Abstract:
A wiring substrate includes an insulating layer, an upper wiring pattern, and a lower wiring pattern, the wiring patterns sandwiching the insulating layer. The lower wiring pattern includes an interlayer connecting conductor integral therewith and projecting toward the upper wiring pattern for electrical connection to the upper wiring pattern. The interlayer connecting conductor is joined to the upper wiring pattern so as to penetrate into the upper wiring pattern beyond a joining interface between the insulating layer and the upper wiring pattern. Thus, the wiring substrate adaptable for a large current is provided without causing degradation of reliability in connection, which may occur by cracking, disconnection, interlayer peeling-off, etc.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a wiring substrate in which a conductor pattern is located in an insulating layer. 
         [0003]    2. Description of the Related Art 
         [0004]    In a wiring substrate, an interlayer connecting conductor (via hole conductor) for electrically connecting wiring patterns in different layers to each other is generally formed by boring a through-hole in the wiring substrate and coating a plating over an inner wall of the through-hole. However, such a method has problems from the viewpoint of productivity, economic efficiency, and so on, because chemicals used in the plating process are expensive and the processing time is relatively long. 
         [0005]    To cope with the above-mentioned problems, there has been proposed, for example, a method of forming an interlayer connecting conductor by filling a conductive paste, which contains metal powder, into a through-hole formed in an insulating layer of a wiring substrate. In the proposed method, after forming the interlayer connecting conductor by filling the conductive paste into the through-hole, the interlayer connecting conductor is compacted by pressing a wiring circuit layer made of a metal foil against the interlayer connecting conductor. At that time, there is a risk that, with the pressing of the wiring circuit layer, the conductor paste in the interlayer connecting conductor may spread out into a gap between the insulating layer and the wiring circuit layer, and adhesion between the insulating layer and the wiring circuit layer may lower, thus causing a conduction failure in the wiring substrate. 
         [0006]    Japanese Unexamined Patent Application Publication No. 2003-8225 discloses a wiring substrate having features to prevent the conductor paste of the interlayer connecting conductor from spreading out to the surroundings of the interlayer connecting conductor, and to prevent an adhesion failure from occurring between the wiring circuit layer and the insulating layer.  FIG. 1  is a schematic sectional view of the wiring substrate disclosed in Japanese Unexamined Patent Application Publication No. 2003-8225. In the wiring substrate illustrated in  FIG. 1 , insulating layers  501 A and  501 B, each containing a thermosetting resin, are laminated, and a wiring circuit layer  502  is buried in the surface of each insulating layer. 
         [0007]    Through-holes for connecting the wiring circuit layers  502  to each other are formed in the insulating layers  501 A and  501 B. An interlayer connecting conductor  503  is formed in each through-hole by filling a conductor material, which contains metal powder, into the through-hole. Furthermore, the wiring circuit layer  502  has a recess  504  into which the interlayer connecting conductor  503  penetrates to be buried therein. The recess  504  serves to prevent the conductor paste of the interlayer connecting conductor  503  from spreading out to the surroundings of the interlayer connecting conductor  503 . 
         [0008]    The substrate described in Japanese Unexamined Patent Application Publication No. 2003-8225 comes with a risk that, if the recess  504  formed in the wiring circuit layers  502  is too shallow, the effect of preventing the spreading-out of the conductor paste would be reduced, and if it is too deep, the thickness of the wiring circuit layers  502  would be reduced and the strength of the wiring circuit layers  502  would be reduced. In other words, the substrate described in Japanese Unexamined Patent Application Publication No. 2003-8225 comes with a risk that connection reliability in the wiring substrate may degrade because the adhesion failure between the wiring circuit layers and the insulating layer cannot be sufficiently avoided. 
         [0009]    Moreover, in Japanese Unexamined Patent Application Publication No. 2003-8225, the interlayer connecting conductor is formed by filling the conductor paste into the through-hole. However, because the conductor paste is made of the resin containing the metal powder, an amount of metal component in the interlayer connecting conductor is relatively small. As a result, a resistance value of the interlayer connecting conductor increases. This raises another problem that the interlayer connecting conductor impedes flow of a current, thus making the wiring substrate not adaptable for a large current. 
       SUMMARY OF THE INVENTION 
       [0010]    Accordingly, preferred embodiments of the present invention provide a wiring substrate that is adaptable for a large current without causing degradation of reliability in connection, which may occur due to cracking, disconnection, interlayer peeling-off, etc. 
