Abstract:
A substrate for power electronics mounted thereon, comprises a middle ceramic layer having a lower surface and an upper surface, an upper metal layer attached to the upper surface of the middle ceramic layer, and a lower metal layer attached to the lower surface of the middle ceramic layer. The lower metal layer has a plurality of millichannels configured to deliver a coolant for cooling the power electronics, wherein the millichannels are formed on the lower metal layer prior to attachment to the lower surface of the middle ceramic layer. Methods for making a cooling device and an apparatus are also presented.

Description:
BACKGROUND 
       [0001]    Many high performance power electronics require cooling devices to prevent them from overheating so as to improve reliability and efficiency thereof. One method for cooling such power electronics is by utilizing heat sinks. The heat sinks operate by transferring the heat away from the power electronics thereby maintaining a lower temperature of the power electronics. There are various types of heat sinks known in thermal management fields including air cooled and liquid cooled devices. 
         [0002]    Typically, the heat sink is made up electrically conductive material. Therefore, the power electronics may be coupled to the heat sink with a substrate disposed therebetween to avoid generation of short circuit, which can damage the power electronics. The substrate generally includes an electrically isolating and thermal conductive layer, such as a ceramic layer. In order to attach the ceramic layer to the heat sink and the power electronics, the substrate further includes metal, such as copper brazed or bonded to upper and lower surfaces of the ceramic layer to perform surface treatment to the ceramic layer. 
         [0003]    The surface treatment process is typically performed at a high temperature, such as 600° C. to 1000° C. Thus, metal patterns and thickness should be controlled to prevent mechanical distortion as the substrate is cooled to room temperature. The cooling may result in mechanical residual stress at the metal to ceramic interfaces due to the difference in coefficient of thermal expansion (CTE) between the metal and the ceramic layer. For example, the ceramic layer includes aluminum nitride ceramic whose CTE is 4 ppm/° C., and the CTE of the copper metal is 17 ppm/° C. 
         [0004]    Additionally, the substrate is formed with a plurality of millichannels on the lower metal for a coolant passing through. However, fabricating the microchannels in the bottom metal of the substrate may relieve some of the residual stress due to removal of the metal. Consequently, the substrate may be deformed. 
         [0005]    It is desirable to have a planar substrate for bonding the substrate to the heatsink and the power electronics. Particularly, when the non-planar substrate is bonded to the heatsink at a higher temperature about 250° C., the substrate deforms more and the bond to the heatsink can not be achieved. 
         [0006]    Therefore, there is a need for new and improved methods for making millichannel substrate, and cooling device and apparatuses using the millichannel substrate. 
       BRIEF DESCRIPTION 
       [0007]    A substrate for power electronics mounted thereon is provided in accordance with one embodiment of the invention. The substrate comprises a middle ceramic layer having a lower surface and an upper surface, an upper metal layer attached to the upper surface of the middle ceramic layer, and a lower metal layer attached to the lower surface of the middle ceramic layer. The lower metal layer has a plurality of millichannels configured to deliver a coolant for cooling the power electronics, and the millichannels are formed on the lower metal layer prior to attachment to the lower surface of the middle ceramic layer. 
         [0008]    A method for making a cooling device is provided in accordance with another embodiment of the invention. The method comprises defining a plurality of millichannels on a first layer and forming a substrate. The forming a substrate comprises attaching the first layer having the millichannels to a lower surface of a second layer, and attaching a third layer to an upper surface of the second layer. The method further comprises attaching the substrate to a base plate so that the first layer is coupled to the base plate, wherein the base plate is configured to cooperate with the substrate to pass a coolant through the millichannels. 
         [0009]    An embodiment of the invention further provides a method for making an apparatus having a substrate with millichannel cooling, wherein the step of making the substrate comprises defining a plurality of millichannels on a first layer, attaching the first layer having the millichannels to a lower surface of a second layer, and attaching a third layer to an upper surface of the second layer. The method further comprises attaching the substrate to a base plate so that the first layer is coupled to the base plate, and attaching a power electronics to the third layer. The base plate is configured to cooperate with the substrate to pass a coolant through the millichannels. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the following detailed description when taken in conjunction with the accompanying drawings in which: 
           [0011]      FIG. 1  is a schematic diagram of an apparatus comprising power electronics, a substrate, and a base plate in accordance with one embodiment of the invention; 
           [0012]      FIG. 2  is an assembled perspective diagram of the apparatus in accordance with one embodiment of the invention; 
           [0013]      FIG. 3  is a perspective diagram of the base plate show in  FIG. 2 ; 
           [0014]      FIG. 4  is a planar diagram of the base plate shown in  FIG. 3 ; 
           [0015]      FIGS. 5(   a )- 5 ( b ) are an top view perspective and a bottom view perspective of the substrate shown in  FIG. 2 ; 
           [0016]      FIG. 6  is a schematic diagram showing cooperation of a lower layer of the substrate and the base plate; 
           [0017]      FIG. 7  is an enlarged schematic diagram showing cooperation of the lower layer and the base plate; 
           [0018]      FIG. 8  is a schematic diagram of the lower metal of the substrate in accordance with another embodiment of the invention; and 
           [0019]      FIG. 9  is a flowchart for forming the apparatus in accordance with one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Various embodiments of the present disclosure are described herein with reference to the accompanying drawings. This description relates generally to methods for making substrates, and cooling devices and apparatuses using millichannel substrates. The description also relates to millichannel substrates and methods for making millichannel substrates. 
