Abstract:
A unique bond-ply structure and associated processes for processing of the bond-ply structure and for joining together circuit layer pairs which makes it possible to create a bond-ply having raised structures of conductive material which compensate for shrinkage during sintering thus creating a stress-free and void-free electrically conductive junction between layer-pairs in an interconnect.

Description:
This application is a divisional of pending application Ser. No. 09/510,574, filed Feb. 22, 2000. 
    
    
     FIELD OF THE INVENTION 
     The field of the invention is integrated circuit packaging. 
     BACKGROUND OF THE INVENTION 
     High density electronic interconnections are required for many important applications including ball grid array packages, high density electronic circuit boards, cell phones and other miniature electronic devices. In one fabrication scheme for high density interconnection, electronic circuitry is fabricated onto both sides of individual layer pairs with through vias connecting the circuitry on the two sides, These layer pairs may then be physically and electronically joined by fabricating a bond ply material with through-holes drilled or lased, filling these holes with sinterable conductive pastes or solders, by stacking the layer pairs and the bond plies in a press, and by curing the resulting structure. The curing process can sinter the paste or solder providing for electronic interconnection between layer pairs and at the same time cure the bond ply material, providing for permanent physical bonding of the structure. However conductive inks and pastes and solders generally shrink significantly during the sintering or soldering process. This shrinkage occurs as the solder or paste sinters, often causing voids or open circuit interconnection. The shrinkage also can force considerable stress into the overall structure, leading to unreliable interconnection or bonding between the layers. Thus, there is a need for material and associated processes which can compensate for this shrinkage providing for stress-free and reliable electronic interconnection between the layer pairs. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a unique bond-ply structure and associated processes for processing of the bond-ply structure and for joining together circuit layer pairs. The disclosed structure and methods makes it possible to create a bond-ply having raised structures of conductive material which compensate for shrinkage during sintering, thus creating a stress-free and void-free electrically conductive junction between layer-pairs in an interconnect. 
     Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawings in which like numerals represent like components. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view of a bond-ply embodying the invention. 
     FIG. 2 is a view of the bond-ply of FIG. 1 with the release sheets removed and nubs exposed. 
     FIG. 3 is a view of the bond-ply of FIG. 1 positioned between layer pairs prior to lamination. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 1 and 2, a bond-ply  1  generally comprises a base layer  10 , adhesive layers  21  and  22 , protective release sheets  31  and  32 , via  40 , and conductive material  50 . Base layer  10  comprises an upper surface  10 A and a lower surface  10 B. Adhesive layers  21  and  22  comprise upper surfaces  21 A and  22 A and lower surface  21 B and  22 B. Protective release sheets  31  and  32  comprise upper surface  31 A and  32 A and lower surfaces  31 B and  32 B. Nubs  51  and  52  shown in FIG. 2 comprise the portions of conductive material  50  located within the portions of via  40  formed by release sheets  31  and  32 . 
     The base layer  10  is preferably a reinforced polymeric core, but may comprise any suitable bond-ply core material. Bond-ply  1  may have two adhesive layers as shown or may have a single adhesive layer. The adhesive layer(s) of bond-ply  1  are preferred to comprise an uncured polymer but may comprise any bond-ply adhesive. Via  40  is preferably filled with sinterable conductive material  50  such as a conductive ink, paste or solder using standard pressure filling methods. Conductive materials particularly suited to the structures and methods disclosed herein are transient liquid sintering pastes such as those described in issued U.S. patents assigned to Ormet. 
     Release sheets  31  and  32  each have a specific thickness chosen so as to leave a measured quantity of excess conductive ink, paste, or solder as raised regions above the plane of the bond ply material, i.e. nubs/nubbins such as nubs  51  and  52  of FIG.  2 . The measured quantity, defined by the thickness of the release layer is chosen to compensate for shrinkage to provide for a highly filled conductive region after sintering is complete. A particularly convenient release layer is provided by copper foil which is readily available in specific thicknesses, which releases readily from polymeric layers or adhesives, which is relatively inexpensive, which may be laser ablated to provide for vias through the bond ply, and which is very convenient for assembly of the bond ply from prefabricated cores and resin coated copper. 
     Nub  51  can be formed by providing a bond-ply having a first adhesive layer  21  and a first release layer  31 , the first release layer  31  having an inner surface  31 B substantially in contact with a surface  21 A of the first adhesive layer, and an outer surface  31 A opposite the inner surface which does not contact the first adhesive layer  21 ; providing a via  40  which passes through the first adhesive layer  21  and first release layer  31  including through both the inner and outer surfaces  31 A and  31 B of the first release layer  31 ; filling the via  40  with a conductive material  50  such that the material  50  substantially fills the entire via  40  including the portion extending out of the first adhesive layer  21  and through the first release layer  31 ; removing the first release layer  31  from the first adhesive layer  21  while leaving at least a portion of the material  50  previously located within the portion of the via  40  extending through the first release layer  31  extending outward from the via  40  and beyond the surface  21 A of the first adhesive layer  21  as a first nub  51 . 
     A bond ply having a second nub  52  can be formed in a fashion similar to that of nub  51 . In particular, the provided bond-ply can have a second adhesive layer  22  and a second release layer  32  with the via  40  passing through the second adhesive layer  22  and second release layer  32  in the same manner that it passes through the first adhesive layer  21  and first release layer  31 , and the second release layer  32  can be removed to form a second nub  52  in the same way as the first release layer  31  was removed to form nub  51 . 
     A refinement of the disclosed methods would include choosing the thickness of the first and second release sheets to correspond to a desired size for the first and second nubs. 
     It should be readily apparent that the disclosed methods are equally applicable when the bond-ply comprises a plurality of vias and that application of the disclosed method in such a case would result in a plurality of nubs, one for each side of each via, remaining on the bond-play after removal of the release sheets. 
     Referring to FIG. 3, a laminated bond-ply/interconnect can be formed by laminating the bond-ply  1  between a first layer pair  100  and a second layer pair  200  such that the first nub  51  is compressed by and against a contact pad  110  on the first layer pair and the second nub  52  is compressed by and against a contact pad  210  on the second layer pair. Such an interconnect would comprise a bond-ply  1  having a core  10  sandwiched between two adhesive layers  21  and  22 , the bond-ply  1  also having a via  40  filled with sintered conductive material  50  extending through the core  10  and adhesive layers  21  and  22 , the conductive material comprising a compressed nub ( 51  and  52 ) in each of the two adhesive layers. 
     It is preferred that such an interconnect could be subjected to heat processing so as to cause sintering of the conductive material and to form a relatively stress free connection between the contact pad on the first layer pair and the contact pad on the second layer pair. 
     Thus in the process provided for this invention, bond ply material is fabricated with specified thickness of copper or other release materials. Through vias are fabricated by mechanical drilling or laser drilling. The vias through both the release layer and the internal bond ply are filled with a conventional pressure filling apparatus. The release films are removed, leaving raised nubbins protruding from each via. The bond ply layers are stacked with corresponding layer pairs in a conventional lamination press and the entire structure is cured under pressure at elevated temperature using a cycle designed to cure the adhesives and to sinter the conductive ink, paste, or solder. 
     It should be apparent, however, to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced.