Patent Publication Number: US-2023156905-A1

Title: Circuit board

Description:
TECHNICAL FIELD 
     The present invention relates to a circuit board. 
     BACKGROUND ART 
     In recent years, the market for a power module on which an IGBT element and the like are mounted has been expanding. The power module is required to have high reliability and high heat resistance. As this kind of technique, various developments have been made so far in circuit boards (also referred to as heat radiating substrates) having a heat radiation function. As this kind of technique, for example, a technique disclosed in Patent Document 1 is known. Patent Document 1 discloses a power module in which a semiconductor element is mounted on a support, such as a lead frame, and the support and a heat radiating plate connected to a heat sink are bonded with an insulating resin layer. 
     RELATED DOCUMENT 
     
         
         Patent Document 
         [Patent Document 1] Japanese Unexamined Patent Publication No. 2011-216619 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     In recent years, better heat radiation properties are required for such a circuit board. The technique disclosed in Patent Document 1 cannot sufficiently satisfy the requirement of the heat radiation function for the power module. 
     The present invention has been made in view of such a situation, and an object of the present invention is to provide a technique of enhancing heat radiation properties in a circuit board having a heat radiation function. 
     Solution to Problem 
     According to the Present Invention, 
     it is possible to provide a circuit board including: 
     an insulating substrate; and 
     a circuit pattern of a metal formed on the insulating substrate in direct contact with the insulating substrate, 
     in which a side surface of the circuit pattern has a region in which an angle formed by a surface of the insulating substrate and a tangential line at a middle portion in a height direction in a cross-sectional view perpendicular to an extending direction of the metal is 80 degrees or more and 100 degrees or less. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide a technique of enhancing heat radiation properties in a circuit board having a heat radiation function. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a plan view of a heat radiating substrate according to an embodiment. 
         FIG.  2    is a cross-sectional view of the heat radiating substrate according to the embodiment. 
         FIG.  3    is an enlarged view showing a circuit pattern for a cross-sectional structure of the heat radiating substrate according to the embodiment. 
         FIG.  4    is an enlarged view showing the circuit pattern for the cross-sectional structure of the heat radiating substrate according to the embodiment. 
         FIG.  5    is an enlarged view showing the circuit pattern for the cross-sectional structure of the heat radiating substrate according to the embodiment. 
         FIG.  6    is a chart showing a production step of the heat radiating substrate according to the embodiment. 
         FIG.  7    is a view showing a cross-sectional structure of the circuit pattern according to the embodiment so as to be comparable with a conventional structure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. 
     &lt;Overview of Heat Radiating Substrate&gt; 
       FIG.  1    is a plan view of a heat radiating substrate  10 .  FIG.  2    is a cross-sectional view of a part of the heat radiating substrate  10 . 
     The heat radiating substrate  10  is a circuit board on which an electronic component of a heat-generating body and the like are mounted, is composed of a metal substrate  12 , an insulating layer  11 , and a circuit pattern  20 , and is a laminate (laminated body) laminated in this order from the bottom as shown in  FIG.  2   . Electronic components and the like are mounted on the circuit pattern  20 . 
     The total thickness TO of the heat radiating substrate  10  is not particularly limited, but is, for example, preferably 300 μm or more and 5000 μm or less and more preferably 1000 μm or more and 4000 μm or less. 
     &lt;Metal Substrate  12 &gt; 
     The metal substrate  12  is a layer composed of a metal material, and in the present embodiment, the insulating layer  11  is formed on an upper surface thereof, and a heat radiating means (not shown), such as heat radiating fins and radiators, is appropriately attached to the lower surface thereof. 
     The metal material constituting the metal substrate  12  is not limited to a specific type, but for example, copper, a copper alloy, aluminum, an aluminum alloy, or the like can be used. 
     The thickness T 1  of the metal substrate  12  is not particularly limited, but is the thickest among the elements (the insulating layer  11 , the metal substrate  12 , and the circuit pattern  20 ) laminated in the heat radiating substrate  10 , and is preferably 10% to 90% with respect to the total thickness TO. 
