AUSTRALIA Patents Act COMPLETE SPECIFICATION (ORIGINAL) Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: Name of Applicant: General Electric Company Actual Inventor(s): Satish Sivarama Gunturi, Madadevan Balasubramaniam, Ramakrishna Venkata Mallina, Richard Alfred Beaupre, Le Yan, Richard S. Zhang, Ljubisa Dragoljub Stevanovic, Adam Gregory Pautsch, Stephen Adam Solovitz Address for Service and Correspondence: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: HEAT SINK AND COOLING AND PACKAGING STACK FOR PRESS-PACKAGES Our Ref : 872649 POF Code: 88428/141848 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): 6-016- HEAT SINK AND COOLING AND PACKAGING STACK FOR PRESS-PACKAGES BACKGROUND [00011 The invention relates generally to power electronics and, more particularly, to advanced cooling for power electronics. 10002] High power converters, such as medium voltage industrial drives, frequency 5 converters for oil and gas, traction drives, Flexible AC Transmission (FACT) devices, and other high power conversion equipment, for example rectifiers and inverters, typically include press-pack power devices with liquid cooling. Non-limiting examples of power devices include integrated gate commutated thyristors (IGCTs), diodes, insulated gate bipolar transistors (IGBTs), thyristors and gate turn-off thyristors (GTOs). Press-pack devices are 10 particularly advantageous in high power applications, and benefits of press-packs include double-sided cooling, as well as the absence of a plasma explosion event during failure. [00031 To construct a high power converter circuit using press-pack devices, heat sinks and press-pack devices are typically sandwiched to form a stack. State-of-the-art power converter stacks typically employ conventional liquid cooled heat sinks with larger diameter cooling 15 channels. The heat sinks and power devices are not integrated in state of the art power converter stacks. In certain applications, thermal grease layers are disposed between respective ones of the press-pack device and the liquid cooled heat sink. In other applications, at least some of the layers are simply held together by pressure, with no thermal grease in between them. This arrangement results in significant contact resistance. Other shortcomings 20 of such power converter stacks include relatively high thermal impedance from the semiconductor junction to the liquid, as well as a relatively complex stack assembly structure and process due to the number of parts involved. [0004] Accordingly, it would be desirable to improve the thermal performance and packaging of power converter stacks using press-pack devices. More particularly, it would be 25 desirable to reduce the thermal impedance from the semiconductor junction to the liquid for high reliability and/or high power density. It would further be desirable to provide a relatively simplified stack assembly structure for high maintainability. 2 [0004a] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims. BRIEF DESCRIPTION [00051 Briefly, one aspect of the present invention resides in a heat sink for directly cooling at least one electronic device package. The electronic device package has an upper contact surface and a lower contact surface. The heat sink comprises a cooling piece formed of at least 5 one thermally conductive material. The cooling piece defines multiple inlet manifolds configured to receive a coolant and multiple outlet manifolds configured to exhaust the coolant. The inlet and outlet manifolds are interleaved. The cooling piece further defines multiple millichannels configured to receive the coolant from the inlet manifolds and to deliver the coolant to the outlet manifolds. The millichannels and inlet and outlet manifolds 10 are further configured to directly cool one of the upper and lower contact surface of the electronic device package by direct contact with the coolant, such that the heat sink comprises an integral heat sink. [00061 Another aspect of the present invention resides in a cooling and packaging stack comprising at least one heat sink. The heat sink comprises a cooling piece defining: multiple 15 inlet manifolds configured to receive a coolant, multiple outlet manifolds configured to exhaust the coolant, and multiple millichannels configured to receive the coolant from the inlet manifolds and to deliver the coolant to the outlet manifolds. The inlet and outlet manifolds are interleaved. The stack further comprises at least one electronic device package comprising an upper contact surface and a lower contact surface. The manifolds and millichannels are 20 disposed proximate to the respective one of the upper and lower contact surfaces for directly cooling the respective surface