Abstract:
A multilayer printed circuit board and a corresponding fabrication method are disclosed, which circuit board achieves a relatively high degree of wiring density and a relatively high degree of wiring design freedom. These advantages are obtained in the inventive printed circuit board by electrically connecting power conductors or ground conductors using through holes. On the other hand, signal conductors in any two adjacent signal wiring layers are electrically connected using via holes extending only through an intervening electrically insulating layer. Preferably, the electrically insulating layer is a layer of photosensitive resin and the via holes are formed using conventional photolithographic techniques.

Description:
This is a divisional of copending application Ser. No. 07/764,733 filed on Sep. 24, 1991 now U.S. Pat. No. 5,451,721. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention pertains to multilayer printed circuit boards and to corresponding fabrication methods. 
     2. Description of the Related Art 
     In prior multilayer printed circuit boards, it is common to dispose power supply and ground layers inside and signal layers outside, and to interconnect those layers by through holes. Inasmuch as the through holes are formed by mechanical drilling and hence tend to be of relatively large size, it is difficult to increase the wiring density of the circuit board. Also, inasmuch as it is necessary to design the signal layers on both sides while always taking through hole connections into consideration, freedom in wiring design is relatively restricted. Moreover, because the through holes pass through the power supply and ground layers, it is necessary to provide clearance holes at those through hole locations in the power supply and ground layers where no power supply or ground connections are required. Accordingly, the higher the through hole density becomes, the smaller the clearance holes have to be formed with still higher accuracy, which in turn entails more accurate alignment of the signal layers with the power supply and ground layers in laminating those layers. Recently, in addition to PIH (Pin In Hole) components which are mounted by inserting their pins into through holes, SMT (Surface Mount Technology) components which are mounted without pins have come to be used. Therefore, there is a need for multilayer printed circuit boards which can be applied to both SMT and PIH components, and which can provide higher wiring density and higher degree of freedom in design of wiring. 
     OBJECTS OF THE INVENTION 
     The primary objects of the invention are to provide a multilayer printed circuit board, and a corresponding fabrication method, which achieves a significantly higher wiring density and a significantly higher degree of wiring design freedom than was previously possible, and in which through hole connections are reduced. Other objects of the invention are to provide a multilayer printed circuit board useful for achieving SMT component mounting and PIH component mounting. 
     SUMMARY OF THE INVENTION 
     The above objects are achieved, in accordance with the invention, by providing a multilayer printed circuit board in which power conductors or ground conductors are electrically connected using through holes. On the other hand, signal conductors in any two adjacent signal wiring layers are electrically connected using via holes extending only through an intervening electrically insulating layer. Preferably, the electrically insulating layer is a layer of photosensitive resin and the via holes are formed using conventional photolithographic techniques. 
     In a first embodiment of the inventive multilayer printed circuit board, a region of electrically conductive material, e.g., copper, overlies, i.e., lies on or over, a surface of an electrically insulating substrate. This region of electrically conductive material is, for example, a layer of electrically conductive material, lying on the electrically insulating substrate, which serves as a power supply layer or ground layer. Two or more signal wiring layers, separated by an electrically insulating layer, overlie the opposite surface of the electrically insulating substrate. The outermost signal wiring layer includes an electrical conductor which serves as a power conductor or ground conductor. A through hole, containing electrically conductive material, serves to electrically connect the power conductor or ground conductor to the power supply layer or ground layer. In addition, one or more via holes, extending only through the electrically insulating layer and containing electrically conductive material, serve to electrically connect two or more signal conductors in the adjacent signal wiring layers. 
     In a second embodiment of the inventive multilayer printed circuit board, two or more signal wiring layers, separated by electrically insulating layers, overlie each surface of the electrically insulating substrate. The outermost signal wiring layers include power or ground conductors which are electrically connected by a through hole containing electrically conductive material. As before, signal conductors in adjacent signal wiring layers are electrically connected by via holes extending through the intervening electrically insulating layers and containing electrically conductive material. 
