WO2023172223A1 - Phase change driven thin flat plate heat spreader with groove instigated, capillary induced, liquid transport for thermal management of conduction cooled electronics - Google Patents

Abstract

The present invention is related with improvement of heat transfer performance of a plug-in unit by implementing phase change mechanism into the cover/frame (definitions are taken from family of ANSI/VITA standards e.g.). Especially for the conduction cooled plug-in unit designs, heat dissipated by the ICs (integrated circuits) and components are conducted to the edge of the unit via frame and covers where it is transferred to the sub-rack or the chassis (11) of the electronics unit.

Description

PHASE CHANGE DRIVEN THIN FLAT PLATE HEAT SPREADER WITH GROOVE INSTIGATED, CAPILLARY INDUCED, LIQUID TRANSPORT FOR THERMAL MANAGEMENT OF CONDUCTION COOLED ELECTRONICS

Technical Field

The present invention is related with improvement of heat transfer performance of a plugin unit by implementing phase change mechanism into the side cover/frame.

Background

General specifications for the physical features of PCBs/plug-in units are defined in different family of ANSI/VITA standards. Main purpose of these standards is to ensure the mechanical interchangeability and enhance the packaging density of plug-in units. Therefore these standards play a key role in the design of various computing systems. Among these standards, particular attention will be given to VITA 48.2. This standard defines detailed mechanical implementation for conduction cooling applications applied to PCBs/plug-in units and widely employed in designing computing units. The proposed invention will be described utilizing the definitions and the structures given in VITA 48.2 standard.

A plug-in unit is composed of a common PCB a PCI or PMC mezzanine card, a guide socket, primary side frame, PMC/XMC cover, two retainers and a secondary side cover as it is defined in ANSI/VITA 48.2. The frame and covers are mainly used for providing mechanical protection, structural robustness and improved thermal characteristics to the design. Especially for the conduction cooled plug-in unit designs, heat dissipated by the IC (integrated circuits)’s and components are conducted to the edge of the unit via frame and covers where it is transferred to the sub-rack or the chassis of the computing unit. Therefore, the enhanced conduction capability of frame and covers will directly improve the thermal performance of the plug-in unit, as long as, the heat conducted to the sub- rack/unit chassis will be removed properly. The heat conduction path of a generic plugin unit is composed of a common PCB and a secondary cover, placed into sub-rack. As mentioned, thermal performance of an electronic circuit board with the given type of arrangement and limitations can be improved by increasing the conductivity of the side covers. The major amount of power consumed by the electronic components and IC’s on the PCB is dissipated as heat. Since overheating or increased temperatures would end up with decreased performance or failure of the ICs and components. The purpose of improving thermal performance is maintaining lower temperatures during operation. More specifically the thermal performance measure can be described as conducting heat through minimum possible temperature difference between source and the sink. In this case, the source will be the heat dissipating component/IC, and the sink will be the fluid flowing through the channels of the chassis.

Especially in aerospace applications, the covers and frames of plug-ins are commonly manufactured from aerospace grade aluminum alloy due to its favorable mechanical properties combined with relatively good thermal characteristics and low density. The use of aerospace grade aluminum in plug-in covers and frames had been well established as a result of to its field proven durability and compatibility. On the other hand, beyond its preferable mechanical properties aerospace grade aluminum has a limited thermal conductivity, and this becomes a limiting factor for the thermal performance of the plug-in unit.

Moreover, parallel to the developments in technology, the power consumption, thus the heat dissipated from electronic circuit boards increased significantly in the last decade. Increased heat loads, result in increased temperature difference between the heat source and sink in conventional conduction cooled designs. Localized high temperature regions can be observed in the vicinity of components which require relatively more power such as RAM modules, microprocessors, FPGA’s, power converters due to the limitations in conductive heat transfer. Consequently, increased temperature of ICs/components would undesirably increase the risk of failure and reduced the reliability and mean time between failures (MTBF).