         [0011]    According to a preferred embodiment of the present invention, a wiring substrate includes an insulating layer, a first conductive pattern and a second conductive pattern disposed in a sandwiching relation to the insulating layer, and an interlayer connecting conductor penetrating through the insulating layer in a direction of thickness thereof and electrically connecting the first conductive pattern and the second conductive pattern to each other, wherein the interlayer connecting conductor is preferably integrally provided with the first conductive pattern, and the interlayer connecting conductor is joined to the second conductive pattern so as to penetrate into the second conductive pattern beyond a joining interface between the insulating layer and the second conductive pattern. 
         [0012]    With the unique features described above, the interlayer connecting conductor electrically connecting the first conductive pattern and the second conductive pattern to each other is provided integrally with the first conductive pattern, and the interlayer connecting conductor penetrates into the second conductive pattern. In other words, a joining interface between the interlayer connecting conductor and the second conductive pattern is not coplanar with the joining interface between the insulating layer and the second conductive pattern. Here, the expression “provided integrally” implies that the interlayer connecting conductor and the first conductive pattern are defined by a single unitary integral metal member, and there is no joining interface therebetween. 
         [0013]    With expansion and contraction caused by temperature changes (heating and cooling), stress is generated in a joining portion between the insulating layer and the conductive pattern due to a difference in expansion coefficients therebetween. If the joining interface between the interlayer connecting conductor and the second conductive pattern is coplanar with the joining interface between the insulating layer and the second conductive pattern, there is a risk that the generated stress may concentrate at the joining interface and peeling-off may occur at the joining interface. This results in a possibility of causing a conduction failure between the first conductive pattern and the second conductive pattern, or generating cracks in the wiring substrate. 
         [0014]    To cope with the above-mentioned problems, the interlayer connecting conductor is arranged to penetrate into the second conductive pattern such that the joining interface between the interlayer connecting conductor and the second conductive pattern and the joining interface between the insulating layer and the second conductive pattern are positioned in different planes. With such an arrangement, the generated stress can be distributed. It is hence possible to prevent the conduction failure caused by the peeling-off and the cracks generated in the wiring substrate. 
         [0015]    Furthermore, when conductive patterns in different layers are connected to each other, as in the related art, by an interlayer connecting conductor that is formed by filling metal powder into a through-hole, a resistance value in a connecting portion between the conductive pattern and the interlayer connecting conductor increases because they are made of different materials. In contrast, according to a preferred embodiment of the present invention, since a connecting conductor defining and functioning as the interlayer connecting conductor is provided integrally with the first conductive pattern, a resistance value in a connecting portion between the first conductive pattern and the interlayer connecting conductor is prevented from increasing, and the above-mentioned problem with the related art can be avoided. 
         [0016]    Moreover, since the interlayer connecting conductor and the first conductive pattern are provided integrally with each other unlike the related-art using the method of filling the conductive paste, an amount of metal component contained in the interlayer connecting conductor is increased so as to reduce a resistance value of the interlayer connecting conductor. 
         [0017]    In the wiring substrate according to a preferred embodiment of the present invention, the insulating layer may be made up of plural layers, and the conductive pattern may be disposed on a surface of an uppermost one of the plural layers constituting the insulating layer. 
         [0018]    With the features described above, a conduction failure is reliably prevented in the surface of the uppermost one of the plural insulating layers to which an electronic component is generally mounted. 
         [0019]    In the wiring substrate according to a preferred embodiment of the present invention, preferably, the interlayer connecting conductor and the second conductive pattern are electrically connected to each other in at least portions thereof through a conductive adhesive. 
         [0020]    With the feature described above, connection reliability between the interlayer connecting conductor and the second conductive pattern is improved because they are electrically connected to each other through the conductive adhesive. 
         [0021]    In the wiring substrate according to a preferred embodiment of the present invention, preferably, an organic coating is provided in at least a portion of a joining interface between the interlayer connecting conductor and the second conductive pattern. 
         [0022]    With the feature described above, since oxidation in a connecting portion between the interlayer connecting conductor and the second conductive pattern is significantly reduced or prevented, a resistance value in that connecting portion is prevented from increasing. 
         [0023]    In the wiring substrate according to a preferred embodiment of the present invention, preferably, at least the interlayer connecting conductor is subjected to surface treatment. The surface treatment is, for example, roughing, blackening, coupling, and formation of an organic coating. 