by direct contact with the coolant, such that the heat sink comprises an integral heat sink. 3 DRAWINGS [00071 These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 5 [00081 FIG. I depicts an electronic device package with upper and lower heatsinks; 10009] FIG. 2 illustrates a heatsink with millichannels and manifolds incorporated into a single cooling piece; [00101 FIG. 3 shows the cross-section of a radial millichannel for the heat sink of FIGS. 2 or 4; and 10 [00111 FIG 4 illustrates a heatsink design, which increases the number of radial channels; [00121 FIG. 5 illustrates a cooling and packaging stack configured for a number of electronic device packages; [00131 FIG. 6 is a cross-sectional view of the manifold arrangement of FIG. 2 or 4; [00141 FIG. 7 illustrates a sealing arrangement for a double-sided heatsink of the present 15 invention; and [00151 FIG. 8 is a cross-sectional view illustrating an offset manifold arrangement for a double sided heat sink embodiment of the present invention. [0016] Although the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. 20 In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 4 DETAILED DESCRIPTION [00171 A heat sink 300 for directly cooling at least one electronic device package 20 is described with reference to FIGS. 1-4. As indicated for example in FIG. 1, an exemplary electronic device package 20 has an upper contact surface 22 and a lower contact surface 24. The heat sink 300 comprises a cooling piece 310 formed of at least one thermally conductive 5 material. As indicated, for example, in FIGS. 2 and 4, the cooling piece 310 defines a number of inlet manifolds 12 configured to receive a coolant and a number of outlet manifolds 14 configured to exhaust the coolant. As shown for example in FIGS. 2, 4, 6 and 8, the inlet and outlet manifolds are interleaved (interdigitated). The cooling piece 310 further defines a number of millichannels 16 configured to receive the coolant from the inlet manifolds and to 10 deliver the coolant to the outlet manifolds. The millichannels 16 and inlet and outlet manifolds 12, 14 are further configured to directly cool one of the upper and lower contact surfaces 22, 24 of the electronic device package 20 by direct contact with the coolant, such that the heat sink comprises an integral heat sink. More particularly, the manifolds 12, 14 and millichannels 16 are disposed proximate to the respective one of the upper and lower contact 15 surfaces 22, 24 for cooling the respective surface by direct contact with the coolant. According to more particular embodiments, the millichannels 16 and inlet and outlet manifolds 12, 14 are configured to deliver the coolant uniformly to the respective one of the upper and lower contact surface 22, 24 of the electronic device package being cooled. [00181 Thus, the heat sink 300 is integral to the electronic device package 20, in that the 20 heat sink 300 cools the electronic device package 20 by direct contact with the coolant. In other words, the millichannels 16 and inlet and outlet manifolds 12, 14 of integral heat sink 300 are open on the side facing the power device package 20, such that the coolant flowing through the millichannels 16 can directly contact the power device package 20. This arrangement is a distinct improvement over the self-contained heat sinks of the prior art, in 25 which the coolant is contained within the heat sink and must thus cool the devices indirectly through the heat sink case. By eliminating these additional thermal layers, the integral heat sink 300 of the present invention provides enhanced cooling of the power devices. [00191 The cooling piece 310 can be formed using a variety of thermally conductive materials, non-limiting examples of which include copper, aluminum, nickel, molybdenum, 30 titanium, alloys thereof, metal matrix composites such as aluminum silicon carbide (AlSiC), 5 aluminum graphite and ceramics such as silicon nitride ceramic. The cooling piece can be cast and/or machined. For example, the cooling piece 310 can be cast and then machined to further define fine features and surface requirements. [00201 Non-limiting examples of the coolant include de-ionized water and other non 5 electrically conductive liquids. [0021] For particular embodiments, the manifolds 12, 14 have relatively larger diameters than the millichannels 16. In one non-limiting example, the width of the millichannels was in a range of about 0.5 mm to about 2.0 mm, and the depth of the millichannels was in a range of about 0.5 mm to about 2 mm. In particular, the thickness of the channels may be determined to 10 ensure pressure uniformity on the semiconductor. By making the pressure distribution on the semiconductor more