Electronic module including a cooling substrate having a fluid cooling circuit therein and related methods

<u>Field of the Invention</u>

0001The present invention relates to the field of electronic modules, and, more particularly, to electronic modules including fluid cooling systems.

<u>Background of the Invention</u>

0002Electronic devices are widely used in many types of electronic equipment. One electronic device is the integrated circuit which may include a silicon or gallium arsenide substrate and a number of active devices, such as transistors, etc. formed in an upper surface of the substrate. It is also typically required to support one or more such integrated circuits in a package that provides protection and permits external electrical connection.

0003As the density of active devices on typical integrated circuits has increased, dissipation of the heat generated has become increasingly more important. In particular, a relatively large amount of heat may be generated in multi-chip modules (MCMs), microwave transmitters, and photonic devices, for example.

0004A typical MCM may include a substrate upon which the electronic devices are mounted. Such substrates are generally made from a material that is suitable for withstanding the heat generated from the electronic devices. For example, ceramics such as low temperatures co-fired ceramics (LTCC) are commonly used for such substrates. Additionally, the MCM may also include a heat sink (e.g., fins) for dissipating heat from the electronic devices, and a fluid cooling circuit within the substrate for transferring heat between the electronic devices and the heat sink. That is, in two-phase cooling circuits, the heat from the electronic devices causes the cooling fluid to vaporize. Heat from the vapor is transferred to the heat sink, and the vapor then condenses to a liquid state. This liquid may then be re-circulated through the cooling circuit.

0005An example of such a substrate is disclosed in U.S. Patent No. 5,870,823 to Bezama et al. entitled "Method of Forming a Multilayer Electronic Packaging Substrate with Internal Cooling Channels". The substrate includes a sintered ceramic body having cooling channels internal to and integral with the sintered ceramic body for the cooling a heat-generating electronic package placed on the sintered body. The flow of cooling fluid into and out of the cooling channels is controlled by manifolds and flow tubes connected to the substrate.

0006With the ever decreasing size of electronic devices and MCMs, it may be desirable to have fluid cooling circuits that do not require flow tubes, mechanical pumps, valves, etc., for circulating the cooling fluid therein. That is, such prior art devices may be relatively large and limit the ability to reduce the size of the substrate. In addition, such devices may be relatively expensive to produce.

<u>Summary of the Invention</u>

0007In view of the foregoing background, it is therefore an object of the invention to provide an electronic module including a cooling substrate having a fluid cooling circuit therein for providing improved fluid flow control and associated methods.

0008This and other objects, features, and advantages in accordance with the present invention are provided by a method for making an electronic module including forming a cooling substrate having a fluid cooling circuit therein having a vertical passageway. The cooling substrate may be formed by forming a plurality of unsintered ceramic layers having passageways therein. The plurality of unsintered ceramic layers and the at least one resistive element may be assembled in stacked relation so that the passageways align to define the fluid cooling circuit and so that at least one resistive element extends in a cantilever fashion into the vertical passageway. Furthermore, the unsintered ceramic layers and the at least one resistive element may be heated to sinter and to cause the at least one resistive element to soften and deform downwardly adjacent vertical sidewall portions of the vertical passageway. The method may also include mounting at least one electronic device on the cooling substrate in thermal communication with the fluid cooling circuit.

0009More specifically, the at least one resistive element may be at least one layer of resistive ink. Additionally, the at least one resistive element may include a plurality of resistive elements each between adjacent unsintered ceramic layers, and the plurality of resistive elements may be positioned adjacent opposing sidewalls of the vertical passageway.

0010The method may also include connecting the at least one resistive element to a voltage source for causing the at least one resistive element to heat and thereby facilitate fluid circulation in the cooling fluid circuit. The plurality of ceramic layers may be low temperature co-fired ceramic layers, for example. Also, a heat sink may be connected in thermal communication with the fluid cooling circuit.

0011An electronic module according to the invention includes a ceramic cooling substrate having a fluid cooling circuit therein having a vertical passageway defined by at least one vertical sidewall. The cooling substrate may include at least one resistive element on portions of the at least one vertical sidewall and extending into the at least one vertical sidewall a predetermined distance. Further, the electronic module may include at least one electronic device mounted on the cooling substrate and in thermal communication with the fluid cooling circuit.