         [0024]    With the feature described above, since the interlayer connecting conductor is subjected to the surface treatment, adhesion between the insulating resin and the interlayer connecting conductor increases, and peeling-off is significantly reduced or prevented which may occur because of stress generated due to a difference in expansion coefficient between two adjacent members. It is hence possible to prevent a conduction failure caused by the peeling-off, cracks generated in the wiring substrate, etc. The peeling-off occurred between the insulating resin and the interlayer connecting conductor leads to the conduction failure with a high probability. Thus, the surface of the interlayer connecting conductor is an important region where the surface treatment is to be performed. In addition to the surface of the interlayer connecting conductor, the surface of the first conductive pattern and the surface of the second conductive pattern may also be subjected to the surface treatment. 
         [0025]    In the wiring substrate according to a preferred embodiment of the present invention, the first conductive pattern may include lands provided integrally therewith on side close to the insulating layer, and the wiring substrate may further include an electronic component mounted to the lands and disposed inside the insulating layer. 
         [0026]    With the features described above, since the electronic component is incorporated inside the insulating layer, the size of the wiring substrate is significantly reduced. 
         [0027]    In the wiring substrate according to a preferred embodiment of the present invention, preferably, the interlayer connecting conductor penetrates into the second conductive pattern preferably by about 10 μm or more, for example, at a most deeply penetrating portion thereof. 
         [0028]    With the feature described above, since an amount by which the interlayer connecting conductor penetrates into the second conductive pattern at the most deeply penetrating portion thereof is preferably about 10 μm or more, for example, the occurrence of a failure due to the peeling-off between the interlayer connecting conductor and the second conductive pattern can be suppressed, and the connection reliability in the wiring substrate is increased. 
         [0029]    According to various preferred embodiments of the present invention, since the joining interface between the interlayer connecting conductor and the second conductive pattern and the joining interface between the second conductive pattern and the insulating layer are positioned in different planes, stress generated due to a difference in expansion coefficient between two adjacent members is distributed. It is hence possible to prevent the interlayer peeling-off and the conduction failure attributable to the stress, and to increase the connection reliability in the wiring substrate. In addition, since the interlayer connecting conductor is provided integrally with the first conductive pattern, an amount of metal component contained in the interlayer connecting conductor is increased, such that a resistance value of the interlayer connecting conductor is significantly reduced. 
         [0030]    The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  is a schematic sectional view of a wiring substrate described in Japanese Unexamined Patent Application Publication No. 2003-8225. 
           [0032]      FIG. 2  is a schematic sectional view of a wiring substrate according to Preferred Embodiment 1 of the present invention. 
           [0033]      FIG. 3  illustrates, in enlarged scale, a circular portion surrounded by a one-dot-chain line in  FIG. 2 . 
           [0034]      FIGS. 4A-4E  successively illustrate a series of manufacturing steps for the wiring substrate according to Preferred Embodiment 1 of the present invention. 
           [0035]      FIG. 5  is a graph depicting the result of carrying out thermal shock tests on the wiring substrate according to Preferred Embodiment 1 of the present invention. 
           [0036]      FIG. 6  is a graph depicting the relationship between the diameter of an interlayer connecting conductor and connection reliability at a penetration amount of about 10 μm, for example. 
           [0037]      FIG. 7  is a graph depicting the relationship between the thickness of an upper wiring pattern and connection reliability at the penetration amount of about 10 μm, for example. 
           [0038]      FIG. 8  is a graph depicting the relationship between the thickness of the upper wiring pattern and the penetration amount. 
           [0039]      FIG. 9  is a schematic sectional view of a wiring substrate according to Preferred Embodiment 2 of the present invention. 
           [0040]      FIG. 10  is a graph depicting the result of carrying out thermal shock tests on the wiring substrate according to Preferred Embodiment 2 of the present invention. 
           [0041]      FIG. 11  is a schematic sectional view of a wiring substrate according to Preferred Embodiment 3 of the present invention. 
           [0042]      FIG. 12A-12D  successively illustrates a series of manufacturing steps for the wiring substrate according to Preferred Embodiment 3 of the present invention. 
           [0043]      FIG. 13  is a schematic sectional view of a wiring substrate according to Preferred Embodiment 4 of the present invention. 
           [0044]      FIGS. 14A-14D  successively illustrates a series of manufacturing steps for the wiring substrate according to Preferred Embodiment 4 of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0045]    The wiring substrate according to various preferred embodiments of the present invention includes an insulating layer made of an insulating resin, and a conductive wiring pattern (conductive pattern) disposed on the insulating layer. The insulating layer may be a single layer or made up of plural layers. The insulating resin of the insulating layer is, for example, a glass epoxy resin, an epoxy resin, a phenol resin, a cyanate resin, or a polyimide resin. Among those examples, the polyimide resin is particularly preferable in exhibiting superior heat resistance after being hardened. 
