Abstract:
The invention is a method of and devices for making electrical connections to an Insulated Metal Substrate (IMS) printed circuit board. The approach is novel. The invention enables IMS type materials to be cost-effectively used with much higher currents. The invention also enables the substrate layer to be used as an active circuit layer.

Description:
BACKGROUND OF INVENTION 
     This invention pertains to power electronics design and manufacture. Specifically, the invention pertains to high current electrical connector designs that provide connections to Insulated Metal Substrate (IMS) circuit boards. 
     IMS circuit board materials are comprised of a metal substrate, usually aluminum or copper with a typical thickness from 0.040″ to 0.125″. A thin insulating material is bonded to the substrate and a layer of copper foil is bonded to the insulating material. 
     The IMS material is processed into printed circuit boards (PCBs) in much the same way as a typical fiberglass PCB where a photo mask is applied to the copper foil and the unwanted copper is chemically etched away, leaving the desired traces and pads. 
     For high power applications, IMS printed circuit boards have only one usable layer and are only suitable for surface mount components. Fiberglass boards can have many layers and a mix of through-hole and surface mount components. The value of the IMS material is in the very low thermal resistance from copper component mounting pad to the metal substrate. In high power applications, the substrate is in turn mounted to a heatsink. This allows surface mount semiconductor components such as transistors, rectifiers and SCRs to operate with low thermal resistance from device junction to ambient air. Low thermal resistance enables higher power to be processed with less silicon die area, which translates to lower costs. 
     There are two problems with the IMS materials for high power applications. First, the mechanical strength of the bond between the copper and insulating material and insulating material and substrate is limited. This weakness precludes the use of large, soldered, surface mounted terminals where high sheer and pull strength is required to reliably hold large cables. The prior art is to use multiple low-current surface mount connectors, pins or headers to make the transition to a fiberglass PCB. A single, high current, high mechanical strength termination could then be made on the fiberglass PCB. Second, the metal substrate layer is typically used only for mechanical support and heat transfer. It is desirable in most high power, high frequency switching applications to have a low impedance DC bus. This requires a two layer circuit board or other laminated bus assembly. The IMS material is limited by having only one easily accessible layer. In lower power applications an IMS board can be configured with a second copper layer but the heat transfer characteristics are compromised and the added cost may be prohibitive. Additionally, the problem of connecting large through-hole electrolytic capacitors to the to the circuit layers to achieve a low AC impedance bus is not solved. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention solves the two problems stated in the background discussion. First, a device will be disclosed that allows a high current connection to be made from an IMS printed circuit board to a wire or to a second printed circuit board. The connection will have high sheer and pull strength and is independent of the IMS insulating material bond strength. Second, a device and method will be disclosed for making a high current, low impedance, electrical connection from both the top copper foil and the IMS substrate to a second printed circuit board. This allows the IMS substrate to be used as an active circuit plane in conjunction with all or part of the top IMS copper foil to create a low AC impedance bus structure. This also allows a low AC impedance, coaxial connection with a fiberglass board where the fiberglass board is able to carry the larger through-hole components, such as electrolytic capacitors, relays and terminal blocks that the IMS board cannot. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIG. 1 illustrates cross sectional, top and a bottom views of an insulated, screw-on, high current PCB terminal for use with IMS circuit boards. 
     FIG. 2 illustrates a cross sectional view and an end view of a high current coaxial connector link for use between two printed circuit boards. 
     FIG. 3 illustrates a cross sectional view and an end view of a high conductivity lock nut to be used in conjunction with the coaxial link illustrated in FIG.  2 . 
     FIG. 4 illustrates a cross sectional view of an insulated, screw-on, high current PCB terminal as used with an IMS circuit board. This figure illustrates how PCB to PCB electrical connections are made. 
     FIG. 5 illustrates a cross sectional view of a high current, coaxial PCB to PCB link as used with an IMS printed circuit board and a second fiberglass PCB. This figure illustrates how two PCB to PCB electrical circuit connections are made using the link illustrated in FIG. 2, the lock nut illustrated in FIG. 3 and a self-clinching machine screw stud. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention consists of two methods of making high current connections to an IMS printed circuit board and three novel hardware devices to facilitate these two methods. The first method, illustrated in FIG. 4 uses the high current PCB terminal shown in FIG.  1 . The second method illustrated in FIG. 5 uses the high current coaxial connector link shown in FIG.  2  and the high conductivity lock nut shown in FIG.  3 . 
