Patent Publication Number: US-2013239617-A1

Title: Methods of forming a glass wiring board substrate

Description:
This application claims the benefit of priority under 35 USC §119 to U.S. Provisional Application Ser. No. 61/417,925 filed Nov. 30, 2010 the content of which is relied upon and incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The present disclosure is related to package-integrated wiring board substrates particularly useful for CPU or GPU packaging, and particularly to methods for forming glass wiring board substrates. 
     BACKGROUND AND SUMMARY 
     Newer generations of high-performance integrated circuits such as central processing units (CPUs) and graphics processing units (GPUs) are getting larger and are being designed to operate over wider operating temperature ranges than past generations. Larger sizes and larger operating temperature ranges cause a need for low coefficient of thermal expansion (low CTE) materials with CTE relatively close to silicon for use as wiring board substrates within the packaging of newer generation integrated circuits. 
     In typical packaging and mounting of a CPU, shown in diagrammatic cross section in  FIG. 1 , an integrated circuit formed on a silicon substrate  60  is mounted within a package that includes a heat spreader  62  in contact with the silicon substrate  60  via a thermal interface material  64 . A wiring board  10  with multiple layers of wiring and insulator material built-up on it provides electrical connection between tight pitch (closely spaced) solder bumps  72  at an interface with the silicon substrate  60  and looser pitch (less closely spaced) solder bumps  74  that provide electrical connection between the integrated circuit in its package and cooperating interfaces, such as a mounting socket on a motherboard  80 . The package and/or the motherboard may also include one or more capacitors  90 . 
     As shown in the cross-section of  FIG. 2 , a wiring board substrate  10  provides the core structural layer upon which integrated circuit packaging wiring layers  102  and insulating layers  104  are built up, forming a built up layer structure  100 . Through-holes  40  in the substrate  10  are plated or filled with conductive material to provide electrical connection between the wiring layers  102  on the two sides (major surfaces or flats) of the wiring board substrate  10 . 
     Substrates in commercial use today are typically formed of fiber reinforced polymer, and the through-holes are produced by mechanical drilling. Polymer CTE is generally undesirably high relative to silicon, and mechanical drilling becomes difficult at smaller hole and pitch sizes. 
     Glass has previously been proposed for use as a wiring board substrate. Certain glasses can provide desirably low CTE. A remaining technical challenge is to provide a cost-effective process for drilling thousands of small holes, close together, while retaining structural strength of the substrate. 
     The present disclosure includes a method or process for forming a glass wiring board substrate for integrated circuit wiring boards, including providing a first molding surface positioned on a first mold having truncated conical pins protruding therefrom, the pins having a diameter at the top end thereof of 150 micrometers or less, and a minimum pitch of 400 micrometers or less, providing a glass sheet having first and second surfaces on opposite major sides thereof, pressing the first surface of the glass sheet against the molding surface, heating the glass sheet and the first molding surface together to a temperature sufficient to soften a glass of which the glass sheet is comprised, such that the pattern of the first molding surface is replicated in the first surface of the glass sheet, thereby producing a formed glass sheet having an array of holes therein, cooling the formed glass sheet and the molding surface together to a temperature below the softening point of said glass, and separating the formed glass sheet from the molding surface. 
     The glass material offers a low CTE well-matched to that of silicon. The forming process offers a dimensional reproducibility based on the use of non stick molds, desirably of graphite, the molds having a CTE close to the material to be formed. The forming process consists of pressing a glass sheet using one mold surface or two mold surfaces simultaneously, each mold surface presenting protrusions corresponding to the through-holes to be formed in the glass. 
     For embodiments of the method in which the through-holes are formed by pressing with a single mold, the holes may be blind holes after pressing, and may then be opened to form through-holes by back side lapping. Alternatively, the glass is pressed up to through-hole formation, avoiding the need of back side lapping. Other embodiments use two molds presenting forming protrusions, pressed on opposite major surfaces of the glass sheet to be formed. 
