Multi-level circuit structure

Abstract: A multi-level circuit structure (50) is provided which includes a plurality of overlying substrates, each having at least one conductive core (10). A plurality of imprinted deformations are formed in each substrate, each deformation having a convex surface (14) and a concave surface (16). Dielectric coatings (18) are utilized to provide insulation between adjacent substrates and by selectively placing a conductive mass between selected overlying imprinted deformations an electrical connection may be formed between adjacent imprinted deformations wherein selected portions of one substrate may be electrically coupled to selected portions of a second substrate.

Description: The present invention relates in general to a multi-level circuit structure for mounting electronic devices thereon. More particularly, the present invention relates to the structure of multi-level circuit cards formed from a plurality of substrates having electrically conductive cores. Still more particularly, the present invention relates to the structure of multi-level circuit cards formed from a plurality of substrates having electrically conductive cores which permit accurate alignment of each level within the multi-level circuit card. In electronic computers, electronic devices or modules housing electronic devices, are mounted on circuit cards which electrically interconnect the various electronic devices and modules. Multi-layer circuit cards are conventionally manufactured by laminating together a number of substrates, i.e. epoxy-fibreglass, having copper circuitry on each side. The multi-layer laminate is then drilled to form the many interconnections or "vias" which are necessary for interlayer connection. Registration of the cores must be extremely precise because misalignment will lead to an unusable circuit card. U.S. Patent No. 3,606,677 describes a multi-layer structure formed by laminating dielectric substrates having conductive patterns thereon with unmetalized substrates therebetween. U.S. Patent No. 3,795,047 describes a multi-layer structure which is formed by laminating subassemblies which are electrically interconnected by uniform spherical particulate powder embedded in an epoxy between adjacent layers. U.S. Patent No. 4,683,653 describes a multi-layer circuit board which is formed from laminated dielectric base plates with circuit patterns disposed thereon. U.S. Patent No. 3,436,819 describes a multi-layer circuit board formed from laminating circuitized dielectric substrates. U.S. Patent No. 4,496,793 describes a multi-layer circuit board which includes one or more metal stabilising sheets for adjusting the thermal coefficient of expansion of the laminate. US-A- 4 663 840 disloses a method of interconnecting conductors of different layers of a multilayer printed circuit board by forming a depression in the relevant conductor of the outermost layer, so that the insulation layer(s) is deformed and contact is made between the outermost layer and the appropriate conductor(s) of the next layer or layers. U.S. Patent Application Serial No. 339,334 discloses a multi-level structure which includes layers which are formed from thermally and/or electrically conductive core materials which are coated with a dielectric material. By utilizing a thermally and/or electrically conductive core each substrate in a multi-level circuit card formed in this manner will have increased structural stability. By forming structures from multiple substrates, each having an enhanced structural stability, the resultant multi-level circuit card will have greatly enhanced structural stability. Additionally, where the core of each substrate is electrically conductive, the core may be used as a ground or power plane, thereby eliminating the need to separately include a conductive layer or sheet as a ground or power plane. A problem which exists with each of the aforementioned prior art structures is the particularly precise registration which is required of each layer during construction so that interconnections which are drilled through multiple layers will intersect the various conductive patterns at precise points. Therefore a need exists for a multi-level circuit card structure which permits accurate registration of multiple levels within the structure. In accordance with the present invention a multi-layer electronic circuit structure comprises: a plurality of overlying non-conducting substrates, each of said substrates having an electrically conductive core; and a printed circuit pattern mounted on at least one of said substrates and in electrical contact with the core of that substrate, electrical connection being provided between the core of one of said overlying substrates and the core of another of said substrates; said electrical connection between said electrically conductive cores being provided by means of said printed circuit