         [0046]    The wiring substrate electrically connects, through the wiring pattern, an electronic component mounted to the surface of the insulating layer and a main substrate (e.g., a mother board) to which the wiring substrate is mounted. The electronic component mounted to the wiring substrate is, for example, an active element such as a silicon semiconductor element or a gallium arsenide semiconductor element, or a passive element such as a capacitor or an inductor. 
       Preferred Embodiment 1 
       [0047]      FIG. 2  is a schematic sectional view of a wiring substrate according to Preferred Embodiment 1.  FIG. 3  illustrates, in enlarged scale, a circular portion surrounded by a one-dot-chain line in  FIG. 2 . It is to be noted that, in  FIG. 3 , though individual members are actually in a state of being closely contacted with each other, the members are drawn with gaps left at joining boundaries between them for convenience of explanation. 
         [0048]    In a wiring substrate  1  according to Preferred Embodiment 1, an upper wiring pattern  11  is located at an upper surface of an insulating layer  10 , and a lower wiring pattern  12  is located at a lower surface of the insulating layer  10 . The upper surface of the insulating layer  10  serves as the above-mentioned surface to which the electronic component is mounted. 
         [0049]    The insulating layer  10  preferably has a thickness of about 0.10 mm, for example, and includes a cylindrical or substantially cylindrical through-via  10 A formed therein. An interlayer connecting conductor  12 A provided integrally with the lower wiring pattern  12 , as described below, is inserted into the through-via  10 A. The upper wiring pattern  11  and the lower wiring pattern  12  located respectively at the upper and lower surfaces of the insulating layer  10  are electrically connected to each other by the interlayer connecting conductor  12 A. 
         [0050]    The wiring pattern  11  is located at the upper surface of the insulating layer  10  and preferably has a thickness of about 0.10 mm, for example. A distal end portion of the later-described interlayer connecting conductor  12 A, projecting out from the insulating layer  10 , penetrates into the upper wiring pattern  11  at each position that is aligned with the through-via  10 A located in the insulating layer  10  when viewed in the direction of thickness thereof. While a joining interface  11 A between the upper wiring pattern  11  and the interlayer connecting conductor  12 A preferably has a circular or substantially circular arc shape in section as illustrated in  FIG. 3 , the joining interface  11 A may have a rectangular or substantially rectangular shape, or other suitable shape, for example. 
         [0051]    The lower wiring pattern  12  is located at the lower surface of the insulating layer  10  and preferably has a thickness of about 0.10 mm, for example. The interlayer connecting conductor  12 A is provided integrally with the lower wiring pattern  12 . In other words, the lower wiring pattern  12  and the interlayer connecting conductor  12 A are defined by a single unitary integral metal member, and there is no joining interface therebetween. 
         [0052]    The interlayer connecting conductor  12 A preferably has a cylindrical or substantially cylindrical shape with a diameter of about 0.6 mm and a length of about 0.2 mm, for example. As illustrated in  FIG. 3 , the distal end portion of the interlayer connecting conductor  12 A preferably has a circular or substantially circular arc shape. The interlayer connecting conductor  12 A is inserted to the through-via  10 A located in the insulating layer  10 . Thus, the distal end portion of the interlayer connecting conductor  12 A projecting out from the upper surface of the insulating layer  10  is burred in the upper wiring pattern  11 . 
         [0053]    By forming the lower wiring pattern  12  and the interlayer connecting conductor  12 A integrally with each other, an amount of metal component contained therein can be increased and a resistance value of the interlayer connecting conductor can be reduced in comparison with the case where the interlayer connecting conductor is formed by filling the conductor paste, which is made of the resin containing the metal powder, into the through-hole. 
         [0054]    While the interlayer connecting conductor  12 A preferably has the cylindrical or substantially cylindrical shape in this preferred embodiment, it may have a rectangular or substantially rectangular columnar shape, or other suitable shape, for example. Furthermore, the distal end portion of the interlayer connecting conductor  12 A may have a planar shape, for example. 
         [0055]    Thus, since the upper wiring pattern  11  is buried in the interlayer connecting conductor  12 A, a joining interface between the interlayer connecting conductor  12 A and the upper wiring pattern  11  is positioned in a plane different from a joining interface between the insulating layer  10  and the upper wiring pattern  11 . It is therefore possible to prevent stress generated due to a difference in expansion coefficient from concentrating at the joining interface, and to prevent a conduction failure caused by peeling-off attributable to the stress, cracks generated in the wiring substrate  1 , etc. 