Different approaches had been followed for reducing the temperature difference between heat source and sinks, for thermal management of electronics. Most common ones are, increasing thermal conductivity by combining or directly using high thermal conductivity materials and using two phase passive heat transfer devices. Manufacturing cover plates from copper would be an example of direct usage of high thermal conductivity material. Its thermal conductivity value (~400W/m²K) is more than 2 times of aluminum and it has comparable characteristics from structural point of view. Therefore copper can be employed for avoiding hot spot formations and reducing the overall temperature differences between heat dissipating components and the sub-rack. However, copper has more than three times higher density from that of aluminum which makes it less favorable for aerospace applications where weight requirements are relatively strict. Additionally, it is not possible to apply dielectric plating to copper surfaces, which could be an unfavorable feature for a material used for manufacturing cold plates, frames or cover plates that are in close contact with electronics.

Some of high thermally conductive materials may have unfavorable mechanical properties, and cannot be directly used as cover plate or frame. As a solution, high thermal conductivity materials are combined with materials with higher strength and durability for providing structural integrity and resistance. Graphite, which have significantly high in-plane thermal conductivity (k = 1600W/m²K) is one of those materials, therefore it is generally combined with metallic plates to improve the conductive heat transfer characteristics of high power density electronics, to provide temperature uniformity. However, the limited amount of heat can be conducted through these sheets as a result of thin layer thickness together with low out-of-plane thermal conductivity. Additionally, the complex and less flexible manufacturing with relatively high cost is another drawback of using graphite sheets in thermal control of electronic devices.

In the above mentioned approaches and applications, heat transfer process is limited by thermal conductivity of the material. However, compelling environmental conditions and increased heat loads emerges the need for alternative methods which can exceed the limits of material properties. Herein, two-phase passive heat transfer devices could become suitable alternatives. There are numerous versions and geometric variants exist for these devices.

In general, these passive two phase heat transfer devices consist of a closed container, a capillary or porous wick structure and small amount of working fluid [6]. The wall/walls of the container is/are lined with the wick structure (porous type, wire mesh) or capillary structure (i.e. channels, grooves) which is wetted by the working fluid. During the operation, working fluid evaporates as a result of received heat form the external heat source. The evaporation takes place at the evaporator section. The vapor flows through the container and condensates at the condenser section. Heat transported by the vapor is rejected to the heat sink. The vapor condenses at the heat sink which is at lower temperature as a result of heat rejection. The condensed liquid, pumped by the capillary forces towards the heat source i.e. evaporator and by this way, the heat transfer cycle is completed [4], The wick structure is the key element in these devices. It provides the required capillary pressure to pump the liquid in absence or against the gravity [12, 13].

Heat Pipe (HP) is used as a general name for the two-phase heat transfer devices. In the earlier phase of development, these devices are manufactured in shape of a pipe or tube which is closed on both ends due to simplicity of processes involved in manufacturing. Therefore, common name “heat pipe” refers to the shape of the earlier versions of these devices. However, today in the literature, the term "heat pipe" may be used for describing a device of any type of geometry designed to function with the same principle [6]. In addition to pipe shaped ones, another widely used version of HPs is Vapor Chamber (VCs). VC is composed of container and this containers inner walls are covered with wick structures. In a VC the evaporator and the condenser are positioned on opposite sides of the container which has much lower thickness value compared to other dimensions. Moreover, VCs are not only used for transferring heat from evaporator to condenser, but also utilized for spreading the localized heat flux using their large condensation area. Therefore, in the last decade VCs widely used in the electronics industry owing to their light weight, geometric flexibility and significantly high thermal conductivity [7,8]. In brief, as a result of their high performance and reliability, heat pipes (HPs) and vapor chambers (VCs) become prominent tools for thermal management of electronics.

As a general description, heat pipes are used for transporting heat from one end to other, while, VC’s are used for spreading the heat input to larger surface area. Different types of HPs and some application specific design examples will be cited here for sake of completeness. Based on these examples, it would be easier to develop an understanding for the motivation behind the proposed invention.

In electronics cooling applications, heat pipes are mainly used for separation of heat source and sink, temperature flattening and temperature control [16]. Different configurations and types of heat pipes exist such as closed thermosyphon, capillary- driven, annular rotating, gas-loaded, loop, capillary, pumped loop, pulsating heat pipe, micro and miniature heat pipes [15]. In the scope of this document, particular interest will be on capillary-driven heat pipes in which capillary driving force is created by the axial grooves on the inner walls. These types of heat pipes are called as grooved HPs. The grooves of the HP can be of triangular, rectangular, or trapezoidal cross-section. Moreover, compound wick structures in which grooves are covered with screen mesh or sintered powder are also used [17],

HP’s with grooved wick structure work efficiently with relatively low power density compared to HP’s with sintered powder/porous or screen mesh type wick. In addition to efficient operation with higher power densities, as a result of being widespread in the market, sintered powder/porous wick HPs are more commonly used in different applications. Compared to sintered wicks groove type wicks especially rectangular ones, can be easily modeled and manufactured by using conventional machining techniques owing to simplicity of the geometry [16].

