Patent Document

CROSS-REFERENCE TO RELATED APPLICATION(s) 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2015-091189, filed on Apr. 28, 2015, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a substrate with embedded component. 
     BACKGROUND 
     Electronic devices such as a personal computer and a server incorporate various kinds of wiring substrates. 
     Among them, the substrate with embedded component such as a resistive element or a capacitor can have a small outer shape, because the component is not exposed on the surface of the substrate, thereby leading to downsizing of an electronic device. 
     Note that technologies related to the present application are disclosed in Japanese Laid-open Patent Publication No. 07-302970 and Japanese Examined Laid-open Patent Publication No. Hei 6-9302. 
     SUMMARY 
     According to one aspect discussed herein, there is provided a substrate with embedded component including: an insulating base member; a conductive pad formed on the insulating base member; a component connected to the conductive pad with a solder; and a resin covering the component, wherein a hole is provided in the insulating base member and the conductive pad, and the insulating base member is exposed on a side surface of the hole. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1E  are cross-sectional views of a substrate with embedded component in the course of manufacturing thereof, which is used in the investigation; 
         FIG. 2  is an enlarged cross-sectional view schematically illustrating a component and its surroundings; 
         FIGS. 3A to 3G  are cross-sectional views of a substrate with embedded component in the course of manufacturing thereof according to a first embodiment; 
         FIGS. 4A and 4B  are enlarged plan views of the substrate with embedded component in the course of manufacturing thereof according to the first embodiment; 
         FIG. 5  is an enlarged plan view for explaining a preferable position of a hole in the first embodiment; 
         FIG. 6  is a cross-sectional view of a model used to calculate thermal expansion amounts of solder and resin in the first embodiment; 
         FIG. 7  is a table obtained by calculating the thermal expansion amounts of solder and resin for electronic components having different sizes in the first embodiment; 
         FIG. 8  is a table obtained by calculating the diameter of the hole which is suitable to the size of each electronic component, the area of the hole in a plan view, and the volume of the hole in the first embodiment; and 
         FIGS. 9A to 9E  are cross-sectional views of a substrate with embedded component in the course of manufacturing thereof according to a second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Prior to the description of the embodiments, matters investigated by the inventors are described. 
       FIGS. 1A to 1E  are cross-sectional views of a substrate with embedded component in the course of manufacturing thereof, which was used in the investigation. 
     As illustrated in  FIG. 1A , in order to manufacture the substrate with embedded component, a copper-clad base member that is made by forming copper film on both surface of an insulating base member  1  is prepared. Then, the copper film is patterned into conductive pad  2 . The insulating base member  1  is, for example, a glass epoxy substrate. 
     Subsequently, as illustrated in  FIG. 1B , solder paste as solder  4  is printed on the conductive pad  2 , and a component  3  is mounted on the solder  4 . Thereafter, the solder  4  is heated to be melted, thereby connecting the component  3  to the conductive pad  2  with the solder  4 . 
     The component  3  may be, for example, a resistive element, a capacitor, or a coil. 
     The material of the solder  4  is preferably a lead-free solder, which is eco-friendly, and a SnAgCu solder is used in this example. The melting point of the SnAgCu solder depends on its composition ratio. For example, a Sn-3Ag-0.5Cu solder having a melting point of about 220° C. is used as the material of the solder  4 . 
     Note that, the conductive pad  2 , to which the component  3  is connected as described above, is also called as a foot print. 
     Next, a process illustrated in  FIG. 1C  is described. 
     Firstly, a first multi-layer wiring base member  7  and a second multi-layer wiring base member  8  are disposed over the insulating base member  1 , to which the component  3  is connected as described above. 
     The multi-layer wiring base members  7  and  8  each include alternate layers of an insulating layer  9  and wiring  10 . A resin sheet of epoxy resin may be used as the insulating layer  9 . Then, a copper-plated layer may be formed as the wiring  10 . 
     Then, prepreg as thermosetting resin  11  is adhered onto a surface of the first multi-layer wiring base member  7 . In addition, an opening  7   a  having a dimension enough to house the component  3  is formed in the resin  11  and the first multi-layer wiring base member  7  by mechanical processing. 
