Abstract:
A method and structure for implementing dense wiring, in printed circuit board or chip carrier applications, which provides superior electrical characteristics while preserving the system resistance and characteristic impedance requirements. The dense wiring is characterized by requiring that all wires have a sufficient cross-sectional area to ensure the longest wires used do not exceed a maximum resistance by either sorting wire lengths and allowing acceptably “short” wires to use denser circuit lines or by providing short lengths of short circuit lines in those areas where necessary and switching to less dense, lower resistance lines where possible. The disclosure also provides for dense wiring in component areas that can then be converted to low resistance wiring with application of a buried via.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a method and structure for implementing dense wiring, in printed circuit board or chip carrier applications, which provides superior electrical characteristics while preserving system resistance and characteristic impedance requirements. 
     2. Related Art 
     Semiconductor chip size continues to decrease, with an attendant increase in component density. The electrical signals which provide communication between chips, via electrically conductive wires or lines, are characterized by increasing operational frequencies. Semiconductor chips mounted on a printed circuit board (“PCB”) are subjected to detrimental effects caused by the inherent resistance of the PCB wiring which interconnects the semiconductor chips. A typical high performance printed circuit board has traditionally not been able to use wiring densities beyond a certain point due to limitations imposed by the DC resistance maximum in interchip wiring networks. Similarly, higher frequency signals also demand wide lines as a means to minimize “skin effect” losses in long lines. Unfortunately, it is problematic to generate dense wiring between a plurality of semiconductor chips on a PCB or chip carrier. 
     The usual solution is to use the typical geometry of wide wire lines and appropriate dielectric layer thicknesses to produce a 50 ohm transmission line characteristic impedance (Z 0 ), and achieve a wiring network with a single wiring layer pair that uses only lower capacitance buried vias and a limited number of higher capacitance plated through hole vias. The result of this approach is that more wiring layers are required, and with a correspondingly thicker printed circuit board structure resulting. Future projections of component input/output (I/O) counts (e.g., approaching 4000 I/O counts with an I/O pitch of 0.8 mm) require a solution other than this usual practice. 
     An alternative is to use fine wire lines that widen when sufficient space is available. However, these wire lines do not maintain the necessary transmission line characteristic impedance (Z 0 ) in all areas. These wire lines also have circuitization yield implications that make them unattractive in most printed circuit board applications. 
     Thus, there is a need for a method and structure that implements dense wiring, in printed circuit board or chip carrier applications, and which provides superior electrical characteristics while preserving the system resistance and characteristic impedance requirements. 
     SUMMARY OF THE INVENTION 
     The present invention discloses a structure and associated method of implementing dense wiring, in printed circuit board or chip carrier applications, which provides superior electrical characteristics while preserving the system resistance and characteristic impedance requirements. 
     In addition, the present invention provides reduced thickness printed circuit boards by providing selective dense wiring layers, and enables dense designs that were heretofore difficult to wire. 
     The present invention provides a multilayered circuit board assembly comprising: a first wiring layer having at least one first conductor, each first conductor having a first cross-sectional area; a second wiring layer having at least one second conductor, the second conductor having a cross-sectional area smaller than a cross-sectional area of said first conductor; said multilayered circuit board assembly having a first area of high component density and a second area of lower component density; said first area of high component density connected to said at least one second conductor. 
     The present invention further provides a method of increasing circuit wiring density in a printed circuit board while maintaining an impedance level and reducing electrical noise, said method comprising: providing a first wiring layer containing at least one first wire type having a first impedance, said first wiring layer having a first conductor; providing a second wiring layer containing at least one second wire type having a second impedance, said second wiring layer having a second conductor; wherein the impedance of said first wire type is equal to the impedance of said second wire type; and wherein said first wire type is used to produce said first conductor, and said second wire type is used to produce said second conductor. 
     The present invention also provides a multilayered circuit board assembly having a plurality of networks, said multilayered circuit board comprising: a first wiring layer having at least one first conductor, the first conductor having a first cross-sectional area; a second wiring layer having at least one second conductor, the second conductor having a second cross-sectional area, said second cross-sectional area being smaller than said first cross-sectional area; said multilayered circuit board having a maximum wire resistance limit; and said second wiring layer having electrical connections to networks such that the total resistance of the network connections remains within a maximum wire resistance limit. 
