Abstract:
A capacitive element for a circuit board or chip carrier having improved capacitance and method of manufacturing the same is provided. The structure is formed from a pair of conductive sheets having a dielectric component laminated therebetween. The dielectric component is formed of two or more dielectric sheets at least one of which can be partially cured or softened followed by being fully cured or hardened. The lamination takes place by laminating a partially cured or softened sheet to at least one other sheet of dielectric material and one of the sheets of conductive material. The total thickness of the two sheets of the dielectric component does not exceed about 4 mils and preferably does not exceed about 3 mils; thus the single dielectric sheets does not exceed about 2 mils and preferably does not exceed about 1.5 mils in thickness. The use of two or more sheets of dielectric material makes it very unlikely that two or more defects in the sheets of dielectric material will align thus greatly reducing the probability of a defect causing a failure in test or field use.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to capacitive elements for circuit boards having improved decoupling capacitance reliability, and more particularly to a circuit board or chip carrier or the like and method of manufacturing the same wherein the board or chip carrier uses multiple layers of dielectric material yet achieves high capacitance. 
     2. Background Art 
     In the manufacture of circuit boards or chip carriers or the like it is desirable to create as high a capacitance as possible between various ground, voltage and signal planes to thereby minimize the amount of discrete decoupling capacitors required on the board surface. The equation for determining capacitance is C=eA/t where C is the capacitance, e is the dielectric constant or relative permittivity of the dielectric material, A is the available area, and t is the thickness of the dielectric material or the spacing between the plates of the capacitor. Thus to increase the capacitance of a capacitor of a given area one can select a material having an increased dielectric constant and/or decrease the thickness of the dielectric material. However, the selection of the dielectric material is often limited by many production and configuration limitations which leaves principal means of increasing the capacitance to a reduction in thickness of the dielectric material. However, a problem encountered with certain types of dielectric material especially epoxy impregnated glass cloth, but other materials as well, at thinner gauges is that any defects in the material in the form of pin holes or voids tend to extend from one surface to the other thus resulting in failure of the structure either under test conditions, or even worse they may pass testing but fail under field use conditions. Therefore it is desirable to form a circuit board or chip carrier having reduced thickness of the dielectric material for improved capacitance but with improved reliability. 
     SUMMARY OF THE INVENTION 
     According to the present invention a capacitive element for a circuit board or chip carrier having improved capacitance and method of manufacturing the same is provided. The structure is formed by selecting a pair of conductive sheets and laminating a dielectric component between the two sheets. The dielectric component is formed of two or more separate sheets of dielectric material at least one of which can be partially cured or softened followed by being fully cured or hardened. The lamination takes place by laminating a partially cured or softened sheet to at least one other sheet of dielectric material and one of the sheets of conductive material. The total thickness of the two sheets of the dielectric component should not exceed about 4 mils and preferably should not exceed about 3 mils; thus the single dielectric sheets should not exceed about 2 mils and preferably should not exceed about 1.5 mils in thickness. The use of two or more sheets of dielectric material makes it very unlikely that two or more defects in the sheets of dielectric material will align thus greatly reducing the probability of a defect causing a failure in test or field use. 
     In one embodiment a pair of ultra thin epoxy impregnated sheets of glass cloth are B-stage (partially) cured and then are laminated between two sheets of conducting material, preferably copper, with the lamination process fully curing the epoxy impregnated cloth to form the laminate structure. In other embodiments one or more sheets of dielectric material are laminated to one or more sheets of conductive material or a film of dielectric material is coated on one or more sheets of conductive material and B-stage cured or softened and then further laminated and fully cured or hardened to form the structure. In other embodiments sheets of various dielectric materials are laminated to each other and to copper sheets by various techniques. In still other embodiments films of dielectric material are coated on copper sheets and B-stage cured, laminated and fully cured. In yet other embodiments different dielectric materials are used to form the dielectric component; for example epoxy is first coated on opposite sides of another dielectric material such as a polyimide sheet, B-stage cured, and then further laminated to two sheets of conducting material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a side elevational view of the components of a circuit board assembled for lamination according to one embodiment of this invention; 
     FIG. 1 b  is the components shown in FIG. 1 a  laminated together to form a circuit board element; 
     FIG. 1 c  is the board of FIG. 1 b  prepared for etching the copper sheets to form clearance holes; 
     FIG. 1 d  is the board of FIG. 1 c  with the copper sheets etched to form the clearance holes; 
     FIG. 2 a  is a side elevational view of the components of a circuit board assembled for lamination according to another embodiment of this invention; 
     FIG. 2 b  is the components of FIG. 2 a  laminated to form a circuit board element; 
     FIG. 3 a  is a side elevational of the first step in forming a circuit board according to yet another embodiment of this invention; 
     FIG. 3 b  depicts the step following that shown in FIG. 3 a  in the manufacture of the circuit board element; 
     FIG. 3 c  depicts the final lamination step after that shown in FIG. 3 b  in the manufacture of the circuit board element; 