     The methods disclosed herein allow for low-cast mass production of many holes simultaneously, using a technology that displaces the formed material, rather than one that removes or adds material. This results in a more efficient and cost effective process. The use of graphite, the presently preferred mold material allows for very good reproducibility of hole position and spacing, due to good match of mold- and material-CTE over the range of molding temperatures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description of specific embodiments of the present disclosure can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which: 
         FIG. 1  is a diagrammatic cross-section of a wiring board substrate  10  within an integrated circuit package; 
         FIG. 2  is a diagrammatic cross-section of a wiring board substrate  10  with built-up layers  100  thereon; 
         FIGS. 3-6  are diagrammatic cross-sections of a glass sheet  30  or formed glass sheet  30 ′ according to various process steps of certain embodiments of the present disclosure; 
         FIGS. 7-9  are diagrammatic cross-sections of a glass sheet  30  or formed glass sheet  30 ′ according to various process steps of certain other embodiments of the present disclosure; and 
         FIG. 10  is a digital image of a portion of an integrated circuit wiring board substrate produced according one or more of the methods disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     With general reference to  FIGS. 3-6 , according to an embodiment of a method of the present disclosure, a glass wiring board substrate  10  for use in integrated circuit packaging is produced by a method including providing a first molding surface  20  positioned on a first mold  22 , having truncated conical pins  24  protruding therefrom, as shown in the diagrammatic cross-section of  FIG. 3 . The pins  24  desirably have a diameter at the top end  26  thereof of 150 micrometers or less, and a minimum pitch  28  of 400 micrometers or less. 
     The method further includes providing a glass sheet  30  having first and second surfaces  32  and  34 , respectively, on opposite major sides thereof, and pressing the first surface  32  of the glass sheet  30  against the molding surface  20  of the mold  22 . The pressing may be performed in part by pressure applied by an active or adjustable means, or by a weight, in either case, desirably applied through a refractory body  29  compatible with the material of the glass sheet  30 , and more desirably of the same material as the first mold  22 . The glass sheet  30  and the first molding surface  20  are then heated together to a temperature sufficient to soften a glass of which the glass sheet  30  is comprised, such that the pattern of the first molding  20  surface is replicated in the first surface  32  of the glass sheet  30 , thereby producing a formed glass sheet  30 ′ having an array of holes  40  therein, as illustrated in the cross-sections of  FIGS. 4 and 5 . The formed glass sheet  30 ′ is then cooled together with the molding surface  20  to a temperature below the softening point of the previously softened glass, after which the formed glass sheet  30  and the molding surface  20  are separated from each other. 
     With proper selection of the material of the mold  22  and molding surface  20 , the separation of the molding surface  20  from the formed glass sheet  30 ′ can be easily performed without any large forces and with little or no damage to the molding surface  20 , allowing many uses of a given mold  22 . This may preferably be achieved by selecting the material of the glass sheet  30  and the material of the mold  22  such that a CTE mismatch between the glass sheet  30  and the first molding surface  20  or the mold  22  is within the range of 0 to less than 15×10−7 at 300° C., and desirably over the entire temperature range from room temperature to above the softening point of the glass of the glass sheet  30 . This also allows for sufficiently accurate hole positioning for wiring board substrate design specifications. Typically −/+20 μm variation in hole position or less can be relatively easily achieved over a 20 mm distance in the final product. 
     The first mold  22  and the first molding surface  20  formed are desirably formed from carbon by machining a carbon block using diamond-coated tools so as to form the first molding surface  20 . This mold material releases well from the formed glass sheet  30 ′. 
     In the particular embodiment shown in  FIGS. 4 and 5 , the array of holes  40  resulting from the pressing and heating of the glass sheet  30  is an array of blind holes. In this case, an additional step may include one or both of grinding and polishing the formed glass sheet  30 ′, on the second surface  34  thereof, to sufficient depth to open the array of holes  40 , resulting in an array of through-holes  40 ′ in the formed glass sheet  30  as shown in  FIG. 6 . According to an alternative embodiment, the pressing process may continue for sufficient time, and with sufficient pressure, such that the originally produced array of holes  40  after the pressing and heating process, is already an array of through holes  40 ′ as in  FIG. 6 . In either case, the resulting formed glass sheet  30 ′ with an array of through holes  40 ′ forms a glass wiring board substrate  10  useful in integrated circuit packaging as explained above with reference to  FIGS. 1 and 2 . 
     According to another alternative embodiment, illustrated generally by the cross-sections shown in  FIGS. 7-9 , a second molding surface  50  may be provided, positioned on a second mold  52 , and the second surface  34  of the glass sheet  30  may be pressed against the second molding surface  50 . The step of heating the glass sheet  30  and the first molding surface  20  together may then further comprise heating the second molding surface  50  also, at the same time, so that both molding surfaces  20 ,  50  and the glass sheet  30  are raised to a temperature sufficient to soften a glass of which the glass sheet  30  is comprised, and such that the pattern of the first molding surface  20  is replicated in the first surface  32  of the glass sheet  30 , and the pattern of the second molding surface  50  is replicated, more or less simultaneously, in the second surface  34  of the glass sheet  30 . 
