Abstract:
The density of plated thru holes in a glass fiber based chip carrier is increased by off-setting holes to positions in which fibers from adjacent holes will not connect. Elongated strip zones or regions having a width approximately the diameter of the holes and running along orthogonal columns and rows of holes, parallel to the direction of fibers, define regions of fibers that can possibly cause shorting between holes. Rotating a conventional X-Y grid pattern of equidistant holes so as to position, for example, alternate holes in one direction between the elongated strip zones running in the opposite direction significantly increases the distance between holes along the elongated strip zones running in each direction. The holes are positioned between elongated strip zones with sufficient clearance to compensate for variations in the linear path of fibers.

Description:
BACKGROUND OF THE INVENTION  
       [0001]      1 . Field of the Invention  
         [0002]     The present invention relates to chip carriers and, more particularly, to high density chip carriers with high wireability for use with flip chip technology, and the like.  
         [0003]     2. Background and Related Art  
         [0004]     As the terminal density of semiconductor chips and, particularly, the density of Input/Output (I/O) connections of chips increases with improved technology, the wireability of chip carriers becomes more problematic. The density of terminals tightly clustered makes it difficult to construct mutually segregated conductors to connect carrier lines to each terminal. Signal carrying terminals and lines are particularly burdensome since they must be segregated from each other as well as from power and ground lines. Signal lines on the chip carrier must have sufficient electrical isolation from other conductors so that undesired coupling and leakage paths are avoided.  
         [0005]     To enable routing in highly dense chip carriers, microvia, as well as other technologies, have been developed. Microvia chip carriers typically use multiple layers to make the required interconnections, particularly in chip packages using flip-chip ball grid array (BGA) technology. In these high pin count technologies, the density of wiring and the wireability of the layers is important, particularly in terms of cost, yield, performance and reliability. “Wireability”, in this regard, can be viewed as the technical possibility of positioning routing lines so that all signals may “escape” (inward or outward) from a given pattern or layer. Constraint considerations for routing include via density, routing line widths and clearances, the terminal sizes and required clearances, the shielding requirements and other design constraints known in the art.  
         [0006]     Microvia chip carrier substrates are generally built around a core with plated thru holes (PTHs). Such high density interconnect (HDI) chip carriers use build-up of layers on each side of a core made of epoxy-glass layers. The glass layers are made of a glass cloth impregnated with epoxy and are laminated at elevated temperatures to make a solid, dimentially stable core. The build up layers on each side of the core are generally non-reinforced epoxy. U.S. Pat. No. 6,518,516B2 describes a typical microvia chip carrier.  
         [0007]     The density constraints of the PTH&#39;s in the core limits the vertical interconnection capability between the front and back of the carrier. For example, high density PTH arrangements can result in reliability failures along glass fibers, from one hole to another, when the holes are placed too close together. The inability to interconnect a large number of signals from the front and back of a chip carrier because of PTH density constraints caused by reliability problems when PTHs are placed too close together, limits the ability to connect higher I/O count chips to a chip carrier or to interconnect such chips to a printed circuit board (PCB).  
         [0008]     Shorting between PTHs in glass reinforced epoxy carriers has been attributed to the fact that the epoxy bond to glass fibers is fairly poor. When PTHs are drilled in the core and plating chemicals are used to plate the PTHs, the poor bonding of epoxy to the glass fibers allows the plating chemicals to penetrate some distance along the fibers. This penetration can result in electrical shorting between holes.  
       SUMMARY OF THE PRESENT INVENTION  
       [0009]     In accordance with the present invention, increased PTH density is achieved in fiber based chip carriers without the risk of fiber induced shorting by off-setting PTHs in a hole jog pattern. Fiber based chip carrier substrates typically arrange the fibers in a matrix pattern where the fibers are woven orthogonal to one another in an X-Y direction. By offsetting alternate rows of PTHs, the distance between holes along the same strands of fibers is substantially increased. Such an arrangement allows for increasing PTH density without decreasing the PTH to PTH spacing along the same strands of fibers. Jog patterns can be obtained by rotating a conventional X-Y grid pattern of PTHs between approximately 15° and 60°, depending upon such factors as the spacing between PTHs, the diameter of PTHs and the desired separation between rows and columns of PTHs.  