Commercially available sintered powder/porous wick HPs are generally manufactured in cylindrical shapes. In some cases these pipes are flattened for being able to integrate HPs on the plates. However, the flattening process limits the surface area of HPs. Another drawback of flattened HPs is the heat transfer performance degradation resulted from bending and flattening process [3].

Furthermore, the flattened HPs or cylindrical ones need to be embedded into the mechanical structures or cold plates in most of the applications due to geometric constraints. These HPs are embedded into the plates by soldering or using thermal epoxy. However, solder or epoxy introduces an additional thermal resistance at the interface of HP and the plate which reduces the overall thermal effectiveness of the design.

Among the commercially available HPs, copper-water is the most common, base material working fluid pair. One of the main reason for the widespread usage of copper- water pair in HPs is the high thermal performance and another reason is the non-toxicity of water. In addition, a non-modifiable factor that affects the thermal performance of a HP is the wettability of the fluid which is directly related with the contact angle between base material and working fluid. The contact angle needs to be zero or very small, for better wetting of the solid surface [14], Copper is hydrophilic base material with non-zero contact angle [15]. Certainly there are other criteria and requirements exist for selecting the working fluid-base material pair such as high latent heat, high thermal conductivity, and high surface tension. Based on these favorable properties copper-water pair is widely used in commercially available HPs [1 , 13].

Other possible working fluid base material pairs used in HPs based on compatibility are summarized in the table below. Experimentally obtained compatibility results of most common base material and working fluids are tabulated by Faghri [15]. The reduced version of these tabulated data is given in Table 1 .

Table 1 : Summarized results of experimental compatibility tests [15].

Except water, other working fluids compatible with copper, such as ammonia, methanol and acetone are all either toxic or flammable. Therefore, water seems to be the most feasible working fluid regarding the design considerations such as flammability and toxicity. However, when used together with sintered wick structures, water may cause damage in the wick structure due to volume expansion upon freezing. As a result of this phenomenon, HP may not be operational after thawing. The storage low temperature limit for subsystems/components, is defined to be lower than -50°C in standards for aerospace/defense industry standards like MIL-STD-810-G, DOE-160. Resultantly, porous/sintered wick type HPs are more prone to failures caused by freezing relative to grooved ones.

It is possible to state that, the grooved HPs are more resistant to freezing failure and has a wider storage temperature range. A, properly designed porous wick absorbs all the fluid in normal operating conditions, therefore it is not expected to be damaged by freezing but there is always a risk. Risk is much lower for a grooved heat pipe. Moreover, grooved wick structure is inherently capable of recovering after dry-out, while the devices with porous or sintered wick requires extra effort to recover. Even though copper has favorable properties, aluminum is a promising alternative that can be used in applications where weight limitations become important such as aircraft, spacecraft etc. since aluminum HP approximately weighs 1/3 of the copper ones. Ease of machining, and relatively lower material cost are the other advantages of aluminum [18]. As a result of those favorable properties and the advantages listed above aluminum flat plate heat pipes have become popular in recent years [9, 10, 1 1].

US Patent 4046190 named as “Flat Plate Heat Pipe” describes, heat spreading, passive phase change device in form of a flat rectangular plate. The flat plate heat pipe composed of two metal grooved plate, which enclose metal wicking structure. The capillary grooves and wicking structure facilitate the transport of liquid from condensation region to evaporation region. The device is claimed to be used for obtaining homogenous temperature distribution and temperature flattening while transferring heat.

US Patent 10295269B2 named as “Flat Plate Heat Pipe with Reservoir Function” describes a flat plate heat pipe design equipped with a collection channel feature which collects the excess liquid and pumps when required. The collection channel is connected to condenser section via narrowing throat to control the liquid flow.