         [0056]    An example of a method of manufacturing the wiring substrate  1  will be described below.  FIGS. 4A-4E  successively illustrate a series of manufacturing steps for the wiring substrate according to Preferred Embodiment 1. 
         [0057]    In a first step ( FIG. 4A ), dry film resists  21  each having a thickness of about 15 μm are pasted to both surfaces of a copper plate  20  having a thickness of about 0.3 mm, which surfaces are opposed to each other in the direction of thickness of the copper plate. It is to be noted that  FIG. 4A  illustrates the copper plate  20  when viewed from a side. 
         [0058]    Next, in a second step ( FIG. 4B ), the copper plate  20  including the dry film resists  21  pasted thereto is etched from one surface side such that the cylindrical interlayer connecting conductors  12 A, each having the diameter of about 0.6 mm and the length of about 0.2 mm, remain on the one surface side. 
         [0059]    In a third step ( FIG. 4C ), holes (not illustrated), each having a diameter of about 0.6 mm, are formed in an insulating resin  23  having a thickness of about 0.15 mm, and the insulating resin  23  is laminated on the copper plate  20  that has been subjected to the etching in the second step. The insulating resin  23  becomes the insulating layer  10 . 
         [0060]    In a fourth step ( FIG. 4D ), a copper plate  24  having a thickness of about 0.1 mm is placed on an upper surface of the insulating resin  23  (i.e., the insulating layer  10 ). In such a state, they are subjected to lamination molding for 1 hour at about 180° C. and about 100 kN, for example. As a result, the distal end portion of the interlayer connecting conductor  12 A is caused to penetrate into the copper plate  24 , and the above-mentioned joining interface  11 A is formed in a circular arc shape. 
         [0061]    In a fifth step ( FIG. 4E ), patterns are formed on both surfaces of a laminate, obtained in the fourth step, by a subtractive process (i.e., a process of removing unnecessary portions while leaving a circuit). As a result, the copper plate is formed into the lower wiring pattern  12 , and the copper plate  24  is formed into the upper wiring pattern  11 . The wiring substrate  1  is thus completed. 
         [0062]    A penetration amount by which the interlayer connecting conductor  12 A is buried in the upper wiring pattern  11  will be described below. The penetration amount is preferably set to be not more than about 50% of the thickness of the upper wiring pattern  11 , for example. That point will be described in more detail below in connection with practical values. 
         [0063]      FIG. 5  is a graph depicting the result of carrying out thermal shock tests on the wiring substrate  1  according to Preferred Embodiment 1. In the graph of  FIG. 5 , the vertical axis represents a resistance change rate (%) of the wiring substrate  1 , and the horizontal axis represents the number of heat cycles in the thermal shock tests. 
         [0064]    As seen from  FIG. 5 , when the penetration amount of the interlayer connecting conductor  12 A into the upper wiring pattern  11  is about 0 μm or about 5 μm, for example, the resistance change rate increases as the number of heat cycles increases. On the other hand, when the penetration amount of the interlayer connecting conductor  12 A into the upper wiring pattern  11  is about 10 μm, about 15 μm or about 20 μm, for example, the resistance change rate hardly changes regardless of the number of heat cycles. Thus, the penetration amount is preferably not less than about 10 μm, for example, for the reason that connection reliability degrades at a smaller value of the penetration amount. 
         [0065]    Moreover, the penetration amount is preferably set to be not less than about 10 μm, for example, regardless of the diameter of the interlayer connecting conductor  12 A and the thickness of the upper wiring pattern  11 .  FIG. 6  is a graph depicting the relationship between the diameter of the interlayer connecting conductor  12 A and the connection reliability at the penetration amount of about 10 μm, for example.  FIG. 7  is a graph depicting the relationship between the thickness of the upper wiring pattern  11  and the connection reliability at the penetration amount of about 10 μm, for example. 
         [0066]    The reason why the connection reliability increases at the penetration amount of not less than about 10 μm, for example, resides in that the joining interface between the upper wiring pattern  11  and the interlayer connecting conductor  12 A is not coplanar with the joining interface between the insulating layer and the upper wiring pattern  11 , which are different in thermal expansion coefficient from each other. Accordingly, the above-mentioned advantageous effect can be obtained regardless of the diameter of the interlayer connecting conductor  12 A and the thickness of the upper wiring pattern  11 . 