     The upper limit value of the thickness T 1  of the metal substrate  12  is, for example, 20.0 mm or less and preferably 5.0 mm or less. The metal substrate  12  having the thickness T 1  equal to or less than the upper limit value is used, so that the heat radiating substrate  10  as a whole can be made thinner. Further, it is possible to improve the workability in routing, cutting, or the like of the heat radiating substrate  10 . 
     The lower limit value of the thickness T 1  of the metal substrate  12  is, for example, 0.1 mm or more, preferably 0.5 mm or more, and more preferably 1.0 mm or more. The metal substrate  12  having the lower limit value or more is used, so that the heat radiation properties of the heat radiating substrate  10  as a whole can be improved. 
     &lt;Insulating Layer  11 &gt; 
     The insulating layer  11  is a layer of a resin substrate mainly composed of a resin material, and has a function of insulating the metal substrate  12  from the circuit pattern  20 . As the insulating layer  11 , a ceramic substrate (an aluminum nitride substrate, a silicon nitride substrate, or the like) may be used instead of the resin substrate. 
     The resin material constituting the insulating layer  11  is not limited to a specific type, but examples thereof include a thermosetting resin, such as an epoxy resin, a phenol resin, a urea resin, a melamine resin, a polyester (unsaturated polyester) resin, or a polyimide resin, a silicone resin, and a polyurethane resin. As the resin material, one or a mixture of two or more of these resins can be used. 
     A filler composed of particles having electrical insulation properties and high thermal conductivity can also be mixed into the resin material constituting the insulating layer  11 . Examples of the constituent material of the particles of the filler include metal oxides, such as alumina, and nitrides, such as boron nitride. 
     The thickness T 2  of the insulating layer  11  is appropriately set according to the purpose, but the thickness T 2  of the insulating layer  11  is preferably 40 μm or more and 400 μm or less from the viewpoint of more effectively transferring heat from the electronic component to the metal substrate  12  while improving the mechanical strength and heat resistance, and it is more preferable that the thickness T 2  of the insulating layer  11  is set to 80 μm or more and 300 μm or less from the viewpoint of further excellent balance between the heat radiation properties and the insulation properties in the entire heat radiating substrate  10 . The thickness T 2  of the insulating layer  11  is set to the above upper limit value or less, so that it is possible to facilitate the transfer of heat from the electronic component to the metal substrate  12 . In addition, the thickness T 2  of the insulating layer  11  is set to the above lower limit value or more, so that the insulating layer  11  can sufficiently alleviate the generation of thermal stress caused by a difference in coefficient of thermal expansion between the metal substrate  12  and the insulating layer  11 . Further, the insulation properties of the heat radiating substrate  10  are improved. 
     &lt;Circuit Pattern  20 &gt; 
     The circuit pattern  20  is composed of a conductive metal material, and is electrically connected to an electronic component (LED or the like) of a heat-generating body by, for example, soldering. For example, copper can be suitably used as the metal material constituting the circuit pattern  20 . With this, the circuit pattern  20  has a relatively small resistance value. The circuit pattern  20  may be at least partially covered with a solder resist layer. 
     The circuit pattern  20  is formed, for example, by working a metal layer laminated on an insulating layer upper surface  11   a  of the insulating layer  11  into a predetermined pattern through cutting and etching. The formation process will be described later in  FIG.  7   , but in the present embodiment, rolled copper is used as a metal layer  20 A in  FIG.  7   . 
     The lower limit value of the thickness T 3  of the circuit pattern  20  is, for example, 0.3 mm or more. In a case where the lower limit value is such a numerical value or more, it is possible to restrain the circuit pattern from generating heat even in a use requiring a high current. The upper limit value of the thickness T 3  of the circuit pattern  20  is, for example, 5.0 mm or less, preferably 4.0 mm or less, and more preferably 3.0 mm or less. In a case where the upper limit value is such a numerical value, the circuit workability can be improved, and the substrate as a whole can be made thinner. 
     &lt;Specific Shape of Circuit Pattern  20 &gt; 
     A specific shape of the circuit pattern  20 , particularly a cross-sectional shape, will be described with reference to  FIGS.  3  to  5   . 
       FIG.  3    is a view showing a cross-sectional structure of the circuit pattern  20 , and is shown here without hatching. This is a structure showing a cross-sectional view perpendicular to the extending direction of the metal material constituting the circuit pattern  20 . 
