Scalable thermal solution for high frequency panel array applications or other applications

An apparatus includes a printed circuit board (PCB) including a surface that has a layer of circuitry. The apparatus also includes a heat sink configured to receive heat from the PCB. The apparatus further includes a thermally-conductive post configured to remove the heat from the PCB to the heat sink via thermal conduction through a thermal path. The thermal path is substantially orthogonal to the surface of the PCB. The post includes an end configured to physically couple to the layer of circuitry.

TECHNICAL FIELD

This disclosure is generally directed to thermal management systems. More specifically, this disclosure relates to a scalable thermal solution for high frequency panel array applications or other applications.

BACKGROUND

A printed circuit board (PCB) typically generates heat during operation of circuitry on or in the PCB. High frequency applications often require small element spacing between circuit components on the PCB, which results from the circuitry spacing dictated by the radio frequency lattice spacing. However, this can create difficulties when trying to cool the circuit components.

One conventional thermal management technique utilizes thermal vias to conduct heat into and through a PCB to a heat sink. However, thermal vias can occupy a relatively large amount of space on a PCB. Moreover, increased thicknesses of PCBs often require vias to increase in size, which again increases the amount of space occupied by the vias on a PCB. In addition, the use of thermal vias typically limits scalability and induces thermal gradients. This complicates the PCB design in addition to the difficulties normally experienced trying to meet the requirements of high frequency applications. Note that using thermal vias within a PCB also requires that the heat be removed from either the other side of the PCB opposite to the circuit element side or at the edges.

Other thermal management techniques include immersion cooling and local boiling of a liquid to a vapor state. However, these techniques often require sealing and could potentially affect both radio frequency (RF) characteristics of devices and affect the devices' long-term reliability. Still other thermal management techniques use internal PCB thermal planes (with and without liquid/vapor chambers), but these also occupy large amounts of space in PCBs,

SUMMARY

This disclosure provides a scalable thermal solution for high frequency panel array applications or other applications.

In a first embodiment, an apparatus includes a printed circuit board (PCB) including a surface that has a layer of circuitry. The apparatus also includes a heat sink configured to receive heat from the PCB. The apparatus further includes a thermally-conductive post configured to remove the heat from the PCB to the heat sink via thermal conduction through a thermal path. The thermal path is substantially orthogonal to the surface of the PCB. The post includes an end configured to physically couple to the layer of circuitry.

In a second embodiment, a system includes one or more PCBs and multiple thermally-conductive posts. The system also includes a heat sink configured to receive heat from the one or more PCBs through the multiple posts. Each PCB includes (i) a surface that has a layer of circuitry and (ii) a surface bonding area within the layer of circuitry. Each post includes an end configured to physically couple to the surface bonding area in the layer of circuitry of a corresponding one of the one or more PCBs. Each post is configured to remove heat from the corresponding PCB to the heat sink via thermal conduction through a thermal path substantially orthogonal to the surface of the corresponding PCB.

In a third embodiment, a method includes printing a layer of circuitry on a surface of a PCB, where the layer of circuitry includes a surface bonding area. The method also includes mounting a heat-generating circuit element on the PCB. The method further includes forming a thermal path substantially orthogonal to the surface of the PCB. The thermal path is formed using a thermally-conductive post thermally connecting the surface bonding area and a heat sink.

DETAILED DESCRIPTION

FIGS. 1 and 2illustrate an example thermal management system100according to this disclosure. In particular,FIG. 1illustrates an assembly view of the thermal management system100, andFIG. 2illustrates an exploded view of the thermal management system100.

As shown inFIGS. 1 and 2, the thermal management system100includes a printed circuit board (PCB)200, a heat sink such as a cold plate300, and one or more heat transfer posts400. The thermal management system100is configured to remove heat from the PCB200and transfer the removed heat to the cold plate300. This can be done to help cool the PCB200and maintain an operating temperature of the PCB200in a specified range. As described below, the cold plate300contacts the PCB200in such a manner that the PCB200need not include any thermal vias or channels through the PCB200in order to transfer heat from the PCB200to the cold plate300. The embodiments of this disclosure are not limited to a heat sink that is a cold plate, as certain embodiments the heat sink can include a convective air stream, a thermo-electric cooler, and the like.

The PCB200includes a top surface205, a bottom surface, and multiple side surfaces including a side surface210. The PCB200also includes one or more heat generating cells215mounted to the top surface205of the PCB200. In the example shown, each heat generating cell215includes a ground layer220and one or more circuit elements225. In each cell215, the ground layer220is disposed between the top surface205of the PCB200and the circuit elements225. Note that the PCB200could include any number of heat generating cells215, and those heat generating cells215may or may not be identical.

As can be seen here, each ground layer220extends beyond the associated circuit element(s)225in order to contact one or more of the posts400. The posts400therefore provide an efficient way to transfer heat from hot surfaces of the PCB200(such as hot surfaces of the ground layers220) to the cold plate300. The posts400therefore provide thermal conduction through short and highly-conductive thermal paths directly to the cold plate300.

The posts400can help to significantly simplify the design of the PCB200by eliminating thermal design features (such as thermal vias) within the PCB200. In some embodiments, there may be no thermal design features internal to the PCB200.

One end of each post400can be placed in physical contact with a hot surface of the PCB200, and an opposite end of each post400is in physical contact with a lower portion of the cold plate300(such as a bottom surface305of the cold plate300). Each post400extends in a direction substantially orthogonal to the hot surface of the PCB200. Depending on the implementation, the posts400could be formed integral with the cold plate300, integral with the PCB200(such as with the ground layers220), or separate from and connected to both the PCB200and the cold plate300.

The posts400can be compatible with future automated assembly processes, and the posts400can be manufactured using simple machining processes. For example, as shown inFIGS. 1 and 2, the cold plate300could include the posts400as an integral part of the cold plate300, such as when the bottom surface305of the cold plate300is machined to form the posts400or when the cold plate300is molded to include the posts400. The posts400can be composed of a high thermal conductive material, such as a metal like copper, aluminum, or a metal alloy. Each post can have any suitable size, shape, and dimensions. In this example, the posts400have a common shape (a rectangular prism), although this is not required.

A bottom surface405of each post400is disposed in thermal communication with a top surface of a ground layer220of a corresponding cell215. In some embodiments, the bottom surface405of the post400can be disposed in direct surface-to-surface contact with the ground layer220. In other embodiments, a thermal interface material (TIM) is disposed between the bottom surface405of the post400and the top surface of the ground layer220. The TIM can help to fill in spaces that may exist between the bottom surface405of the post400and the top surface of the ground layer220. The TIM also forms a high thermally-conductive path where heat moves from the ground layer220to the post400. The TIM can be a rigid material, such as epoxy or solder. In particular embodiments, the TIM may be a pliable gap filling material, such as a BERGQUIST thermal interface material. The TIM is usually thin, typically 1-5 mils, but could be thinner or thicker.

The vertical height of each post400provides clearance for the associated circuit element225. For example, the vertical height of each post400could represent the height of the associated circuit element225plus an additional distance to keep the cold plate300from directly contacting the circuit element225.

As shown inFIG. 2, the heat generating cells215can form an array on the PCB200. In this example, the heat generating cells215are disposed in multiple rows, where the heat generating cells215in adjacent rows are offset from one another. This arrangement could be used, for instance, in a high frequency application (for example, 33 GHz), such as high frequency planar phased array antennas. For example, this arrangement could be used with a high frequency phased array antenna that includes sixteen rows each containing sixteen heat generating cells215, where the generating cells215represent Q-band unit cells for an antenna. However, this arrangement of heat generating cells215is for illustration only, and any other suitable arrangement of heat generating cells215could be used and for any other frequency or frequency range as required by the design application. For example, other heat generating cells215could be used to implement a driver of a radiator.