         [0067]    For example, as seen from  FIG. 6 , when the diameter of the interlayer connecting conductor  12 A is any of about 0.05 mm, about 0.6 mm and about 1.0 mm, for example, the resistance change rate hardly changes regardless of the number of heat cycles. Furthermore, as seen from  FIG. 7 , when the thickness of the upper wiring pattern  11  is any of about 0.1 mm, about 0.3 mm and about 1.0 mm, for example, the resistance change rate hardly changes regardless of the number of heat cycles. 
         [0068]    While the penetration amount is preferably not less than about 10 μm, for example, the thickness of the upper wiring pattern  11  is preferably set to be not less than about 100 μm, for example, on that condition.  FIG. 8  is a graph depicting the relationship between the thickness of the upper wiring pattern and the penetration amount. In the graph of  FIG. 8 , the vertical axis represents the penetration amount, and the horizontal axis represents the thickness of the upper wiring pattern  11 . As seen from  FIG. 8 , when the penetration amount is about 10 μm, the thickness of the upper wiring pattern  11  is about 100 μm, for example. When the thickness of the upper wiring pattern  11  is not less than about 100 μm, for example, the penetration amount is held substantially constant. Thus, the penetration amount can be obtained as a preferred value of not less than about 10 μm, for example, when the thickness of the upper wiring pattern  11  is not less than about 100 μm, for example. 
       Preferred Embodiment 2 
       [0069]    A wiring substrate according to Preferred Embodiment 2 will be described below. The wiring substrate  1  according to Preferred Embodiment 2 differs from that according to Preferred Embodiment 1 in making the interlayer connecting conductor  12 A and the upper wiring pattern  11  electrically connected through a conductive adhesive. Only different point will be described below. 
         [0070]      FIG. 9  is a schematic sectional view of the wiring substrate  1  according to Preferred Embodiment 2. In the wiring substrate  1  according to Preferred Embodiment 2, as illustrated in  FIG. 9 , a conductive adhesive  13  is coated on the distal end portion of the interlayer connecting conductor  12 A. The conductive adhesive  13  is, for example, a low-resistance conductive paste made of nano-silver or nano-copper, for example. It is to be noted that the conductive adhesive  13  may be made of a resin composition containing metal powder, for example. 
         [0071]    Since the conductive adhesive  13  is coated on the distal end portion of the interlayer connecting conductor  12 A, the upper wiring pattern  11  and the interlayer connecting conductor  12 A are electrically connected to each other through the conductive adhesive  13 . With the presence of the conductive adhesive  13  coated on the interlayer connecting conductor  12 A, the connection reliability of the interlayer connecting conductor  12 A can be further increased. 
         [0072]    An example of a method of manufacturing the wiring substrate  1  according to Preferred Embodiment 2 preferably is substantially the same as that in Preferred Embodiment 1. The wiring substrate  1  according to Preferred Embodiment 2 can be manufactured by coating the conductive adhesive  13  on the distal end portion of the interlayer connecting conductor  12 A in the third step illustrated in  FIG. 4C . 
         [0073]    While the penetration amount is preferably set to be not less than about 10 μm in Preferred Embodiment 1, the penetration amount can be set to be not less than about 5 μm in the case where the conductive adhesive  13  is coated.  FIG. 10  is a graph depicting the result of carrying out thermal shock tests on the wiring substrate  1  according to Preferred Embodiment 2. As seen from  FIG. 10 , when the penetration amount is not less than about 5 μm, the resistance change rate hardly changes regardless of the number of heat cycles. Thus, the penetration amount is preferably not less than about 5 μm in Preferred Embodiment 2 for the reason that the connection reliability degrades at a smaller value of the penetration amount. 
       Preferred Embodiment 3 
       [0074]    A wiring substrate according to Preferred Embodiment 3 will be described below. The wiring substrate  1  according to Preferred Embodiment 3 is different from that according to Preferred embodiment 1 in that the insulating layer  10  has a multilayered structure, and a wiring pattern is located between the adjacent layers. Only those different points will be described below. 
         [0075]      FIG. 11  is a schematic sectional view of the wiring substrate  1  according to Preferred Embodiment 3. As illustrated in  FIG. 11 , the insulating layer  10  of the wiring substrate  1  according to Preferred Embodiment 3 preferably is constituted as a laminate including a first layer  101 , a second layer  102 , a third layer  103 , and a fourth layer  104 , which are successively positioned in the mentioned order from the surface side of the insulating layer  10 . 