     The resin  11  is, for example, a thermosetting epoxy resin, and is disposed also between the first multi-layer wiring base members  7  and  8 . Note that the resin  11  is not yet cured at this step and is in an uncured state. 
     Subsequently, as illustrated in  FIG. 1D , the insulating base member  1 , the first multi-layer wiring base member  7 , the second multi-layer wiring base member  8 , and the resin  11  are stacked and pressed while heating these elements, thereby thermally curing the resin  11 . 
     The highest temperature of the resin  11  in this process is in a range of about 180° C. to 200° C. at which the resin  11  is thermally cured. Since this temperature is lower than the melting point of the solder  4 , the solder  4  is not melted in this process. 
     By pressing in this manner, the resin  11  penetrates into the opening  7   a,  and the opening  7   a  is filled with the resin  11 . 
     Subsequently, as illustrated in  FIG. 1E , a semiconductor element  17  is mounted on the uppermost wiring  10  via a solder bump  16 . In this example, the solder bump  16  is a Sn-3Ag-0.5Cu solder having the same composition ratio and melting point as those of the solder  4 . 
     Then, the solder bump  16  is subjected to reflow by heating it, thereby connecting the semiconductor element  17  to the wiring  10  with the solder bump  16 . 
     Thus, the basic structure of the substrate  18  with embedded component according to this example is completed. 
     The substrate  18  can have a small outer shape because the component  3  is not exposed on the substrate surface, thereby leading to downsizing of an electronic device such as a server in which the substrate  18  is incorporated. 
     Here, according to the method of manufacturing the substrate  18  with the embedded component, the solder  4  and the resin  11  are also heated by heat for melting the solder bump  16  in the process of  FIG. 1E . 
       FIG. 2  is an enlarged cross-sectional view schematically illustrating the surroundings of the component  3  at this time. 
     Since the solder bump  16  and the solder  4  have the same melting point as described above, the solder  4  is also melted when the solder bump  16  is melted by heating. In addition, the resin  11  around the solder  4  thermally expands by this heat. 
     Accordingly, the melted solder  4  is pressurized by the resin  11  around the solder  4  and spreads along an interface between the resin  11  and the insulating base member  1  in the lateral direction. Then, in the worst case, the conductive pads  2  adjacent to each other in the lateral direction are electrically short-circuited via the solder  4 . 
     Such a phenomenon is called solder flash, which adversely contributes to the reduction in the yield of the substrate  18  with embedded component. 
     Although the inventors investigated some methods for preventing the solder flash, these methods have difficulties. 
     For example, it is considered that the material having higher melting point than that of the Sn-3Ag-0.5Cu solder, which is the material of the solder bump  16 , is employed as the material of the solder  4 . In this case, the solder  4  having the higher melting point does not melt even when subjected to the reflow in the process of  FIG. 1E , thereby preventing the above described solder flash. 
     In this method, however, the solder  4  needs to be heated at high temperature to be melted in the process of  FIG. 1B , which causes a significant difference in the thermal expansion between the insulating base member  1  and the conductive pad  2  due to the heating, so that damage such as crack occurs to the insulating base member  1 . 
     In contrast, it is also considered that the solder having lower melting point than that of the Sn-3Ag-0.5Cu solder, which is the material of the solder  4 , is employed as the material of the solder bump  16 , for the purpose of preventing the solder  4  from melting when the solder bump  16  is subjected to the reflow. However, the solder having lower melting point than that of the Sn-3Ag-0.5Cu solder is mechanically fragile. Therefore, when such a solder is used as the solder bump  16 , connection strength between the wiring  10  and the semiconductor element  17  is lowered. 
     Alternatively, the material, whose melting point rises once melted, is considered to be used for the solder  4 . 
     For example, in the solder paste in which copper powder is added to the solder, since a part of the copper powder is melted into the solder at the first melting, composition ratio of copper in the solder becomes high. Thus, a higher temperature than that in the first melting is required to melt the solder at the next melting, and thus the solder  4  is considered to be not melted in the reflow of the solder bump  16  in the process illustrated in  FIG. 1E . 