     Desirably, the second molding surface  50  includes a first pin  54   a  thereon which is positioned in a mirror image location relative to a corresponding pin  24   a  on the first molding surface  20 , as shown in  FIG. 7 . In an embodiment further represented by the cross section of  FIG. 8 , the resulting formed glass sheet  30 ′ includes two arrays of blind holes  40  and  42 , on the first a second surfaces  32 ,  34 , respectively. The holes formed by corresponding pins  54   a  and  24   a  are divided by a thin layer or web of glass  31 , so the method desirably further includes etching the formed glass sheet  30 ′ sufficiently to join a hole formed in the second side  34  of the formed glass sheet  30 ′ by the first pin  54   a  with a hole formed in the first side  32  of the formed glass sheet  30 ′ by the corresponding pin  24   a.  This provides one or more open through holes. Desirably, an entire array of open through holes is produced, by using a second molding surface  50  that comprises multiple pins  54  arranged in a second-molding-surface pattern that is a mirror image of a first-molding-surface pattern of pins  24  on the first molding surface ( 20 ), with the resulting formed glass sheet with through-hole array  40 ′ as shown in  FIG. 9 . According to yet another alternative embodiment, the pressing process with two molding surfaces  20 ,  50  may continue for sufficient time, and with sufficient pressure, such that the originally produced arrays of holes  40 ,  42 , after the pressing and heating process, is already an array of through holes  40 ′ as in  FIG. 9 . Regardless of the particular embodiment, the resulting formed glass sheet  30 ′ with an array of through holes  40 ′ again forms a glass wiring board substrate  10  useful in integrated circuit packaging as explained above with reference to  FIGS. 1 and 2 . 
     The glass of the glass sheet  30  desirably has a CTE in the range of 30 to 90×10 −7 /° C., more desirably in the range of 30 to 40×10 −7 /° C., so as to be relatively close to that of silicon. 
     EXAMPLES 
     For Corning Code 0211 glass (available from Corning Incorporated of Corning, N.Y., USA and/or their distributors) a desirable graphite material may be EDM4 (available from Poco Graphite, Inc., Decatur, Tex., USA or their distributors) having a CTE of 78×10 −7 /° C. Mold surfaces comprising EDM4 were used to press Corning Code 0211 glass at 740° C. in a nitrogen atmosphere. 
     For actual production molds, diamond-tool machining is the preferred method for their formation. For the testing reported here, an EDM4 graphite mold was machined by wire EDM to produce 10 000 pins on 40×40 mm molding surface, the pins having a mean pitch 400 μm, a mean height of 230 μm, and a diameter at the bottom of the pins of 250 μm and 150 μm at the top. This mold was then used to press a sheet of Corning Code 0211 glass at 740° C. in a nitrogen atmosphere. 
     For back polishing, the molded glass was secured on a polishing support with adhesive wax melted at 70° C., then ground and polished to the level of the holes. For a 230 μm pin height, a 210 μm final thickness was targeted. A digital image of the resulting substrate  10  with array of holes is shown in  FIG. 10 . 
     As a second example, for Corning Eagle XG® glass (available from Corning Incorporated, Corning N.Y., USA, and/or their distributors), a desirable graphite material may be Ref 2020 (available from MERSEN [formerly Carbone Loraine] of Paris, France and/or their distributors), having a CTE of 38×10 −7 /° C. CTE. Mold surfaces comprising Ref 2020 were successfully used to press sheets of Eagle XG® glass at 1040° C., also in a nitrogen atmosphere. 
     It is noted that terms like “preferably,” “commonly,” and “typically,” when utilized herein, are not utilized to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to identify particular aspects of an embodiment of the present disclosure or to emphasize alternative or additional features that may or may not be utilized in a particular embodiment of the present disclosure. 
     Having described the subject matter of the present disclosure in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present disclosure are identified herein as preferred or particularly advantageous, it is contemplated that the present disclosure is not necessarily limited to these aspects. 
     It is noted that one or more of the following claims utilize the term “wherein” as a transitional phrase. For the purposes of defining the present invention, it is noted that this term is introduced in the claims as an open-ended transitional phrase that is used to introduce a recitation of a series of characteristics of the structure and should be interpreted in like manner as the more commonly used open-ended preamble term “comprising.”