         [0021]    As illustrated in  FIGS. 1-2 , an apparatus  10  comprises a base plate  11 , a substrate  12  coupled on the base plate  11 , and at least one heat source  100 , such as power electronics coupled on the substrate  12 , together forming a stack configuration. The base plate  11  and the substrate  12  cooperate with each other to direct one or more coolants to cool the heat source  100 . 
         [0022]    In certain embodiments, non-limiting examples of the coolant comprise de-ionized water and other non-electrically conductive liquids. Non-limiting examples of the power electronics or heat source  100  may include Insulated Gate Bipolar Transistors (IGBT), Metal Oxide Semiconductor Field Effect Transistors (MOSFET), Diodes, Metal Semiconductor Field Effect Transistors (MESFET), and High Electron Mobility Transistors (HEMT). The millichannel substrate embodiments find applications to the power electronics manufactured from a variety of semiconductors, non-limiting examples of which include silicon (Si), silicon carbide (SiC), gallium nitride (GaN), and gallium arsenide (GaAs). 
         [0023]    In the arrangement in  FIG. 1 , the substrate  12  may be attached to the baseplate  11  and the power electronics  100  using a number of techniques, including but not limited to, brazing, bonding, diffusion bonding, soldering, or pressure contact such as clamping, which provides a simple assembly process, which reduces the overall cost of the apparatus  10 . As indicated in  FIG. 1 , in some non-limiting examples, the apparatus  10  may further comprise a first solder  13  configured to attach the low layer  122  to the base plate  11 , and a second solder  14  configured to attach the power electronics  100  to the upper layer  121 . According to certain applications, the solders  13 ,  14  may not be employed. 
         [0024]      FIG. 3  illustrates a perspective diagram of the base plate  11 .  FIG. 4  illustrates a planar diagram of the base plate  11  shown in  FIG. 3 . As illustrated in  FIGS. 3-4 , the base plate  11  is a cubic shape, and defines a plurality of inlet manifolds  20  and a plurality of outlet manifolds  21 , each is recessed downwardly from an upper surface (not labeled) of the base plate  11 . In some examples, the base plate  11  may be other shape. In embodiments of the invention, the inlet manifolds  20  are configured to receive a coolant, and the outlet manifolds  21  are configured to exhaust the coolant. In one non-limiting example, the inlet manifolds  20  and the outlet manifolds  21  are interleaved. 
         [0025]    For the embodiment in  FIGS. 3-4 , there is one more inlet manifold  20  than the outlet manifold  21  in order to preserve symmetry of the coolant flow. Alternatively, there may be one more outlet manifold  21  than the inlet manifold  20 . In the exemplary embodiment, the base plate  11  further comprises an inlet plenum  22  and an outlet plenum  23 . The inlet plenum  22  is configured to communicate fluidly with the inlet manifolds  20 , and the outlet plenum  23  is configured to communicate fluidly with the outlet manifolds  21 . Thus, the coolant can enter into the base plate  11  from the inlet plenum  22  and flow out of the base plate  11  from the outlet plenum  23 . 
         [0026]    In some non-limiting examples, diameters of the inlet and outlet plenums  22 ,  23  may be larger than diameters of the inlet and outlet manifolds  20 ,  21 . Thus, there is no significant pressure-drop in the plenums. Other discussion of the channels is disclosed in U.S. Pat. No. 7,353,859, which is incorporated herein by reference. 