     The high current PCB terminal, illustrated in FIG. 1 comprises three parts; metal cap  6  with a threaded machine screw stud on the top of the cap, electrically insulating material  7  and threaded machine screw insert  8 . Threaded machine screw insert  8  has knurls around the outside diameter of the insert. Also, the preferred embodiment would use a potting compound for insulating material  7 . To simplify the discussion on how this terminal is applied; the composite device of elements  6 ,  7  and  8  will be referred to as terminal  1 . 
     FIG. 4 illustrates the first method of making IMS connections with a typical application for the high current PCB terminal  1  (FIG. 1) where a low resistance connection is made between IMS PCB  40  and fiberglass PCB  30 . Top copper foil layer  41  of IMS PCB  40  is etched to remove the copper foil within a prescribed radius to provide voltage clearance between self-clinching machine screw stud  50 , stud  50  is installed into metal substrate  43  of IMS PCB  40 , flush with the bottom surface of IMS PCB  40 . Terminal  1  is screwed onto stud  50  making electrical contact with top foil  41  of IMS PCB  40 . In the preferred embodiment, terminal  1  is soldered to top foil  41  of IMS PCB  40  around the bottom outside circumference of terminal  1  or under the mating surface ring area using reflow solder paste. This arrangement provides a high current, high strength terminal that is electrically isolated from the IMS substrate material. Fiberglass PCB  30  has a clearance hole for the machine stud on terminal  1  and is fastened to terminal  1  with standard flat washer  23 , lock washer  22  and hex nut  21  thus making a solid electrical compression contact between terminal  1  and bottom foil  32  of fiberglass PCB  30 . This assembly provides a low resistance high current connection between top foil  41  of the IMS PCB  40  and bottom foil  32  of fiberglass PCB  30  and also provides a means of mounting fiberglass PCB  30 . Additionally, a wire with a ring terminal may be fastened to the machine stud on terminal  1 , with or without the inclusion of PCB  30 . 
     The high current coaxial connector link illustrated in FIG. 2 comprises two parts; outside metal ring  9  and bushing  10 . Bushing  10  is made of an electrically insulating material and has a center clearance hole. 
     The high conductivity locknut illustrated in FIG. 3 is fabricated in one piece with an outside width and thickness much larger than standard hex nuts in proportion to the thread size to extend the bearing surface without using a fender washer. The threads are slightly deformed to provide lower electrical contact resistance and to lock the nut in position. The deformed threads are specifically designed to enable an electrical connection with significantly greater contact area, on the microscopic level, between the machine screw threads and the nut threads. This self-locking nut design also reduces the number of interfaces that current must flow through from three; nut-to-lock washer, lock washer-to-flat washer, flat washer-to-PCB foil to one; nut-to-PCB foil. The base metal and plating materials in the preferred embodiment are selected for high electrical conductivity and corrosion resistance. 
     FIG. 5 illustrates the second method of making IMS connections with a typical application for high current coaxial connector link  2  (FIG. 2) and the high conductivity locknut  3  (FIG. 3) where the substrate  43  of IMS PCB  40  is used as an active electrical conductor and where two, high current connections are brought from IMS PCB  40  to fiberglass PCB  30  with very low AC impedance between the two conductors. Coaxial link  2 , the composite assembly of elements  9  and  10 , is placed over self-clinching machine stud  50 , fiberglass PCB  30  is placed over machine stud  50  and the assembly is fastened together with high conductivity nut  3 . The resulting electrical circuit is a low resistance connection between top foil  41  of IMS PCB  40  through the coaxial link  2  to bottom foil  33  of fiberglass PCB  30  and a second autonomous circuit with a press-fit metal connection between IMS PCB substrate  43  and self-clinching machine stud  50 , through stud  50 , then through nut  3 , to top foil  31  of fiberglass PCB  30 . The AC impedance between these two autonomous current paths is very low due to the geometry of conductor link  2 . 
     This second method allows the use of IMS substrate layer  43  as an active power plane to form a low AC impedance DC bus structure on IMS PCB  40  and allows this low AC impedance characteristic to be maintained between IMS PCB  40  and energy storage capacitors on adjacent fiberglass PCB  30  and without stressing the mechanical bonds of IMS insulating layer  42 .