In addition to heat dissipation, the posts400provide an array structure support to the PCB200and provide a platform for including RF absorbing material. For example, the posts400could be formed as solid posts, so the posts400can remain intact when a force (such as a weight or fastener load) is applied against the posts400in a direction toward the PCB200.

In this way, the system100provides a low cost thermal solution that does not require heat to flow into and through a PCB. Moreover, various implementations of the system600allow for scalability, minimize thermal gradients, and reduce the complexity of PCB design. In addition, this approach can be done economically. The cost impact to the PCB is minimal since no internal cooling features may be required, and only the incorporation of a surface pad area245for each post may be needed. Standard and inexpensive processes can be used to fabricate or attach the posts, and improved thermal paths can be obtained.

AlthoughFIGS. 1 and 2illustrate one example of a thermal management system100, various changes may be made toFIGS. 1 and 2. For example, the relative sizes, shapes, and dimensions of the various components shown inFIGS. 1 and 2are for illustration only. Each component inFIGS. 1 and 2could have any other size, shape, and dimensions. Also, the use of a cold plate300in conjunction with posts400that contact portions of heat generating cells215could be used with any homogenous or heterogeneous heat generating cells215. Further,FIGS. 1 and 2show a single post400contacting each ground layer220, but multiple posts400could contact each ground layer220. In addition, other types of posts400could be used in the thermal management system100, such as the posts described inFIG. 5below. Also, the bottom surface of the PCB200can be similar to the top surface205by including another set of heat generating cells. That is, the bottom surface of the PCB200can include a ground layer to which one or more circuit elements are mounted. The bottom surface of the PCB200can further include posts (such as the posts400or the posts described inFIG. 5below) that provide thermal conduction through short and highly-conductive thermal paths directly to a heat sink, such as another cold plate disposed below the PCB200.

FIG. 3illustrates an example heat generating cell215of the system100ofFIG. 1according to this disclosure. Although certain details will be provided with reference to the components of the heat generating cell215, it should be understood that other embodiments may include more, less, or different components. In this example, the heat generating cell215represents a Q-band unit cell for an antenna in a high frequency planar phased array, although any other suitable heat generating cell215could be used.

As shown inFIG. 3, the heat generating cell215includes the ground layer220, which is composed of an electrically-conductive material like copper or gold. The heat generating cell215also includes a first capacitor230, a second capacitor235, a surface bond pad240, and a surface pad area245configured to contact a post400. The heat generating cell215further includes a first RF pad250, an application specific integrated circuit (ASIC) or other processing device255, a monolithic microwave integrated circuit (MMIC) or other integrated circuit260, and a second RF pad265. The capacitors230and235, the processing device255, and the integrated circuit260can be disposed over the ground layer220. Note, however, that other components do not overlap or block the surface pad area245, which allows a post400to make thermal contact with the ground layer220via the surface pad area245. The surface bond pad240can be composed of the same material as the ground layer220and can be disposed coplanar with the ground layer220. The integrated circuit260could represent any suitable integrated circuit, such as a MMIC high power amplifier (HPA).

AlthoughFIG. 3illustrates one example of a heat generating cell215, various changes may be made toFIG. 3. For example, the relative sizes, shapes, and dimensions of the various components shown inFIG. 3are for illustration only. Each component inFIG. 3could have any other size, shape, and dimensions. Also, the heat generating cell can include a different type of circuitry.

FIG. 4illustrates an example thermal analysis of an operation of the thermal management system100ofFIG. 1according to this disclosure. In some embodiments, a Q-Band application requires an MMIC mounting temperature of less than about 80° C. As shown inFIG. 4, the thermal management system100provides a MMIC mounting temperature of about 59° C., which provides a substantial margin between the specified requirements and the performance of the system100. In the example shown, the entire cold plate300has a temperature of about 20° C. or less.