US Patent 2007204646A1 named as “Cold Plate Incorporating a Heat Pipe” describes implementation of flat heat pipe into cold plates which have been used for cooling beverages. One side of the described flat cold plate is cooled by ice. The beverage that needs to be cooled is circulated in channels at the opposite side of the plate. The heat transfer between two sides of the plate is enhanced with the incorporation of heat pipe technology.

US Patent 2014146475A1 named as “Flexible Thermal Interface for Electronics” describes a planar heat pipe where heat is received from one side of the plate and rejected to opposite side. The design also includes a serpentine portion which flexes with three degree of freedom to compensate the expansion and contraction of the plate. These serpentine portions also move with the vibrations to reduce the stress imposed on to BGA or CGA.

The application numbered W003107728A1 named as “High Performance Cold Plate” describes a planar cold plate, in which heat is removed from the system by a circulating fluid. In this design, HP is utilized for receiving heat from electronic components and conducting it to the heat exchanger. Then heat is rejected to the circulating fluid with the use of a heat exchanger. However, the design is neither involved with a phase change heat transfer mechanism nor a passive heat transfer device.

All of those documents describe inventions different than the proposed one, in terms of application area, design features and implementation. The proposed invention differs from these applications with the following features:

• Custom designed grooves with tailored dimensions (width, depth) according to heat dissipation.

• T ransverse groove that intersects the longitudinal ones at the evaporator location which delays the dry out and enhances the liquid transport.

• Capillary grooves are placed to the one side of the plate to reduce the negative effects of restricted vapor flow space.

• The proposed invention is cover/frame as defined in different family of ANSI/VITA standards.

• In the proposed design metal wicking structure is not used, to eliminate the risk of getting damaged upon freezing.

• Heat will be transferred in planar direction in the proposed invention.

The scientific papers published in the journals related with the issue are not limited with the ones in the reference section. However, as far, the ones cited there, do not directly violate the inventive step and the novelty of the proposed invention.

Aim of the Invention

The present invention is related with improvement of heat transfer performance of a plugin unit by implementing phase change mechanism into the side cover/frame (definitions are taken from family of ANSI/VITA standards e.g.). Especially for the conduction cooled plug-in unit designs, heat dissipated by the ICs (integrated circuits) and components is conducted to the edge of the unit via frame and covers where it is transferred to the subrack or the chassis of the electronics unit.

Since it is possible to describe the thermal performance as conducting heat through minimum possible temperature difference between source and the sink, increasing thermal conductivity of the frame/cover plate of a plug-in units, increases the overall thermal performance. The invention intends to propose an approach for improvement of thermal conductivity of the side covers/frames of plug-in units by combining flat plate heat pipes and the covers/frames of these units. The flat heat pipe which will be integrated into the covers/frames will have a novel grooved wick structure design. The details of invention are as follows:

• Phase change mechanism is used for improvement of thermal conductivity of covers or frames of 3U or 6U plug-in units. The limits of the dimensions and implementation alternatives for these covers/frames are defined in related standards [2],

• Invention is employed in conductive cooled plug-in units in which, heat dissipated by the electronic components on the PCB is transferred to the edge of the unit via conduction and removed from the chassis of the unit by forced or natural convection.

• The passive phase change heat transfer mechanism implemented into the cover/frame to improve heat transfer performance between heat dissipation spot and the heat rejection spot. The heat dissipated by the electronic component on the PCB will be conducted to the cover/frame. The heat addition to that spot will evaporate the working fluid and the evaporated fluid will be transported to the cooler part of the cover/frame where it is condensed. The condensed liquid will be transported back to the heat dissipation spot (evaporator) with the help of capillary forces. In brief, the effective conductivity of cover/frame will be increased by integrated passive phase change cycle. This is simply a flat plate grooved heat pipe, with custom and novel groove design, integrated into cover/frame of a plugin unit.

• Among the various design criteria/limitations thermal concerns are the ones which less attention is paid during PCB design. Since, PCB design limitations and concerns are not totally based on thermal considerations, in most of the cases, electronic components are placed disregarding the thermal perspective. Therefore, thermal design is mostly remains limited with the features and implementations on mechanical components such as cover or frame, in plug-in units.

• In the proposed approach, customized design of the vapor and liquid flow paths of the working fluid enables to adjust thermal conductivity of the cover/frame as desired. As a result of the integrated phase change cycle, the designer will have control over the temperature distribution across the cover/frame and thus PCB of the plug-in unit.