         [0076]    The upper wiring pattern  11  is located at an upper surface of the first layer  101 , and a lower wiring pattern  121  is located at a lower surface of the first layer  101 . An interlayer connecting conductor  121 A is provided integrally with the lower wiring pattern  121 . A distal end portion of the interlayer connecting conductor  121 A projects toward the upper wiring pattern  11  from the first layer  101  such that the distal end portion thereof penetrates into the upper wiring pattern  11 . 
         [0077]    A lower wiring pattern  122  is located at a lower surface of the second layer  102 . An interlayer connecting conductor  122 A is provided integrally with the lower wiring pattern  122 . A distal end portion of the interlayer connecting conductor  122 A projects toward the lower wiring pattern  121  from the second layer  102  such that the distal end portion of the interlayer connecting conductor  122 A penetrates into the lower wiring pattern  121 . 
         [0078]    A lower wiring pattern  123  is located at a lower surface of the third layer  103 . An interlayer connecting conductor  123 A is provided integrally with the lower wiring pattern  123 . A distal end portion of the interlayer connecting conductor  123 A projects toward the lower wiring pattern  122  from the third layer  103  such that the distal end portion of the interlayer connecting conductor  123 A penetrates into the lower wiring pattern  122 . 
         [0079]    A lower wiring pattern  124  is located at a lower surface of the fourth layer  104 . An interlayer connecting conductor  124 A is provided integrally with the lower wiring pattern  124 . A distal end portion of the interlayer connecting conductor  124 A projects toward the lower wiring pattern  123  from the fourth layer  104  such that the distal end portion of the interlayer connecting conductor  124 A penetrates into the lower wiring pattern  123 . 
         [0080]    In the wiring substrate  1  according to Preferred Embodiment 3, as described above, the wiring patterns have such a multilayered structure that, looking at the wiring pattern of each layer, the distal end portion of the interlayer connecting conductor provided integrally with the wiring pattern in the underlying layer is buried in the relevant wiring pattern. Therefore, as in Preferred Embodiment 1, it is possible to avoid peeling-off between the laminated layers, which may occur by stress generated due to a difference in expansion coefficient, and to prevent the occurrence of a conduction failure, cracking, etc. 
         [0081]      FIGS. 12A-12D  successively illustrates a series of manufacturing steps of the wiring substrate  1  according to Preferred Embodiment 3. 
         [0082]    As illustrated in  FIG. 12A , a copper plate  204  is etched to form the interlayer connecting conductors  124 A, and an insulating resin  234  is laminated on the copper plate  204  such that the distal end portion of each interlayer connecting conductor  124 A projects out from the insulating resin  234  by about 0.05 mm, for example. The insulating resin  234  becomes the fourth layer  104  in  FIG. 11 . 
         [0083]    A copper plate  203  having a thickness of about 0.3 mm is placed on an upper surface of the insulating resin  234 , and they are subjected to lamination molding for 1 hour at about 180° C. and about 100 kN, for example. As a result, the distal end portion of the interlayer connecting conductor  124 A is caused to penetrate into the copper plate  203 . 
         [0084]    Next, as illustrated in  FIG. 12B , a dry film resist (not illustrated) having a thickness of about 15 μm is pasted to an upper surface of the copper plate  203 . The copper plate  203  is then etched such that the cylindrical interlayer connecting conductors  123 A, each having the diameter of about 0.6 mm and the length of about 0.2 mm, remain on the upper surface side of the copper plate  203 . 
         [0085]    Next, as illustrated in  FIG. 12C , patterns are formed on both surfaces of a laminate, obtained as illustrated in  FIG. 12B , by the subtractive process. As a result, the copper plate  204  is formed into the lower wiring pattern  124 , and the copper plate  203  is formed into the lower wiring pattern  123 . 
         [0086]    Next, as illustrated in  FIG. 12D , holes (not illustrated), each having a diameter of about 0.6 mm, are formed in an insulating resin  233  having a thickness of about 0.15 mm, and the insulating resin  233  is laminated on the fourth layer  104  such that the interlayer connecting conductors  123 A formed in  FIG. 12C  are inserted to the holes, respectively. The insulating resin  233  becomes the third layer  103  in  FIG. 11 . 
         [0087]    The wiring substrate  1  according to Preferred embodiment 3, illustrated in  FIG. 11 , can be formed by successively repeating the steps described above with reference to  FIGS. 12A-12D . Thus, the size of the wiring substrate  1  can be further reduced by forming the wiring patterns inside the insulating layer  10  in the multilayered structure. 
