Patent Publication Number: US-2004050586-A1

Title: Circuit board where circuit terminals are interconnected and method of fabricating the same

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001] This application claims priority from Korean Patent Application No. 2002-32334, filed on Jun. 10, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The present invention relates to a circuit board where circuit patterns are formed on a top surface and a bottom surface of a substrate, and more particularly, to a circuit board where circuit terminals on the top surface and the bottom surface are interconnected, and to a method of fabricating the same.  
       [0004] 2. Description of the Related Art  
       [0005] In a case where many electric devices or semiconductor chips are mounted on a small circuit board, it is advantageous to form two circuit patterns, one on a top surface and the other on a bottom surface of the substrate and connect at least one of the circuit terminals on the top surface to at least one of the circuit terminals of the bottom surface so as to reduce the size of the circuit board.  
       [0006] In a conventional circuit board where circuit terminals on the top surface and the bottom surface are interconnected, such circuit terminals are electrically connected to one another by a conductive filler filled in a through hole formed in the substrate or by a plating layer formed at an inner wall of such a through hole.  
       [0007]FIG. 1A illustrates a circuit board  1  disclosed in U.S. Pat. No. 6,337,037, where an upper terminal  21  and a lower terminal  22  are electrically interconnected by a conductive filler  40  filled in a through hole  35 . However, the conductive filler  40  is often filled disconnectedly in the through hole  35 , and this causes a failure in connection. In addition, an additional cost for filling the hole of the substrate with the conductive filler  40  may be required.  
       [0008]FIG. 1B illustrates a circuit board  1  disclosed in U.S. Pat. No. 4,622,107, where a conductive plating layer  50 , instead of the conductive filler  40 , is formed at the inner wall of the through hole  35 . However, the conductive plating layer  50  of the circuit board  1  of FIG. 1B has an inferior strength and/or durability under thermal expansion thereof because the conductive plating layer  50  is thin.  
       SUMMARY OF THE INVENTION  
       [0009] The present invention provides a circuit board, which is efficient to produce, and has at least an upper terminal and a lower terminal securely interconnected and a method of fabricating the circuit board.  
       [0010] According to an aspect of the present invention, there is provided a circuit board where circuit terminals on the top and bottom surfaces of the substrate are interconnected. The circuit board comprises a substrate having a top surface and a bottom surface and including circuit patterns disposed on the top and bottom surfaces thereof, the substrate defining a connection hole therein; an upper terminal electrically connected to a circuit pattern formed on the top surface of the substrate; and a lower terminal electrically connected to a circuit pattern formed on the bottom surface of the substrate, the lower terminal being located opposite to the upper terminal, wherein the upper terminal and the lower terminal are bonded inside the connection hole of the substrate for electrical connection there-between.  
       [0011] Preferably, end portions of the upper and lower terminals are bonded inside the connection hole. It is also preferable that a conductive plating layer is formed on at least one opposite sides of the ends to facilitate the electrical connection.  
       [0012] According to another aspect of the present invention, there is provided a method of fabricating a circuit board where circuit terminals on the top and bottom surfaces of the substrate are interconnected. The method comprises the steps of: (a) preparing a circuit board which includes a substrate, having a top surface and a bottom surface and including circuit patterns disposed on the top and bottom surfaces thereof, the substrate defining an upper terminal which is electrically connected to a circuit pattern formed on the top surface of the substrate, the substrate further defining a lower terminal which is electrically connected to a circuit pattern formed on the bottom surface of the substrate, the lower terminal being located opposite to the upper terminal; (b) forming at least one connection hole in the substrate adjacent to the upper and lower terminals by etching the substrate; and (c) electrically connecting an end of the upper terminal to an end of the lower terminal inside the connection hole.  
       [0013] It is preferable that the above-identified step (a) further comprises the step of forming a notch adjacent to the ends of the upper and lower terminals, and the step (c) is performed by thermocompression bonding. Preferably, the step (c) further comprises forming a conductive plating layer on at least one opposite surface of the ends of the upper and lower terminals.  