     However, since a rise in the melting point of the solder  4  is small, the solder  4  is potentially melted in the reflow of the solder bump  16  in the process illustrated in  FIG. 1E , which causes the aforementioned solder flash. 
     Instead of changing the material of the solder  4  in this manner, it is considered that the opening  7   a  may be filled with an adhesive agent before the process of  FIG. 1D , and the adhesive agent is cured in advance. Thus, the surroundings of the solder  4  are harden with the adhesive agent, which prevents the solder  4  from spreading in the lateral direction even when the solder  4  is melted in the process illustrated in  FIG. 1E . 
     However, this method requires an additional process of filling the adhesive agent into the opening  7   a,  resulting in an increase in the number of processes and also in manufacturing cost of the substrate  18 . Moreover, it is technologically difficult to measure the adhesive agent having the same volume as that of the opening  7   a  and fill this adhesive agent into the opening  7   a.    
     In the followings, embodiments capable of preventing the solder flash are described. 
     First Embodiment 
     In the present embodiment, the solder flash in a substrate with embedded component is prevented as described below. 
       FIGS. 3A to 3G  are cross-sectional views of a substrate with embedded component in the course of manufacturing thereof according to the present embodiment.  FIGS. 4A and 4B  are enlarged plan views of the substrate with embedded component in the course of manufacturing thereof according to the present embodiment. 
     Note that in  FIGS. 3A to 3G and 4A to 4B , the same element as that illustrated in  FIGS. 1A to 1E and 2  is denoted by the same reference numeral as that in  FIGS. 1A to 1E and 2 , and description thereof is omitted in the following. 
     First, as illustrated in  FIG. 3A , a copper-clad base member, which is made by forming copper films having a thickness of about 12 μm to 35 μm on both surfaces of the insulating base member  1 , is prepared. After that, the copper films are patterned into conductive pads  2 . 
     The material and thickness of the insulating base member  1  are not particularly limited. In this example, a glass epoxy substrate having a thickness of about 0.06 mm to 0.2 mm is used as the insulating base member  1 . 
     Next, as illustrated in  FIG. 3B , a hole  1   a  is formed in each of the conductive pad  2  and the insulating base member  1  by drilling. 
     Note that the holes  1   a  may be formed by laser processing instead of drilling. In this case, a portion of the conductive pad  2  corresponding to the holes  1   a  may be previously removed in the patterning of  FIG. 3A  to allow the holes  1   a  to be formed only in the insulating base member  1  by laser processing. 
     In the present embodiment, metal film and the like is not formed on the side surface of the hole  1   a , so that the material of the insulating base member  1  is left exposed in the hole  1   a.    
       FIG. 4A  is an enlarged plan view of the hole  1   a  and its surrounding when this process is ended. 
     As illustrated in  FIG. 4A , the hole  1   a  has an approximately circular plane shape, and the all portions of the hole  1   a  is included in the conductive pad  2  in a plan view. 
     Next, as illustrated in  FIG. 3C , a metal mask  19  is disposed over the insulating base member  1 . 
     An opening  19   a  is formed at a position of the metal mask  19  that corresponds to the conductive pad  2 , and the position of the metal mask  19  is adjusted so that the opening  19   a  overlaps the conductive pad  2 . 
     Then, by a printing method, solder paste is printed as the solder  4  on the conductive pad  2  in the opening  19   a.    
     The solder  4  is preferably a lead-free solder, which is eco-friendly, and is a SnAgCu solder in this example. The melting point of this SnAgCu solder is also not particularly limited. In the present embodiment, Sn-3Ag-0.5Cu, which has a low melting point of about 220° C. and thus causes no damage on the insulating base member  1  when melted, is used. 
       FIG. 4B  is an enlarged plan view of the hole  1   a  and its surrounding when this process is ended. 
     As illustrated in  FIG. 4B , an island  19   b  is provided to the metal mask  19 , and the hole  1   a  is closed by the island  19   b.  Thus, the solder  4  can be printed only around the hole  1   a , while preventing the solder  4  from entering into the hole  1   a.    