         [0010]     In one arrangement, X—rows of PTHs are rotated by about 30° to move alternate Y—row vias between Y—rows, thereby more than doubling straight pitch (in-line pitch) in both the X and Y direction, depending upon the drill bit size used for the PTH. Such rotation provides a good comprise between competing parameters, such as drill bit size diameter and the spacing between holes, both in a straight line and in lateral separation.  
         [0011]     Accordingly, it is an object of the present invention to provide an improved chip carrier.  
         [0012]     It is a further object of the present invention to provide a chip carrier with improved wireability.  
         [0013]     It is yet a further object of the present invention to provide a chip carrier with increased PTH density.  
         [0014]     It is yet still a further object of the present invention to provide an improved method and layout arrangement for positioning PTHs in a multilayer chip carrier using fiber based technology.  
         [0015]     It is another object of the present invention to provide a high density PTH layout method and arrangement for use in epoxy impregnated glass fiber multilayered chip carriers.  
         [0016]     It is still another object of the present invention to provide high density PTH fiber-based chip carriers without decreasing PTH to PTH space in the X-Y direction. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0017]      FIG. 1  shows a top view of a typical microvia X-Y matrix or grid arrangement of holes used to form PTHs in chip carriers.  
         [0018]      FIG. 2  shows a top view of an arrangement of holes, in accordance with the present invention, formed by rotating the matrix or grid arrangement of  FIG. 1  with respect to the X-Y plane to offset the holes.  
         [0019]      FIG. 3  depicts the cross-section layers of a typical eight layer chip carrier.  
         [0020]      FIG. 4  shows one arrangement in which the holes of a rotated signal layer may be connected to a non-rotated array of holes.  
         [0021]      FIG. 5  shows another arrangement in which the holes of a rotated signal layer may be connected to a non-rotated array of holes. 
     
    
     DETAILED DESCRIPTION  
       [0022]     With reference to  FIG. 1 , there is shown a pattern of circles or holes  1  each representing a typical PTH with drill bit diameter Dim. Typically, the PTHs are plated to form a PTH grid arrangement. As shown, the holes  1  are arranged in an X-Y grid arrangement. In such arrangement, the X and Y lines of PTHs  1  are aligned with the direction of the glass fibers in the layers of the multilayer chip carrier. The fibers in the layers of such structure are typically woven m a mesh-like pattern with the fibers running parallel to one another and in both the X and Y direction generally intersecting orthogonally or at approximately at 90°. The actual pitch Xμm of holes is shown for both the X and Y directions. As used herein “orthogonal” means 90° or some minor variation one way or another.  
         [0023]     The PTHs  1  in  FIG. 1  would typically have a diameter of 150 μm and would be 300 μm or higher apart in both the X and Y direction. This results in a 450 μm or higher pitch which gives a density of about 5 holes per square mm. This limits the density of HDI chip carrier substrates since most of the wiring is on the top side of the substrate (close to the chip) and only a limited number of signals can be routed to the bottom side of the carrier in the region under the chip. It is clear that this becomes more of a problem as chip signal density increases. A typical high signal count density under a chip is 2000 signals in a 10 mm chip or 20 signals per square mm. Since, typically, a maximum of 5 holes per square mm can be routed to the bottom half of the chip carrier through the core, approximately 75% of the wiring, then, must be contained on the top side. This increases the number of layers needed for HDI chip carriers which increases cost and also lengthens the path between the chip and bottom of the chip carrier, thereby affecting performance. In accordance with the present invention, a solution to the above problem is provided in the form of a hole layout pattern that allows the holes to be placed closer to each other while at the same time avoiding shorting between holes.  
         [0024]      FIG. 2  shows a top view one arrangement of a PTH off-set layout pattern for a chip carrier, in accordance with the present invention. The layout uses nine PTHs to aid in describing the off-set pattern, but it is clear that such pattern would be repeated many times over with the same spacing being repeated along the same lines in both the X and Y directions. The same actual pitch Xμm as shown in  FIG. 1  is used. The glass fibers in  FIG. 2  run in the X and Y directions, as in  FIG. 1 , and thus along X and Y elongated strip zones or regions  3 ,  5 ,  7  and  9 . These strip zones or regions define the regions in the chip carrier where glass fibers could interconnect holes. In this regard, the diameter of the fibers may be many times smaller than the width of the zone.  
