Abstract:
An arrangement of a flex circuit ( 7 ) is provided which includes a first substrate ( 10 ) with a connected first conductive membrane ( 16 ) having a first gap ( 20 ). A second substrate ( 44 ) with a connected conductive membrane ( 50 ) is spaced away from the first substrate ( 10 ). At least one of the substrates ( 10 ), ( 44 ) is flexible. A first surface mounted device ( 24 ) with a first end cap termination ( 36 ), bridges over the first gap ( 20 ). The end cap termination ( 36 ) electrically connects with the second conductive membrane ( 50 ) to complete an electrical circuit between the first and second conductive membranes ( 16 ), ( 50 ) when the substrates ( 10 ), ( 44 ) are displaced toward one another.

Description:
FIELD OF THE INVENTION 
     The field of the present invention is that of flexible circuits. More particularly the field of the present invention is that of flexible circuits utilized in the automotive industry which can additionally incorporate the terminals of surface mounted devices as contacts for various electrical components. 
     BACKGROUND OF THE INVENTION 
     In the most recent quarter century there has been a significant increase in the utilization of electronics in automotive vehicles. For example virtually all automotive vehicles are now controlled by an engine control module to optimize the performance of the engine with regard to fuel efficiency and environmental emissions. Inflatable restraints have been added to enhance occupant safety in an event of a crash situation. Advanced braking systems have been provided such as in anti-skid braking and traction control to further enhance safe operation of the vehicle occupants. Other electrical systems have been added or enhanced to add to the comfort of the vehicle. The above mentioned additions and enhancements have added to the complexity of the wiring system which supplies power and signal transfer to and from various electrical components and controllers. Currently most power requirements and signaling requirements in a vehicle are supplied by hard conductor wiring. The required wiring is provided in a bundle which is commonly called a wiring harness. Fabrication of a wiring harness is a very labor intensive operation. Additionally installation and/or repair of a wiring harness can also be very time consuming and expensive. Another disadvantage associated with the hard copper conductive wiring currently provided in most vehicles is in the cost associated with the copper. For certain applications the copper wire needed to provide for signal transfer at a low electrical load can be provided by a copper wire of a very small cross-sectional area. However, a minimum diameter of copper wiring is not determined not by its electrical requirements. The minimum diameter of the wiring is determined by its frailty during the assembly process of the wiring harness and of the wiring of the vehicle. 
     In an attempt to reduce the cost of automotive wiring harnesses a new type of electrical conduit has been provided. This new conduit is typically referred to as a flexible circuit. In a flexible circuit a polymeric (or insulated metallic ribbon) substrate which is flexible provides support for the various electrical conductor lines. The electrical conductors needed are then supplied by a foil which is affixed to the flexible substrate. Another flexible layer of polymeric material can cover the conductors on the flexible substrate. 
     The utilization of flexible circuits as a replacement for the prior wiring harnesses has been limited due to the particular problems faced by flexible circuits at the various component interfaces. For instance, the interface of the flexible circuit with a fuse box has been a particular limiting factor. To overcome those limitations an inventive contact method has been provided as explained in U.S. patent application Ser. No. 09/298,240 filed Apr. 22, 1999. Other examples of connector interfaces for flexible circuits can be found in a review of U.S. Pat. Nos. 5,885,091 and 5,969,418, commonly assigned. 
     Another problem which comes about with the utilization of flexible circuits is in the use of relays. Relays are typically switches that have two contacts which are either spring biased towards or away from each other, which are activated (closed or opened) by an electromagnetic force by a selectively excitable electromagnetic coil. Relays are typically provided for high current applications which require momentary application. Relays are commonly self-contained entities that contain a coil and contacts. Relays are typically pre-packaged to a pre-determined physical footprint. The design of relays is mostly controlled by their manufacturers and users of such relays adapt their circuits to pre-designed physical specifications. Two major elements in the design of relays is the generation of high current contact heat between the contacts, and the amount of current which is allowed in the exciting coil which is typically of a much lower current. Because of the current capabilities of the contacts of the relay, most relays have a solid post with a relatively high mass. The relay coil is typically specified to be excited by a low current and therefore has very low mass copper wires. When connecting the relay to a conventional circuit board the high relative mass of the relay terminals provide a relatively large heat sink so that lots of heat is required to assemble the relay to the remainder of the circuit. This heat required for assembly is typically within the limits of a rigid circuit board. However, with a flexible circuit no such rigid circuit board is provided and items such as relays which require a high heat input are difficult to assemble to the flexible circuit. An attempt to provide the heat required for many relays will cause the flexible circuit to be warped and deformed. Prior to the present invention to accommodate the heat of assembly the flexible circuit would often have to be redesigned to be a non-planar structure with a much larger mass then what was desirable. Such a modification of the flexible circuit diminishes its many advantages. Flexibility of the flexible circuit is especially important in applications of engine control modules which often have physically large circuits which are folded over for placement within a mounting box to conserve space within the engine compartment of the vehicle. It is desirable to provide freedom of utilization of a flexible circuit which allows for the relatively higher current contacts in connecting one circuit to another that is typically required in the utilization of a relay without having a flexible circuit with an increased mass which will diminish its characteristics of flexibility. 
