Abstract:
A printed circuit board and a manufacturing method of a printed circuit board which ensure high mechanical bonding strength and reliable electrical connection and permit ready mounting are provided. The printed circuit board has: n (n: an integer) conducting pattern layers on each of which a wiring pattern is formed; (n+1) insulating patterns formed alternately with the conducting pattern layers; conducting pads formed on the wiring patterns of the conducting pattern layers; and openings continuous from the conducting pads to a surface of the topmost of the insulating layers, penetrating the insulating layers.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a printed circuit board, on whose surface a ball grid array element is mounted. 
     DESCRIPTION OF THE PRIOR ART 
     A ball grid array (hereinafter abbreviated as BGA), a sort of semiconductor element, is a multi-terminal semiconductor element, on which many solder balls are formed on its bottom surface. 
     Mounting of this BGA on a printed circuit board is basically accomplished by melting solder balls by heating, and solidifying the solder balls onto conductor pads formed opposite to the solder balls on the printed circuit board, respectively. 
     However, the BGA and the printed circuit board are only connected by many solder balls and conductor pads having small size. Therefore mechanical stresses resulting from bending, vibration, impact and the like working on the printed circuit board after its mounting tend to invite poor contact between solder balls and conductor pads or peeling of conductor pads. Many mounting methods have already been devised to solve this technical problem. 
     One of these mounting methods is an under-fill method. In this method, liquid resin is injected between a BGA and the printed circuit board after the BGA is mounted as described above. Then the liquid resin is hardened to adhere the BGA to the printed circuit board in an integrated manner and thereby to increase the bonding strength. 
     For mounting a BGA on a printed circuit board configured by stacking a plurality of conductor layers, there has been a method disclosed in the Japanese Patent Laid-open No. Hei 9-246684 (disclosed on Sep. 19, 1997). This method will be described below as an example of the prior art. 
     FIG. 7 is a longitudinal sectional view of the BGA mounting structure of the prior art. 
     A circuit board  130  has of an opening  130   d  and pads  140   a  to  140   f.  The opening  130   d  has a stepwise structure increasing in depth toward the center, provided in the part where a BGA  110  is to be mounted. The pads  140   a  to  140   f  connected to patterns  130   e  to  130   j  in different exposed parts of a multi-layered circuit board  130  and to be mounted with solder balls  120   a  to  120   f.  The BGA  110  has the solder balls  120   a  to  120   f,  growing in size toward the center so as to be mounted on the pads  140   a  to  140   f  provided on the circuit board  130 . The solder balls  120   a  to  120   f  are melted on the pads  140   a  to  140   f  to connect the circuit board  130  and the BGA  110 . 
     However, the conventional under-fill method referred to above involves the extra steps of resin injection and hardening, resulting in the problems of slower production and higher manufacturing costs. 
     Furthermore, the injection resin, once hardened, is extremely difficult to remove, making it impossible to replace any BGA by itself whose contact has become poor and rendering the printed circuit substantially irreparable. Therefore many BGA-mounted printed circuit boards have to be discarded in whole, entailing extremely high repair costs. 
     The BGA mounting structure disclosed in the Japanese laid-open patent also involves the problem of inadequate mechanical connective strength between the BGA  100  and the circuit board  130  and poor electrical reliability of their connection. Because the solder balls  120   a  to  120   f  are only bonded to the pads  140   a  to  140   f,  each of a very small size, resulting in insufficient bonding strength and easy peeling of the pads  140   a  to  140   f.    
     Moreover, since the opening  130   d  can only be formed in a stepwise structure, there is the problem that the solder balls  120   a  to  120   f  cannot be electrically connected to the patterns  130   e  to  130   j  as desired, therefore strictly limiting the freedom of wiring and inhibiting sufficient enhancement of the wiring density. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a printed circuit board and a manufacturing method of a printed circuit board which ensure high mechanical bonding strength and reliable electrical connection and permit ready mounting. 