         [0027]    In certain embodiments, the baseplate  11  may comprise at least one thermally conductive material, non-limiting examples of which may include copper, aluminum, nickel, molybdenum, titanium, and alloys thereof. In some examples, the baseplate  11  may also comprise at least one thermally conductive material, non-limiting examples of which may include thermo pyrolytic graphite (TPG). In other examples, the baseplate  11  may also comprise at least one thermally conductive material, non-limiting examples of which may include metal matrix composites such as aluminum silicon carbide (AlSiC), aluminum graphite, or copper graphite. Alternatively, the baseplate  11  may also comprise at least one thermally conductive material, non-limiting examples of which may include ceramics such as aluminum oxide, aluminum nitride, or silicon nitride ceramic. In certain examples, the baseplate  11  may include at least one thermoplastic material. 
         [0028]      FIGS. 5(   a )- 5 ( b ) show a top view perspective and a bottom view perspective of the substrate  12 . As illustrated in  FIGS. 5(   a )- 5 ( b ), the substrate  12  comprises the lower layer  122  (a first layer), a middle layer  120  (a second layer), and the upper layer  121  (a third layer). The middle layer  120  comprises an upper surface  123  and a lower surface  124  opposite to the upper surface  123 . The upper layer  121  is coupled to the upper surface  123 , and the lower layer  122  is coupled to the lower surface  124 . For the arrangement in  FIG. 1 , the substrate  12  is coupled to the base plate  11  by attaching the lower layer  122  to the base plate  11 . The power electronics  100  are coupled to the substrate  12  by attaching the power electronics  100  to the upper layer  121 . 
         [0029]    In some embodiments, for the arrangements in  FIGS. 5(   a )- 5 ( b ), the middle layer  120  may comprises at least one electrically isolating and thermally conductive layer. The upper layer  121  and lower layer  122  may comprise at least one conductive material, respectively. In one non-limiting example, the middle layer  120  is a ceramic layer, and the upper and lower layers  121 ,  122  may comprise metal, such as copper attached to the upper surface  123  and the lower surface  124  of the ceramic layer  120 . Thus, the substrate  12  may have either a direct bonded copper (DBC), or an active metal braze (AMB) structure. The DBC and AMB refer to processes which copper layers are directly bonded to a ceramic layer. 
         [0030]    Non-limiting examples of the ceramic layer  120  may comprise aluminum oxide (AL 2 O 3 ), aluminum nitride (AlN), beryllium oxide (BeO), and silicon nitride (Si 3 N 4 ). Both the DBC and the AMB may be convenient structures for the substrate  12 , and the use of the conductive material (in this case, copper) on the ceramic layer  120  may provide thermal and mechanical stability. Alternatively, the conductive layer  121 ,  122  may include other materials, but not limited to, aluminum, gold, silver, and alloys thereof according to different applications. 
         [0031]    For the arrangement in  FIG. 5(   b ), the lower layer  122  defines a plurality of millichannels  125  arranged parallel to each other and configured to communicate fluidly with the inlet and outlet manifolds  20 ,  21  (shown in  FIG. 7) . In one non-limiting example, the millichannels  125  may pass through the lower layer  122  to form through-hole configuration (shown in  FIG. 7 ) on the lower layer  122 . That is, the millichannel depth may be equal to the thickness of the lower layer  122 . Alternatively, in other examples, the millichannels  125  may not pass through the lower layer  122  so as to form a plurality of trenches on the lower layer  122 . 
         [0032]      FIG. 6  illustrates a schematic diagram showing cooperation of the lower layer  122  and the base plate  11 .  FIG. 7  illustrated an enlarged diagram of a portion of the cooperation of the lower layer  122  and the base plate  11 . It should be noted that this configuration is only illustrative, and in certain examples, the lower  122  may contact the base plate  11  after the substrate  12  is formed. As illustrated in  FIGS. 6-7 , the millichannels  125  are oriented substantially perpendicular to and in fluid communication with the inlet and outlet manifolds  20 ,  21 . As used herein the term “oriented substantially perpendicular” should be understood to mean that the millichannels  125  may be oriented at angles of about ninety degrees plus/minus about thirty degrees (90+/−30 degrees) relative to the inlet and outlet manifolds  20 ,  21 . In other examples, the millichannels  125  may be oriented at angles of about ninety degrees plus/minus about fifteen degrees (90+/−15 degrees) relative to the inlet and outlet manifolds  20 ,  21 . 
         [0033]    In operation, for the arrangement in  FIGS. 6-7 , the coolant can enter the inlet manifolds  20  from the inlet plenum  22 , then flow through the millichannels  125 , and finally enter the outlet manifolds  21  to flow out of the base plate  11  from the outlet plenum  23 . Thus, the heat generated from the power electronics  100  (shown in  FIG. 1 ) may be removed by the coolant. 
