Abstract:
An optimal microelectronic semiconductor device mount area on a printed circuit board is provided. A novel mount area includes a plurality of collinear arrangements of attach pads and collinear arrangements of vias so that, at a minimum, at least one signal trace may be routed directly through the mount area. Additionally, capacitors may be coupled directly within the mount area on a bottom surface of the printed circuit board. The mount area includes a plurality of collinear arrangements of attach pads and a plurality of collinear arrangements of vias. Each of the collinear arrangements of attach pads are preferably separated from the nearest adjacent collinear arrangements of attach pads by an equivalent distance. A plurality of collinear arrangements of vias are separated from adjacent collinear arrangements of vias by a first distance. At least two mutually adjacent collinear arrangements of vias define a trace routing channel through the mount area.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention relates to assembly methods of printed circuit boards and to printed circuit boards obtained thereby and, in particular, to optimal via and microelectronic semiconductor device attach pad configurations providing improved signal trace routings on a printed circuit board.  
         BACKGROUND OF THE INVENTION  
         [0002]    Printed Circuit Board (PCB) manufacturers are under continual pressure to provide reduction in board sizes. Production of reduced size PCBs is often complicated by the concurrent market demand for increasingly sophisticated electronic devices and the corresponding requisite density of circuitry and semiconductor devices for providing increased levels of functionality for a PCB.  
           [0003]    While the seemingly conflicting demands for reduced PCB size and increased PCB capabilities are partially addressed by improvements in miniaturization made by the semiconductor industry, PCB manufacturers nevertheless find demand for reduced PCB size and consequential increases in PCB component densities. Furthermore, improvements in semiconductor fabrication techniques resulting in improved miniaturization often complicate PCB manufacture. As I/O signal densities of semiconductor devices increase, it becomes more technologically challenging to accommodate these devices and the associated signal traces on smaller PCBs that are required by today&#39;s electronic devices.  
           [0004]    Numerous integrated circuit packaging schemes have been developed over the years for addressing I/O and package sizing requirements. Dual in-line packages (DIP) are still common although DIPs are inefficient from a miniaturization standpoint. A DIP package typically features a ceramic body having leads along two opposing edges. Generally, DIPs are utilized for low complexity devices where minimization of the surface mount area is not critical.  
           [0005]    Pin grid arrays (PGAs) were developed for addressing rising I/O requirements of semiconductor chips and are generally implemented in larger chips with more leads than found in DIPs. Pin grid arrays feature connections, in the form of pins, covering the entire bottom surface area of the chip as opposed to peripheral leads found on most chips.  
           [0006]    Ball grid arrays (BGAs) are generally configured much like a PGA. Rather than pins, BGAs feature solder balls on the package bottom for mounting with a PCB. By utilizing the entire chip surface for electrical connections, the chip package surface area, as well as the weight, are significantly reduced compared to chips featuring only chip peripheral leads.  
           [0007]    A consequence of increased I/O signal densities associated with chips such as BGAs is an increase of trace population and trace density on the PCB. In order to minimize the PCB profile, or surface area, more narrow trace lines or additional PCB layers are often required. The ability to minimize trace line widths is limited. Adding additional PCB layers is, therefore, often the only practical solution. Because the number of layers of a PCB generally impacts the PCB manufacturing cost more than gains from reducing the surface area, it is clearly advantageous to minimize the number of PCB layers by effectively utilizing the surface area of a PCB by providing efficient, dense trace routing.  
           [0008]    One of the more common and frustrating problems encountered when designing trace routings for PCBs is finding routing channels that pass through BGA mount areas. Redesign of the trace routings is often required. Barring available PCB area, the addition of PCB layers may be required. When board space is available on another existing PCB layer, additional routing vias may be added to the PCB to utilize this space. However, the routing vias themselves consume valuable routing space.  
           [0009]    Therefore, it may be seen from the foregoing that a solution for routing trace lines on a PCB densely populated with chips such as BGAs and PGAs while minimizing the need for routing vias is desired.  