     A printed circuit board according to the present invention has: n (n: an integer) conducting pattern layers on each of which a wiring pattern is formed; (n+1) insulating patterns formed alternately with the conducting pattern layers; conducting pads formed on the wiring patterns of the conducting pattern layers; and openings continuous from the conducting pads to a surface of the topmost of the insulating layers, penetrating the insulating layers. 
     A manufacturing method of a printed circuit board according to the present invention has the steps of: forming alternately (n+1) (n: an integer) insulating patterns with n conducting pattern layers on each of which a wiring pattern is formed, and forming openings continuous from the surface of the topmost of the insulating layers to conducting pads formed on the wiring patterns of the conducting pattern layers. 
     According to the present invention, an internal conducting pattern layer not exposed on the surface and soldered parts of electronic parts are both mechanically bonded and electrically connected in a printed circuit board in which conducting pattern layers and insulating layers are alternately stacked. As the insulating layer that is located closer to the top surface than this conducting layer supports sideways of the soldered members of the electronic parts, the bonding strength of the electronic parts and the printed circuit board is enhanced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a plan view of a printed circuit board according to the first embodiment of the present invention. 
     FIG. 1B is a cross sectional view showing along the line I-I′ in the FIG. 1A embodiment. 
     FIG. 1C is a cross sectional view of the first step of the manufacturing method showing along the line I-I′ in the FIG. 1A embodiment. 
     FIG. 1D is a cross sectional view of the second step of the manufacturing method showing along the line I-I′ in the FIG. 1A embodiment. 
     FIG. 1E is a cross sectional view of the third step of the manufacturing method showing along the line I-I′ in the FIG. 1A embodiment. 
     FIG. 1F is a cross sectional view of the fourth step of the manufacturing method showing along the line I-I′ in the FIG. 1A embodiment. 
     FIG. 2A is a plan view of a printed circuit board according to the second embodiment of the present invention. 
     FIG. 2B is a cross sectional view showing along the line II-II′ in the FIG. 2A embodiment. 
     FIG. 3A is a plan view of a printed circuit board according to the third embodiment of the present invention. 
     FIG. 3B is a cross sectional view showing along the line III-III′ in the FIG. 3A embodiment. 
     FIG. 4A is a plan view of a printed circuit board according to the fourth embodiment of the present invention. 
     FIG. 4B is a cross sectional view showing along the line IV-IV′ in the FIG. 4A embodiment. 
     FIG. 5A is a plan view of a printed circuit board according to the fifth embodiment of the present invention. 
     FIG. 5B is a cross sectional view showing along the line V-V′ in the FIG. 5A embodiment. 
     FIG. 6A is a plan view of a printed circuit board according to the sixth embodiment of the present invention. 
     FIG. 6B is a cross sectional view showing along the line VI-VI′ in the FIG. 6A embodiment. 
     FIG. 7 is a longitudinal sectional view of the BGA mounting structure of the prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (1st Embodiment) 
     FIG. 1A is a plan view of a printed circuit board according to the first embodiment of the present invention. FIG. 1B is a cross sectional view showing along the line I-I′ in the FIG. 1A embodiment. 
     In FIGS. 1A and 1B, a BGA (ball grid array)  2  is mounted on a printed circuit board  1 . These printed circuit board  1  and BGA  2  are both mechanically and electrically connected each other by pad openings  31  formed on the upper surface of the printed circuit board  1  and solder balls  21  formed on the bottom surface of the BGA  2 . 
     In FIG. 1B, the printed circuit board  1  is a six-layered board formed by alternately stacking six conducting pattern layers  4 - 1  to  4 - 6  and five insulating layers  5 - 1  to  5 - 5 . Each of the conducting pattern layers  4 - 1  to  4 - 6  is formed in a wiring pattern of a specific shape. Desired combination of these conducting pattern layers  4 - 1  to  4 - 6  is further electrically connected by throughholes (not shown). 