     Subsequently, as illustrated in  FIG. 3D , the component  3  is mounted on the solder  4 . Then, the solder  4  is melted by heating it at a temperature of about 220° C., thereby connecting the component  3  to the conductive pad  2  with the solder  4 . The component  3  may be, for example, a resistive element, a capacitor, or a coil. 
     Even when the solder  4  is melted in this manner, the solder  4  hardly enters into the hole  1   a , because the insulating material of the insulating base member  1  having a low solder wettability is exposed on the side surface of the hole  1   a.    
     Subsequently, as illustrated in  FIG. 3E , the first multi-layer wiring base member  7 , the resin  11 , and the second multi-layer wiring base member  8  explained in  FIG. 1C  are disposed in this order over the insulating base member  1 . 
     As explained in  FIG. 1C , prepreg is adhered as the resin  11  onto the surface of the first multi-layer wiring base member  7 . Also, the opening  7   a  having a size enough to house the component  3  therein is formed in the resin  11  and the first multi-layer wiring base member  7 . 
     Prepreg used as the resin  11  is, for example, a thermosetting epoxy resin, and is in the uncured state at this time. Although the thickness of the resin  11  is not particularly limited, the thickness of the resin  11  is about 0.06 mm to 0.2 mm in this example. 
     In addition, the thicknesses of the insulating layer  9  and the wiring  10  are not particularly limited. The insulating layer  9  is, for example, a resin sheet of epoxy resin having a thickness of about 0.06 mm to 0.2 mm. The wiring  10  is, for example, a copper-plated layer having a thickness of about 12 μm to 35 μm. 
     Then, as illustrated in  FIG. 3F , the insulating base member  1 , the first multi-layer wiring base member  7 , the resin  11 , and the second multi-layer wiring base member  8  are stacked. Thus, the opening  7   a  is closed by the second multi-layer wiring base member  8 , while housing the component  3  in the opening  7   a.    
     Thereafter, the insulating base member  1 , the first multi-layer wiring base member  7 , the resin  11 , and the second multi-layer wiring base member  8  are pressed while heating these elements, thereby thermally curing the resin  11 . 
     The highest temperature of the resin  11  in this process is in the range of about 180° C. to 200° C. at which the resin  11  is thermally cured. Since this temperature is lower than the melting point of the solder  4 , the solder  4  is not melted in this process. 
     Moreover, by pressing in this manner, the resin  11  penetrates into the opening  7   a,  and the opening  7   a  is filled with the resin  11 . 
     Subsequently, as illustrated in  FIG. 3G , the semiconductor element  17  is mounted on the uppermost wiring  10  via the solder bump  16 . 
     The material of the solder bump  16  is not particularly limited. However, the solder having lower melting point than that of the solder  4  is mechanically fragile, and thus the connection strength between the semiconductor element  17  and the wiring  10  would be reduced when such a solder of low melting point is used as the solder bump  16 . 
     In order to enhance the connection strength between the semiconductor element  17  and the wiring  10 , it is preferable to use the solder having the melting point equal to or higher than that of the solder  4  for the material of the solder bump  16 . In view of this, Sn-3Ag-0.5Cu, which is the same material having the same melting point (about 220° C.) as that of the solder  4 , is used for the material of the solder bump  16 . 
     Then, the solder bump  16  is subjected to reflow under heating at a temperature of about 220° C. so as to connect the semiconductor element  17  to the wiring  10  with the solder bump  16 . 
     In this reflow, solder  4  having the same melting point of that of the solder bump  16  melts, and the resin  11  around the solder  4  thermally expands. Thus, the solder  4  is subjected to the pressure generated by the resin  11  around the solder  4 . In the present embodiment, however, the melted solder  4  escapes into the hole  1   a , and hence the melted solder  4  does not spreads in the lateral direction. As a result, the solder flash, in which the conductive pads  2  adjacent to each other in the lateral direction are electrically short-circuited via the solder  4 , can be suppressed. 