         [0025]     Although the arrangement of  FIG. 2  shows a rotation of 26.6° about the middle hole If of the bottom row of holes to form the off-set pattern, it is clear that variations in the extent of this rotation may be made in accordance with the particular design choice and/or ground rules being employed. Rotations of 26.6° provides the same in-line spacing between holes in the X and Y directions. “In-line spacing” is the spacing between holes along the same X and Y lines, shown as bit-to-bit spacing in  FIG. 2 . Thus, the spacing between holes  1   a  and  1   b  along elongated strip zones or region  3  is the same as the spacing between holes  1   c  and  1   d  along elongated strip zone or region  5 . Similarly, the same spacing exists between holes along elongated strip zones or regions  7  and  9 . Although the strip zones or regions  3 ,  5 ,  7  and  9  represent zones or regions within which glass fibers in the X and Y directions could provide a shorting path between holes, fiber paths may have some slight linear variation requiring some separation between these strips and adjacent holes.  
         [0026]     It is clear that any rotation that is about 30° will give substantially the same spacing between holes in the X and Y directions. It can be seen that the purpose of rotating the pattern of holes it to rotate aligned adjacent holes in the X and Y direction to an off-line or off-set position to thereby increase the distance between in-line holes. Thus, holes  1   c ,  1   e  and  1   f , which would otherwise be aligned along strip region  9 , are off-set from one another thereby substantially increasing the distance between new in-line holes  1   c  and  1   d . Thus, the bit-to-bit spacing along a line of holes, as shown in  FIG. 2 , is increased from X minus D to the square root of 5 times X minus D, where X is the actual pitch shown in  FIG. 1  and D is the PTH drill bit diameter. Similarly, the straight pitch (or straight line pitch) is increased from X to the square root of 5 times X.  
         [0027]     As further shown in  FIG. 2 , the glass cross gap distance for this arrangement is the square root of 5 over 5 times X minus D. The glass cross gap represents the distance between elongated strip zones and, as hereinabove indicated, the strip zones define linear regions within which fibers could potentially interconnect holes. The glass cross gap distance provides a margin of safety against some linear irregularity in fiber paths that might otherwise provide opportunity for shorting between holes.  
         [0028]     In the arrangement of  FIG. 2 , the angle of rotation used provides an effective compromise among the possible angles of rotation. Although different applications may allow rotations of between 15° to 60°, the angle shown not only provides the same distance between holes in the X and Y direction, but it also provides the same glass cross gap distance in the X and Y directions. In addition, “same pitch”defined by the square root of 5 over 5 times X, exists uniformly between adjacent lines of holes, as rotated, in both the X and Y direction. Thus, the same pitch exists between holes  1   c  and  1   g  as exists between holes  1   c  and  1   e  in the X direction. The same pitch is present along adjacent holes in the Y direction. The following table shows some typical examples of how the variables shown in  FIG. 2  interrelate. All values are in μm and are rounded.  
                                                                         Drill                   Actual Pitch   Bit   Straight Pitch   Bit to Bit Pitch   Glass Cross Cap       X   D   P   SP   GG                                212   50   470   424   45       212   90   470   384   5       225   50   503   453   51       225   100   503   404   0       250   50   559   509   62       250   100   559   459   12       300   50   670   620   84       300   100   670   570   34                  
 
         [0029]     The above table shows how the glass cross gap distance, in particular, varies with several examples of actual pitch value X, in μm, for drill bit sizes generally between 50 and 100 μm. As can be seen, the glass cross gap distance for a drill bit diameter of 100 μm and actual pitch value of 212 μm would go negative, meaning there would be overlap between adjacent strips or zones of fibers. Similarly, other pitch values have points at which a drill bit size will cause the glass cross gap distance to go negative. Accordingly, the cross gap distance can be adjusted to meet the conditions of the particular application employed.  