     In FIG. 1B, in a plurality of specific positions on the second conducting pattern layer  4 - 2 , not exposed on the top surface of the printed circuit board  1 , conducting pads  3  to be electrically connected to this second conducting pattern  4 - 2  are formed. The plurality of specific positions are directly underneath the solder balls  21  of the BGA  2 . 
     The insulating layer  5 - 1  immediately next to this conducting pattern layer  4 - 2  toward the top surface of the printed circuit board  1  is formed pad openings  31  to accommodate the solder balls  21  therein. 
     No wiring pattern of the conducting pattern layer  4 - 1  is formed in the positions on the conducting pattern layer  4 - 1 , which is toward the top surface of the printed circuit board  1 , directly above these pads  3 . 
     Furthermore, the peripheral parts of the pad openings  31  closer to the top surface of the printed circuit board  1  than the conducting pads  3  are formed by extending the insulating layers  5 - 1  and  5 - 2  to support the conducting pads  3  from around so they may not come off toward the pad openings  31 . 
     These conducting pads  3  and pad openings  31  are circularly shaped, and the diameter of the former should desirably be greater by 50 to 100 μm than that of the latter, but these dimensions need not be so strictly specified. 
     To mount the BGA  2  onto the printed circuit board  1  in this embodiment, the BGA  2  can be soldered, together with other surface-mounted electronic parts, by a collective reflowing method onto the top surface of the printed circuit board  1  as shown in FIG.  1 B. 
     In this process, the solder balls  21  are melted by heating and, after evenly spreading within the pad openings  31 , are cooled to solidify, keeping to fill the inside of the pad openings  31 . 
     Therefore, while the solders  21  are both mechanically bonded and electrically connected to the conducting pads  3  each other, the solder balls  21  are caught by the vertical faces of the insulating layers  5 - 1  and  5 - 2  facing the pad openings  31 , resulting in substantially enhanced mechanical bonding strength. 
     In this embodiment, the solder balls  21  of the BGA  2  are both mechanically and electrically connected to the conducting pattern layer  4 - 2  each other inside the printed circuit board  1  and at the same time the insulating layer  5 - 2  holds them from their surroundings. Therefore a high mechanical bonding strength is achieved between the BGA  2  and the printed circuit board  1 . 
     Next will be described the manufacturing method of a printed circuit board according to the present invention. Here is described a case in which a build-up method is applied, which is well known to those skilled in the art. 
     FIG. 1C is a cross sectional view of the first step of the manufacturing method showing along the line I-I′ in the FIG. 1A embodiment. FIG. 1D is a cross sectional view of the second step of the manufacturing method showing along the line I-I′ in the FIG. 1A embodiment. FIG. 1E is a cross sectional view of the third step of the manufacturing method showing along the line I-I′ in the FIG. 1A embodiment. FIG. 1F is a cross sectional view of the fourth step of the manufacturing method showing along the line I-I′ in the FIG. 1A embodiment. 
     In advance, the conducting pattern layers  4 - 2  to  4 - 5  and the insulating layers  5 - 2  to  5 - 4  are alternately stacked by a known subtractive method or the like to form the conducting pattern layers  4 - 2  to  4 - 5  as wiring patterns having respectively required shapes as shown in FIG.  1 C. 
     Next, epoxy-based liquid insulating resin is applied onto the surfaces of the conducting pattern layers  4 - 2  and  4 - 5  and of the insulating layers  5 - 2  and  5 - 4  and hardened, the insulating layers  5 - 1  and  5 - 5  are formed as shown in FIG.  1 D. 
     Then, the pad openings  31  are formed as shown in FIG. 1E by a known laser method, photoetching or the like in the parts of the wiring pattern of the conducting pattern layer  4 - 2 , to constitute connecting points to the solder balls  21  of the BGA  2  so that these parts are exposed on the top surface of the printed circuit board  1  toward the insulating layer  5 - 1  side. 
     Next, throughholes for mutual electrical connection of the conducting pattern layers  4 - 2  to  4 - 5  are formed by penetrating through all the insulating layers  5 - 1  to  5 - 5 . 