• The groove geometric dimensions are determined according to the amount of heat that will be carried via phase change in that custom designed grooved path. The depth, width and the number of channels can be chosen according to the heat dissipation rate from unit area. The groove placement and arrangement will be based on the position of heat dissipation spots. As a novel increment, a transverse groove intersecting the longitudinal ones interconnects the grooves which does not pass through the heat dissipation spot. This feature aims to delay, prevent the dry out at the evaporator section by supplying liquid from the channels that are not passing through the heat dissipating region.

• The proposed invention will be manufactured from aluminum, since it can also be used in airborne equipment, where weight management is important. Moreover, aluminum can be easily machined with the conventional methods such as milling, which enables the customization of each cover/plate. Also, by horizontal milling process flat grooves can be easily and cost effectively manufactured.

• As mentioned previously, thermal conductivity of cold plates or covers/frames can be improved in desired directions by embedding HPs (in some cases flattened HPs). However, both embedding and flattening processes reduces the thermal performance of HPs. The HPs which are embedded into plates are generally sintered/porous wick type. In military applications, standards may require lower storage temperatures as low as -55^SC. Sintered/porous wick type HPs could be damaged at the lower limits of the storage temperatures cited here. On the other hand, with proposed grooved path design, the plates will survive through freezing temperatures without any damage and will operate normally after thawing.

In order to perform the aims which are mentioned above, a conduction cooled plug-in unit thermally connected to a sub-rack is improved and said conduction cooled plug-in unit comprises the components below:

• cover plate comprising a lower casing and upper casing, to form a hermetically sealed casing that is evacuated and then filled with certain amount of working fluid, conducting dissipated heat from electronic components on a PCB to chassis of the sub-rack unit,

• transverse grooves: o of which the dimension, orientation, and placement are specified according to the heat load distribution of the PCB that will be used together with the lower casing, o which intersects with longitudinal grooves to provide extra spot for evaporation since the evaporation takes place at the liquid vapor interface which is attached to the corners of the grooves. o which are machined/etched into the lower casing,

• said longitudinal grooves: o of which the dimension, orientation, and placement are specified according to the heat load distribution of the PCB that will be used together with the lower casing, o which intersect and connect the transverse grooves that are not passing through evaporator region where the working fluid evaporates as a result of received heat form the electronic components, o which feeds the excess liquid accumulated in the condenser to the evaporator region by interconnecting channels surrounding the evaporator to enhance liquid balance between the evaporator and condenser regions, o which improves the stability of the two phase heat transfer cycle to delay the drying at the evaporator region, and which provides an alternative liquid flow path between condenser and evaporator, o which intersects with transverse grooves to provide extra spot for evaporation since the evaporation takes place at the liquid vapor interface which is attached to the corners of the grooves. o which are machined/etched into the lower casing,

The conduction cooled plug-in unit comprises groove bosses machined/etched onto the inner surface of the lower casing, supporting the upper casing, and keeping the spacing between top of the transverse grooves and longitudinal grooves and lower surface of the upper casing to enable vapor flow, in a preferred embodiment. The conduction cooled plug-in unit comprises a fill tube in which working fluid which is filled in casing after hermetically sealed casing is evacuated, is placed, in a preferred embodiment.

The conduction cooled plug-in unit comprises retainers which conduct the heat dissipated by the electronic components, to the chassis , in a preferred embodiment.

Brief Description of the Figures

Figure 1 shows perspective view of the plug-in unit with 6U standard PCB and its placement in electronics/avionics/computing unit. The figure aims to demonstrate mechanical features and general environment of usage for the grooved heat pipe integrated cover plate.

Figure 2 shows exploded view of a plug-in unit, showing components, subassemblies and connection elements of a plug-in unit including screws and retainers.

Figure 3 shows the details of the transverse and longitudinal grooves together with the fluid flow direction.

Figure 4 shows details of the grooves and vapor flow passage on the section of cover plate.

