Abstract:
A stacked via structure for reducing vertical stiffness includes: a plurality of stacked vias, each via disposed on a disc-like structure. The disc-like structure includes a platted through hole landing supporting the plurality of stacked vias. The platted through hole landing includes an etched pattern.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of, and claims priority to, commonly-owned and co-pending application filed under U.S. Ser. No. 12/020,534, filed on Jan. 26, 2008; and contains material similar to that disclosed in commonly-owned, co-pending applications. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED-RESEARCH OR DEVELOPMENT 
     None. 
     INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
     None. 
     FIELD OF THE INVENTION 
     The invention disclosed broadly relates to the field of chip design and more particularly relates to the field of electronic substrates in chip design. 
     BACKGROUND OF THE INVENTION 
     Integrated circuits (chips) are generally made of silicon on which electronic circuits are fabricated. These chips are placed on substrates. A substrate is made of organic materials embedded with copper interconnects. The substrate helps to join the chip to external circuits on a motherboard.  FIG. 1   a  shows a cross-section of a chip  110  on a substrate  120 . These are the two key components of an electronic module. 
       FIG. 2  shows a cross-section of the substrate  120 . The density of connection points (controlled collapse chip connect, or C4s)  130  between the chip  110  and the substrate  120  is a critical parameter. An increased number of C4s  130  requires multiple buildup layers  150  to facilitate electrical connections to the external motherboard. Buildup layers  150  are fabricated in stages on the top and bottom of a fiber reinforced core  155 . 
       FIG. 2  shows stacked vias  140  as well as staggered vias  145  needed to complete the interconnection. Stacked vias  140  help achieve more than 20% connection density compared to a staggered via  145 .  FIG. 3  shows a conventional stacked via  140  and a platted through hole (PTH)  160 . A PTH  160  allows electrical connectivity between the top and bottom buildup layers. 
     The coefficient of thermal expansion (CTE) of various materials used to construct a module is not matched and is known to drive thermomechanical stresses within a module. Repeated thermal cycling of an electronic module exhibits failure at via interface regions due to thermomechanically driven accumulated strain. 
     There is a need for a system to reduce thermomechanical stresses on electronic modules. 
     SUMMARY OF THE INVENTION 
     Briefly, according to an embodiment of the invention a stacked via structure for reducing vertical stiffness includes: a plurality of stacked vias. Each via is disposed on a disc-like structure which includes a platted through-hole landing. The platted through-hole landing: a multi-part compliant center zone; and spring-like stiffness-reducing connectors for connecting parts of the multi-part compliant center zone of the platted through hole landing. The compliant center zone includes: an outer zone; an intermediate zone; and a center zone. The three zones are electrically conducting and mechanically facilitates the compliant center zone. 
     In another embodiment of the present invention, a substrate via structure includes: a plurality of stacked vias. Each via is disposed on a disc-like structure including: an etched platted-through landing. The disc-like structure may be etched with a spoke-like pattern. The etched pattern may be concentric circles. The concentric circles may form a gimbal pattern. 
     Further, the platted through-hole landing may have a thickness of substantially 3 μm. This thickness is achieved by controlled grinding of the copper top surface of the platted through-hole landing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To describe the foregoing and other exemplary purposes, aspects, and advantages, we use the following detailed description of an exemplary embodiment of the invention with reference to the drawings, in which: 
         FIG. 1  shows a cross-section of the two key components of an electronic module, a chip and a substrate, according to the known art; 
         FIG. 2  shows a cross-section of the substrate, indicating the stacked and staggered vias, according to the known art; 
         FIG. 3  shows a close-up view of stacked vias and an exploded view of the stacked vias and the platted through hole; 
         FIG. 4   a  shows a close-up view of stacked vias built on a platted through hole landing, according to the known art; 
         FIG. 4   b  shows a close-up view of stacked vias built on a soft landing, according to an embodiment of the present invention; 
         FIG. 4   c  shows another view of the stacked vias of  FIG. 4   a , according to the known art; 
         FIG. 4   d  shows another view of the stacked vias of  FIG. 4   b , according to an embodiment of the present invention; 
         FIG. 5  shows an example of a spoke-like construction etched into the substrate layer, according to an embodiment of the present invention; 
         FIGS. 6A ,  6 B,  6 C, and  6 D show concentric circles connected to each other at overlapping points, according to an embodiment of the present invention; 
         FIG. 7  shows a PTH landing with substantially reduced thickness, according to an embodiment of the present invention; 
         FIG. 8   a  shows a 30× magnification of deformation of a stacked via with a PTH cap; and 
         FIG. 8   b  shows a 30× magnification of deformation of a stacked via with the PTH cap removed. 
     
    
    
     While the invention as claimed can be modified into alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the present invention. 