     Moreover, since the insulating base member  1 , which has a low solder wettability, is exposed on the side surface of the hole  1   a , the hole  1   a  can be prevented from being filled with the solder  4  before this process, which allows the solder to escape into the hole  1   a  in the process. 
     By these steps, the basic structure of the substrate  25  with embedded components completes. 
     According to the present embodiment, since the melted solder  4  can escape into the hole  1   a , solder flash can be suppressed. 
     Although the number of the holes  1   a  provided in the single conductive pad  2  is one in this example, a plurality of the holes  1   a  may be provided in the single conductive pad  2  to increase the amount of the solder  4  escaping into the hole  1   a.    
     Next, explanation is given for the preferable positions of the hole  1   a  that effectively suppress the solder flash. 
       FIG. 5  is an enlarged plan view for explaining the preferable positions of the hole  1   a , which illustrate the state immediately after the component  3  is connected to the conductive pad  2  with the solder  4  in the process illustrated in  FIG. 3D . 
     Unlike the example in  FIG. 4A , in an example illustrated in  FIG. 5 , a part of the hole  1   a  is located outside the conductive pad  2  in the plan view. 
     The component  3  and the solder  4  are not present at the part of the hole  1   a  located outside the conductive pad  2 . Therefore, when the resin  11  is pressurized in the process of  FIG. 3F , the resin  11  enter the part of the hole  1   a  that is located outside the conductive pad  2 , and hence the hole la is closed by the resin  11 . As a result, the solder  4  melted in the process illustrated in  FIG. 3G  cannot escape into the hole  1   a , leading to an increased risk of generation of the solder flash. 
     On the other hand, in the present embodiment, all portions of the hole  1   a  is positioned inside the conductive pad  2  as illustrated in  FIG. 4A . Therefore, all portions of the hole  1   a  is covered with the solder  4 , thereby preventing the resin  11  from entering the hole  1   a.    
     The inventor further investigated the diameter of the hole  1   a  which can effectively suppress the solder flash. 
     In this investigation, the thermal expansion amounts of the solder  4  and the resin  11  was calculated. 
       FIG. 6  is a cross-sectional view of a model used in this calculation. 
     In this model, the thickness of the insulating base member  1  is set as T, and the diameter of the hole  1   a  is set as D. Also, the height of the component  3  is set as Z, and the length of the component  3  is set as Y. 
     The size of the component  3  is represented by YxZ as a combination of the length Y and the height Z. Here, four types of the dimension YxZ were used, namely, 0.6 mm×0.3 mm, 1.0 mm×0.5 mm, 1.6 mm×0.8 mm, and 2.0 mm×1.2 mm. 
       FIG. 7  is a table obtained by calculating (1) the thermal expansion amount ΔV s  of the solder  4  and (2) the thermal expansion amount ΔV r  of the resin  11 , for each of the electronic components  3  having these sizes. Note that each thermal expansion amount was calculated when the temperature was risen from 20° C. to 225° C. This is substantially equals to the thermal expansion amount when the temperature of each of the solder  4  and the resin  11  is risen from room temperature (20° C.) to a temperature (about 220° C.) in the reflow of  FIG. 3G . 
     Here, the thermal expansion rate of the solder  4  was set 21 ppm/° C., and the thermal expansion amount of the resin  11  was set 60 ppm/° C. 
     Then, the sum (ΔV s +ΔV r ) of (1) the thermal expansion amount ΔV s  of the solder  4  and (2) the thermal expansion amount ΔV r  of the resin  11  is equal to (3) the amount ΔV h  of the solder  4  entering the hole  1   a.    
     Note that, for reference,  FIG. 7  also lists the area S c  of the conductive pad  2  in the plan view which is suitable for each size of the electronic component  3 . 
     On the other hand,  FIG. 8  is a table obtained by calculating the diameter D of the hole  1   a  which is suitable for each of the aforementioned sizes of the electronic component  3 , the area Sh of the hole  1   a  in the plan view, and the volume V of the hole  1   a . In this calculation, the thickness T of the insulating base member  1  was set to 0.06 mm, which is a smallest thickness for practical use. 