     It should be noted that SMT components are readily mounted on (appropriately dimensioned and configured) signal conductors, which serve as mounting terminals, in the outermost wiring layers of the first and second embodiments. In addition, PIH components are readily mounted in the through holes employed in the two embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described with reference to the accompanying drawings, wherein: 
     FIGS. 1A-1J depict the steps involved in fabricating a first embodiment of the inventive multilayer printed circuit board; and 
     FIGS. 2A-2I depict the steps involved in fabricating a second embodiment of the inventive multilayer printed circuit board. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1A-1J depict the fabrication of a first embodiment of the inventive multilayer printed circuit board, which first embodiment is, for example, a four-layer printed circuit board. 
     Referring now to FIG. 1A, there is shown an electrically insulating substrate  10 , e.g., a glass epoxy substrate, both sides of which are laminated with layers  12  and  14  of electrically conductive material, e.g., copper. Each of the layers  12  and  14  has a weight of, for example, 0.5 ounces and a thickness of, for example, 18 micrometers. 
     As shown in FIG. 1B, in fabricating the first embodiment, the copper layer  12  on one side is patterned by conventional selective etching to form a first wiring layer or wiring level which includes signal wiring conductors  16 . The copper layer  14  on the other side is used as a power supply layer in this embodiment. 
     Next, in FIG. 1C, a layer  18  of electrically insulating material, preferably a layer of photosensitive resin, is applied to cover the signal wiring conductors  16  in the first wiring layer, and the photosensitive resin is then exposed and developed to form via holes  20  at selected locations. The photosensitive resin insulating layer used in a reduction to practice of the invention was Probimer 52, a negative type photosensitive epoxy resin marketed by Ciba-Geigy Ltd. This epoxy resin was applied to the surface of the substrate by curtain coating, and was then precured at 80° C. for one hour, then exposed with a mercury lamp, then developed with developer DY90 (a mixture of propylene carbonate, cyclohexanone, and gamma-butyrolactone) also marketed by Ciba-Geigy Ltd. 
     After being developed, the surface of the epoxy resin layer  18  was roughened by etching with a solution of potassium permanganate, and was then activated with Activator 180, a seeding chemical marketed by DYNACHEM Inc. This seeding chemical is a solution including colloidal tin and palladium. 
     As shown in FIG. 1D, a layer  22  of electrically conductive material, e.g., copper, of second level is deposited by electroless plating on the entire surface of the insulating layer  18  formed with via holes. The copper layer  22  of the second level is connected to the signal wiring conductors  16  of the first level by plated via holes  24 . The copper deposited by the electroless plating is also deposited on the lower copper layer  14  in FIG.  1 C and thus thickens the copper layer  14  a little. The lower copper layer thus thickened is indicated by reference number  14 ′ in FIG.  1 D. The copper layer may be further thickened by electroplating after the electroless plating, if necessary. 
     In FIG. 1E, the copper layer  22  is patterned by selective etching to form a second wiring layer or wiring level which includes signal wiring conductors  26 . 
     As shown in FIG. 1F, in the same manner as the step shown in FIG. 1C, a second photosensitive resin insulating layer  28  is applied on the signal wiring conductors  26  in the second level, and via holes  30  are formed at selected locations by exposure and development. 
     As shown in FIG. 1G, a through hole  32  is formed by drilling, e.g., mechanical drilling, at a location where a power supply connection is to be established. Although only one through hole  32  is shown in FIG. 1G, a plurality of through holes may be provided as required. The through hole  32  is used to provide a power supply connection from the power supply layer on the lower side of the circuit board to a power supply conductor or power supply pad on the upper side of the circuit board. 
     As shown in FIG. 1H, in the same manner as the step shown in FIG. 1D, copper is deposited by electroless plating on the entire surface inclusive of the through hole  32 . The copper is also deposited on the lower copper layer  14 ′ shown in FIG.  1 G and thus thickens the lower copper layer  14 ′. The lower copper layer thus thickened is indicated by reference number  14 ″ in FIG.  1 H. The deposited copper layer may be further thickened by electroplating after the electroless plating, if necessary. On the upper surface of the circuit board, there is formed a surface conductor layer  36 , which is connected through the vias  34  to the signal wiring conductors  26  in the second level. A through hole connection  38  is formed in the through hole  32 , which connects the upper surface and the lower surface of the circuit board. 
