METHOD FOR MAKING COAXIAL INTERCONNECTION BOARDS AND BOARDS MADE THEREBY

Abstract: The invention relates to a method for making coaxial con­ ductor interconnection boards including, on the upper side of the board, signal conductors surrounded by a dielectric and by a conductive shield. To prevent shorting of the signal conduc­ tors with the shield, the conductors are step-stripped and the unshielded conductor end with the dielectric exposed is in­ serted in drilled, non-metallized holes. The completely ex­ posed signal conductor extends to the lower side of the board and is connected conductively to terminals pads, while the shields on the upper side are connected by a conductive layer which is connected to the ground plane.

Description: This invention relates to wire scribed circuit boards and methods for their production wherein at least some of the conductors are coaxial conductors. Coaxial conductors are the preferred transmission medium for high frequency signals, particularly high speed digital signals. The signal conductor of coaxial conductors is protected by a shield which is electrically connected to the reference potential or "ground". The shield isolates the signal conductor from the surrounding environment, thus providing a clean uniform environment for signal travel. The shield prevents radiation of energy from the signal conductor which improves signal propagation and reduces electrical interference which may affect other signals in the vicinity. Although coaxial conductors are recognized as a preferred interconnect medium for high speed signals, they are not widely used in electrical circuit boards because of the problems incurred in properly terminating the ends of the coaxial conductor. The signal conductor and shield must be kept electrically isolated from each other at the terminal points. In manual installations, the signal conductor is usually terminated by stripping back or removing the shield and insulation and then connecting it to the terminal point by soldering, welding or the like. Termination of the shield is conventionally made by connecting a wire, often called a drain wire, between the shield and a central ground point. This approach, however, is not practical in automatically produced interconnection boards or where there are large numbers of interconnections. Wire scribed circuit boards can be produced automatically using the method described in US patents 3 671 914 and 3 674 102. In this process, an insulated wire is scribed onto an insulating base by feeding a continuous strand of wire onto the surface of a base while simultaneously affixing the wire to the base thereby producing a wire image of a predetermined interconnect pattern. Through holes are subsequently drilled at the terminal points intersecting the end points of the conductors. The hole walls are then metallized to electrically connect the ends of the conductors to surface terminal pads or later added components. The above cited patents describe a method for adhering insulated wire to an insulating base or substrate and for making terminations at the ends of the conductors. The process, however, does not work if coaxial conductors are used, since the signal conductor and shield would become electrically connected and thus "shorted" at the terminal points. In US patent 3 969 816 discrete coaxial conductors are placed at locations on a substrate. Lengths of coaxial conductors are bridged between discrete point locations determined by a matrix apertured substrate. The conductors are anchored within the substrate apertures and the ends then milled flush with the board surface to expose the shield and signal conductor. In order to prevent shorting of the signal conductor to the shield, a ring of dielectric material is placed on the surface, covering the shield end. In another embodiment, a selective etch (employing a photo-resist mask) is used to recess the shield from the surface plane. The patent also discloses terminal points interconnected by insulated conductors which are metallized to create a shield for the conductors. The conductor termination techniques disclosed are all based on having conductor terminations flush with the board surface. This invention provides a method for making a coaxial interconnection board wherein said coaxial conductors include a signal conductor surrounded by a dielectric, in turn surrounded by a conductive shield, characterized by the steps of drilling holes through the substrate of the board at selected terminal points; placing a plurality of preformed coaxial conductors on one surface of the substrate by a practice comprising stripping the ends of said coaxial conductors to provide exposed signal conductor sections and exposed dielectric sections; inserting said stripped ends of the coaxial conductors through said holes at the terminal points so that said exposed dielectric sections substantially reside in said holes, said conductive shields being on one side of the board, and said exposed signal conductors on the other side of the board. The holes at the terminal points are non-metallized. The circuit board is constructed having one side provided with a ground plane and having the other side provided with terminal pads and signal conductors. Holes are drilled through the board at the terminal points. The ends of the scribed coaxial conductors are step-stripped to expose a length of bare signal conductor followed by a bare insulation section. The ends of the coaxial conductor are then inserted so that the bare insulation sections fill the holes. The bare signal conductor emerges from one side of the board and is connected to a respective terminal pad whereas the complete, shielded coaxial conductor resides on the other side. Since the coaxial conductor shields are all on the side of the board which does not include any signal conductor terminations, the shields can be interconnected by applying a plated copper layer or a conductive encapsulating layer or by reflow soldering. Figs. 1A through 1E are a series of sequential illustrations showing the process according to the invention for making coaxial interconnections boards. Figs. 2A through 2C illustrate a variation of the process of Figs. 1A through 1E. Figs. 3A through 3C illustrate another variation of the process of Figs. 1A through 1E. Figures 1A through 1E illustrate the sequential steps used in creating a coaxial circuit board according to one embodiment of the invention. The ends of the coaxial conductors are step-stripped and inserted through holes in the board so that the shields are on one side and the exposed signal conductors are on the other side of the board. By stripping back the coaxial conductors and locating the shields and exposed signal conductors on opposite sides, a clearance area is provided which permits termination of the signal conductors without creating an electrical short circuit to the shields. With this arrangement, the shield(s) can then be interconnected using a common ground plane. The initial board shown in Fig. 1A includes a substrate 100 with signal conductors on one surface including a terminal pad 102. The other side of the board is coated with an activatable adhesive layer 104. In addition, the board may contain internal conductors such as conductor 106, as well as ground planes and power planes (not shown). Holes 108 are drilled through the board at the terminal points in the area of the terminal pads, here pad 102. A coaxial conductor 110 is prepared as shown in Fig. 1B by step-stripping the end. The shield 112 of the coaxial conductor 110 is stripped to expose the dielectric insulation 114, and then the end portion of the exposed insulation is further stripped to expose a section of signal conductor 116. The length of the exposed insulation section 114 is approximately equal to the thickness of the board 100. The step-stripping can be accomplished mechanically using notched knife blades. One such knife blade would be dimensioned with a notch which accommodates the insulation 114 and used to strip the shield from the end of coaxial conductor 110. Another such knife blade would be dimensioned with a notch which accommodates the signal conductor 116 and used to strip insulation 114 from the signal conductor 116 in the area where shield material has previously been removed. Alternatively, other known stripping techniques can be used such as grinding, abrading, chemical removal and laser vaporization. As shown in Fig. 1C, the stripped coaxial conductor end is inserted in hole 108 so that the exposed insulation section 114 fills the hole with the bare signal conductor 116 extending from the side of the board including terminal pad 102 and the shield 112 extending from the other side of the board. The coaxial conductor 110 is then tacked in place in the adhesive coating 104 as shown in Fig. 1D. This can be accomplished using the general method disclosed in US patent 3 671 914 where the board is mounted on an X-Y table and the coaxial wire dispensed from a scribing head. Adhesive layer 104 is activated along the conductor path by applying energy locally, such as ultrasonic energy. After the conductor run has been tacked in place, the other end of the coaxial conductor 110 is similarly step-stripped and inserted through a terminal hole (not shown). The bare signal conductor 116 can be bent over and soldered to terminal pad 102, as shown in Fig. 1D, or can be connected by floating the board in a solder bath to interconnect the signal conductor and the terminal pad. Thereafter, the board side with the coaxial conductor(s) 110 is encapsulated with a conductive epoxy resin 118 to form a ground plane interconnecting all of the conductive shields 112. A suitable material is EPO-TEL 17® which is epoxy filled with silver flakes (4 parts metal to 1 part epoxy). Alternatively, this side of the board can be plated with a copper layer to provide interconnection to the shields and a ground plane. The copper plated board side may then be encapsulated with a non-conductive resin, if desired. A variation of the process according to the invention is shown in Figures 2A through 2C. The circuit board substrate 201 is provided with holes 202 having non-metallized walls which are located to provide coaxial conductor termination points or component lead connections. The upper side of board 201 includes a surface conductor pattern 204 produced by conventional printed circuit techniques. The conductor pattern 204 includes