     Here, when the volume V of the hole  1   a  is smaller than the amount ΔV h  (see  FIG. 7 ), the all of the hole  1   a  is filled with the solder  4 , which makes it difficult to absorb the thermal expansion of the solder  4  by the hole  1   a . This problem becomes significant, when the hole  1   a  on a side opposite to the component  3  is closed. 
     In order to prevent this problem, the volume V of the hole  1   a  is preferably set to be equal to or larger than the amount ΔV h  (ΔV h ≦V) to prevent the hole  1   a  from being fully filled with the solder  4 . 
     According to  FIGS. 7 and 8 , ΔV h ≦V holds when the diameter D of the hole  1   a  is equal to or larger than 0.07 mm. Thus, by setting the diameter D to be equal to or larger than 0.07 mm, the hole  1   a  is prevented from being fully filled with the solder  4 . This is equivalent to setting the area Sh of the hole  1   a  to be equal to or larger than 4% of the area S c  of the conductive pad  2 . 
     In contrast, when the diameter D is too large, the area of the portion of the conductive pad  2  which is connected to the solder  4  becomes small, which results in a reduction in the connection strength between the conductive pad  2  and the component  3 . In order to prevent this reduction in the connection strength, the diameter D is preferably set to be equal to or smaller than 0.3 mm. As for an area ratio of the hole  1   a  and the conductive pad  2 , such a reduction in the connection strength can be prevented by setting the area Sh of the hole  1   a  to be equal to or smaller than 20% of the area S c  of the conductive pad  2 . 
     Second Embodiment 
     In the present embodiment, wiring is densely formed in a substrate with embedded component as described below. 
       FIGS. 9A to 9E  are cross-sectional views of the substrate with embedded component in the course of manufacturing thereof according to the present embodiment. In  FIGS. 9A . to  9 E, the same element as that described in the first embodiment is denoted by the same reference numeral as that in the first embodiment, and description thereof is omitted in the following. 
     First, as illustrated in  FIG. 9A , a copper-clad base member, which is made by forming copper films on both surfaces of the insulating base member  1 , is prepared. Then, the copper film on one of the surfaces of the insulating base member  1  is patterned into conductive pads  2 , and the copper film on the other surface of the insulating base member  1  is patterned into conductive patterns  2   a.    
     In the present embodiment, the hole  1   a  is formed in the conductive pad  2  at this patterning. 
     Subsequently, as illustrated in  FIG. 9B , a resin sheet as the insulating layer  9  is adhered onto the other surface of the insulating base member  1 , and then a copper-plated film is formed on this insulating layer  9  and is patterned to form wiring  21  having a thickness of about 12 μm to 35 μm. 
     As the insulating layer  9 , a resin sheet of epoxy resin having a thickness of about 0.06 mm to 0.2 mm can be used, for example. 
     Then, as illustrated in  FIG. 9C , the hole  1   a  is formed in the insulating base member  1  by irradiating the insulating base member  1  with excimer laser and the like through the hole  1   a  of the conductive pad  2 . 
     The power of the laser is set enough to evaporate the insulating base member  1  made of resin, and is insufficient to evaporate the conductive pattern  2   a.  Thus, the conductive pattern  2   a  is not opened in this process, and hence the structure in which the hole  1   a  is closed by the conductive pattern  2   a  is obtained. 
     Subsequently, as illustrated in  FIG. 9D , the component  3  is connected to the conductive pad  2  with the solder  4 , similarly to the first embodiment. 
     Thereafter, the processes illustrated in  FIGS. 3E to 3G  in the first embodiment are performed, thereby obtaining the basic structure of a substrate  30  with embedded component according to the present embodiment illustrated in  FIG. 9E . 
     According to the present embodiment described above, the conductive pattern  2   a  closes the hole  1   a  as illustrated in  FIG. 9C . When the hole  1   a  is formed in the insulating layer  9  below the conductive pattern  2   a,  the hole  1   a  would be an obstacle to route the wiring  21  on the insulating layer  9 . However, by closing the hole  1   a  in this manner, the wiring  21  can be flexibly routed, and hence the wiring  21  can be formed at high density. 
     All examples and conditional language recited herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.

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