       [0014] According to the present invention, a circuit board which is efficient to produce and has an upper terminal and a lower terminal securely interconnected and a method of fabricating the circuit board are provided. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0015] The above and other features and objectives of the present invention will become more apparent by the following embodiments thereof with reference to the attached drawings in which:  
     [0016]FIG. 1A is a sectional perspective view of a conventional circuit board where circuit terminals on a top surface and a bottom surface are interconnected by a conductive filler filled in a through hole;  
     [0017]FIG. 1B is a sectional perspective view of a conventional circuit board where circuit terminals on a top surface and a bottom surface are interconnected by a conductive plating layer formed at an inner wall of a through hole;  
     [0018]FIG. 2 is a sectional perspective view of a circuit board of the invention where circuit terminals on a top surface and a bottom surface are interconnected according to the present invention;  
     [0019]FIG. 3A is a sectional perspective view of a circuit board of the invention having at least one upper terminal on a top surface and one lower terminal on a bottom surface;  
     [0020]FIG. 3B is a cross-sectional view taken along line A-A′ of FIG. 3A;  
     [0021]FIG. 4A is an exploded, partially perspective view showing a portion of a top surface and a bottom surface of a circuit board of the invention where an etching stopper is formed at both surfaces of the circuit board;  
     [0022]FIG. 4B is a cross-sectional view showing a circuit board of the invention in which the top surface and the bottom surface are covered with the etching stopper of FIG. 4A;  
     [0023]FIG. 5 is cross-sectional view showing a circuit board of the invention in which a connection hole is formed therein;  
     [0024]FIG. 6 is a cross-sectional view showing a circuit board of the invention in which the upper terminal and the lower terminal are covered with a plating layer;  
     [0025]FIG. 7 is a cross-sectional view showing a circuit board of the invention from which an etching stopper is removed;  
     [0026]FIG. 8 is a cross-sectional view illustrating a process of connecting an end of an upper terminal to an end of a lower terminal by thermocompression bonding, in accordance with the invention; and  
     [0027]FIG. 9 is a cross-sectional view showing a circuit board whose bonding portions of the upper terminal and the lower terminal are further secured by filling the connection hole with an insulating bonding material.  
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
     [0028] The present invention will now be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown.  
     [0029]FIG. 2 shows a circuit board I according to the present invention where a circuit pattern (not shown) is formed on a top surface and a bottom surface of a substrate  10 . An end  23  of an upper terminal  21  on the top surface of the substrate  10  and an end  24  of a lower terminal  22  on the bottom surface of the substrate  10  are bonded inside a connection hole  130 .  
     [0030] The substrate  10  supports two circuit patterns formed respectively on the top surface and the bottom surface, and insulates the circuit patterns.  
     [0031] The upper terminal  21  and the lower terminal  22  each may be a part of the circuit pattern respectively formed at the top surface and the bottom surface or a separate element which is electrically connected to the circuit pattern. The upper terminal  21  and the lower terminal  22  are connected preferably in an orthogonal direction to the circuit board  1 .  
     [0032] The connection hole  130  is used to connect the end  23  of the upper terminal  21  to the end  24  of the lower terminal  22 . It is preferable that the size of the connection hole  130  is as small as possible, but large enough to provide a space in which the upper terminal  21  and the lower terminal  22  can be firmly connected. The connection hole  130  can be formed, for example, by etching a connection area  100  shown in FIG. 3B where the end  23  of the upper terminal  21  and the end  24  of the lower terminal  22  are interconnected therein, or by forming the substrate  10  with the connection area  100  empty, or by blanking the substrate  10 . If the connection hole  130  is formed by a method other than etching the upper terminal  21  and the lower terminal  22  have to be configured to support the ends  23  and  24  over the connection hole  130 . In addition, it is preferable that the end  23  of the upper terminal  21  and the end  24  of the lower terminal  22  are arranged in a predetermined area of the circuit board  1  so as to minimize the number of the connection hole  130 .  