     Then the pad openings  31  and other parts are masked as required. 
     Next, by a known additive process, the conducting pattern layer  4 - 1  is formed over the surface of the insulating layer  5 - 1 , and the conducting pattern layer  4 - 6  is formed over the surface of the insulating layer  5 - 5  as wiring patterns having respectively required shapes. 
     Finally, the masking is removed to complete the mounting. 
     (2nd Embodiment) 
     FIG. 2A is a plan view of a printed circuit board according to the second embodiment of the present invention. FIG. 2B is a cross sectional view showing along the line II-II′ in the FIG. 2A embodiment. 
     In FIGS. 2A and 2B, the same elements as shown in FIGS. 1A and 1B have the same reference numbers, and their description is omitted. 
     In FIGS. 2A and 2B, conducting pads  3  are provided in specific positions on the third conducting pattern layer  4 - 3 . 
     The pad openings  31  are formed for accommodating the solder balls  21  within in the positions on the insulating layers  5 - 1 ,  5 - 2  and the conducting pattern layer  4 - 2 , which are closer to the top surface of a printed circuit board  1  than the conducting pattern layer  4 - 3 . 
     No wiring pattern of the conducting pattern layers  4 - 1  and  4 - 2  is formed in the positions on the conducting pattern layers  4 - 1  and  4 - 2 , which are closer to the top surface of the printed circuit board  1  than the conducting pads  3 , directly above these pads  3 . 
     Furthermore, the peripheral parts of the pad openings  31  closer to the top surface of the printed circuit board  1  than the conducting pads  3  are formed by extending the insulating layers  5 - 1  and  5 - 3  to support the conducting pads  3  from around so they may not come off toward the pad openings  31 . 
     In this embodiment, the size of supporting the solder balls  21  from around is greater than in the first embodiment. Therefore not only can mechanical bonding strength be further enhanced, but also can the solder balls  21  of the BGA  2  be electrically connected to the conducting pattern layer  4 - 3  within the printed circuit board  1 . 
     (3rd Embodiment) 
     FIG. 3A is a plan view of a printed circuit board according to the third embodiment of the present invention. FIG. 3B is a cross sectional view showing along the line III-III′ in the FIG. 3A embodiment. 
     In FIGS. 3A and 3B, the same elements as in FIGS. 1A to  2 B have the same reference numbers, and their description is omitted. 
     In FIGS. 3A and 3B, conducting pads  3  are provided in specific positions on the second and third conducting pattern layers  4 - 2  and  4 - 3 . 
     The pad openings  31  are formed for accommodating the solder balls  21  within in the positions on the insulating layer  5 - 1  and the conducting pattern layer  4 - 1 , which are closer to the top surface of a printed circuit board  1  than the conducting pattern layer  4 - 2 . 
     No wiring pattern of the conducting pattern layer  4 - 1  is formed in the positions on the conducting pattern layer  4 - 1 , which are closer to the top surface of the printed circuit board  1  than the conducting pads  3  of the conducting pattern layer  4 - 2 , directly above these pads  3 . 
     Furthermore, the peripheral parts of the pad openings  31  closer to the top surface of the printed circuit board  1  than the conducting pads  3  of the conducting pattern layer  4 - 2  are formed by extending the insulating layers  5 - 1  and  5 - 2  to support the conducting pads  3  from around so they may not come off toward the pad openings  31 . 
     Similarly, the pad openings  31  are formed for accommodating the solder balls  21  within in the positions on the insulating layers  5 - 1  and  5 - 2  and the conducting pattern layers  4 - 1  and  4 - 2 , which are closer to the top surface of a printed circuit board  1  than the conducting pattern layer  4 - 3 . 
     No wiring pattern of the conducting pattern layers  4 - 1  and  4 - 2  is formed in the positions on the conducting pattern layers  4 - 1  and  4 - 2 , which are closer to the top surface of the printed circuit board  1  than the conducting pads  3  of the conducting pattern layer  4 - 3 , directly above these pads  3 . 