uniform, the performance of the semiconductor is not compromised. Further, it should be noted that the millichannels 16 and manifolds 12, 14 could have a variety of cross-sectional shapes, including but not limited to, rounded, circular, trapezoidal, triangular, and square/rectangular cross sections. The channel shape is selected based on the 15 application and manufacturing constraints and affects the applicable manufacturing methods, as well as coolant flow. Beneficially, the incorporation of millichannels 16 into the heat sink 10 significantly increases the surface area of heat conduction from the semiconductor device 20 to the coolant. [00221 In the illustrated arrangements, the inlet and outlet manifolds 12, 14 are disposed in 0 a circular (also referred to herein as axial) arrangement, and the millichannels 16 are disposed in a radial arrangement. In another example (not illustrated), the inlet and outlet manifolds 12, 14 are disposed in a radial arrangement, and the millichannels 16 are disposed in a circular (axial) arrangement. As used herein, the phrases "circular arrangement" and "axial arrangement" should be understood to encompass both curved and straight "circular" passages 25 (either manifolds or millichannels, depending on the embodiment) connecting the radial passages (millichannels or manifolds, depending on the embodiment). [0023] For the example arrangements shown in FIGS. 2 and 4, cooling piece 310 further defines a groove 302 for receiving a gasket 304 (see FIG. 7), such as an O-ring. As indicated in FIG. 7, the gasket 304 is used to seal the heat sink 300 to a neighboring electronic device 30 package 20. As indicated in FIG. 6, for example, the coolant is supplied to heat sink 300 via an inlet plenum 3. The coolant then flows into the inlet manifolds (alternate concentric 6 manifold sections) 12 via inlet ports 11, as indicated in FIG. 6. After passing through the millichannels 16 (see FIGS. 2 or 4), the coolant is exhausted from the outlet manifolds 14 (the other alternate concentric manifold sections) via outlet ports 13 to the outlet plenum 5, as indicated in FIG. 6, for example. 5 [00241 FIG. 3 shows the cross-section of a radial channel 16, for example a radial millichannel. FIG. 4 illustrates a design to increase the number of radial channels to facilitate a reduction in pressure drop with a corresponding improvement in cooling efficiency. More particularly, for the example arrangement shown in FIG. 4, the number of radial channels is larger near the circumference of the cooling piece 310 relative to the number of radial 10 channels near the center of the cooling piece 310. This arrangement permits the inclusion of additional radial channels for given spatial and machining constraints. [00251 Beneficially, by incorporating the millichannels and inlet/outlet manifolds into a single cooling piece 310 as illustrated in FIGS. 2-4, for example, the assembly process is simplified. In particular, the use of a single cooling piece 310 eliminates the need to bond two 15 components. Instead, heat sink 300 can be sealed to an adjacent device package 20 using a gasket 304, such as an O-ring assembly, which provides a robust and simple seal between the adjacent components. [0026] For the exemplary embodiments described above with reference to FIGS. 1-4, the upper contact surface 22 and lower contact surface 24 can be circular in cross-section, and the 20 cooling piece 310 can be cylindrical (i.e., a disk or hockey-puck arrangement). However, other geometries can be employed, including without limitation, square and rectangular cross sections. For the example arrangement depicted in FIG. 1, the electronic device package 20 is a press-package 20. Although the invention is not limited to any specific device structure, the following example press-package configuration is provided for illustrative purposes. In the 25 example, the press-package 20 comprises at least one semiconductor device 21 formed on a wafer 23, upper and lower coefficient of thermal-expansion (CTE) matched plates 25, 27, and upper and lower electrodes 28, 29. The wafer 23 is disposed between the CTE plates 25, 27, the upper electrode 28 is disposed above the upper CTE plate 25, and the lower CTE plate 27 is disposed above the lower electrode 29, as shown for example in FIG. 1. For the press 30 package embodiment, each of the wafer 23, CTE plates 25, 27 and electrodes 28, 29 may have a circular cross-section. Non-limiting examples of semiconductor devices include IGCTs, GTOs and IGBTs. The present invention finds application to semiconductor devices 7 manufactured from a variety of semiconductors, non-limiting examples of which include silicon (Si), silicon carbide (SiC), gallium nitride (GaN), and gallium arsenide (GaAs). The press-package typically includes an insulating (for example, ceramic) housing 26, as indicated for example in FIG. 1. Although FIG. I shows the heat sinks as extending outside the housing 5 26, in other embodiments, the cooling pieces 310 of the heat sinks 300 are disposed within the housing 26. Moreover, electrodes 28, 29 can extend vertically beyond the bounds of housing 26, for example with a compliant seal disposed between the outer circumference of electrodes 28 (and 29) and the housing 26. In addition, the heat sinks 300 can extend out of the housing (as shown) to enable electrical connections and for placing other devices that need to be 10 cooled. Therefore, the cooling pieces 310 can have a larger diameter than housing 26. 100271 The heatsink 300 can be single-sided or double-sided. For example, for the arrangement depicted in FIG. 5, the upper and lower heat sinks 300 are depicted as being one sided, whereas the middle heat sink 300 is double-sided. A double-sided heat sink is configured for directly cooling multiple (in particular, two) electronic device packages 20. As 15 indicated in FIG. 5, for a double sided heat sink 300, the cooling piece 310 has a first surface 2 and a second surface 4. A first subset of the millichannels 16, inlet manifolds 12 and outlet manifolds 14 are formed in the first surface 2 of the cooling piece 310. (See, for example, the arrangements of FIGS. 2 and 4.) Similarly, a second subset of the millichannels 16, inlet manifolds 12 and outlet manifolds 14 are formed in the second surface 4 of the cooling piece 20 310. (See, for example, the arrangements of FIGS. 2 and 4.) The first subset of millichannels is configured to directly cool an upper contact surface 22 of one of the electronic device packages 20 by direct contact with the coolant, and the second subset of millichannels is configured to directly cool a lower contact surface 24 of another of the electronic device packages 20 by direct contact with the coolant. 25 [0028] FIG. 8 depicts an offset arrangement of inlet and outlet manifolds for a double-sided heat sink. As indicated in FIG. 8, the inlet manifolds 12 in the first subset are offset from the inlet manifolds in the second set. Similarly, the outlet manifolds 14 in the first subset are offset from the outlet manifolds in the second set. More particularly, the inlet manifolds in the first subset are positioned opposite to the outlet manifolds in the second subset. Similarly, the 30 outlet manifolds in the first subset are positioned opposite to the inlet manifolds in the second subset. This offset arrangement has several advantages. By positioning the manifolds carrying hot fluid (outlet manifolds) on a first cooling face opposite to the manifolds carrying cold fluid (inlet manifolds) on the other cooling face, better "heatspreading" in the manifolds 8 is achieved as compared to the case of aligned inlet and outlet manifolds. In addition, the offset arrangement improves the velocity and pressure distribution of the coolant, all of which contribute to improved heatsink performance. [00291 Beneficially, heat sinks 300 provide enhanced heat transfer relative to conventional 5 cooling of power devices. Conventional heat sinks are not integral to the press-packages but rather are self-contained, in that the coolant does not contact the power devices but rather is encased within the heat sink. Thus, conventional heat sinks include cases, which are typically coupled to press-packages via thermal grease layers, which increase the thermal resistance. Thus, conventional heat-sinks contain additional thermal layers (the case), which impede heat 10 transfer. In contrast, heat sinks 300 are disposed integral to the press-packages, directly cooling the power devices with direct contact by the coolant, thereby enhancing the heat transfer. Further, the interleaved inlet and outlet channels deliver coolant uniformly to the surface of the device being cooled, and the millichannels increase the surface area of heat conduction from the power device to the coolant in this integral heat sink. For the 15 embodiments illustrated in FIGS. 1-4, the heat sinks 300 are adapted for use with existing electronic packages 20, such as press-packages 20. Accordingly, heat sinks 300 can be used to integrally cool conventional press-pack power devices without modification of the device packages. Moreover, incorporating the millichannels and inlet/outlet manifolds into a single cooling piece 310 simplifies the assembly process. 