Part References

1. PCB

2. Lower casing

3. Upper casing

4. Retainer

5. Bolt

6. Fastener

7. Transverse groove

8. Longitudinal groove

8.1 ., 8.2 Direction of liquid flow on transverse grooves

8.3., 8.4 Direction of liquid flow on longitudinal grooves

8.5. Direction of vapor flow

9. Groove boss 10. Electronic component

1 1. Chassis

12. Chassis-plug-in unit interface

13. Chassis cover

14. Fill tube

15. Cover plate

Detailed Description

As stated in the previous chapters, the proposed invention is related with the implementation of the passive phase change heat transfer mechanism into the cover/frame of a plug-in unit with novel features. General specifications of a plug-in unit are defined in different family of ANSI/VITA standards [2], The art disclosed, mainly concerns improvement of dry-out resistance of the passive two phase heat transfer mechanism which is integrated into the cover of a plug-in unit. This design is composed of a hermetically sealed shell containing working fluid and wick structure. The wick structure transports the fluid from condenser region to evaporator region with the help of capillary forces. These features describe a flat plate heat pipe or heat spreader for the persons skilled in art. However, the proposed invention, describes a customizable cover plate/frame which can be designed specific to the PCB it will be used together. The dimensions, orientation and placement of grooves can be designed according to the heat load distribution of the PCB. Thus, the heat transfer characteristics and the temperature distribution of the cover plate together with the PCB become a controllable parameter up to an extent.

In conduction cooled plug-in unit designs heat dissipated by the IC (integrated circuits)’s and components are conducted to the edge of the unit via frame/cover and then it is conducted to the sub-rack or the chassis of the computing unit through retainers or direct contact. Therefore, generally heat is transferred from center to the edges of the cover/frame in transverse direction. There is no need for improving the heat transfer performance of the cover/frame in-plane directions since heat rejection will be made from the edges of the cover/frame by conduction to the chassis of the unit.

As the heat flux increases, the rate of evaporation, may become higher than liquid flow rate to the evaporator. This imbalance causes drying at the evaporator and heat transfer associated with the phase change is interrupted. As a result, the equivalent or effective thermal conductivity of the heat pipe reduces significantly. Disclosed invention aims to improve the stability of operation by improving the management of the condensation and evaporation rates of the two phase cycle. The interconnecting, longitudinal grooves which intersect and connect the transverse grooves that are not passing through the heat dissipation (evaporator) region, help to delay/prevent dry-out and balance the amount of liquid accumulated at the condenser. The interconnecting grooves feed the evaporator region with the liquid from grooves at the periphery of evaporator region. These grooves enhance the liquid balance between evaporator and condenser regions by feeding the excess liquid accumulated in the condenser to the evaporator region. Evaporation takes place in a micro region, near the top corner of the grooves (The liquid vapor interface is attached this location). Therefore it can be said that the corners at the intersection of longitudinal and transverse channels (7, 8) provide extra spot for evaporation. Thus, these intersecting grooves also enhances the evaporation performance of the design. Additionally it should be noted that the longitudinal intersecting grooves are easy to manufacture since they can be machined by conventional processes such as milling, etching, EDM.

Referring to Figure 1 , the proposed invention is intended to be integrated into the cover of a plug-in unit. Figure 1 is given for demonstrating area of application as an example. As it is obvious for the persons skilled in the art, the application area and the size are not limited with the one disclosed herein. In particular basic conduction cooled plug in unit which is composed of a primary or secondary side cover plate (15), a PCB (1 ) and retainers (4). A conduction cooled plug-in unit can be installed into a sub-rack which is composed of a metallic chassis (11 ) and chassis cover (13). Conduction cooled plug-in unit, thermally connected to the sub-rack through a chassis-plug-in unit interface (12). Heat dissipated by the electronic components on PCB (1 ), transferred to the cover plate (15), and then conducted to the chassis (1 1 ) of the sub-rack through retainers (4).

In Figure 2, components of a plug-in unit and the grooved cover plate (15) is shown in detail. The transverse and longitudinal grooves (7, 8) are machined/etched into lower casing (2) of the hermetically sealed shell. Grooves (8) provide the necessary capillary action that is required for facilitating liquid flow from condenser region to evaporator region. In addition to grooves (7, 8), bosses (9) are also machined/etched onto the inner surface of lower casing (2). The groove bosses (9) act as structural support for the upper casing (3) of the hermetically sealed shell of the cover plate. Since the operating pressure inside the hermetically sealed shell is usually lower than the environment, the upper casing (3) is expected to be deflected towards lower casing (2). Those bosses (9) implemented for keeping the spacing between top of the groove and lower surface of the upper casing (3) to enable vapor flow. As mentioned above, plug-in unit, mechanically fixed and thermally connected to the chassis via retainers (4). Retainers (4) connected to the cover plate by using screws or bolts (5). The hermetically sealed casing of the cover plate is evacuated and then filled with certain amount of working fluid. The casing is evacuated and filled from the fill tube (14). Fill tube (14) is bent and sealed after the filling process completed. As mentioned, main function of the cover plate is to conduct heat dissipated from electronic components (10) on the PCB (1 ) to the chassis (1 1 ) of the sub-rack unit. Generally in a plug-in unit, PCB (1 ) is mechanically connected to the cover plate (15) via fasteners (6) such as screws or bolts.

