Abstract:
An organic land grid array having multiple built up layers of metal sandwiching non-conductive layers, having a staggered pattern of degassing holes in the metal layers. The staggered pattern occurs in two substantially perpendicular directions. Traces between the metal layers have reduced impedance variation due to the degassing hole pattern.

Description:
FIELD 
     The present invention relates generally to computer board and chip packaging, and more specifically to organic land grid array (OLGA) design and manufacturing. 
     BACKGROUND 
     As input/output (I/O) speed and the total number of I/Os required for high performance semiconductor chips have increased dramatically, the need for increased numbers of interconnect lines with low line impedance variation in chip packages has increased as well. To address this need, manufacturers have used multi-layered packages where several layers of conductors are separated by layers of dielectric material. 
     In printed circuit board (PCB) and integrated circuit (IC) manufacture, often silicon dies are to be connected to a mother board. This connection of a die to a mother board is known as a package. The die may be flip mounted to a piece of substrate called an Organic Land Grid Array (OLGA). The OLGA is typically formed of a core of FR 4  material used commonly in the manufacture of printed circuit boards. 
     On two sides of the OLGA board are typically a series of built-up layers, formed from alternating layers of dielectric material and conductive material. Patterns may be built in the metal or conductive layer through various etching processes such as wet etching which are known in the art and will not be described further herein. Plated through holes called vias are used to make interconnects between various layers of metal. Using these layers and vias, several layers of interconnections may be built up. 
     In an OLGA packaging technology, input/output functions are accomplished using metal traces between the layers. These traces are typically grouped. Each trace has an impedance generated by its geometry and location on the OLGA. Due to the manufacturing technology and material requirements, OLGA packages require a number of degassing holes to be formed in the metal layers to allow for proper operation. Degassing holes allow gas to be evaporated so that bubbles do not form in the package. 
     Traces may be routed over or under the degassing holes, or around the degassing holes, or a combination thereof. Since the traces are not in the same location on the OLGA, the traces have an impedance variation, or mismatch. OLGA trace impedance variation arises from two separate origins, manufacture variation and design variation. Manufacture variation and design variation add statistically to yield overall impedance variation, or mismatch. 
     Manufacture variation arises from geometry variations of traces, including trace width, trace thickness, dielectric thickness, and variation of the dielectric constant of a dielectric. Design variation is introduced from package design. When traces are run in an OLGA, they have a routing direction and a fan-out direction. Traces must be routed from the die to the package. When the traces are routed, the direction of the trace is referred to as the routing direction. The fan-out direction is typically 45 degrees from the routing direction, either plus 45 degrees or minus 45 degrees. 
     A typical degassing hole pattern has a grid-like array of degassing holes aligned vertically between two layers, as is shown in FIG.  1 . In FIG. 1, the degassing holes  102  of the top and bottom layers are exactly aligned in the x and y directions. When traces such as trace  1  and trace  2  are used with a degassing hole alignment scheme as shown in FIG. 1, trace  1  has less metal from the conductive layers both above and below the trace than trace  2 . The difference in the amount of metal above and below traces  1  and  2  continues when the traces are run in the fan-out direction. The degassing hole pattern of FIG. 1 leads to design impedance variations alone being on the order of 20%. 
     Another degassing hole pattern shown in FIG. 2 has another grid-like array of degassing holes  202  staggered from the degassing holes  204  of the next layer. In FIG. 2, the degassing holes are staggered in the x direction between layers. The distance between degassing holes is known as pitch. The degassing holes of the layers in FIG. 2 are staggered by a half pitch in the x direction. When traces such as trace  3  and trace  4  are used with a degassing hole alignment scheme as shown in FIG. 2, trace  3  has less metal from the conductive layers both above and below the trace than trace  4 . The variation in the amount of metal above and below the traces is lowered in fan-out at a 45 degree angle from the x direction because of the staggering of the holes. Still, significant design impedance variations are present with the degassing hole pattern of FIG.  2 . 
     For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a degassing hole pattern that reduces design variation in an OLGA package. 
     SUMMARY 
     An OLGA package includes a pair of conductive layers, each layer having a number of degassing apertures therethrough, the apertures of the layers being staggered in both a first direction and a second direction, a non-conductive layer located between conductive layers, and a pair of metal traces between the pair of conductive layers, the traces having approximately the same impedance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a prior art degassing hole configuration; 
     FIG. 2 is a top view of another prior art degassing hole configuration; 
     FIG. 3 is an isometric view of a degassing hole configuration according to one embodiment of the invention; 
     FIG. 3A is a top view of the embodiment shown in FIG. 3; 
     FIG. 3B is a section view of the embodiment shown in FIG. 3A, taken along lines  3 B— 3 B thereof; 
     FIG. 4 is a block diagram of an integrated circuit according to one embodiment of the present invention; and 
     FIG. 5 is a perspective view of a package embodiment of the present invention. 
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following detailed description of the embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
     FIG. 3 shows a degassing hole configuration according to one embodiment  300  of the present invention. Embodiment  300  comprises an OLGA having at least two built up layers of metal  302  and  304  sandwiching a dielectric layer  306  (FIG.  3 B). Each of the metal layers  302  and  304  has therein a plurality of degassing apertures or holes  308  and  310 , respectively. When the metal layers  302  and  304  are aligned vertically, such that the layers are stacked, embodiment  300  has degassing holes  308  in layer  302  and degassing holes  310  in layer  304  which are staggered in both the x direction and the y direction, as referenced in FIG.  3 . While x and y are chosen, any coordinate system may be used without departing from the scope of the invention. 