pattern and a deformation of said cores, said deformation forming a protrusion on a surface of that core. In order that the invention will be fully understood preferred embodiments thereof will now be described with reference to the accompanying drawings in which: Figures 1 to 11 depict a series of sectional views illustrating the construction of one layer of a multi-level circuit card, in accordance with an embodiment of the present invention; Figure 12 depicts a portion of a multi-level circuit card constructed in accordance with the embodiment of the present invention shown in Figures 1 to 11 with an electronic device mounted thereupon; and Figure 13 depicts a portion of another embodiment of a multi-level circuit card with an electronic device mounted thereupon. Figure 1 , depicts a sectional view illustrating the construction of one layer of a multi-level circuit card in accordance with a preferred embodiment of the present invention. The construction of a single layer of a multi-level circuit card begins by utilizing an electrically conductive core 10. Preferably conductive core 10 is constructed of a metallic material, such as copper, aluminum, iron or the like, or a composite metallic material such as copper-invar-copper. In accordance with the preferred embodiment of the present invention, conductive core 10 is chosen to give each substrate a thermal coefficient of expansion which closely matches that of the material from which an electronic device mounted to the substrate is fabricated from. Typically, electronic devices are fabricated from a material like silicon, or gallium arsenide. In the case of a silicon electronic device, conductive core 10 can provide a thermal coefficient of expansion which closely matches that of silicon by utilizing copper-invar-copper or molybdenum. By providing conductive core 10 with a thermal coefficient expansion which closely matches that of the electronic devices to be mounted thereon, it is possible to permit such devices to be directly attached to the multi-layer circuit card without reliability concerns which may result from a thermal mismatch between the electronic device and the multi-layer circuit card. Conductive core 10 Includes a plurality of imprinted deformations 12 which are constructed utilizing a punch or die. Each imprinted deformation 12 preferably includes a convex surface or protrusion 14 and a concave surface or depression 16 . In the depicted embodiment of the present invention, conductive core 10 is preferably provided utilizing a material which is between about 2.5 x 10⁻⁵ and 5.0 x 10⁻⁵ metres (one and two mils) in thickness. Each imprinted deformation 12 is preferably extruded from the surface of conductive core 10 to a distance of approximately 1.0 x 10⁻⁴ metres (four mil), or approximately twice the thickness of conductive core 10 . Next, Figure 2, which depicts the next step in the construction of a layer of a multi-level circuit card which is constructed in accordance with the preferred embodiment of the present invention. After imprinting conductive core 10 with a plurality of imprinted deformations 12, the side of conductive core 10 having dimples protruding therefrom is knife coated with a dielectric material. The dielectric material selected is preferably a polymer, a ceramic material or any other suitable dielectric material. Examples of polymer materials are polyimide, Mylar, polyetherimide, polytetrafluoroethylene, epoxy (FR4) and bismaleimide resins (Mylar is a trade mark owned by E.I du Pont de Nemours and company). In the preferred embodiment of the present invention, dielectric material 18 is implemented utilizing polyimide. The layer of dielectric material 18 is then knife coated onto conductive core 10 to ensure that the coating thickness will be limited to the height of convex surface 14 of each imprinted deformation 12 . Thereafter, dielectric material 18 is cured, utilizing any technique known in the art. Alternatively, a photo-imageable dielectric material may be utilized and "curtain coated" onto conductive core 10 . Thereafter, the photo-imageable material may be exposed and developed to expose each convex surface 14 . With reference now to Figure 3 , the next step in the process of constructing one layer of a multi-level circuit card is depicted. As is illustrated in Figure 3 , the lower surface of conductive core 10 is now coated with a suitable photo-resist 20 which is exposed, developed and rinsed in a manner well known in the art to expose rings around selected imprinted deformations 12 which will be utilized to create isolated interconnections or "vias." As discussed above with respect to Figure 2 , in an alternative embodiment a photo-imageable dielectric material may be curtain coated onto the lower surface of conductive core 10 and exposed and developed to expose rings around each imprinted deformation 12 . Referring now to Figure 4 , the next