     As shown in  FIG.  3   , the circuit pattern  20  is formed on the insulating layer upper surface  11   a  of the insulating layer  11 . Here, the height (that is, the distance to a metal layer upper surface  21  from a metal layer lower surface  22  which is an interface with the insulating layer upper surface  11   a ) of the circuit pattern  20  is denoted by D. At this time, in a metal layer side surface  23 , in a case where an angle formed by a surface (that is, the insulating layer upper surface  11   a ) of the insulating layer  11  and a tangential line L 1  at a position X 1  of the middle portion (height of 0.5D) in the height direction is denoted by θ (hereinafter, referred to as “inclination angle θ”, a region in which the inclination angle θ is 80 degrees or more and 100 degrees or less is provided. That is, the metal layer side surface  23  of the circuit pattern  20  has a region that is formed substantially perpendicularly at the position X 1  of the middle portion in the height direction. 
     A specific shape of the circuit pattern  20  will be described with reference to  FIG.  4   . Here, the shape of the metal layer side surface  23  will be particularly described.  FIG.  4    shows the cross-sectional structure of the same cross-sectional view as that of  FIG.  3   . 
     In the cross-sectional view shown in  FIG.  4   , the metal layer side surface  23  has a trailing-tailed part  23   a , a straight line part  23   b , and a rounded part  23   c  in this order from the lower side to the upper side. 
     The trailing-tailed part  23   a  has a structure formed in a boundary region (a region from the interface to a predetermined height) with the insulating layer  11 , and has a trailing-tailed shape in which a surface becomes gentler (a gentle curve in the cross-section) toward the insulating layer upper surface  11   a  side. The position where the trailing-tailed part  23   a  is formed is, for example, a region to the position of 0.4D in the perpendicular direction from the insulating layer upper surface  11   a  of the insulating layer  11 , and is a region preferably to the position of 0.3D and more preferably to the position of 0.2D therefrom. 
     The straight line part  23   b  is, for example, a region that is continuously formed in a straight line having the above inclination angle θ with a surface roughness equal to or less than an average roughness of the surface in the cross-section. The straight line part  23   b  is formed in a range from the position of 0.4D to the position of 0.6D in the perpendicular direction from the insulating layer upper surface  11   a  of the insulating layer  11 . The straight line part  23   b  is formed in a range preferably from the position of 0.3D to the position of 0.7D and more preferably from the position of 0.2D to the position of 0.8D. The straight line part  23   b  can also be said to be a region (that is, a perpendicular part) in which the tangential line of the region is formed substantially perpendicularly at the above-described inclination angle  9  (for example, 80 degrees or more and 100 degrees or less). 
     The rounded part  23   c  is a region connected to the metal layer upper surface  21  of the circuit pattern  20 , and exhibits a curved surface that becomes gentler toward the metal layer upper surface  21  side. The rounded part  23   c  is formed in a range to the boundary with the metal layer upper surface  21  from the position of 0.6D in the perpendicular direction from the insulating layer upper surface  11   a  of the insulating layer  11 . The rounded part  23   c  is formed in a range to the boundary with the metal layer upper surface  21  (the position of the height D) preferably from the position of 0.7D and more preferably from the position of 0.8D. 
     The intervals between adjacent circuit patterns  20 X and  20 Y will be described with reference to  FIG.  5   . The circuit patterns  20 X and  20 Y shown here satisfy the conditions of the circuit pattern  20  illustrated in  FIG.  4   . 
     Here, the height D of each of the circuit patterns  20 X and  20 Y is denoted by b. Further, in the metal layer side surface  23  of the circuit pattern  20 X, the position of the middle portion (height of 0.5D) in the height direction is denoted by X 1   x . Similarly, in the metal layer side surface  23  of the circuit pattern  20 Y, the position of the middle portion (height of 0.5D) in the height direction is denoted by X 1   y . The distance between the positions X 1   x  and X 1   y  of respective middle portions (that is, the width between patterns at the middle portion in the height direction) is denoted by a. In this condition, a region in which the aspect ratio b/a is 0.2 or more and 5 or less is provided. In other words, the region is a region in which the height D (b) is relatively high with respect to the distance between the patterns. 