In Figure 3, the details of the grooves that are machined onto the hermetically sealed casing of the cover plate (15) are given. As stated earlier, main design feature disclosed herein is the longitudinal groove (8) that intersects the transverse grooves (7). This longitudinal groove (8) helps managing the excess liquid accumulation at the condenser region and also delays/prevents dry-out at the evaporator region by providing additional liquid flow towards evaporator region from the surrounding grooves. Moreover, implementation of this groove creates extra spots for liquid-vapor interface to form. As a result, the heat transfer performance of the system is increased and the two phase heat transfer cycle becomes more stabilized. Those longitudinal grooves (8) are positioned at the mid-plane of the evaporator region which is expected to be aligned with the heat dissipating electronic components (10). An electronic component (10) and the alignment of the longitudinal groove (8) can also be seen in Figure 3. The direction of liquid flow on transverse grooves (8.1 , 8.2) under the effect of capillary forces is shown on Figure 3 via arrows. The direction of liquid flow on longitudinal groove (8.3, 8.4) is also shown on Figure 3 via arrows. The liquid, flowing to the evaporator region, evaporated and flow through the vapor passage between the top of the grooves and the inner surface of the upper part of the casing (3). The vapor flow direction for the configuration described here is shown in Figure 4. The direction of vapor flow (8.5) is shown via arrows on Figure 4. The liquid is evaporated at the evaporator , and the vapor travels towards the edge of the hermetically sealed shell, and condenses there by rejecting heat to the chassis (11 ) of the sub-rack. References

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Claims

CLAIMS A conduction cooled plug-in unit thermally connected to a sub-rack, characterized in that:

• cover plate (15) comprising a lower casing (2) and upper casing (3), to form a hermetically sealed casing that is evacuated and then filled with certain amount of working fluid, conducting dissipated heat from electronic components (10) on a PCB (1 ) to chassis (11 ) of the sub-rack unit,

• transverse grooves (7): o of which the dimension, orientation, and placement are specified according to the heat load distribution of the PCB (1 ) that will be used together with the lower casing (2), o which intersects with longitudinal grooves (8) to provide extra spot for evaporation since the evaporation takes place at the liquid vapor interface which is attached to the corners of the grooves. o which are machined/etched into the lower casing (2),

• said longitudinal grooves (8): o of which the dimension, orientation, and placement are specified according to the heat load distribution of the PCB (1 ) that will be used together with the lower casing (2), o which intersect and connect the transverse grooves (7) that are not passing through evaporator region where the working fluid evaporates as a result of received heat form the electronic components (10), o which feeds the excess liquid accumulated in the condenser to the evaporator region by interconnecting channels surrounding the evaporator to enhance liquid balance between the evaporator and condenser regions, o which improves the stability of the two phase heat transfer cycle to delay the drying at the evaporator region, and which provides an alternative liquid flow path between condenser and evaporator, o which intersects with transverse grooves (7) to provide extra spot for evaporation since the evaporation takes place at the liquid vapor interface which is attached to the corners of the grooves. o which are machined/etched into the lower casing (2), The conduction cooled plug-in unit according to claim 1 and characterized in that: groove bosses (9) machined/etched onto the inner surface of the lower casing (2), supporting the upper casing (3), and keeping the spacing between top of the transverse grooves (7) and longitudinal grooves (8) and lower surface of the upper casing (3) to enable vapor flow. The conduction cooled plug-in unit according to claim 1 and characterized in that: a fill tube (14) in which working fluid which is filled in casing after hermetically sealed casing is evacuated, is placed. The conduction cooled plug-in unit according to claim 1 and characterized in that: retainers which conduct the heat dissipated by the electronic components (10), to the chassis (11 ).