step in the process is illustrated. A plurality of isolation rings 22 have been etched into conductive core 10 to isolate selected imprinted deformations 12 . In this manner, individual imprinted deformations may be electrically isolated from the remainder of conductive core 10 in order to provide an isolated via, when a multi-level circuit card is constructed, in a manner which will be explained in greater detail herein. Next, with reference to Figure 5 , another step in the process of the construction of a layer of a multi-level circuit card is illustrated. While photo-resist layer 20 is still in place, the layer illustrated and constructed thus far is placed in an electrophoretic bath and the etched isolation rings 22 are electro-deposition coated to seal and give strength to isolation rings 22 . By using electro-deposition techniques, isolation rings 22 are filled partially with epoxy 24 and during a subsequent cure cycle each isolation ring 22 will achieve a smooth although somewhat recessed fill of epoxy 24 , thereby adding additional strength to the layer thus constructed. Referring now to Figure 6 , the next step in the process of constructing a single layer of a multi-layer circuit card is depicted. Photo-resist layer 20 has been removed by conventional means and a photo-sensitive solder mask 26 , is applied to the underside of conductive core 10 . Photo-sensitive solder mask layer 26 is then exposed and developed utilizing conventional photolithographic techniques to selectively expose concave surface 16 of selected imprinted deformations 12 . With reference now to Figure 7 , the next step in the process of creating one layer of a multi-level circuit card is illustrated. Dielectric layer 18 is reactive ion etched (RIE) to diminish the thickness of dielectric layer 18 . In this manner, each convex surface 14 of an imprinted deformation 12 is exposed. This technique is a preferred step wherein each convex surface 14 of an imprinted deformation 12 may be utilized to provide electrical contact between adjacent conductive cores upon the assemblage of multiple conductive cores into a multi-layer circuit card. Referring now to Figure 8 , the next step in the process of constructing one layer of a multi-level circuit card is depicted. The upper surface of conductive core 10 is now coated with a uniform plating of a metallic material such as copper. This is accomplished by seeding dielectric material 18 with a suitable seedable material such as platinum, palladium, nickel, or the like. The preferred seed material is palladium. U.S. Patent Application Serial No. 290,486, entitled "Method for Conditioning on Organic Polymeric Material" describes electrochemical and chemical methods for seeding a polymeric material, in particular a polyimide material with a seed material such as palladium. The teaching contained within the aforementioned patent application is incorporated herein by reference. The process according to U.S Patent Application serial No. 290,486. is a method for conditioning at least one surface of an organic polymeric material that is capable of transporting electrons and counter ions in a reversible manner. The process comprises applying electrons to redox sites, i.e., sites which may undergo reduction and oxidation, of the polymeric material with concurrent uptake of counter ions from the electrolyte. The reduced polymeric material is then placed into contact with a solution that contains cations of a metal which will diffuse into the organic polymeric material and contact the redox sites thereof. Metal deposited in such a manner can mediate continued electron transfer of the polymer resulting in further metal deposition under the previously deposited metal. The redox sites thereby transfer electrons to the cations which is energetically disposed to receiving electrons from the redox sites to thereby reduce the cation to metal atoms in the zero oxidation state. Next, a second metal is deposited from the electrolysis plating bath onto the zero oxidation state metal. The electrons are supplied to the redox sites of the polymeric material by either means of a cathode in an electrochemical circuit, the potential applied to the cathode being equal to or negative of the reduction potential of the polymer or preferably by means of a reducing agent and solution, the oxidation potential of the reducing agent being negative with respect to the reduction potential of the polymer. Alternatively, the electrons may be supplied to the redox sites of the polymeric material by contacting the polymeric material with tetrakis (dimethylamino) ethylene. The reducing agent may be in a charged or neutral form. The polymer must possess chemical functionality who's reduction potential is negative relative to the reduction potential of the metal ions. A suitable reducing agent for conditioning the polymer surface is a benzophenone radical, an ion in