         SUMMARY OF THE INVENTION  
         [0010]    In an embodiment of the present invention, an optimal microelectronic semiconductor device mount area on a printed circuit board is provided. The optimal mount area includes a plurality of collinear arrangements of attach pads and collinear arrangements of vias so that, at a minimum, at least one signal trace may be routed directly through the mount area. Additionally, capacitors may be coupled directly within the mount area on a bottom surface of the printed circuit board. Each of the collinear arrangements of attach pads are preferably separated from the nearest adjacent collinear arrangements of attach pads by an equivalent distance. A plurality of collinear arrangements of vias are separated from adjacent collinear arrangements of vias by a first distance. At least two mutually adjacent collinear arrangements of vias have a separating distance approximately equal to two times the first distance, thereby defining a trace routing channel through the mount area. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    A more complete understanding of the method and apparatus of the present invention may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein:  
         [0012]    [0012]FIGS. 1A and 1B are, respectively, an upper and lower view of a conventional attach pad and via configuration associated with a BGA mount area of a printed circuit board;  
         [0013]    [0013]FIGS. 2A and 2B are, respectively, an upper and lower view of an attach pad and via configuration associated with a BGA mount area of a printed circuit board according to an embodiment of the present invention;  
         [0014]    [0014]FIGS. 3A and 3B are, respectively, an upper and lower view of an attach pad and via configuration associated with a BGA mount area of a printed circuit board according to an alternative embodiment of the present invention;  
         [0015]    [0015]FIGS. 4A and 4B are, respectively, an upper and lower view of a printed circuit board incorporating a BGA mount area according to the teachings of the present invention; and  
         [0016]    [0016]FIG. 5 is an upper view of a printed circuit board including a plurality of BGA mount areas according to the teachings of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    The preferred embodiment of the present invention and its advantages are best understood by referring to FIGS.  1 - 5  of the drawings, like numerals being used for like and corresponding parts of the various drawings.  
         [0018]    As is known in the art, vias provide electrical connections from one layer of a printed circuit board to another. Vias can be created by a number of processes including mechanical drilling, laser drilling and dry etching techniques. A via is simply a hole through one or more layers of the printed circuit board and is typically plated to assure good electrical conduction between layers. Vias may pass through only a single layer of the PCB, for example in a two-layered board, or they may pass through multiple layers when signal routing is required between non-adjacent PCB layers. Other vias may pass completely through a PCB, for example to provide routing for adjacent source voltage pads or ground pads, and are commonly referred to as clearance vias. By using clearance vias, a single source voltage pad can be used to provide a voltage source for all layers of a multi-layered PCB.  
         [0019]    Routing vias are used to provide signaling between PCB elements when providing a direct signal trace is either not possible or is logistically unattractive. The use of routing vias typically results from trace densities so large that accommodation of additional traces is not possible. Areas proximate high input/output semiconductor chips typically have large trace densities. Consequently, these areas often require routing vias.  
         [0020]    Ball grid arrays, and similar microelectronic semiconductor devices, often require a substantial number of vias due to the large, local trace density associated with the BGA mounting area of the PCB. While efforts continue to accommodate larger trace densities by shrinking trace dimensions, there is inarguably a limit to miniaturization of the trace itself. Furthermore, semiconductor chip miniaturization continues to outpace advances in reductions to trace dimensions, a scenario further complicating the task of PCB manufacturers.  
         [0021]    An exemplary attach pad and via configuration associated with a BGA mount area  100  of a PCB is illustrated in FIGS. 1A and 1B. This configuration includes rows  50 - 57  of attach pads, for example attach pads  57   a - 57   h  of attach pad row  57 , adjacent to rows  60 - 67  of offset vias, for example vias  67   a - 67   h  of via row  67 , forming respective attach pad and via columns. The mount area  100  is thereby formed of a plurality of collinear arrangements of attach pads and a plurality of collinear arrangements of vias, each collinear arrangement respetively including a plurality of attach pads and vias. Each collinear arrangement of attach pads has adjacent thereto at least one collinear arrangement of a plurality of vias. Thus, in this typical arrangement, the PCB is designed with a via placed on the board for each attach pad of the BGA. The channel diameter is typically minimized to provide the lowest BGA profile, or footprint, attainable. Generally, each attach pad of a collinear arrangement, for example each of the attach pads of row  57 , are equally spaced from adjacent attach pads in the same row, that is the distance d 1  is typically equivalent to the distance d 2 . Furthermore, each row of attach pads generally share a common attach pad separating distance d 1 . Each collinear arrangement of vias are generally spaced an equivalent distance from adjacent collinear arrangements of attach pads. For example, via row  67  is separated from attach pad row  57  by a distance d 3 . The distance d 4  separating via row  66  from attach pad row  56  is typically equivalent to d 3 .  
