Other than the material strength limiting its minimum wall thickness the processes used to make the container (stamping, etching, etc.) all have their own manufacturing limits as does the wick material chosen.
Q: how to connect the copper wick with the body inside the vapor chamber?
A: Wicks can be applied to the wall simply by holding them in place with the internal structure or by other methods such as diffusion bonding or sintering.
Q: 1) Do you have guidance for modeling vapor chambers in a CFD environment? i.e. do you model with isotropic or anisotropic conductivities?
A: For CFD code that does not support actual heat pipe or vapor chamber modeling like Boyd’s SmartCFD we would opt to model as anisotropic. The thru Z conductivities would be lower than the X-Y.
Q: What thermal interface materials need to be used between the device and the vapor chamber?
A: The thermal interface material (TIM) used would be no different than those required for other thermal solutions. If your component is flat and pressed up against the vapor chamber then a high-performance grease or phase change material may be used. If you have multiple components at varying heights then a gap pad or thermal gel or putty may be the better choice.
Q: 2) Do the Cu, Stainless Steel, and Titanium at their constructed at their thinnest dimension net out to the same thermal conductivity?
A: Yes, effective thermal conductivities would be similar given the same vapor space regardless of material type. The stainless and titanium offer the ability to have reduced wall thicknesses thus reducing their overall thickness for the same vapor space.
Q: Condenser design I mean Condenser cooling design.
A: Other than the heat source input area the condenser may be on either side of the vapor chamber. If there is space available to add fins to the vapor chamber on top (opposite the heat source) as well as on either side of the heat source this will maximize your condenser and reduce your airside resistance.
Q: Is any special surface treatment performed on the wick to be able to use water as a working fluid?
A: Boyd assures fluid, wick, and container are compatible and able to meet the customer’s life requirements based on well-known material sets and reliability testing.
Q: What makes vapor chamber advantageous over heat pipes?
A: Vapor chambers are more efficient at spreading heat over a larger X-Y plane vs. multiple heat pipes. The evaporator can more effectively pick up heat over a heat source vs. multiple heat pipes that are side by side and the vapor is not limited to a single path to get from warm to cooler regions.
Q: What is typical pressure that may be applied to the vapor chamber without causing malfunction or deformation.
A: The pressure applied will depend on the vapor chamber material, wall thickness, and internal structure. We see applications with very little pressure ~5 PSI as well as applications that are over 200 PSI.
Q: Any concern with bulging of the vapor chamber under pressure due to thin walls?
A: Ultra-thin vapor chambers can be designs to support higher external pressure by optimization of the internal support geometry. If water is used as the working fluid the vapor chamber will see internal pressure above 100C. For these cases, the wall thickness may be increased or the number of top to bottom mechanical connections may need to be increased.
Q: Could you please comment on Ti VC configuration, and the reason for high k as compared to Cu VC?
A: Both Titanium and Stainless Steel have higher strength compared to copper allowing for designs with thinner walls and a larger space for vapor to flow.
Q: What is the limiting factor in effective thermal conductivity? The mass flow of the wick or the mass flow of the vapor?
A: Both must balance in order to meet the designed power. The dT of the vapor then is used to determine the effective thermal conductivity.
Q: Do you provide thermal models of vapor chambers for use in Flotherm or Flotherm XT? or what is the best way to model these for thermal simulation?
A: Boyd has included vapor chamber modeling within our SmartCFD software that is commercially available. The resulting dT predicted in SmartCFD can be used to build more simplistic models in other CFD modeling packages.
Q: comment about how the ratio between the heat source area over the chamber area affects the performance of the device
A: Since there is a dT to get heat into the vapor chamber through the wall and wick the larger the size of the chamber the higher the effective thermal conductivity will be (similar dT but greater distance).
Q: What are the min and max environmental conditions in which vapor chambers are effective?