     Furthermore, the peripheral parts of the pad openings  31  closer to the top surface of the printed circuit board  1  than the conducting pads  3  of the conducting pattern layer  4 - 3  are formed by extending the insulating layers  5 - 1  to  5 - 3  to support the conducting pads  3  from around so they may not come off toward the pad openings  31 . 
     Since this embodiment is configured in this manner, not only is enhanced mechanical bonding strength as in the second embodiment, but also can the solder balls  21  of the BGA  2  be electrically connected to any of the plurality of conducting pattern layers  4 - 2  to  4 - 5 . 
     (4th Embodiment) 
     FIG. 4A is a plan view of a printed circuit board according to the fourth embodiment of the present invention. FIG. 4B is a cross sectional view showing along the line IV-IV′ in the FIG. 4A embodiment. 
     In FIGS. 4A and 4B, the same elements as in FIGS. 1A to  3 B have the same reference numbers, and their description is omitted. 
     In FIGS. 4A and 4B, conducting pads  3  are provided in specific positions on the first and second conducting pattern layers  4 - 1  and  4 - 2 . 
     The pad openings  31  are formed for accommodating the solder balls  21  within in the positions on the insulating layer  5 - 1  and the conducting pattern layer  4 - 1 , which are closer to the top surface of a printed circuit board  1  than the conducting pattern layer  4 - 2 . 
     No wiring pattern of the conducting pattern layer  4 - 1  is formed directly above these conducting pads  3 . 
     Furthermore, the peripheral parts of the pad openings  31  closer to the top surface of the printed circuit board  1  than the conducting pads  3  of the conducting pattern layer  4 - 2  are formed by extending the insulating layers  5 - 1  and  5 - 2  to support the conducting pads  3  from around so they may not come off toward the pad openings  31 . 
     Since this embodiment is configured in this manner, not only is enhanced mechanical bonding strength as in the first embodiment, but also can the solder balls  21  of the BGA  2  be electrically connected to the conducting pattern layer  4 - 1  on the top surface of the printed circuit board  1 . 
     (5th Embodiment) 
     FIG. 5A is a plan view of a printed circuit board according to the fifth embodiment of the present invention. FIG. 5B is a cross sectional view showing along the line V-V′ in the FIG. 5A embodiment. 
     In FIGS. 5A and 5B, the same elements as shown in FIGS. 1A to  4 B have the same reference numbers, and their description is omitted. 
     In FIGS. 5A and 5B, there are only two conducting pattern layers and insulating layers each, respectively  4 - 1 ,  4 - 2  and  5 - 1 ,  5 - 2 , and conducting pads  3  are provided in specific positions on the second conducting pattern layer  4 - 2 . 
     The pad openings  31  are formed for accommodating the solder balls  21  within in the positions on the insulating layer  5 - 1  and the conducting pattern layer  4 - 1 , which are closer to the top surface of a printed circuit board  1  than the conducting pattern layer  4 - 2 . 
     No wiring pattern of the conducting pattern layer  4 - 1  is formed directly above these conducting pads  3 . 
     Furthermore, the peripheral parts of the pad openings  31  closer to the top surface of the printed circuit board  1  than the conducting pads  3  of the conducting pattern layer  4 - 2  are formed by extending the insulating layers  5 - 1  and  5 - 2  to support the conducting pads  3  from around so they may not come off toward the pad openings  31 . 
     Since this embodiment is configured in this manner, mechanical bonding strength is enhanced as in the first embodiment. 
     (6th Embodiment) 
     FIG. 6A is a plan view of a printed circuit board according to the sixth embodiment of the present invention. FIG. 6B is a cross sectional view showing along the line VI-VI′ in the FIG.  6 A embodiment. 
     In FIGS. 6A and 6B, the same elements as shown in FIGS. 1A to  5 B have the same reference numbers, and their description is omitted. 