acetonitrile containing tetrabutylammonium fluoroborate as an electrolyte. The foregoing is only one example of a technique which may be utilized. Other seeding techniques known in the art may also be utilized. Next, by means known in the art, another metal layer is electrolessly deposited onto the seed layer to form a plating base. Metals such as copper, gold, silver, nickel and the like may be electrolessly deposited onto the seed layer. The thickness of the electrolessly deposited metal is typically from about .1 micron to about 10 microns. In accordance with the preferred embodiment of the present invention, copper plate 28 will be utilized to form the circuit lines and points of interconnection between this layer of the multi-level circuit card of the present invention and a second layer. Preferably copper plate 28 is approximately eight microns in thickness. The next step in the process of constructing one layer of a multi-level circuit card is illustrated in Figure 9 . Copper layer 28 is coated with an appropriate photo-resist material 30 . Photo-resist material 30 is then exposed and developed to generate desired circuit lines and points of interconnection within copper plate 28 . Of course, pattern plating or electroless plating may also be utilized to generate desired circuit lines and points of interconnection. Referring now to Figure 10 , the resultant circuit lines and points of interconnection which were formed with copper plate 28 are illustrated. Those skilled in the art will appreciate that copper plate 28 is etched into the desired circuit lines and points of interconnection by applying an appropriate etchant to those areas of copper plate 28 which are not protected by photo-resist layer 30 , in accordance with known techniques in this art. Finally, as depicted in Figure 11 , the circuitry side of conductive core 10 is coated with a photo-sensitive solder mask 32 and each section of copper plate 28 which covers a convex surface 14 of an imprinted deformation 12 is exposed through photo-sensitive solder mask 32 . Similarly, a conductive paste 34 is utilized to fill selected concave surfaces 16 of certain imprinted deformations 12 . Thus, those skilled in the art will appreciate that it will be possible to generate a conductive path from conductive paste 34 through that portion of conductive core 10 which forms imprinted deformation 12 , and thereafter through a portion of copper plate 28 which has been plated onto the convex surface 14 of an imprinted deformation 12 . Referring now to Figure 12 , depicted is portion of a multi-level circuit card which as been constructed in accordance with a preferred embodiment of the present invention with an electronic device mounted thereon. This higher level structure has been created by stacking a plurality of single layers which have been constructed in accordance with the method illustrated in Figures 1-11 . A plurality of conductive pads 40 may be mounted to the upper surface of multi-level circuit card 50 , in electrical contact with selected convex surfaces 14 of imprinted deformations 12 . Next, one technique for electrically interconnecting an electronic device or chip 38 to multi-level circuit card 50 is described. The technique for connecting chip 38 to multi-level circuit card 50 utilises a plurality of solder balls 36 in a manner which is commonly known in the art of solder reflow as controlled collapse chip connection (C4) technology. U.S. Patent Nos. 3,401,126 and 3,429,040 disclose a technique whereby the controlled collapse chip connection technique of face down bonding of a semiconductor chip to a carrier may be established. The technique described in U.S patent No's 3,401,126 and 3,429,040 is directly applicable to the mounting technique depicted in Figure 12 and the teachings of those patents are incorporated herein by reference. Additionally, those skilled in the art will appreciate that conductive paste may be utilized to attach chip 38 to multi-level circuit card 50 . The techniques described in the U.S patent No's 3,401,126 and 3,429,040 disclose the formation of a malleable pad of metallic solder on the semiconductor device contact site and the solder joinable sites on the conductors of the chip carrier. The device carrier solder joinable sites are surrounded by non-solderable barriers such that when the solder on the semiconductor device contacts sites melts, surface tension will hold the semiconductor device suspended above the carrier. Similarly, a conductive pad 40 may be mounted to the lower surface of multi-level circuit card 50 so that surface mount devices may be mounted to that layer. One key characteristic of a multi-level circuit card which has been constructed in accordance with the preferred embodiment of the present invention is that the compressive nature of conductive paste 34 within each concave surface 16 of an imprinted deformation 12 in combination