A: With water as the working fluid ultra-thin vapor chambers are effective in ambient temperatures of ~5 – 95C. Above 100C there is a positive pressure with the vapor chamber and this must be known at the time of vapor chamber design. At less than 0C, the water is frozen and the vapor chamber temperature will need to be raised before it will function properly. Other working fluids may be considered for temperatures outside of this range.
Q: How construction material affects overall Conductivity, given that heat spread and the transfer takes place at isothermal (2-phase equilibrium)?
A: With ultra-thin vapor chambers, the wall thickness is thin enough that it does not significantly influence the effective thermal conductivity that is mainly dominated by the 2 phase vapor flow.
Q: Are there hybrid constructions with perhaps one side Cu and one side stainless steel?
A: Not currently but may be possible depending on bonding type and fluid used.
Q: What value is the pressure inside the vapor chamber?
A: The pressure inside a vapor chamber will depend on the vapor pressure of the fluid and the temperature. For example, above 100C a water-based vapor chamber will have a positive pressure and under 100C it will have a vacuum inside the vapor chamber.
Q: Could you compare a little bit the diffusion bonding and brazing?
A: Diffusion bonding typically requires a slightly higher temperature than brazing and additional applied pressure. Unlike brazing, diffusion bonding does not require any additional alloy to be added within the joint.
Q: How about the Qmax that VCs can transport? is there any rule of thumb?
A: The Qmax of a vapor chamber varies widely based on heat source location, overall vapor chamber size, influences from gravity or other forces, etc. The only real rule is all 2 phase solutions have a Qmax.
Q: 4) I assume the Titanium chamber is the most expensive by a wide margin. What is the relative cost difference between Cu and SS chambers?
A: Titanium has a higher cost per Kg than Cu or SS. Depending on the thickness of the VC and if there are other components as part of the assembly the cost delta is likely less than you would imagine.
Q: What is the ratio between the water inside the vapor chamber over the empty space?
A: This will vary based on the wick thickness used and overall vapor space. But in general, the volume of fluid is much much smaller than the volume of vapor.
Q: Can you speak to current industrial applications shipping today in products: e.g. Auto, Aerospace to Power or Consumer Electronics.
A: Vapor chambers are used in nearly every application and industry where electronics cooling is needed.
Q: Will you talk about bonding the VC’s to Al heatsinks?
A: Bonding of vapor chambers to an aluminum heat sink is typically done by solder (Ni plate and solder to aluminum) or by using thermal epoxy another TIM with screws.
Q: For SUS VC, please comment on the working fluid used?
A: Boyd assures fluid, wick, and container materials are compatible and able to meet the customer’s life requirements based on established material sets and reliability testing.
Q: What is the current cost delta between copper and titanium vapor chamber everything else being equal?
A: Titanium has a higher cost per Kg than Cu or SS. Depending on the thickness of the VC and if there are other components as part of the assembly the cost delta is likely less than you would imagine and must be reviewed on a case by case basis.
Q: In the transient analysis, after 60s, I would think that the Copper VC is performing best because the hotspot is the lowest (57.4C). Correct?
A: After 30 seconds the dT of the TiVC was less than that of the copper VC. The absolute temperature difference part to part may be due to minor ambient effects and test setup.
Q: Are ultra-thin vapor chambers cost-effective and suitable for high volume production/consumer goods.
A: Ultra-thin vapor chambers are produced in the millions for very cost-sensitive mobile devices. Boyd has high volume production lines in place for these products.
Q: Have these ultra-thin vapor chambers used in space? How do they stand up to vacuum at a higher temperature (higher internal pressure)?
A: Vapor chambers have been used in space applications. For use of ultra-thin vapor chambers in space, we would need to evaluate the fluid type, internal pressure @ temperature, and external opposing pressure (zero) to determine what wall thickness and what internal structure would be required.
Q: With either Cu or Ti VC, can these be treated for electrical isolation?
A: Coatings can be applied for electrical isolation. Adhesively applied films are the most common but spray type coatings may also be applied depending on process temperature.
Q: Thanks for the excellent presentation! You mentioned that the chambers are typically fused or bonded onto heatsinks but what about the heat source? What kind of TIMs are usually used on that side of the chamber?