20 [0030] A cooling and packaging stack 100 embodiment of the invention is described with reference to FIG. 5. The cooling and packaging stack 100 includes at least one heat sink 300 comprising a cooling piece 310 defining a number of inlet manifolds 12 configured to receive a coolant, a number of outlet manifolds 14 configured to exhaust the coolant, and a number of millichannels 16 configured to receive the coolant from the inlet manifolds and to deliver the 25 coolant to the outlet manifolds. Heat sinks 300 are described above, and various details of the various embodiments of the heat sink will thus not be repeated. The cooling and packaging stack 100 further includes at least one electronic device package 20 comprising an upper contact surface 22 and a lower contact surface 24. The manifolds 12, 14 and millichannels 16 are disposed proximate to the respective one of the upper and lower contact surfaces 22, 24 for 30 directly cooling the respective surface by direct contact with the coolant, such that the heat sink comprises an integral heat sink. 9 [0031] Exemplary materials for heat sink 300 are discussed above. Inlet and outlet manifolds 12, 14 are described above with reference to FIGS. 2, 6 and 8. For the embodiment shown in FIG. 6, coolant is supplied to the heat sinks 300 through inlet plenum 3 and exhausted via outlet plenum 5. Various configurations of millichannels 16 are discussed 5 above with reference to FIGS. 2-4. The relative arrangements of the manifolds and millichannels are described above with reference to FIGS. 2 and 4. In one embodiment, the inlet and outlet manifolds 12, 14 are disposed in a radial arrangement, and the millichannels 116 are disposed in a circular (or more generally, in an axial) arrangement. In the illustrated embodiments of FIGS. 2 and 4, the millichannels 16 are disposed in a radial arrangement, and 10 the inlet and outlet manifolds 12, 14 are disposed in a circular arrangement. 100321 To enhance cooling, the heat sinks 300 in stack 100 may employ the millichannel design illustrated in FIG. 4. As discussed above, FIG. 4 illustrates a design to increase the number of radial channels to facilitate a reduction in pressure drop with a corresponding improvement in cooling efficiency. More particularly, for the example arrangement shown in 15 FIG. 4, the number of radial channels is larger near the circumference of the cooling piece 310 relative to the number of radial channels near the center of the cooling piece 310. This arrangement permits the inclusion of additional radial channels for given spatial and machining constraints. [00331 The example arrangement shown in FIG. 5 illustrates a cooling configuration with 20 single-sided upper and lower heat sinks 300 and a double-sided, middle heat sink 300. For the illustrated embodiment, the stack 100 comprises multiple heat sinks 300 and multiple electronic device packages 20. As indicated, the heat sinks 300 and electronic device packages 20 are alternately arranged. More particularly, and as noted above, for the embodiment illustrated by FIG. 5, at least one of the heatsinks 300 comprises a double sided 25 heat sink. The double sided heat sink comprises a cooling piece 310 having a first surface 2 and a second surface 4. A first subset of the millichannels 16, inlet manifolds 12 and outlet manifolds 14 are formed in the first surface 2 of the cooling piece 310. Similarly, a second subset of the millichannels 16, inlet manifolds 12 and outlet manifolds 14 are formed in the second surface 4 of the cooling piece 310. The first subset of millichannels 16 is configured to 30 cool an upper contact surface 22 of one of the electronic device packages 20 with the coolant. Similarly, the second subset of millichannels 16 is configured to cool a lower contact surface 24 of another of the electronic device packages 20 with the coolant. As discussed above with 10 reference to FIG. 8, the double-sided heat sink may incorporate an offset arrangement of inlet and outlet manifolds, to further enhance heat sink performance. 100341 For the cooling and packaging stack 100, the heat sink(s) 300 is (are) sealed to respective ones of the upper and lower contact surfaces 22, 24 of the electronic device package 5 20, such that the coolant flowing through the millichannels directly cools the respective one(s) of the upper and lower contact surfaces of the electronic device package 20 by direct contact with the coolant. For the example arrangement depicted in FIGS. 2 and 7, each cooling piece 310 further defines at least one groove 302. As indicated, for example, in FIG. 7, the stack 100 further comprises at least one gasket 304 disposed in the groove(s) 302 for sealing the 10 heat sink 300 to the respective electronic device package(s) 20. In particular embodiments, the gasket 304 comprises an O-ring. Beneficially, this assembly process is relatively simple and robust, requiring few manufacturing steps and eliminating the need for more complex assembly techniques, such as soldering or other metal bonding techniques. [00351 It should be noted that the specific arrangement shown in FIG. 5 is merely 15 illustrative, and cooling and packaging stack 100 may include any number of electronic device packages 20 and corresponding heat sinks 300 for cooling the electronic device packages. One of the many benefits of the present invention is its flexibility and modularity for cooling a desired number of device packages. 