     FIG. 4 a  is a side elevational view showing the components ready for lamination of still a further embodiment of this invention; 
     FIG. 4 b  shows the lamination of the components depicted in FIG. 4 a;    
     FIG. 5 a  is a side elevational view depicting the structure of one component of a circuit board according to still another embodiment of this invention; 
     FIG. 5 b  depicts the laminate structure of a circuit board using the component of FIG. 5 a ; and 
     FIG. 6 is a longitunal sectional view of two capacitive elements used to form a completed circuit board. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, and for the present to FIGS. 1 a  and  1   b  one embodiment of the present invention is shown. FIG. 1 a  depicts a lay-up of the components of a circuit board according to one embodiment of this invention which components include a pair of conducting sheets of material  10 , preferably copper. The copper can be either preferably ½ oz. or 1 oz. copper sheets, although thicker or thinner copper sheets could be used, suitable sheets being available from Gould Corp. other conductive material such as, but not limited to copper-Invar-copper, or copper on an aluminum carrier can be used. The sheets preferably have one surface  11  roughened to improve adhesion to other materials as is well known in the art. A pair of sheets  12   a  of dielectric material are provided, located between the copper sheets  10 . The dielectric sheets are preferably ultra thin sheets of glass cloth which have been impregnated with an epoxy and partially (B-stage) cured. This B-stage curing is accomplished by heating to about 100° C. for about 5 to about 20 minutes. Preferably the cloths are 101 or 104 style glass cloths manufactured by Clark-Schwebel Inc. The 101 style cloth is about 1.0 to about 1.1 mil thick and the 104 glass cloth is about 1.5 mils thick. The epoxy resin preferably can be any one of several resins such as a phenolically hardened epoxy resin. Glass cloths impregnated with this type of resin are sold by IBM under the trade mark Driclad™. (As used herein the dielectric resinous material in its partially cured or softened stage is designated by the number eg “ 12 ” followed by the letter “a” thus “ 12   a ”, and in the fully cured or hardened state by the number e.g. “ 12 ” followed by the letter “b” thus “ 12   b ”.) The sheets of copper  10  and the sheets of dielectric material  12   a  are then laminated together in a laminating press at a temperature of from about 180° C. to about 200° C. at between about 300 psi to about 500 psi for about 1 to 2 hours. This will result not only in laminating the components but also in fully curing the impregnated B-stage cured cloth  12   a  resulting in a pair of fully cured glass cloths  12   b  laminated between a pair of copper sheets  10  to thus form the basis of a circuit board or chip carrier  14  as shown in FIG. 1 b . The thickness of the two glass cloths  12   b  will be less than about 4 mils, and if style 101 cloth is used the thickness will be less than about 3 mils. 
     As indicated earlier one advantage of using two sheets of impregnated glass cloth is to greatly reduce the potential for failure of the structure due to defects such as pin holing in the manufacturing of the epoxy impregnated glass cloth. If a defect were to occur in one of the cloths it is highly unlikely that a similar defect would appear in the other cloth aligned or coincident with the defect in the first cloth; hence there is much less likelihood of failure of the part under testing or field conditions. 
     The laminate structure  14  forming the capacitive element can be further processed into a completed circuit board by any of a number of processes well known in the art. For example the copper sheets  10  may be used as voltage planes, e.g. a power plane and a ground plane. In such case clearance holes need to be provided in the copper sheets  10 . This can be accomplished, for example, by photolithography and subtractive etching, as shown in FIGS. 1 c  and  1   d.    
     Referring to FIG. 1 c  both of the copper sheets  10  are coated with a photoresist  16 ; and, using photolithography the photoresist is exposed and developed to reveal the underlying copper sheets  10  at the location that the clearance holes are to be formed. The revealed copper is then etched with conventional etchant to form holes  18  and the remaining photoresist stripped as seen in FIG. 1 d . This structure can then be laminated with other structures, or be used as a base on which to form signal planes or connections to produce a completed circuit board or chip carrier using well known techniques. 
     Referring to FIGS. 2 a  and  2   b  another embodiment of the present invention is shown. In this embodiment a pair of copper sheets  10  are coated each on one side thereof with a dielectric material  13   a . The material may be epoxy of the type previously described, or other types of dielectric material could be used, such as, a cyanate ester, a polyimide, or polytetraf luoroethlyene (PTFE). The dielectric materials, other than the impregnated glass cloth may be applied as liquids or in the case of polyimide and PTFE be in the form of free standing films of material. The material is partially cured as described previously, or in the case of films or glass cloth may be applied to the copper in the partially cured form, and the sheets of copper  10  with the dielectric material  13   a  thereon are laminated together to form structure  20 , shown in FIG. 2 b  comprised of two sheets of copper  10  separated by two sheets of fully cured dielectric material  13   b . The total thickness of the two dielectric layers  13   b  together should not exceed about 4 mils and preferably should not exceed about 3 mils. The structure can be further processed into a circuit board or chip carrier as previously described. The term partially cured and fully cured material is used herein to denote specifically the curing of thermoset resins by cross linking, the partial cure referring to less than complete cross linking and full cure referring to a more complete cross linking. (The terms partial cure and full cure may also sometimes be used herein in referring to thermoplastics. In such case heating of the thermoplastic causes a softening of the material which sometimes is referred to herein as partial cure and cooling causes the material to harden sometimes referred to herein as complete cure; although sometimes the more appropriate terms “softened” and “hardened” are used when referring specifically to thermoplastics.) 