InFIG. 4, two of the posts400are connected to the ground layer220of the heat generating cell215. A majority of each post400has a temperature of about 20° C. or less. Only a small portion410of each post400near the bottom of each post400has a warmer temperature, typically in the range of about 20° C. to about 22.9° C. The posts400can provide thermal paths with uniform heat transfer rates, and the majority of the height of each post400is as cold as the cold plate300.

Here, a TIM415is used between the posts400and the ground layer220. The TIM415has temperatures in the range of about 24.9° C. (near the bottom of each post400) to about 39.6° C. (near the ground layer220).

In the example shown, the PCB200exhibits different temperatures that are related to (i) proximity to a circuit element225that generates heat and transfers the heat to the PCB200and (ii) proximity to the posts400that remove heat from the PCB200. The PCB200exhibits temperatures T1-TNin various areas, where T1>T2, T2>T3, . . . , TN-1>TN. The hottest area of the PCB200here is located directly under the circuit element225and near the top surface205of the PCB. The coolest areas of the PCB200can be located directly under the surface pads245of the ground layer220that contact the posts400.

FIG. 5illustrates a thermal analysis of an operation of another thermal management system501according to this disclosure. The thermal management system501here includes one or more heat transfer posts500, which are attached to the PCB200via soldering or other mechanism. The heat transfer posts500can also be physically connected to a cold plate, such as through a surface-to-surface connection or through a highly-conductive thermal interface. The thermal management system501also includes a layer of TIM505that connects a top surface510of the post500with a bottom surface of the cold plate. In the event of rework or disassembly, removing/replacing a layer of TIM505is a simpler, faster, and less destructive process than removing/replacing solder.

The post500here has a variable cross-sectional shape, namely the top of the post500is wider than a bottom of the post500. This enables the post500to occupy a smaller surface area on the heat generating cells215, while at the same time providing a larger surface area for conducting heat to the cold plate. In comparison to a post400that has uniform lateral cross-sections, the larger surface area at the top of the post500provides more thermal conduction and increases the rate of heat removal. As the layer of TIM505may have a lower thermal conductivity than the post500, the larger top surface510can help to improve the rate of transferring heat across the TIM505. The embodiments of this disclosure are not limited to posts that have uniform lateral cross-sections, as certain embodiments include posts that are not uniform in cross-section. The embodiments of this disclosure are not limited to posts that are identical to each other, as certain embodiments include posts that are not identical to each other. Non-identical posts can vary in dimensions, shape, materials, or another characteristic.

In this particular embodiment, the post500includes a nail head shape, with a narrower-radius cylindrical lower portion515and a wider-radius cylindrical upper portion520. In other embodiments, the post500could have another shape, such as a pyramid, a truncated pyramid, a cone, or a truncated cone.

In the example shown, the post500exhibits temperatures in a range of about 20° C. to about 29.1° C. The areas of transition from the lower portion515to the upper portion520exhibit a temperature that is in a range of about 20° C. to about 29.1° C.

AlthoughFIGS. 4 and 5illustrate examples of thermal analyses of thermal management systems, various changes may be made toFIGS. 4 and 5. For example, the relative sizes, shapes, and dimensions of the various components shown inFIGS. 4 and 5are for illustration only. Each component inFIGS. 4 and 5could have any other size, shape, and dimensions. Also, the temperatures shown inFIGS. 4 and 5are specific examples only and do not limit this disclosure to any particular temperatures or temperature ranges.

FIG. 6illustrates an example of the scalability of the thermal management system100ofFIG. 1according to this disclosure. To take advantage of economies of scale, a manufacturer can produce multiple PCBs200a-200m, each of which could be identical to the PCB200ofFIG. 1. The manufacturer can form a multi-PCB thermal management system600by coupling the multiple PCBs200a-200mto each other in any suitable arrangement. The PCBs200a-200mcan be coupled to each other in any suitable manner, such as by using an adhesive. The manufacturer can also produce a single cold plate or multiple cold plates configured to contact the PCBs200a-200m.