     The present invention additionally provides a method of maintaining a characteristic impedance level in wires of a printed circuit board, said method comprising: providing a first wiring layer containing at least one first wire of a first type, wherein said first wire has a first cross-sectional area; providing a second wiring layer containing at least one second wire of a second type, wherein said second wire has a second cross-sectional area; wherein said first cross-sectional sectional area is not equivalent to said second cross-sectional area; and wherein said first wire and said second wire have the same electrical characteristic impedance. 
     The present invention therefore provides a method and structure that implements dense wiring, in printed circuit board or chip carrier applications, which provides superior electrical characteristics, while preserving the system resistance and characteristic impedance requirements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a cross-sectional view of a related art multilayered printed circuit board having a conventional structure. 
     FIG. 2 depicts a cross-sectional view of a multilayered printed circuit board in accordance with a first embodiment of the present invention. 
     FIG. 3 depicts a cross-sectional view of a multilayered printed circuit board in accordance with a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention solves the issue of requiring all wires to have a large cross-sectional area so as to ensure that the longest wires used do not exceed a given maximum resistance, by either sorting wire lengths and allowing acceptably “short” wires to use dense circuit lines, or by providing short lengths of wire lines in those areas where necessary and switching to less dense lower resistance wire lines where possible. Statistics for large printed circuit boards indicate that over fifty percent of the wired networks on a typical printed circuit board have total wire lengths that are one-half as long as the longest wires. Often, fifty percent of the wired networks on a high performance board are only one-third the length of the longest wired networks. 
     Referring now to the drawings and beginning with FIG. 1, a cross-sectional view of a related art multilayered printed board structure (PCB)  100  is shown. The multilayer printed circuit board structure  100  is comprised of a plurality of wire cores or wire layers  120 , which are stacked on top of each other in a dielectric medium, and separated by reference plane  140 , to form the multilayered printed board structure  100 . Each wire layer  120  contains a plurality of individual wires  130 ,  150 . In the conventional multilayered printed board structure  100 , each wire  130 ,  150  in each wire layer  120  has approximately the same geometry, and therefore, approximately the same inherent electrical characteristics (i.e., resistance, capacitance, impedance, etc.). The same geometry is employed for two reasons. First, to preserve the transmission line impedance of the wires  130 ,  150 . Second, to meet a specified maximum DC resistance based upon the length of the longest interconnect wire on the multilayered printed board structure  100 . Thus, the conventional multilayered printed board structure  100  yields a high performance interconnect environment, but one which lacks dense circuitization due to the relatively wide (i.e., &gt;75 um, typically) wires  130 ,  150 . 
     This lack of dense circuitization results in a need for more layers of wire cores  120  in order to accomplish the required wire interconnects, which in turn produces thicker printed circuit board structures. Interconnections between the wires in individual wire cores  120  are accomplished with a device such as a plated through hole via  110 . However, the plated through hole via  110  has the drawback of an inherent capacitance. This capacitance has detrimental effects on propagated signals, caused by the transmission-line-related performance characteristics of the wires  130 ,  150 , especially in high performance circuit applications. Therefore, PCB structures which require additional capacitance-producing plated through hole vias to interconnect an increased number of wiring layers are not desirable. 
     The deterioration of a propagated signal is expressed in terms of either the “rise-time” or the “fall-time” of the signal&#39;s response to a step change. The deterioration of the propagated signal can be quantified with the formula (Z 0  * C)/2, where Z 0  is the transmission line characteristic impedance, and C is the amount of the via capacitance. In a wire having a typical 50 ohm transmission line impedance, a plated through hole via having a capacitance of 4 pico farad (pf) would represent a 100 pico-second (ps) rise-time (or fall time) degradation, as compared to a 12.5 ps degradation with a 0.5 pf buried via of the present invention, as discussed below. This difference is significant in systems operating at 800 MHz or faster, where there are associated signal transition rates of 200 ps or faster. 