         [0034]      FIG. 8  illustrates a schematic diagram of the lower layer  122  in accordance with another embodiment of the invention. In the illustrated embodiment, the lower layer  122  defines a plurality of millichannels  126  arranged into more than one, such as four groups to cooperate with the base plate  11  to cool the power electronics. The different groups of the millichannels  126  are arranged parallel to and separated with each other, and the millichannels  126  in the same one group are arranged parallel to each other. Lengths and widths of the millichannels  126  in different groups may be the equal. In other examples, the lengths and widths of the millichannels  126  in one group may be different from the lengths and widths of the millichannels  126  in the other group. In certain applications, the millichannels  125 - 126  may be arranged in different patterns such as angular, arc, zigzag and other patterns depending upon the design criteria. According to more particular embodiments, the channels  20 - 23 ,  125 - 126  are configured to deliver the coolant uniformly to improve thermal removal performance. 
         [0035]    In embodiments of the invention, the millichannels  125 - 126  may include square/rectangular cross sections. Non-limiting examples of cross sections of the millichannels  125 - 126  may include u-shaped, circular, triangular, or trapezoidal, cross-sections. The millichannels  125 - 126  may be cast, machined, or etched in the lower layer  122 , and may be smooth or rough. The rough millichannels may have relatively larger surface area to enhance turbulence of the coolant so as to augment thermal transfer therein. In non-limiting examples, the millichannels may employ features such as dimples, bumps, or the like therein to increase the roughness thereof. Similarly to the millichannels  125 - 126 , the manifolds  20 - 21  and the plenums  22 - 23  may also have a variety of cross-sectional shapes, including but not limited to, round, circular, triangular, trapezoidal, and square/rectangular cross-sections. The channel shape is selected based on the applications and manufacturing constraints and affects the applicable manufacturing methods, as well as coolant flow. 
         [0036]    In some embodiments, the substrate  12  may be planar so as to be coupled to the base plate  11  and the power electronics  100  securely. Therefore, when fabricating the substrate  12 , deformation of the substrate  12  should be restrained. In some non-limiting examples, the lower layer  122  may be formed with the millichannels  125 ,  126  before being coupled to the middle layer  120  to prevent the substrate  12  from deformation. 
         [0037]    In one non-limiting example, the substrate  12  may have the direct bonded copper or the active metal braze structure. Accordingly, as described in a flowchart  200  illustrated in  FIG. 9 , when fabricating the substrate, in step  205 , the millichannels are formed on the lower copper layer. Then, in step  210 , the lower and upper copper layers are attached to the middle ceramic layer, which is one example is done simultaneously at a high temperature, such as 600° C. to 1000° C. 
         [0038]    Alternatively, the lower layer and the upper layer may not be attached to the middle layer simultaneously but serially. Additionally, in certain examples, the upper layer may also define millichannels similar to the lower layer millichannels  125 - 126 . As described herein, the upper and lower layers may include other conductive material such as aluminum, gold, silver, and alloys thereof according to different applications. 
         [0039]    Subsequently, in step  215 , the substrate is attached to the base plate. In certain embodiments, such as indicated in  FIG. 1 , a variety of the first solders  13  may be employed to complete the attachment. In some examples, the first solder  13  may comprise a eutectic lead-tin solder including 63 wt % tin and 37 wt % lead. In other examples, the first solder  13  may comprise a hi-lead solder including 92.5 wt % lead, 2.5 wt % silver, and 5 wt % tin. An attachment temperature using the lead-tin solder may be about 210° C., and an attachment temperature using the hi-lead solder may be about 310° C. to complete the attachment of the substrate  12  to the base plate  11 . 
         [0040]    Finally, in step  220 , the power electronics are attached to the upper layer using some type of the second solder  14 , which can be implemented by one skilled in the art. Alternatively, in some embodiments, the power electronics may be attached to the substrate before the substrate is attached to the base plate. Thus, in certain embodiments, the millichannels are first formed on the lower layer before the lower layer and the upper layer are coupled to the middle layer so that the deformation of the substrate in the fabrication process may be reduced or eliminated. 
         [0041]    In operation, as shown in step  225 , the coolant flows through the base plate and also through the millichannels in the substrate thereby cooling the electronics. 
         [0042]    While the disclosure has been illustrated and described in typical embodiments, it is not intended to be limited to the details shown, since various modifications and substitutions can be made without departing in any way from the spirit of the present disclosure. As such, further modifications and equivalents of the disclosure herein disclosed may occur to persons skilled in the art using no more than routine experimentation, and all such modifications and equivalents are believed to be within the spirit and scope of the disclosure as defined by the following claims.