     [0033] On the other hand, the end  23  of the upper terminal  21  and the end  24  of the lower terminal  22  can be bonded by thermocompression bonding, resistance welding, or any other conceivable methods of bonding metals. In the present disclosure, a bonding method performed particularly by using thermocompression bonding is described. In this case, it is preferable that conductive plating layers  140  as shown in FIG. 6 are formed respectively on at least one opposite side of the ends  23  and  24  to facilitate and secure the thermocompression bonding. The conductive plating layers  140  may be formed of a metal such as gold, tin, or the like.  
     [0034] In addition, after the end  23  of the upper terminal  21  and the end  24  of the lower terminal  24  are bonded together, the connected portion of the ends  23  and  24  are preferably covered with an insulating substance to protect or prevent separation of the ends  23  and  24 , which may be caused by an unintended contact by foreign objects.  
     [0035] Hereinafter, a method of fabricating the circuit board is described.  
     [0036] The circuit board can be prepared by a conventional method. For example, a metal layer is first formed on a top surface and a bottom surface of a substrate and photoresist is coated on its insulating portion. Then, a series of exposure, development, etching, and exfoliation processes are performed to provide circuit patterns on the top surface and the bottom surface of the substrate.  
     [0037]FIGS. 3A and 3B show a circuit board  1  where an upper terminal  21  is formed on a top surface of the substrate  10  and a lower terminal  22  is formed on a bottom surface of the substrate  10 . The upper terminal  21  and the lower terminal  22  may be a part of circuit patterns on the top and bottom surfaces or separate elements which are electrically connected to the circuit patterns. Whether the upper terminal  21  and the lower terminal  22  are to be a part of the circuit patterns or separate elements can be determined based on characteristics of the circuit board, integrity of the circuit, and/or characteristics of the materials constituting the circuit patterns, etc.  
     [0038] After the circuit board  1  is prepared, a connection hole  130  is formed in the substrate  10 , particularly, in an area  100  between the end  23  of the upper terminal  21  and the end  24  of the lower terminal  22 . Location and size of the connection hole  130  are determined based on the locations of circuit elements mounted on the circuit board  1  and materials constituting the substrate  10 . The shape of the connection hole  130  is not limited to a straight volumn as shown in FIG. 3B, and it may be formed along any curvature or as a diagram such as a polygon or a circle.  
     [0039] The connection hole  130  functions as a path connecting the top surface of the substrate  10  with the bottom surface of the substrate  10 . Thus, the location of the connection hole  130  is not necessarily limited to the outer circumference (i.e., edges) of the substrate  10 . In other words, the connection hole  130  may be formed either at an intermediate or inner portion of the substrate  10  or at an outer circumference of the substrate  10 . If the connection hole  130  is formed at an intermediate portion of the substrate  10 , the circumference of the connection hole  130  is enclosed by the substrate  10 . However, if the connection hole  130  is formed at the outer circumference of the substrate  10 , at least one side of the connection hole  130  is open from the side of the substrate  10 .  
     [0040] The location of the connection hole  130  is further related to the location and/or arrangement of elements mounted on the substrate  10 , such as semiconductor chips and connection terminals arranged thereon. For example, if the semiconductor chip is mounted at an intermediate area of the substrate  10  and the connection terminal is disposed under the semiconductor chip, it is preferable that the connection hole  130  is formed under the semiconductor chip. In contrast, if the connection terminal is formed around the semiconductor chip, it is preferable that the circuit pattern, connected with the connection terminal, is formed in the circumference of the substrate  10  and the connection hole  130  is formed in the circumference of the substrate  10 .  
     [0041] Meanwhile, if there are a plurality of upper terminals and lower terminals that must be connected with one another, it is preferable that the upper and lower terminals are connected through the least possible number of connection holes so as to reduce the size of the circuit board.  