     &lt;Summary of Features of Heat Radiating Substrate  10 &gt; 
     As described above, in the metal layer side surface  23 , the trailing-tailed part  23   a  is provided to a height of 0.4D, and the straight line part  23   b  that is formed substantially perpendicularly from the position of a height of 0.4D to the position of a height of 0.6D is provided. That is, since the region of the trailing-tailed part  23   a  is small, a sufficient interval between the circuit patterns can be secured in the insulating layer upper surface  11   a  even in a case where the distance a between the positions X 1   x  and X 1   y  of the middle portions of the heights of the circuit patterns  20 X and  20 Y or the distance between the boundary portions in the metal layer upper surface  21  is narrowed. In other words, the circuit patterns  20 X and  20 Y can be made dense. 
     &lt;Method of Producing Heat Radiating Substrate  10 &gt; 
       FIG.  6    is a chart showing the production process of the heat radiating substrate  10 . A method of producing the heat radiating substrate  10  will be described with reference to  FIG.  6   . 
     (S 10 : Laminated Body Preparation Step) 
     A laminate  10 A in which the metal substrate  12 , the insulating layer  11 , and the metal layer  20 A are laminated in this order from the bottom is prepared. The metal layer  20 A is worked by the following steps to form the circuit pattern  20 . 
     As the method of producing the laminate  10 A, a known method can be used. For example, the metal substrate  12  is used as a carrier, and a liquid material (varnish-like material) as a constituent material of the insulating layer  11  is applied onto the metal substrate  12  having the thickness T 1  by, for example, a spray method. 
     Then, the liquid material applied on the metal substrate  12  is dried by natural drying or forced drying. With this, the insulating layer  11  having the thickness T 2  is obtained. At this time, the insulating layer  11  may not be completely cured (so-called B stage state). 
     Next, the metal layer  20 A having a thickness T 3 ′ is formed on the insulating layer  11 . That is, the metal layer  20 A to be the circuit pattern  20 , for example, rolled copper is laminated on the insulating layer upper surface  11   a  of the insulating layer  11  by a hot press or the like. With this, the laminate  10 A is obtained. The thickness T 3 ′ of the metal layer  20 A is set in consideration of an etching step, which will be described later. 
     (S 12 : Circuit Pattern Cutting Step) 
     Subsequently, the metal layer  20 A of the above-described laminate  10 A is cut so as to have a desired pattern by using a router. A provisional circuit pattern  20 B is formed on the insulating layer  11  by leaving a metal layer (thin copper portion  20 B 1 ) having a predetermined thickness for a portion that is not a pattern. That is, there is a concern that the insulating layer  11  may be damaged in a case where all the patterns are formed by cutting. Therefore, the metal layer (thin copper portion  20 B 1 ) having a thickness is partially left as a margin. With this, a laminate  10 B having the provisional circuit pattern  20 B is obtained. 
     (S 14 : Etching Step) 
     Subsequently, the etching process is performed on the laminate  10 B having the provisional circuit pattern  20 B to melt the remaining metal layer (thin copper portion  20 B 1 ) and to form a desired pattern so that the final circuit pattern  20  can be obtained. With this, the heat radiating substrate  10  is obtained. 
     Effect of Embodiment 
     The features and effects of the embodiment are summarized as follows. 
     (1) The heat radiating substrate  10  includes: 
     the insulating layer  11  (insulating substrate); and 
     the circuit pattern  20  of the metal formed on the insulating layer  11  in direct contact with the insulating layer  11 , 
     in which the side surface (that is, the metal layer side surface  23 ) of the circuit pattern  20  has a region in which the inclination angle θ formed by the surface (insulating layer upper surface  11   a ) of the insulating layer  11  (insulating substrate) and a tangential line L at the middle portion (X 1 ) in the height direction in a cross-sectional view perpendicular to the extending direction of the metal is preferably 80 degrees or more and 100 degrees or less. 
     With this, the circuit pattern  20  can be made dense. 
     (2) In the heat radiating substrate  10 , the insulating layer  11  (insulating substrate) is a resin substrate. 