     As shown in FIG. 1I, the surface conductor layer  36  and the lower copper layer  14 ″ are patterned by selective etching to form signal wiring conductors  40  of third wiring level, a ground conductor  42  on the upper surface, a power supply conductor or power supply land  44  on the upper surface, and a power supply conductor  46  on the lower surface. The signal wiring conductors  40  on the upper surface provide SMT component mounting terminals. The lower copper layer  14 ″ in FIG. 1H may be patterned as necessary when such components as resistors and capacitors are to be mounted on the lower side of the substrate for supply voltage regulation, decoupling and so on. 
     Finally, if so desired, and as shown in FIG. 1J, the conductor areas not to be exposed to solder are covered with solder masking layers  48  of a polyamide or epoxy resin and the through hole filled with solder using conventional wave soldering techniques. Prior to wave soldering, or even in the absence of wave soldering, a PIH component is readily mounted on the circuit board by inserting the pin or pins of the component into the through hole or through holes of the circuit board. 
     In the above-mentioned embodiment, the copper layer  14  on the lower surface of the insulating substrate is used as a power supply layer. However, the copper layer  14  may be used as a ground layer and the copper layer  42  on the upper surface of the circuit board may be used as a power supply layer. The electrical characteristics are almost the same in both cases. 
     Although a four-layer printed circuit board has been illustrated in the above, it is also possible to manufacture a printed circuit board of still more wiring layers by repeating similar steps. 
     FIGS. 2A to  2 I depict the fabrication of a second embodiment of the inventive multilayer printed circuit board in which wiring layers are stacked on both sides of an insulating substrate. The fabrication process is basically the same as that shown in FIGS. 1A to  1 I, except that wiring layers are formed as well on the lower side of the substrate  10 . The elements on the lower side of the substrate corresponding to those formed on the upper side of the substrate are designated by reference numbers with primes attached to the reference numbers for the elements on the upper side of the substrate. 
     With regard to FIG. 2, both the upper copper layer  12  and the lower copper layer  14  are patterned as shown in FIG.  2 B and are used as signal wiring layers. The subsequent substeps are similar to those described with regard to FIGS. 1C to  1 I. A through hole  32  is formed as shown in FIG.  2 G. The entire surface is then deposited with copper by electroless plating as shown in FIG.  2 H. As shown in FIG. 2I, the surface conductor layers  36  and  36 ′ are etched to desired patterns. As shown in FIG. 2I, on the upper side of the circuit board there are formed a power supply conductor or power supply land  44  connected to a lower power supply layer  46  through a plated through hole  38 , signal wiring conductors  40  and a ground layer  42 . Also, the power supply layer  46  and signal wiring conductors  40 ′ are formed on the lower side of the circuit board. The signal wiring conductors  40  and  40 ′ may be used as SMT component mounting terminals. 
     With regard to FIG. 2 as well, the conductor layer  42  may be used as a power supply layer and the conductor layer  46  may be used as a ground layer. 
     The circuit board shown in FIG. 2I has no ground layer on its lower side. Hence, when a component which requires a ground connection is mounted on the lower side of the circuit board, it is sufficient to provide on the lower side of the circuit board a ground land (not illustrated) similar to the power supply land  44 , and to provide another through hole through which to connect the ground layer  42  on the upper side of the circuit board to the ground land on the lower side. Needless to say, the ground land may be formed by the patterning used in FIG. 2I at the same time as the other conductor patterns on the surfaces of the circuit board. 
     The present invention has the following advantages: 
     (1) Because interconnection between signal layers is made through small via holes formed by photoprocessing and the use of larger through holes is minimized, the signal wiring density of the four-layer printed circuit board shown in FIG. 1 can be increased to about twice as much as that of conventional four-layer printed circuit board of the through hole connection type, with the result of greatly reduced circuit package size. 
     (2) Because through hole connections for signal wiring conductors are reduced, parasitic inductances and capacitances of signal wires are notably decreased, and the performance of multilayer printed circuit boards is improved. 
     (3) Because signal wiring can be designed substantially without regard to through hole connections, the degree of freedom for signal wiring design is increased such that design is facilitated and wiring design time can be shortened. 
     (4) Because the surface regions of a printed circuit board with no signal wiring conductors formed are covered with a power supply layer or ground layer, a higher shielding effect can be obtained. 
     (5)The present invention can be very economically implemented by utilizing existing processes and materials. 
     (6) The present invention can be applied not only to SMT component mounting but also to PIH component mounting.