0012More specifically, the at least one resistive element may include a plurality of resistive elements each extending into the at least one vertical sidewall the predetermined distance. For example, the predetermined distance may be greater than about 10 mils. Furthermore, the at least one vertical sidewall may include a plurality of sidewalls, and the plurality of resistive elements may be positioned adjacent at least two opposing vertical sidewalls of the vertical passageway. The ceramic substrate may include a low temperature co-fired ceramic, and the electronic module may further include a heat sink in thermal communication with the fluid cooling circuit.

<u>Brief Description of the Drawings</u>

0013<ul id="ul0001" list-style="none" compact="compact"><li>FIG. 1 is a perspective view of an electronic module including a cooling substrate according to the invention.</li><li>FIG. 2 is a cross-sectional view of the package of the electronic module of FIG. 1 taken along line 2-2.</li><li>FIGS. 3-5 are cross-sectional views of the vertical passageway of the cooling substrate of FIG. 2 illustrating a method for making the electronic module of the present invention.</li></ul>

<u>Detailed Description of the Preferred Embodiments</u>

0014The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and the dimensions of layers and regions may be exaggerated in the figures for greater clarity.

0015Referring to FIGS. 1-2, an electronic module <b>10</b> in accordance with the invention is now initially described. The electronic module <b>10</b> includes a package <b>11</b> surrounding an electronic device <b>12</b>. The package <b>11</b> includes a cooling substrate <b>13</b> and a lid <b>14</b> connected thereto. The cooling substrate <b>13</b> includes a base <b>15</b> and at least on resistive element <b>16</b>, and it has a fluid cooling circuit <b>19</b> therein, as will be described further below. The lid <b>14</b> defines a cavity <b>17</b> over the electronic circuit <b>12</b> for protection of the electronic circuit and its interfaces (not shown).

0016The lid <b>14</b> may be attached by brazing using a seal ring, for example, as will be appreciated by those of skill in the art, though other configurations are also possible. The base <b>15</b> and lid <b>14</b> may include a low temperature co-fired ceramic (LTCC) material, for example. This material offers advantages in terms of ruggedness, and an ability to form recesses and small stable passageways therein, as well as to provide electrical paths therethrough. Of course, other similar materials may be used as well.

0017Furthermore, one or more electronic devices <b>12</b> may be carried by the package <b>11</b>, as will be appreciated by those skilled in the art. The electronic device <b>12</b> may include semiconductor devices or integrated circuits, heat coils, resistors, etc., for example. Of course, other electronic devices may also be included in the electronic module <b>10.</b> The package <b>11,</b> as best seen in FIG. 1, may carry electrical connectors <b>18</b> on at least one of its surfaces. For example, the electrical connectors <b>18</b> may be pins in a pin grid array, as illustratively shown. In other embodiments, edge connectors may be provided to connect to a ribbon type cable, for example, as will be appreciated by those skilled in the art.

0018A heat sink <b>20</b> (FIG. 1) may be in thermal communication with the cooling substrate <b>13</b> and may include fins <b>21,</b> for example. Of course, other heat sinks known to those of skill in the art may also be used. For example, the heat sink <b>20</b> maybe a rack or metal chassis in which the electronic module is mounted. Thus, heat will be transferred from the electronic device <b>12</b> to the fluid cooling circuit <b>19</b> through the base <b>15,</b> and from the fluid cooling circuit to the heat sink <b>20</b> also through the base, as illustratively shown with arrows in FIG. 2. Furthermore, the cooling substrate <b>13</b> may optionally include vias or passageways (not shown) to provide enhanced thermal communication between the electronic device <b>12</b> and the fluid cooling circuit <b>19</b> and between the fluid cooling circuit and the heat sink <b>20</b> in some embodiments, as will be understood by those of skill in the art.

0019Turning now to FIGS. 3-5, the electronic module <b>20</b> will now be described in further detail with reference to a method for making the same. The method begins by forming the cooling substrate <b>13</b> so that it has the fluid cooling circuit <b>19</b> therein having the vertical passageway <b>24</b>. More specifically, the cooling substrate <b>13</b> may be formed using a plurality of unsintered ceramic layers <b>30-34</b> having passageways therein. It should be noted here that only the unsintered ceramic layers <b>30-34</b> are shown in FIGS. 3-5 for clarity of illustration, but that additional unsintered ceramic layers may also be used in the fabrication the cooling substrate <b>13</b>.