with the exposed convex surface 14 of each imprinted deformation 12 means that the multiple layers of multi-level circuit card 50 will fit together much like a ball and socket. Thus, the problem encountered in prior art multi-level circuit cards of accurate registration is reduced by providing the structure of the present invention in a manner which not only does not require drilling through multiple layers but also makes accurate registration much more simple to accomplish in view of this ball and socket arrangement. Thus, it should be apparent to those skilled in the art upon reference to Figure 12 that a multi-level circuit card 50 has been created which includes three signal planes and three power planes. Of course, different combinations of signal and power planes are possible. Finally, with reference to Figure 13 , there is depicted a multi-level circuit card 50 which may be constructed in accordance with another embodiment of the present invention. A multi-level circuit card 50 is again constructed utilizing three single layers which have been constructed in accordance with the embodiment depicted in Figures 1-11 . However, unlike multi-level circuit card 50 depicted in Figure 12 , in which each conductive core 10 is situated with the convex surfaces 14 of each imprinted deformation 12 on the upper surface thereof, in the multi-level circuit card 50 of Figure 13 , one or more conductive cores 10 are positioned with the convex surface 14 of imprinted deformation 12 oriented in a downward manner. In this embodiment, a mass of conductive paste 34 is preferably utilized to create a conductive path between two concave surfaces 16 of imprinted deformations 12 , between the two lowermost conductive cores 10 . An additional layer of dielectric material 44 is provided at several levels to create additional insulation between adjacent conductive cores 10 . A second conductive mass 42 is also utilized to create a conductive path between these portions of copper plate 28 which are mounted to two convex surfaces 14 of two imprinted deformations 12 . In this manner, solder balls 36 may be simply and easily placed within conductive paste in the uppermost concave surfaces 16 of imprinted deformations 12 to provide a manner of mounting chip 38 . A multi-level circuit card structure has been described which includes a plurality of overlying substrates, each having at least one conductive core disposed therein. One or more imprinted deformations are formed in each substrate, each deformation having a protusion i.e. convex surface and a depression i.e concave surface. Dielectric coatings are utilized to provide insulation between adjacent substrates and by selectively placing a conductive mass between selected imprinted deformations an electrical connection is formed with an adjacent imprinted deformation wherein selected portions of one substrate can be electrically coupled to selected portions of a second substrate. The improved multi-level circuit card structure is formed from a plurality of substrates having thermally and/or electrically conductive cores which permits accurate registration between cores during construction thereof.

Claims: A multi-layer electronic circuit structure comprising: a plurality of overlying non-conducting substrates, each of said substrates having an electrically conductive core (10); and a printed circuit pattern (28) mounted on at least one of said substrates and in electrical contact with the core of that substrate, electrical connection being provided between the core of one of said overlying substrates and the core of another of said substrates; said electrical connection between said electrically conductive cores being provided by means of said printed circuit pattern and a deformation (12) of said cores, said deformation forming a protrusion (14) on a surface of that core. A multi-layer electronic circuit structure as claimed in claim 1 wherein said deformation forms a deformation (16) on the reverse of said surface corresponding to said protrusion (14). A multi-layer electronic circuit structure as claimed in any of claims 1 or 2 wherein conductive means (42) is disposed between a protrusion of one of said cores and a protrusion of another of said overlying cores. A multi-layer electronic circuit structure as claimed in claim 2 wherein conductive means (34) is disposed between a deformation of one of said cores and a protrusion of another of said cores. A multi-layer electronic circuit structure as claimed in any of claims 3 or 4 wherein said conductive means is a conductive paste. A multi-layer electronic circuit structure as claimed in any of the preceding claims further comprising an electronic component (38) mounted on said structure.

Inventors: FRANKENY RICHARD FRANCES, HERMAN KARL, FRANKENY, RICHARD FRANCES, HERMAN, KARL
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: H05K3
IPC codes: H05K3, H01L23