     DETAILED DESCRIPTION 
     Embodiments of the present invention relate to a stacked via structure for electronic substrates such that the thermomechanical stresses on the vias are reduced. This stacked via structure reduces the vertical stiffness inherent in current via structures. Referring to  FIG. 4   a  there is shown an optimized configuration for chip modules, according to the known art. The vias of  FIG. 4   a  (Vial)  140  are built on the platted through hole (PTH) landing  162  and are conventionally supported by this disc-like structure, preferably made of copper (Cu). Although other materials could be used, copper is ideal because of its electrical and thermal properties. 
     Each via member of the three-stack via  140  is about 20 μm thick. Because of the difference in the coefficient of thermal expansion (CTE) between copper and the build-up layers  150  which occurs during a thermal cycle (125 degrees C. to −55 degrees C.), the build-up layers  150  as shown in  FIG. 3  (with a CTE of approximately 20 ppm/degrees C.) shrink much faster than the Cu-via  140  (with a CTE of approximately 16 ppm/degrees C.). As this occurs, the stacked via  140  is compressed in the Z direction against the PTH landing  162  by the surrounding build-up layers  150  as they compress. 
     The key advantage of a preferred embodiment of the present invention is that reducing the stiffness of the PTH landing  162  in the Z direction reduces the compression stress on the copper vias  140 . This solution also allows a stacked via  140  to pitch with greater ease as its bending stiffness is reduced by the compliant PTH landing  162 . 
       FIG. 4   b  illustrates this concept. Consider that the PTH landing  462  of  FIG. 4   b  has three distinct zones. The inner zone  462  is a disc that supports the via stack  460 , the outer zone  464  is a circular ring and the intermediate zone  470  provides the extra compliance represented by spring-like elements. These spring-like elements  470  provide compliance to the center landing  462  by allowing increased flexibility of movement when force is applied in the Z direction. The functional operation of this embodiment can be compared to that of a trampoline where the center zone is allowed to move compliantly along the Z-direction by means of springs holding the canopy along its periphery. 
     The compliant spring-like connectors  470  are preferably constructed from the same etching process that is employed to generate the circuit pattern on the first layer of Cu present on both sides of the core  155 . The conventional disc-like structure of the PTH  462  is innovatively etched with patterns (as discussed later) so that they are electrically conducting but also mechanically compliant along the Z axis. 
     A finite element (FE) analysis of a three-stack via configuration reveals that the cumulative strain of a conventional stacked via of 1.7% can be reduced to 1.3% (25% reduction) by providing a compliant PTH landing  462  for a stacked via  460 .  FIGS. 4   c  and  4   d  show the configurations used in the FE estimates. 
       FIG. 4   c  shows a schematic illustration of the stacked vias  140  of  FIG. 4   a.    
       FIG. 4   d  shows a schematic illustration of the stacked vias of  FIG. 4   b . This is the optimal structure wherein the bottom stack is completely disconnected from the PTH structure. The stiffness of this structure in the Z direction is substantially zero. 
       FIGS. 5 ,  6 A,  6 B,  6 C,  6 D, and  7  show various embodiments which also minimize the Z-stiffness of the PTH landing  560  within the scope of the present invention.  FIG. 5  shows a spoke-like construction that can be achieved using the subtractive etching process used to generate the first circuit layer. Compared to a solid disc-like PTH landing  162 , removal of copper material by etching (in order to form a spoke-like structure) introduces a reduction in the load carrying area of the modified PTH  560 . The Z-stiffness is accordingly reduced. The three distinct zones ( 462 ,  464  and  470 ) discussed in  FIG. 4   b  are identified as  562 ,  564  and  570  in  FIG. 5  of the invention. 
       FIG. 6A  shows another embodiment of the invention wherein concentric circles connected to each other at non-overlapping points are used to reduce Z-stiffness. Notice that a gimbal-like structure shown in  FIG. 6B  is a subset of this configuration in which the pitching stiffness can be reduced to very low levels. A gimbal has at least two rings mounted on axes which are at right angles to each other. In this embodiment, the concentric circles will be mounted at acute and/or obtuse angles in order to accommodate the via in the center, as shown in  FIGS. 6C and 6D . The three distinct zones ( 462 ,  464  and  470 ) discussed in  FIG. 4   b  are identified as  662 ,  664  and  670  in  FIGS. 6A through 6D  of the invention. 
     A multitude of Z-stiffness reducing patterns on PTH landings can be envisaged without increasing the electrical resistance of an interconnect.  FIG. 7  shows a PTH landing with substantially reduced thickness (reduced from 10 um to 3 um) within the PTH region. Such a configuration is achieved by means of controlled grinding of the copper top surface. In this configuration the intermediate and center zones merge into a single zone. 
       FIG. 8   a  shows a 30× magnification of deformation of a stacked via with a PTH cap.  FIG. 8   b  shows a 30× magnification of deformation of a stacked via with the PTH cap removed. You will note that the deformation is lessened without the PTH cap. 
     Therefore, while there has been described what is presently considered to be the preferred embodiment, it will be understood by those skilled in the art that other modifications can be made within the spirit of the invention.