0020The plurality of unsintered ceramic layers <b>30-34</b> and resistive elements <b>16a-16c</b> may be assembled in stacked relation so that the passageways align to define the fluid cooling circuit <b>19,</b> and so that the resistive elements extend in a cantilever fashion into the vertical passageway <b>24</b>, as seen in FIG. 3. More specifically, the resistive elements <b>16a-16c</b> may be formed by applying one or more layers of resistive ink on respective unsintered ceramic layers <b>31-33</b>. Of course, other suitable materials known to those of skill in the art are also contemplated by the invention, such as metal plates, etc.

0021After the unsintered ceramic layers <b>30-34</b> and the resistive elements <b>16a-16c</b> are aligned, the cooling substrate <b>13</b> may be laminated using common laminating processes, for example, which causes the resistive elements <b>16a-16c</b> to begin deforming toward respective vertical sidewalls <b>25</b> of the vertical passageway <b>24</b>, as illustratively shown in FIG. 4. The lamination process will vary depending upon the amount of deformation desired and the materials used for the resistive elements <b>16a-16c,</b> as will be appreciated by those of skill in the art. As noted above, varying materials other than resistive inks may be used for the resistive elements <b>16a-16c</b>, some of which may not deform as readily as others.

0022The unsintered ceramic layers <b>30-34</b> and the resistive elements <b>16a-16c</b> are heated to sinter and form the body <b>15</b> of the cooling substrate <b>13,</b> as illustratively shown in FIG. 5. The heating also causes the resistive elements <b>16a-16c</b> to soften and deform downwardly adjacent portions of the vertical sidewalls <b>25</b> and thus form the resistive elements <b>16</b>. The heating may be performed according to typical LTCC process parameters, which will be appreciated by those of skill in the art.

0023It will also be appreciated by those of skill in the art that different geometries may be used for the vertical passageway <b>24</b>. That is, if the vertical passageway <b>24</b> is generally circular, then there will be only a single vertical sidewall <b>25</b>. Of course, if the vertical passageway <b>24</b> is generally rectangular, then there will be opposing pairs of vertical sidewalls <b>25</b>. As shown in FIGS. 3-5, the resistive elements <b>16a-16c</b> are positioned between adjacent ceramic layers <b>31, 32; 32, 33;</b> and <b>33, 34</b>, respectively, and also positioned adjacent opposing sidewalls <b>25</b> of the vertical passageway <b>24</b>. The resistive elements <b>16a-16c</b> preferably extend into the vertical sidewalls <b>25</b> a predetermined distance <b>A</b>, which is preferably greater than about 10 mils, for example.

0024The resistive elements <b>16a-16c</b> may be connected to a voltage source <b>36</b> for causing the resistive elements <b>16</b> to heat and, in conjunction with other geometric and topological features of the cooling fluid circuit <b>19</b>, thereby facilitate fluid circulation in the cooling fluid circuit <b>19</b>, as will be readily appreciated by those of skill in the art. For example, the voltage source <b>36</b> may be external to the electronic module <b>10</b>, or the electronic device <b>12</b> may provide the voltage to the resistive elements <b>16.</b> By way of example, vias filled with a conductive ink may be used for connecting the voltage source <b>36</b> with the resistive elements <b>16</b>. Of course, a termination ink layer may be required for such connections, and other suitable connections may also be used, as will be appreciated by those of skill in the art.

0025More specifically, heating the resistive elements <b>16</b> may vaporize fluid adjacent thereto to define a vapor region of flow in the two-phase fluid cooling circuit <b>19</b>. This is particularly advantageous when the electronic module <b>10</b> is "cold", i.e., when most of the fluid is in a liquid state. The resistive elements <b>16</b> may be used to establish the two-phase fluid flow before the electronic device <b>12</b> begins operation, which may not otherwise be possible using heat solely from the electronic device. Of course, those of skill in the art will appreciate that the resistive elements <b>16</b> may also be used in a single phase fluid circuit as well.

0026Accordingly, it will be appreciated by those of skill in the art that the cooling substrate <b>13</b> of the present invention provides relatively small and inexpensive embedded resistive elements <b>16</b> that are controllable and reliable. Further, thermally efficient, localized (e.g., within about 100□m or less), and relatively precise fluid flow control may be achieved without the need for prior art control devices, such as mechanical pumps and valves. Also, the electronic module 10 according to the present invention may advantageously be fabricated using current materials and processes known to those of skill in the art.

0027Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed, and that other modifications and embodiments are intended to be included within the scope of the appended claims.