     [0042] In order to form the connection hole  130  by etching, an etching stopper  120  is disposed on the top and bottom surfaces of the substrate  10  except the area  100  where the connection hole  130  is to be formed as shown in FIGS. 4A and 4B. After the etching stopper  120  is disposed, the connection hole  130  is formed by applying an etching solution to the area  100  where the connection hole  130  is to be formed. Thereby, etching starts from the surface of the circuit board  1  where the etching stopper  120  and a terminal are not formed at the substrate  10  and proceeds inwards the substrate between the end  23  of the upper terminal  21  and the end  24  of the lower terminal  22 . After the connection hole  130  is formed, the etching solution is cleaned or removed from the circuit board  1 . The upper terminal  21  and the lower terminal  22  are composed of materials that are not to be etched by the etching solution.  
     [0043] After the connection hole  130  is formed, the circuit board  1  has a cross-section as shown in FIG. 5. Then, the upper terminal  21  and the lower terminal  22  are electrically connected inside the connection hole  130 .  
     [0044] The upper terminal  21  and the lower terminal  22  can be connected by first cutting the ends  23  and  24  of the upper and lower terminals  21  and  22  (adjacent to notch  110 ), there-after bending or drawing the upper terminal  21  and the lower terminal  22  to join inside the connection hole  130  of the circuit board  1 , and bonding the ends  23  and  24  of the upper and lower terminals  21  and  22  together. The upper terminal  21  and the lower terminal  22  can alternatively be connected by directly bending or drawing the upper terminal  21  and the lower terminal  22  inwards to join inside the connection hole  130  and then bonding them together, without first cutting their ends  23  and  24 , if the substrate  10  is thin and the upper and lower terminals  21  and  22  are sufficiently malleable.  
     [0045] With reference to FIG. 2, if the ends  23  and  24  of the upper and lower terminals  21  and  22  are cut, a total of four ends are respectively formed in the left and right sides of the connection hole  130  (two on the top and two on the bottom of the substrate  10 ). In this case, it is not necessary to bond the right end  23  of the upper terminal  21  and the right end  24  of the lower terminal  22  as shown in FIG. 2. It is possible to bond the left side end of the upper terminal  21  and right side end of the lower terminal  22 . However, if the ends of the upper and lower terminals of the same side are bonded, it is possible to bond the other sides together as shown in the drawing figure. In that case, it is advantageous in that multiple ends of upper terminals and lower terminals can be bonded within one connection hole.  
     [0046] The end  23  of the upper terminal  21  and the end  24  of the lower terminal  22  can be cut by an additional cutting process. However, they can be cut at the same time when thermocompression bonding (to be described herein-after) is performed if a notch  110  is formed adjacent to the end  23  of the upper terminal  21  and the end  24  of the lower terminal  22  to facilitate cutting during compression by the bonding device. The notch  110  can be formed easily before the connection hole  130  is formed in the circuit board. Alternatively, the notch  110  may be formed after the connection hole  130  is formed.  
     [0047] The upper terminal  21  and the lower terminal  22  can be bonded by a method of metal bonding known in the art. If the upper terminal  21  and the lower terminal  22  are bonded by thermocompression bonding, it is preferable that a conductive plating layer  140  usable for thermocompression bonding is formed on at least one opposite surface of the ends  23  and  24 . It is preferable that the conductive plating layer  140  is formed in an area where the end  23  of the upper terminal  21  and the end  24  of the lower terminal  22  are to be bonded before the etching stoppers  120  are removed, as shown in FIG. 6. After the conductive plating layer  140  is formed, thermocompression bonding is performed by using a thermocompression bonding device  200  as shown in FIG. 8. However, it is possible to remove the etching stopper before or after performing thermocompression bonding.  
     [0048] If the circuit board  1  is used with the open connection hole  130  as described above, a bonding portion of the upper terminal  21  and the lower terminal  22  may be separated by unintended contact by foreign objects. In order to prevent the terminals  21  and  22  from being separated from each other, it is preferable that a connected portion of the end  23  of the upper terminal  21  and the end  24  of the lower terminal  22  is covered with an insulating adhesive substance  150  as shown in FIG. 9.  
     [0049] Since the end  23  of the upper terminal  21  and the end  24  of the lower terminal  22  are bonded inside the connection hole  130  of the circuit board, the circuit board can be compacted and effectively manufactured, and the bonding portion of the upper terminal and the lower terminal has a superior strength and durability.  
     [0050] While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.