100361 For the exemplary embodiments discussed above with reference to FIG. 5, each of 20 the upper contact surface 22 and lower contact surface 24 can be circular in cross-section, and each of the cooling pieces 310 can cylindrical in cross-section (i.e., a disk or hockey-puck arrangement). However, and as noted above, other geometries can be employed, including without limitation, square and rectangular cross-sections. According to more particular embodiments, the electronic device package 20 is a press-package 20. As noted above, the 25 invention is not limited to any specific device structure. However, the following example press-package configuration is provided for illustrative purposes. In the example, the press package 20 comprises at least one semiconductor device 21 formed on a wafer 23, as shown for example in FIG. 1. The press package 20 further includes upper and lower thermal expansion coefficient (CTE) matched plates 25, 27 and upper and lower electrodes 28, 29, as 30 indicated in FIG. 1, for example. The wafer 23 is disposed between the CTE plates 25, 27. The upper electrode 28 is disposed above the upper CTE plate 25, and the lower CTE plate 27 is disposed above the lower electrode 29. Each of the wafer 23, CTE plates 25, 27 and 11 electrodes 28, 29 has a circular cross-section. For the illustrated press-pack embodiments, the stack 100 further includes an insulating (for example, ceramic) housing 26, as shown for example in FIGS. 1 and 5. Although FIGS. 1 and 5 show the heat sinks as extending outside the housing 26, in other embodiments, the cooling pieces 310 of the heat sinks 300 are 5 disposed within the housing 26, as discussed above. [0037] Beneficially, cooling and packaging stack 100 provides enhanced heat transfer relative to conventional cooling of power devices, with a robust and simple sealing arrangement. For example, the heat sinks can be sealed to adjacent device packages using relatively simple O-ring assemblies. In addition, in particular embodiments, heat transfer can 10 be enhanced by contacting the coolant directly on the upper and lower contact surfaces 22, 24 of the press-pack 20. Further, by locating narrow and deep millichannels 16 directly under the power devices, the heat transfer surface area from the junction of the device to the liquid can be maximized. Relative to a conventional stack assembly of press-pack devices and liquid cooled heat sinks, the thermal resistance is greatly reduced with relatively low pressure drop 15 and flow rate. By achieving a relatively high flow speed with a low pressure drop, cooling is enhanced. [0038] By providing higher reliability and a larger operating margin due to improved thermal performance, the heat sink 300 and cooling and packaging stack 100 are particularly desirable for applications demanding very high reliability, such as oil and gas liquefied natural ?0 gas (LNG) and pipeline drives, oil and gas sub-sea transmission and distribution, and drives. In addition, the heat sink 300 and cooling and packaging stack 100 can be employed in a variety of applications, non-limiting examples of which include high power applications, such as metal rolling mills, paper mills and traction, etc. [0039] Although only certain features of the invention have been illustrated and described 25 herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. [0040] Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of 30 the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof. 12 Parts List 2 first surface of the heat sink 3 inlet plenum 4 second surface of the heat sink 5 outlet plenum 10 heat sink 11 inlet ports 12 inlet manifolds 13 outlet ports 14 outlet manifolds 16 millichannels 20 electronic device package (press-package) 21 semiconductor device 22 upper contact surface of electronic device package 23 wafer 24 lower contact surface of electronic device package 25 upper thermal-expansion coefficient (CTE) matched plate 26 press-package housing 27 lower thermal-expansion coefficient (CTE) matched plate 28 upper electrode 29 lower electrode 100 cooling and packaging stack 300 heat sink 302 groove for receiving an O-ring 304 gasket (o-ring) 310 cooling piece 13