A: The thermal interface material (TIM) used would be no different than those required for other thermal solutions. If your component is flat and pressed up against the vapor chamber then a high-performance grease of phase change material may be used. If you have multiple components at varying heights then a gap par or thermal gel or putty may be the better choice.
Q: On a Ti Ultra-thin vapor chamber, what’s the minimum width and Z step (height step ratio)? Minimum edge width?
A: The wall thickness needed before the vapor cavity and the edge of the VC and width needed for the weld zone is ~2mm but will depend on the total vapor chamber thickness, the thickness of the top/bottom cover, and other details that would be examined as part of our quoting and DFM process.
Q: Would you please add more details to Optics application (QSFP)?
A: Ultra-thin vapor chambers and in some cases heat pipes have been used to cool custom designs for optical modules. These solutions typically bring the heat past the back edge of the cage to allow for more fin area to be added and capture additional airflow compared to traditional solutions.
Q: How to verify the VC capacity in Turbo without dry out (transient state)?
A: Boyd’s SmartCFD can predict the percentage of Qmax and if over 100% then the user will know that dry out is likely and use that information to make design changes.
Q: What is the price range of a 100×100 Cu/Cu alloy vapor chamber?
A: Pricing depends on production volume and specific features that are included on the vapor chamber or vapor chamber assembly. Our commercial team is best suited to answer this question.
Q: Have you tried made vapor champers in plastics?
A: We are aware of experimental flexible VC’s that used copper-coated plastic walls. It was possible to make a functioning VC using water as the working fluid but the copper layer always developed micro-cracks when the VC was flexed resulting in a short working life.
Q: How are vapor chambers charged, and what goes into determining the charge amount?
A: Boyd uses multiple techniques to insert the working fluid and remove the air to create the needed vacuum within a vapor chamber. The method chosen depends on the type of vapor chamber, material set, and production volume. The amount of fluid change typically depends on the saturation limit for the wick type and thickness selected. That selection will be based on the amount of power that the vapor chamber needs to support.
Q: Is there any way to calculate the balance between the evaporation limit and condenser limit of the vapor chamber? How to calculate the evaporation limit so that proper condenser design can be done?
A: The heat flux limit in the evaporator depends on the wick properties and is typically determined by experiments. There is no similar limitation on the condenser side. A low thermal resistance at the condenser can be maintained as long as the rate of liquid condensation can be quickly cleared away by the wick. Any liquid build on the surface of the condenser will increase its thermal resistance.
Q: Have you done any designs for optical cages, such as QSFP, SFP cages?
A: Yes, we have integrated ultra-thin vapor chambers and heat pipes into custom designs for cooling optical modules. These solutions typically bring the heat past the back of the cage. This allows for more fin area to be added and to capture additional airflow compared to a local extruded or die case heat sink solution.
Q: You have shown plate-like vapor chambers, can you produce different shapes like additional pins?
A: This presentation was focused on traditional and ultra-thin vapor chambers so all were of a planar format. Boyd also offers 3D vapor chambers that may have extensions added to the vapor chamber to assist in bringing heat throughout a heat sink volume improving its fin efficiency.
Q: How does operating temperature affect VC performance? For example, if the component maximum is less than 100°, how do you ensure that the fluid boils? Conversely, if you are greater than 100°C, how do you ensure no dry out?
A: The boiling point of the fluid is based on the pressure inside the system (negative or positive pressure) and the fluids vapor pressure.
Q: Graphene vs Graphite? Same properties?
A: In this presentation, we are using the words graphite and graphene interchangeably. These materials are commercially available and typically have high in-plane thermal conductivities in the 1000W/mK range (depending on thickness) while having far lower thru conductivities on the over of 2 – 20 W/mK.
Q: Do you have an automotive application approved that can be used from -40C to 105C
A: Boyd has vapor chamber designs that have been tested over this temperature range for many applications including Aerospace & Defense, Telecommunications, and Automotive.