     (3) In the heat radiating substrate  10 , the metal of the circuit pattern  20  is rolled copper. 
     It is possible to form the metal layer  20 A of rolled copper provided on the insulating layer  11  into the desired circuit pattern  20  by efficiently working the metal layer  20 A through cutting and etching. 
     (4) In the heat radiating substrate  10 , in a case where the height of the circuit pattern  20  is denoted by D, the metal layer side surface  23  of the circuit pattern  20  exhibits a straight line in the cross-sectional view (for example, in the cross-sectional structure in  FIG.  4   ) perpendicular to the extending direction of the metal, in a height range of 0.4D or more and 0.6D or less in the perpendicular direction from the insulating layer  11  (the insulating layer upper surface  11   a ). That is, since the straight line part  23   b  that can be regarded as substantially perpendicular is provided in the height range of 0.4D or more and 0.6D or less in the perpendicular direction from the insulating layer upper surface  11   a , it is possible to form a circuit pattern  20  having good occupancy efficiency. 
     (5) The metal layer side surface  23  of the circuit pattern  20  exhibits a trailing-tailed shape at the interface with the insulating layer  11 . That is, the trailing-tailed part  23   a  is provided in the vicinity of the metal layer lower surface  22 , more specifically, in the range from the insulating layer upper surface  11   a  to a height of 0.4D. With such a trailing-tailed shape, it is possible to make the circuit pattern  20  dense while realizing the desired adhesiveness of the circuit pattern  20  to the insulating layer  11 . 
     (6) In a case where the height D of the circuit pattern  20  is denoted by b and the distance (distance from X 1   x  to X 1   y ) from the adjacent circuit pattern in the middle portions X 1   x  and X 1   y  of the height of the circuit pattern  20  is denoted by a, a region in which an aspect ratio b/a is 0.2 or more and 5 or less is provided. In this way, the region has the configuration in which the aspect ratio b/a is 0.2 or more and 5 or less, that is, the configuration in which the distance a between the circuit patterns  20  is narrowed with respect to the thickness T 3  (height D=b) of the circuit pattern  20 , so that the circuit pattern  20  can be made dense. 
     Although the embodiment of the present invention has been described above, the embodiment is examples of the present invention, and various configurations other than the above embodiment can be adopted. 
     Examples 
       FIG.  7    shows photographs of cross-sectional structures of Example and Comparative Example.  FIG.  7 ( a )  is the heat radiating substrate  10  produced by forming the circuit pattern  20  by using the cutting and etching shown in the above-described embodiment (Example).  FIG.  7 ( b )  is a heat radiating substrate produced by forming the circuit pattern  20  by using only conventional general etching (Comparative Example). Here, the photographs of the circuit cross-sections are arranged side by side up and down so as to be comparable with each other. In these photographs, the width of the circuit pattern  20 , more specifically, the width of the metal layer lower surface  22  (the interface with the insulating layer  11 ) is formed to be 1 mm. 
     In Comparative Example shown in  FIG.  7 ( b ) , the side surface portion has a trailing-tailed shape (substantially Mt. Fuji shape) as a whole, and the area of the upper surface of the circuit pattern  20  is narrowed. On the other hand, in Example shown in  FIG.  7 ( a ) , the region of the side surface portion with the trailing-tailed shape (trailing-tailed part  23   a  in  FIG.  3   ) is small, and most of the region forms a straight line (straight line part  23   b  in  FIG.  3   ). Therefore, as described above, it is possible to make the circuit pattern  20  dense as compared with the related art. 
     This application claims priority based on Japanese Patent Application No. 2020-050891 filed on Mar. 23, 2020, all of its disclosures are incorporated herein. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10 : heat radiating substrate 
               10 A,  10 B: laminate 
               10 B: laminate 
               11 : insulating layer 
               11   a : insulating layer upper surface 
               12 : metal substrate 
               20 ,  20 X,  20 Y: circuit pattern 
               20 A: metal layer 
               20 B: provisional circuit pattern 
               20 B 1 : thin copper portion 
               21 : metal layer upper surface 
               22 : metal layer lower surface 
               23 : metal layer side surface 
               23   a : trailing-tailed part 
               23   b : straight line part 
               23   c : rounded part