Each of the PCBs can be configured to operate or remain operable while one or more of its lateral side surfaces are in physical contact with one or more other PCBs. In the event that a portion of the multi-PCB thermal management system600is damaged, that portion can be repaired/replaced, as opposed to repairing/replacing the entire multi-PCB thermal management system600. For example, when a subset of the PCBs200a-200mis damaged, the damaged subset of PCBs can be replaced without repair/replacement of undamaged components of the system600.

Note that the system600is not limited to using identical heat generating cells on the PCBs. Any number(s) and type(s) of heat generating cells could be used in the system600. Also note that the600could use any suitable posts with one or more cold plates, such as posts400integrated with the cold plate(s) or posts500attached to the cold plate(s).

AlthoughFIG. 6illustrates one example of the scalability of the thermal management system, various changes may be made toFIG. 6. For example, the relative sizes, shapes, and dimensions of the various components shown inFIG. 6are for illustration only. Each component inFIG. 6could have any other size, shape, and dimensions. Also, the multiple PCBs200a-200mcan be arranged in any suitable fashion.

FIG. 7illustrates a hybrid embodiment of a thermal management system700according to this disclosure. As shown inFIG. 7, the hybrid thermal management system700includes a PCB705with an arrangement of uniform narrow posts707soldered or otherwise connected to a circuitry layer710, as well as heat generating cells709. The uniform narrow posts707can be the same as or similar to the narrow radius cylindrical lower portion515or the post400. The hybrid thermal management system700also includes a cold plate715with a corresponding arrangement of uniform wider posts717machined to its bottom surface720. A layer of TIM730is placed between the top surfaces of each of the uniform narrow posts707of the PCB705and a bottom surface of each of the uniform wider posts717of the cold plate715or between the bottom surface of the cold plate715and the uniform narrow posts707. The uniform wider posts717can be the same as or similar to the wider radius cylindrical upper portion520or a truncated post400. Accordingly, the thermal management system700includes a layer of TIM730between the surfaces where the cold plate715and the PCB705meet, whether the post is integral to the PCB or the cold plate.

AlthoughFIG. 7illustrates one example of a hybrid embodiment of a thermal management system, various changes may be made toFIG. 7. For example, the relative sizes, shapes, and dimensions of the various components shown inFIG. 7are for illustration only. Each component inFIG. 7could have any other size, shape, and dimensions.

FIG. 8illustrates an example method800of manufacturing a scalable thermal solution for high frequency panel array applications or other applications according to this disclosure. A layer of circuitry is printed on a surface of a PCB at step810. This could include, for example, printing the ground layer220and other wiring using a high thermally and electrically conductive material (such as copper or gold). As part of this step, one or more surface bonding areas245can be formed in the layer of circuitry. Each surface bonding area245is configured to physically couple to a post400or500. One or more heat generating components are mounted to the surface of the PCB at step820. This could include, for example, mounting circuit elements225to the ground layer220without covering the surface bonding areas245.

A cold plate is obtained at step830, and posts are formed at step840. This could include, for example, manufacturing or otherwise obtaining a cold plate having integrated posts, or manufacturing or otherwise obtaining a cold plate and separate posts. If the posts are separate, the posts are integrated with the cold plate or the PCB at step850. This could include, for example, soldering or otherwise attaching the posts to the PCB or the cold plate. Optionally, TIM can be deposited at step860. This could include, for example, depositing the TIM at the exposed ends of the posts. The cold plate is connected to the PCB through the posts and the optional TIM at step870. This could include, for example, placing ends of the posts400on the ground layers220or placing the bottom surface of the cold plate300on the posts400.

AlthoughFIG. 8illustrate one example of method of manufacturing a scalable thermal solution for high frequency panel array applications or other applications, various changes may be made toFIG. 8. For example, while shown as a series of steps, various steps inFIG. 8could overlap, occur in parallel, occur in a different order, or occur any number of times.