         [0022]    One of the more frustrating problems encountered during PCB design is the placement of a PCB mount area  100  in the path of a needed signal trace. For example, on a board having already limited space for laying signal traces, the requirement for mounting a semiconductor chip may obstruct desired trace routes. The physical placement of the chip on the PCB may often be critical. Moving a chip away from an already trace dense area proximate a data bus, for example, may cause undesirable system performance degradations. It is the PCB designer&#39;s task to find alternative trace routes or, when no alternative routes are available, to drop a routing via on the PCB. In the latter event, if the trace route terminates on the same PCB layer on which it originates, a second routing via is then required to return the trace to the originating layer. On PCBs having large trace populations and large numbers of chips mounted on the PCB, routing space becomes even more critical. It is clearly advantageous to minimize the number of routing vias because each routing via reduces the available routing area by the dimensions of the via.  
         [0023]    The present invention provides a technique for minimizing, or even eliminating in some cases, the need for routing vias used for averting BGA mount areas during trace routing. In FIGS. 2A and 2B, there are respectively illustrated a top and bottom view of a BGA mount area  200  having a novel attach pad and via configuration according to an embodiment of the present invention. Rows  150 - 157  of attach pads have offset rows of vias  160 - 167  respectively adjacent thereto. By offsetting rows of vias adjacent to each row of attach pads, each attach pad, for example attach pad  153   a,  has a via, for example via  163   a,  associated therewith for routing signals from each of the attach pads included in the BGA mount area  200 .  
         [0024]    As mentioned hereinabove, the high trace densities associated with BGA mount area  200 , and mount areas for other similar high I/O signal devices, generally preclude the possibility for routing a trace through the mount area. This results from miniaturization trends in semiconductor device manufacturing techniques to shrink the device packages as much as possible. The resulting compact packages have leads densely packed together. The channel width between leads is generally large enough to accommodate a single trace—typically used by the trace from the attach pad to the associated via.  
         [0025]    Heretofore, a routing via, through which a signal trace is passed to another PCB layer, is typically placed in close proximity to the mount area  200  when a route to a desired termination point is obstructed by the BGA mount area  200 . The present invention overcomes the need for placement of routing vias in these circumstances. As depicted in FIG. 2A, the mount area  200  is divided into multiple quadrants having channels between any two adjacent quadrants. In this illustrative example, four via and attach pad quadrants  115 - 118  (illustratively encompassed with dashed lines) are formed by a novel arrangement of the vias in relation to the associated attach pads. Two perpendicularly oriented channels, one generally horizontal channel  130  and one generally vertical channel  135 , result from the formation of the via and attach pad quadrants  115 - 118 . In the present example, the quadrants are defined, in effect, by removing the innermost column and row of vias from the traditional attach pad and via configuration and respectively replacing them along the most peripheral row and column of attach pads. Constructed in this manner, channels  130  and  135  can be used for trace routing directly through the BGA mount area.  
         [0026]    Notably, the attach pad rows  150 - 157  as oriented in the mount area  200  of the present invention (FIG. 2A) are identically arranged as the attach pad rows  50 - 57  (FIG. 1A) of the prior art mount area  100 . Therefore, no modification to the semiconductor ball-grid arrays is required to make use of the present invention.  
         [0027]    Two distinct advantages are provided by the present invention. With reference again to FIG. 2A, channels  130  and  135  may be used for routing traces on the upper side of the PCB directly through the mount area  200 , thereby avoiding the use of a routing via when a BGA, for example, obstructs a desired trace route. Furthermore, on the underside of the PCB the widths of the channels  130  and  135  are larger (as can be seen in FIG. 2B) than the channel widths on the upper side. This results from the underside channel widths being defined by the distance between vias, whereas on the upperside the channel widths are defined by the distance between adjacent attach pads. Because the channel is created by eliminating an intermediate collinear arrangement of vias, for example a row or column of vias, the channel width on the upperside of the PCB will be equivalent to the distance between two adjacent collinear arrangements of attach pads whereas on the bottom side of the PCB, the channel width will be equal to approximately twice the distance between two adjacent via rows (or columns) in conventional PCBs. More specifically, the width of channel  130  on the upperside is equal to the distance between any two attach pads adjacent to channel  130  and located in a common attach pad column, for example attach pads  153   a  and  154   a  respectively contained within quadrants  116  and  117  and attach pad column  170 . The width of channel  135  on the upperside of the PCB is likewise determined from the distance between any two attach pads adjacent to channel  135  and located in a common attach pad row, for example attach pads  154   d  and  154   e  respectively contained within quadrants  117  and  118  and attach pad row  154 . It is understood that microelectronic semiconductor devices generally are manufactured with uniform lead spacings and channels  130  and  135  would, consequently, have equivalent widths. However, the present invention would be equally applicable for designing mount areas for non-uniform arrays of chip leads.  