       Preferred Embodiment 4 
       [0088]    A wiring substrate according to Preferred Embodiment 4 will be described below. The wiring substrate  1  according to Preferred Embodiment 4 is different from that according to Preferred Embodiment 3 in mounting electronic components inside the insulating layer  10  of the wiring substrate  1  according to Preferred Embodiment 3. Only such a different point will be described below. 
         [0089]      FIG. 13  is a schematic sectional view of the wiring substrate  1  according to Preferred Embodiment 4. As in Preferred Embodiment 3, the insulating layer  10  of the wiring substrate  1  according to Preferred Embodiment 4 is constituted as a laminate including the first layer  101 , the second layer  102 , the third layer  103 , and the fourth layer  104 , which are successively positioned in the mentioned order from the surface side of the insulating layer  10 . 
         [0090]    In addition to the interlayer connecting conductors  121 A,  122 A,  123 A and  124 A, lands  121 B,  122 B,  123 B and  124 B are formed integrally with the lower wiring patterns  121 ,  122 ,  123  and  124  that are formed at lower surfaces of the first layer  101 , the second layer  102 , the third layer  103 , and the fourth layer  104 , respectively. 
         [0091]    Electronic components  151 ,  152 ,  153  and  154  are mounted respectively to the lands  121 B,  122 B,  123 B and  124 B in the individual layers. The electronic components  151 ,  152 ,  153  and  154  are disposed respectively in the first layer  101 , the second layer  102 , the third layer  103 , and the fourth layer  104 . Thus, with the electronic components  151 ,  152 ,  153  and  154  incorporated in the insulating resin, the size of the wiring substrate  1  is significantly reduced. 
         [0092]    The electronic components  151 ,  152 ,  153  and  154  are, for example, active elements such as silicon semiconductor elements or gallium arsenide semiconductor elements, or passive elements such as capacitors or inductors. 
         [0093]      FIGS. 14A-14D  successively illustrates a series of manufacturing steps for the wiring substrate  1  according to Preferred Embodiment 4. 
         [0094]    In a first step ( FIG. 14A ), dry film resists (not illustrated) each having a thickness of about 15 μm are pasted to both surfaces of a copper plate  204  having a thickness of about 0.3 mm, which surfaces are opposed to each other in the direction of thickness of the copper plate. The copper plate  204  including the dry film resists pasted thereto is etched from one surface side such that the cylindrical interlayer connecting conductors  124 A, each having the diameter of about 0.6 mm and the length of about 0.19 mm, remain on the one surface side. 
         [0095]    In a second step ( FIG. 14B ), the copper plate  204  having been subjected to the etching in  FIG. 14A  is further etched to form lands  124 B each having a thickness of about 0.01 mm. With the step of forming the lands  124 B, the interlayer connecting conductor  124 A having been formed in the length of about 0.19 mm is caused to have the length of about 0.2 mm. 
         [0096]    In a third step ( FIG. 14C ), the electronic component  154  is mounted to the formed lands  124 B. Thereafter, in a fourth step ( FIG. 14D ), an insulating resin  234  is laminated on the copper plate  204 . In Preferred Embodiment 4, the insulating resin  234  is in a liquid phase. After coating the insulating resin  234  over the copper plate  204 , the insulating resin  234  is subjected to debubbling under vacuum and then heated to such an extent that the insulating resin  234  in a semi-hardened state encapsulates the electronic component  154 . At that time, the insulating resin  234  has a thickness of about 0.15 mm. 
         [0097]    Thereafter, as in the step (see  FIGS. 12A-12D ) described above in Preferred Embodiment 3, a copper plate  203  having a thickness of about 0.3 mm is placed on the insulating resin  234 , and they are subjected to lamination molding for 1 hour at about 180° C. and about 100 kN, for example. The wiring substrate  1  illustrated in  FIG. 13  is formed by subsequently repeating the above-described steps from the step illustrated in  FIG. 14A . Thus, with the electronic components incorporated in the insulating resin, the size of the wiring substrate  1  can be reduced. 
         [0098]    While the wiring substrate according to various preferred embodiments of the present invention has been described in detail above, practical structures of the wiring substrate  1  can be modified in design as required. The operations and the advantageous effects stated in the foregoing preferred embodiments are merely explained as most preferable operations and advantageous effects obtained with the present invention. In other words, the operations and the advantageous effects of the present invention are not limited to those described above in the preferred embodiments. 
         [0099]    While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.