     The present invention discloses two approaches to alleviating the deterioration of the propagated signal, which may be used singly or in combination. The first approach is to use circuit wires for shorter network interconnections, wherein these wires are characterized by having a higher resistance and a controlled transmission line characteristic impedance (Z 0 ). This first approach is illustrated in FIG.  2 . The second approach is to use densely spaced wires in areas of the PCB where components are mounted, and to convert these densely spaced wires to less densely spaced, low resistance wires (where space allows) with one or more buried vias. The second approach is illustrated in FIG.  3 . 
     Referring now to FIG. 2, a multilayered printed circuit board structure  200  is shown. The multilayered printed circuit board structure  200  is composed of a plurality of wire cores or wire layers  220 ,  260 ,  270 , separated by a reference planes  240 . The wire layers  220 ,  260 ,  270  contain wires  230 ,  250 ,  280  of various dimensions. 
     More specifically, there are two coarse wiring layers  220 , each of which contains closely spaced wires  230  having a relatively large cross-sectional area. Next, intermediate wire layers  260  are formed on the uppermost coarse wiring layer  220 . These intermediate wire layers  260  contain intermediate-sized wires  250 . Since these intermediate-sized wires are of smaller cross-sectional area than the coarse wires  230 , more of these intermediate-sized wires  250  can be accommodated in wire layer  260  than if coarse wires were used. And, wire layer  260  can be thinner than the corresponding coarse wire layer  220 . 
     Finally, dense wire layers  270  are formed on the uppermost intermediate wire layer  260 . Each dense wire layer  270  contains a plurality of closely spaced dense wires  280 , each of which has a relatively small cross-sectional area (i.e., typically about 1*10 −9  m 2 ). Because each dense wire  280  has such a small cross-sectional area, and they are closer to the reference plane, they can be placed in close proximity to each other, and dense circuitization results. The dense wires  280  are contained completely within the dense wire layer  270 , which facilitates complete testing of the dense wire layer  270  prior to assembly of the full printed circuit board. This process provides for a printed circuit board assembly yield that is not influenced by the anticipated lower yield of the dense wire layer  270 . 
     Electrical interconnections between dense wires  280  in different dense wire layers  270  are accomplished by constructing a buried via  290  using techniques known in the art. To facilitate access to individual dense wires  280  in the dense wire layer  270  can be accomplished by forming a blind via  285  in the capping layer  275 . The blind via  285  may be formed using laser ablation techniques, or other techniques known in the art, such as photo-imageable processes, plasma etching, or controlled-depth mechanical drilling. Blind via  285  may also extend through one or more of the wire layers, thus permitting access to other wires on other wire layers. 
     Referring now to FIG. 3, the second approach is illustrated which uses circuit wires for shorter network interconnections, wherein these wires are characterized by having a higher resistance and controlled transmission line characteristic impedance (Z 0 ). 
     The multilayered printed circuit board structure  300  is again composed of a plurality of wire cores or wire layers  330 ,  331 ,  375  separated by reference planes  305 ,  310 , and mounted on a substrate layer  305 . The wire layers  330 ,  331 ,  375  contain wires  340 ,  350  of various dimensions. 
     More specifically, there are coarse wiring layers  330 , each of which contains coarse wires  340  having a relatively large cross-sectional area. Next, fine wire layers  375  are formed on the coarse wiring layer  331 . These fine wire layers  375  contain a plurality of fine wires  350 . Since these fine wires  350  are of smaller cross-sectional area than the coarse wires  340 ,  331 ,  341 , more of the fine wires  350  can be accommodated in wire layers  355  and  375  than if coarse wires were used. And, wire layer  375  can be thinner than the corresponding coarse wire layer  330 . 
     A buried via  370  can also be used to convert from a fine wire  351  in a component-dense area to a low resistance coarse wire  341  in a different coarse wire layer. The buried via  370  is formed using techniques known in the art. Access to individual fine wires  355  in the dense wire layer  375  can be accomplished by forming a blind via  360  in the capping layer  385 . The blind via  360  may also extend through one or more of the wire layers, permitting access to other wires as necessary. Blind via  360  may be a laser-ablated via, or may be formed using other techniques known in the art. 
     While embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.