     In FIGS. 6A and 6B, two BGAs  2  are mounted on both surfaces of the printed circuit board  1 . 
     The conducting pads  3  are provided in specific positions on the second and fifth conducting pattern layers  4 - 2  and  4 - 5 . 
     The pad openings  31  are formed for accommodating the solder balls  21  within in the positions on the insulating layer  5 - 1  and the conducting pattern layer  4 - 1 , which are closer to the top surface of a printed circuit board  1  than the conducting pattern layer  4 - 2 . 
     No wiring pattern of the conducting pattern layer  4 - 1  is formed directly above these conducting pads  3 . 
     Furthermore, the peripheral parts of the pad openings  31  closer to the top surface of the printed circuit board  1  than the conducting pads  3  of the conducting pattern layer  4 - 2  are formed by extending the insulating layers  5 - 1  and  5 - 2  to support the conducting pads  3  from around so they may not come off toward the pad openings  31 . 
     Similarly, the pad openings  31  are formed for accommodating the solder balls  21  within in the positions on the insulating layer  5 - 5  and the conducting pattern layer  4 - 6 , which are closer to the top surface of a printed circuit board  1  than the conducting pattern layer  4 - 5 . 
     No wiring pattern of the conducting pattern layer  4 - 6  is formed directly above these conducting pads  3 . 
     Furthermore, the peripheral parts of the pad openings  31  closer to the bottom surface of the printed circuit board  1  than the conducting pads  3  of the conducting pattern layer  4 - 5  are formed by extending the insulating layers  5 - 4  and  5 - 5  to support the conducting pads  3  from around so they may not come off toward the pad openings  31 . 
     Since this embodiment is configured in this manner, not only can BGAs be mounted on both sides of the printed circuit board, but also is enhanced mechanical bonding strength as in the first embodiment. 
     Although a BGA  2  were cited as an example of an electronic part to be mounted on the printed circuit board  1  in every embodiment described, the electronic part(s) to be mounted may be any other appropriate semiconductor element(s), another printed circuit board or the like. 
     In every embodiment described above, the solder balls  21  of the BGA  2  were cited as an example of members to be soldered to the conducting pads  3 , but the members to be soldered may as well be solder bumps or the like. 
     Though what constitute a conducting pattern layer are cited as an example of conducting pads to be electrically connected to soldered electronic parts in every one of the foregoing embodiments, these conducting pads may as well form a metallic layer on the surface of a conducting pattern layer. 
     In every embodiment described, it is sufficient for the sizes of the pad openings in the conducting pattern layer located closer to the top surface than the conducting pads to be greater than the sizes of the conducting pads. In other words, it is sufficient for the conducting pattern layer to be exposed in the pad openings. 
     In every one of the foregoing embodiments, it is sufficient for the sizes of the pad openings in the insulating layer located closer to the top surface than the conducting pads to be smaller than the sizes of the conducting pads. In other words, it is sufficient for the insulating layer to be exposed in the pad openings. 
     Although insulating layers are extended to cover the inside of the openings in every embodiment described above, the inside may as well be covered with another insulator, which may be of either the same material as that of said insulating layers or something else. 
     As hitherto described, according to the present invention, internal conducting pattern layers not exposed to the surface and soldered members of electronic parts are both mechanically bonded and electrically connected each other in a printed circuit board in which the conducting pattern layers and insulating layers are stacked alternately. 
     Furthermore, these connecting parts are formed as openings in the surface of the printed circuit board, and the strength of connection between conducting pads and the conducting pattern layers is substantially increased by providing insulating layers on the inner walls of these openings to prevent the conducting pads from coming off. 
     Therefore, a printed circuit board can be realized of which mechanical bonding strength is remarkably high, electrical connection is reliable, and mounting can be readily accomplished. 
     In this manner, poor contact between solder balls and conducting pads and damages, such as peeling of conductor pads, due to bending, vibration, impact and the like working on the printed circuit board can be significantly reduced.