conductive areas or lands 205 adjacent or surrounding the holes 202. The lower side of the board is copper coated to provide a ground plane 206. The coaxial conductors 210 are cut to length and step-stripped at both ends as shown in Fig. 2A. Thus, shield 212 is removed from each end of the coaxial conductors 210 to expose a bare insulation section 214, and the insulation is removed to expose a bare signal conductor section 216. The coaxial conductors 210 are pre-cut to lengths according to the distance to be spanned and according to the desired transmission line characteristics. The ends of the thus prepared coaxial conductors 210 are then inserted in the appropriate holes, as shown in Fig. 2B. This can be done manually or semi-automatically using equipment for appropriately locating the coaxial conductor ends and inserting them into the holes 202. The bare insulation sections 214 correspond in length to the thickness of the substrate 201 and, thus, substantially fill the holes 202 when the coaxial conductors 210 are in place. The signal conductor ends 216 emerging above the board are bent over to make mechanical contact with the appropriate surface conductor areas 205. The signal conductors 216 may then be soldered or welded in place. The loose coaxial conductors 210 on the lower side of board 201 are then dressed down to lay flat on the board surface and the board is coated with a conductive layer 218 (Fig. 2C) which interconnects the shields 212 of the various coaxial conductors 210 and connects to the ground plane. The conductive coating may be either an epoxy-filled conductive resin or may be a plated copper layer. Where desired, a non-conductive surface lamination layer may thereafter be applied. (Not shown). Another variation according to the invention is shown in Figs. 3A through 3C. The lower surface of the substrate is provided with an additional solder coating 220. Furthermore, the surface of the shield 212 of the coaxial conductors 210 is provided with a solder coating 222 surrounding said shield 212. The coaxial conductors 210 are inserted as shown in Fig. 3B and the signal conductor ends connected in a manner similar to the one previously described in connection with Fig. 2B. When the coaxial conductors 210 are in place, they are dressed down to lay fiat on the board surface and solder 222 on the coaxial conductors 210 and on the board surface 220 is reflowed. If necessary, additional solder may be added during the solder reflow process. When completed, all of the coaxial conductor shields 212 are interconnected by means of the formed solder coating 224.

Claims: A method of making a coaxial conductor interconnection board wherein said coaxial conductors include a signal conductor surrounded by a dielectric in turn surrounded by a conductive shield, characterized by the steps of drilling holes (108, 202) through the substrate of the board (100, 201) at selected terminal points; placing a plurality of preformed coaxial conductors (110, 210) on one surface of the substrate by a process comprising stripping the ends of said coaxial conductors (110, 210) to provide exposed signal conductor sections (116, 216) and exposed dielectric sections (114, 214); inserting said stripped ends of the coaxial conductors through said holes at the terminal points so that said exposed dielectric sections (114, 214) substantially reside in said holes, said conductive shields (112, 212) being on one side of the board, and said exposed signal conductors (116, 216) on the other side of the board. The method of claim 1 wherein the coaxial conductors (110, 210) are scribed on said one surface of the board by placing an adhesive coating on the board and activating said adhesive upon contact with the coaxial conductor being scribed. The method of claim 1 wherein the coaxial conductors are precoated with an adhesive and scribed by activating said adhesive upon contact with the board surface. The method of claim 1 further including the step of coating the side of the circuit board provided with said coaxial conductors (110, 210) with a conductive material (118, 218) to interconnect said conductive shields. The method of claim 4 wherein said coating (118, 218) is a copper layer formed by metal plating. The method of claim 4 wherein said coating (118, 218) is a conductive resin layer. The method of claim 4 wherein said coating is solder (224, Fig. 3C). The method of claim 1 wherein said coaxial conductors (110, 210) and one side of said board provided with said coaxial conducters are coated with solder (220, Fig. 3B) prior to inserting said stripped ends in said holes, and wherein said solder (220) is formed into a conductive coating (224) interconnecting the coaxial conductor shields by reflowing said solder after said coaxial conductor ends have been inserted. The method of claim 1 wherein said circuit board includes conductive terminal pads (102, 205) on the side free of coaxial conductors, and wherein said exposed signal conductors (116, 216) emerging from said holes (108, 202) are electrically connected thereto.

Inventors: PLONSKI, PHILIP J.
Applicants: ADVANCED INTERCONNECTION TECHNOLOGY, INC.
Application year: 1986
Main classification: H05K1
IPC codes: H05K3, H05K7, H01R9, H05K1