     In one embodiment, shown also in FIG. 3A, the degassing holes are arranged in a lattice pattern, with the degassing holes in layers  302  and  304  forming a lattice pattern having a lattice constant in one direction, and a second lattice constant in another, substantially perpendicular, direction. For purposes of description, the lattice pattern is shown in FIG. 3 with a first lattice constant in the x direction, and a second lattice constant in the y direction. In this embodiment, the degassing hole lattice pattern has degassing holes alternating from the layer  302  to layer  304  in both the x direction and the y direction. In other words, in the degassing hole lattice pattern as viewed from above, that is looking down on the stacked layers  302  and  304 , has degassing holes  308  of layer  302  alternating with degassing holes  304  of layer  310  in both the x direction and the y direction. 
     Each degassing hole  308  or  310  has a position in x and y coordinates, as well as a z coordinate position. The z direction is best shown in FIG.  3 . The z coordinate of all degassing holes on each layer is the same. The z coordinate of all degassing holes  308  on layer  302  is different from that of degassing holes  310  on layer  304 , but the same for all degassing holes  308  on layer  302 . As may be seen from FIG. 3A, in embodiment  300 , each degassing hole is adjacent in the x direction and in the y direction to a degassing hole on a different layer and having a different z coordinate. The degassing holes alternate z coordinates from layer  302  to layer  304  in both the x direction and the y direction. Each degassing hole in embodiment  300  has a unique set of x, y, and z coordinates. 
     In one embodiment, the first lattice constant, that is the lattice constant in the x direction, has a pitch, or spacing  316 , of twice the diameter of the degassing holes  308  and  310 . In this embodiment, the second lattice constant, in the y direction, has a pitch  318  equal to the diameter of the degassing holes  308  and  310 . In another embodiment, the first lattice constant is 500 microns, and the second lattice constant is 250 microns. In this embodiment, the diameter of the degassing holes is 250 microns. Therefore, the minimum spacing between adjacent degassing holes in either of the layers  302  or  304  is about 500 microns. 
     Traces, as has been mentioned, are typically grouped. Traces  312  and  314  are shown in FIGS. 3 and 3B. It should be understood, however, that more than two traces may be grouped, and that such additional traces are within the scope of the invention. Each trace may run in a first, routing direction, and may be fanned out in a second, fan-out direction. Typically, the fan-out is accomplished at an angle of plus or minus 45 degrees from the routing direction. As has been mentioned above, different traces having different amounts of metal from the layers above and below the traces will have an impedance mismatch. With the staggered pitch configuration of degassing holes shown in the embodiment of FIG. 3, a signal trace such as trace  312  or  314  routed between the metal layers  302  and  304  will have roughly the same number of degassing holes from each layer in its trace path. Further, each signal trace, such as trace  312  or  314 , will also have an amount of metal above and below the trace that is more closely matched with the amount of metal above and below another trace routed in the same fashion. 
     In the trace configuration shown in FIG. 3, trace  312  and trace  314  are positioned to run in the worst possible trace lines, that is, to create a trace line pair having the largest impedance variation between the traces. As can be seen from the figure, trace  314  crosses the centers of degassing holes in each layer at approximately their centers when running in the routing direction. Trace  312 , on the other hand, crosses the edges of the degassing holes in each layer while running in the routing direction. In this worst case scenario, impedance variation due to design considerations may be significantly reduced over impedance variation due to design in different degassing hole configurations. 
     Provided that the main routing direction for traces in the configuration of FIG. 3 is substantially in the x direction, a trace routed in the x direction will have no restrictions as to its y coordinate. In other words, a trace routed in the x direction could freely be moved up or down in terms of its y coordinate position. If multi-signal lines are routed closely along the x direction, the degassing hole configuration will reduce the amount of wasted space. 
     The reductions in impedance due to design variations of the embodiment shown in FIG. 3 are even more pronounced in the fan-out direction, 45 degrees from the routing direction. This may be seen in FIG.  3 A. When traces are routed in the x direction and fan-out at 45 degrees from the x direction, the traces cross degassing holes both above and below the traces. No matter where the trace runs, if it is approximately 45 degrees, either positive or negative, from the routing direction, it will cross degassing holes. As can be seen from the Figure, each trace  312  and  314  crosses degassing holes in approximately the same number, and covering approximately the same degassing hole area. In other words, each trace  312  and  314  has much less of a variation of the amount of metal above and below the trace when running in the fan-out direction. This further reduces impedance due to design considerations. 
     Worst case impedance variation due to design factors is significantly reduced by the degassing hole configuration shown in embodiment  300 . The elimination of large variations in the amount of metal above and below traces reduces the impedance due to design factors. Traces may be routed with fewer concerns for exacting placement due to the degassing hole configuration of embodiment  300 . 
     FIG. 4 illustrates a block diagram of an integrated circuit  400  of the present invention. The integrated circuit  400  receives an input or control signal. The signal can be coupled to an OLGA package  402  for processing by internal circuitry  404 . The OLGA  402  can be arranged as described above with respect to FIG.  3 . The integrated circuit may be any type of integrated circuit, including but not limited to a processor, memory, memory controller, or application-specific integrated circuit (ASIC). 
     FIG. 5 shows a circuit package embodiment  500  of the present invention. In embodiment  500 , a connection of a silicon die  502  is made to an OLGA portion of a motherboard  504 . In one embodiment, the silicon die is flip mounted to the board. The OLGA can be arranged as described above with respect to FIG.  3 . Also, in another embodiment, an OLGA as described above with respect to FIG. 3 could be mounted to a socket on a motherboard. Such a circuit package could also include a processor or other electronic components known to those of ordinary skill in the art. 
     In other embodiments, the OLGA package of the present invention could be used in such configurations as for chipset and processor packaging, and the like. Any processor product using OLGA packaging technology is capable of implementing one of the embodiments of the present invention. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.