         [0030]     It should be understood that FIGS.  2  and above table set forth a specific example of how the spacing between holes along a line or region of fibers may be increased. It is clear that by rotating the grid arrangement off axis, from the direction of strands of fiber, the spacing is increased by both the diagonal dimension and the potential for skipping holes along the same zone of fiber strands. Since the zones of fiber strands are defined by the diameter of the holes, smaller diameter holes increase the potential of skipping holes alone these zones of fiber strands. The glass or fiber cross gap separation, however, must also be maintained to some degree, although configurations may be possible which would allow very minimal or negative cross gap for holes that are sufficiently distant from one another in the direction of fibers. It should also be understood that placement of holes in the direction of fibers does not necessarily require that the holes be exactly aligned as long as appropriate cross gap separation is maintained.  
         [0031]      FIG. 3  shows a cross-sectional view of a typical  8  layer chip carrier  10  used, for example, for carrying flip chips, such as, C4 chips, in a BGA chip carrier arrangement. Layer  11  attaches to chip  12  through solder ball connections  14  in a manner well known to those skilled in the art. It is clear that although one chip is shown for purposes of illustration, that more than one chip may be attached to chip carrier  10 . It is also clear that layer  11  could also attach to other electrical components. As is also well known to those skilled in the art, the chip(s) act to provide a signal processing arrangement. Layer  13  acts as the build up layer on the chip side of the core. Signal layers  15  and  21 , along with Voltage/Ground Layers  17  and  19 , comprise the core. Layer  25  is the BGA layer which attaches, through solder bumps  27 , to printed wiring board (PWB)  29  to transmit signals thereto from signal layer  23 . The PWB  29  may be any circuitized substrate. Signal layers  13  and  15  provide fan out of signals from the chip. In chip carrier arrangements like  FIG. 3 , the number of signal layers required may be determined by the net count between the chip to BGA connections and the chip solder balls to BGA bump pitches. It is understood that the various layers include dielectric material separating conductive layers.  
         [0032]     Thus, as 10 mm chips move toward providing up to 2000 signals, limitations on increased microvia density in the core due to fiber shorting would be necessarily required more signal layers. However, in accordance with the PTH layout pattern of the present invention, increased density of PTHs in the core layers  15 ,  17 ,  19  and  21  allows more signals to be vertically transmitted beneath the chip thereby limiting the number of total layers required. Thus, the hole layout pattern may begin on either signal layer  13  or  15 , and the holes made to extend through the core and end in either layer  21  or  23 .  
         [0033]      FIG. 4  shows one arrangement for connecting holes that have been rotated to holes that have not been rotated. This may be used, for example, where a pattern of rotated electrical contacts of a hole is required to interface with the non-rotated contacts. Thus, for example, the rotated hole contacts through layer  13  and the core could be connected to non-rotated contacts on layer  23 . Connections through metal lines  33 ,  35 ,  37  and  39  are made in the X—direction.  FIG. 5  shows a similar arrangement with somewhat different metal line connections  41  and  43 .  
         [0034]     Although reference has been made to PTHs in fiber based materials, such as glass fibers and woven glass fibers impregnated with epoxy, it is clear that the pattern layout of PTH electrical connectors through the chip carrier, in accordance with the present invention, may be employed with other technical approaches to connecting chips or other electrical components to substrates. Thus, for example, where conductive pins or other electrical contacts may be employed in a substrate, such as, a chip carrier reinforced with some form of strand or fiber having potential for shorting, the off-set pattern of the present invention may be employed to increase in-line spacing along the fiber between such pins or contacts to allow increased density. In this regard, the term “connection point”, as used herein, may comprise any of a variety of connection point technologies used to enable routing in highly dense integrated circuit packages, such as, microvia, blind via, burried via, staggered via, bond pad, and other similar technologies.  
         [0035]     Similarly, the substrate need not be a chip carrier per se but could be any fiber-based substrate material for carrying electronic components having conductors formed thereon or extending therein that have potential for shorting. Such substrate may be a single layer or multilayer substrate. Where conductive vias are employed in a multilayer substrate, the vias may extend through any one or all of the layers.  
         [0036]     It will be understood from the foregoing description that various modifications and changes may be made in the preferred embodiment of the present invention without departing from its true spirit. It is intended that this description is for purposes of illustration only and should not be construed in a limiting sense. The scope of this invention should be limited only by the language of the following claims.