         [0028]    Because the attach pads only appear on one surface of the PCB, the widths of the channels may be greater on the PCB surface on which the vias terminate. As illustrated in FIG. 2B, the widths of channels  130  and  135  are defined by the spacing of vias adjacent to the respective channels and located in quadrants respectively adjacent to the channel. For example, the width of channel  130  on the underside of the PCB is equal to the distance between vias  163   e  and  164   e  respectively contained within quadrants  115  and  118  and attach pad column  184 . The width of channel  135  is similarly obtained and will be equivalent to the width of channel  130  if the mount area  200  is designed for a typical semiconductor device having a uniform lead configuration. While the present illustrative example has shown a mount area  200  designed for an 8×8 lead array semiconductor device, it should be clear that the present invention is equally applicable for PCB mount areas designed to accommodate semiconductor devices having more or less leads. Many BGAs today have hundreds of I/O leads. The present example describes an 8×8 array for simplification of discussion only.  
         [0029]    Numerous arrangements of the attach pad and via configuration as taught by the present invention are possible. In FIGS. 3A and 3B, there are respectively illustrated an upper and lower view of a PCB mount area  300  for mounting a BGA and incorporating the teachings of the present invention. The mount area  300  is divided into two quadrants  215  and  216  (illustratively encompassed with dashed lines) of attach pads and associated vias with a single channel therebetween. The embodiment illustrated is designed to accommodate a semiconductor chip having an 8×8 array of leads. Accordingly, the mount area  300  would be suitable for mounting a chip with an equivalent lead configuration as the previously discussed embodiment, with reference to FIG. 2, and the prior art mount area configuration, as discussed with reference to FIG. 1.  
         [0030]    With reference now to FIGS. 4A and 4B, there are respectively illustrated top and bottom views of a printed circuit board  350  having a semiconductor device mount area  400  incorporating the teachings of the present invention. Semiconductor device mount area  400  includes a novel arrangement of rows of attach pads, for example attach pad row  360 , and rows of vias, for example via row  370  (and consequently columns of both attach pads and vias). Two channels, one horizontally oriented channel  330  and one vertically oriented channel  335 , result from the formation of the via and attach pad quadrants  316  and  317 . In the present example, the quadrants are defined, in effect, by omitting an intermediate row and column of vias from a traditional attach pad and via configuration and respectively replacing them along the peripheral row and column of attach pads. Channels  330  and  335  are thereby formed at the respective positions where the via row and column were omitted. These channels can then be used for routing a trace, or multiple traces, directly through the semiconductor mount area  400 . Furthermore, on the bottom side of the PCB  350  (FIG. 4B), the channel  330  and  335  dimensions are large enough to accommodate mounting capacitors directly therein. Ground and power vias can advantageously be located adjacent to channels  330  and  335 . Decoupling capacitors can then be mounted directly in the channels on the bottom side of the PCB  350  in close proximity to the ground and power vias. Consequently, power decoupling can be performed directly at the component pins.  
         [0031]    In FIG. 5, there is illustrated a printed circuit board  450  having multiple semiconductor device mount areas. Semiconductor mount area  410  is a four quadrant mount area having two channels similar to those discussed with reference to FIGS. 2 and 4. Semiconductor mount areas  420  and  430  incorporate attach pad and via arrangements embodying the principles taught by the present invention as described hereinabove but designed for semiconductor device mounts having alternative lead arrangements. For example, mount area  420  comprises four quadrants  421 - 424  that are placed internally to four surrounding quadrants  425 - 428 . The attach pad and via arrangements depicted, as well as those aforedescribed, are illustrative only. It should be understood that any number of arrangements are possible and the invention is not intended to be limited to those described herein.  
         [0032]    Although one or more embodiments of the method and apparatus of the present invention has been illustrated in the accompanying drawings and described above, it will be understood that the invention is not limited to the embodiment disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.