     Referring to FIGS. 3 a - 3   c  yet another embodiment of the invention is shown. In this embodiment a layer of dielectric material  13   b  such as an epoxy is applied to a copper sheet  10  and fully cured to form the component structure  22  as shown in FIG. 3 a . Following this a layer of dielectric material  13   a  preferably also an epoxy is applied to the layer  13   b  of dielectric material and partially cured to form component structure  24  as shown in FIG. 3 b . Thereafter a sheet of copper  10  is laminated to the layer of dielectric material  13   a , the lamination fully curing the layer  13   a  of FIG. 3 b  to form the composite structure  26  shown in FIG. 3 c  of two sheets of dielectric material  13   b  laminated between two sheets of copper  10 . 
     Referring now to FIGS. 4 a  and  4   b  still another embodiment of the invention is shown. FIG. 4 a  shows the components ready to be laminated, which components include a sheet of copper  10  having coated thereon a coating of epoxy  13   b  which is fully cured. (It should be noted that the epoxy  13   b  could be only partially cured if desired.) A sheet of glass cloth impregnated with epoxy resin and partially cured  12   a  is provided as well as a second sheet of copper  10 . (The second sheet of copper  10  can optionally be coated with a dielectric material also.) The components of FIG. 4 a  are then laminated to form the structure  28  shown in FIG. 4 b . This embodiment illustrates how different forms of the epoxy can be used. 
     Referring to FIGS. 5 a  and  5   b  still another embodiment of the invention is shown which depicts one way in which several different dielectric materials are combined. As shown in FIG. 5 a  a sheet of polyimide  30  is coated on each side with a layer of epoxy  13   a  which epoxy is partially cured to form a dielectric component  32 . A pair of copper sheets  10  are laminated one to each of the layers of epoxy and as described previously the epoxy fully cures during lamination to form composite structure  34  of a dielectric material formed of a sheet of polyimide  30  and two layers of fully cured epoxy  13   b  sandwiched between a pair of copper sheets  10  as shown in FIG. 5 b.    
     In the embodiments shown the capacitance, according to the formula given above, should be at least about 500 pico farads per square inch for elements having a dielectric thickness of about 2 mils, and at least about 250 pico farads per square inch for elements having a thickness of about 4 mils. 
     It should be noted that in all of the embodiments shown one or more of the layers of dielectric material may be filled with particulate matter having a high dielectric constants, such as for example, but without limitation, barium titanate, strontium titanate, lead-zirconium titanate, and tantalum oxide. The particle size can range from about 0.1 micron up to about one-half the thickness of the layer containing the particulate material. It is preferred that there be a range of sizes of the particles to promote increased volumetric packing efficiency and hence an increased dielectric constant e. Such loading will increase the capacitance somewhat. 
     FIG. 6 shows in cross section a circuit board utilizing two capacitive elements of the type shown in FIG.  1 D. It is to be understood, however, that other capacitive elements of this invention such as those shown in FIGS. 2B,  3 C,  4 B or  5 B could also be utilized. 
     Shown in FIG. 6 are a pair of capacitive elements  14  of the type shown in FIG. 1D each having laminated to the copper sheet  10  on each side thereof, a layer of dielectric material  40 . A signal core comprised of dielectric material  42  and circuitry  44  formed on both sides of a dielectric material  42  is placed between a pair of capacitive elements  14 , and adjacent to dielectric material  40 . Also circuitry  46  is formed on the exposed surfaces of each of the sheets of dielectric material  40 . Plated through holes  48  are formed to connect the circuitry  44 ,  46  and appropriate copper sheets  10  which form voltage and power planes. 
     In a preferred technique for forming the circuit board a pair of dielectric layers  42  with the circuitry  44  thereon are provided as preformed signal planes, and the dielectric layers  40  are all provided as b stage cured sticker sheets of fiber glass impregnated with epoxy. These are stacked up with the capacitive elements and a sheet of copper foil applied to each of the exposed surfaces of the sticker sheets  40  where the circuitry  46  is to be formed. The stack is laminated together under heat and pressure sufficient to fully cure the sticker sheets and bond all of the components together. The through holes are then drilled and the holes plated with copper. The surface of the copper foil is also plated. The external circuit features  46  are then defined by standard photo lithographic and copper etch techniques. These processes to form the circuit board are all well known in the art. 
     It also should be understood that additional capacitive elements and signal planes could be employed, as well as the other configurations of the capacitive elements. Also other techniques well known in the art could be employed to form the circuit board. 
     Accordingly, the preferred embodiments of the present invention have been described. With the foregoing description in mind, however, it is understood that this description is made only by way of example, that the invention is not limited to the particular embodiments described herein, and that various rearrangements, modifications, and substitutions may be implemented without departing from the true spirit of the invention as hereinafter claimed.