     SUMMARY OF THE INVENTION 
     To make manifest the above noted and other desires a revelation of the present invention is brought forth. In a preferred embodiment circuit arrangement of the present invention, the contact members of the relay are separated from the coil mechanism. The contact members are brought into contact by displacement of two flexible substrates which are normally separated from each other. The contacts of the substrates can be one of the circuit conductive traces with an end cap of a surface mounted device on the other substrate. The device which brings the separated substrates in the contact with each other can be an aforementioned coil or can be due to physical displacement such as in a keyboard. With the utilization of surface mounted devices which are already utilized on the flexible circuits the end caps of the surface mounted devices can be utilized as the contacts. The present invention is even more particularly advantageous since most end caps are made from a silver alloy conductive material which provides a low interface resistance. When used in the embodiment of a relay, the flexible circuit arrangement of the present invention can have a coil that can be post connected with one of the substrates on a ferritic carrier. The coil need not be exposed to the heat of assembly of the surface mounted devices. 
     Other features of the invention will become more apparent to those skilled in the art upon a reading of the following detailed description and upon reference to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an enlarged side elevational view of a flex circuit arrangement according to the present invention. 
     FIG. 2 is an electrical diagram of the circuit shown in FIG.  1 . 
     FIG. 3 is an enlargement of an end cap of a surface mounted device with an alternate termination to that shown in FIG.  1 . 
     FIG. 4 is a sectional view of a flex circuit relay arrangement according to the present invention. 
     FIG. 5 is a perspective view of a ferritic insert and relay coil which is shown in the flex circuit relay arrangement of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, an arrangement of a flex circuit  7  of the present invention is provided. The flex circuit  7  includes a first generally polymeric substrate  10 . The first substrate  10  is approximately 0.75 mm thick and preferred values have been found between 0.25 and 1.5 mm. Although the 0.25 and 1.5 mm. range of thicknesses has been found to be preferable, the present invention can be utilized on substrates which are lower or greater than the aforementioned preferred thickness range. A preferred material for the first substrate  10  is polyimide. Polyimide is often preferable wherein soldering may be utilized to attach electrical devices to the substrate. Where a conductive adhesive is utilized to connect electrical devices to the substrate in lieu of soldering, polyester has been found to be suitable. The substrate can also include other material such as PVDF and Teflon. Furthermore, the substrate can be a metal ribbon which is insulated. The insulation may be provided by a post-lamination process, or by use of an adhesive which is used to bond a conductor to the substrate. If a metal ribbon substrate is provided, it can be utilized as part of an electrical circuit as a ground plane. 
     Adhesively connected with the first substrate  10  along its surface  12  is a first conductive membrane  16 . The first conductive membrane typically can be a foil or powder copper which has been etched out in a circuit desired pattern. Other conductive membrane materials will include aluminum or an un-solderable material such as aluminum or an aluminum alloy which is cladded over copper. The copper allows an electrical component to be attached to the connective membrane by solder, although aluminum is being relied upon to perform the conductive function. The first conductive membrane  16  has a gap  20  therein. The gap serves to prevent shorts between the termination points of electrical components. 
     Mounted over the first gap  20  is a first surface mounted device  24 . The first surface mounted device  24  provides a thick film resistor  28 . The surface mounted device  24 , as shown, is a flat chip resistor. However, other suitable surface mounted devices can also be utilized such as capacitors, inductors, fuses, shunts and other various electrical elements. The surface mounted device  24  has an alumina substrate  30 . Extending on top of the alumina substrate  30  is a film resistive element  32 . A typical resistive film material is Ruthenium Oxide RuO 2 . The resistive element  32  additionally has a protective coating on its side  34  which is not shown. The surface mounted device  24  has two outer post terminals or end caps  36 . There are two ways to fabricate the end cap of the chip resistor surface mounted device  32 . In FIG. 3 the end cap construction illustrates a pre-termination of the conductor  31 . The conductor  31 , sometimes referred to as inner termination, is applied first, then the resistor material  33  is applied. In the FIG. 1 the end cap conductor is post terminated, being applied after the resistor  32  is applied to the substrate  30 . A 0.0003 mm thick termination barrier  35 , extends over the inner termination  31  and is typically provided by nickel (shown in FIG. 3 only). The barrier  35  termination is provided for leaching control. The barrier termination  35  is covered by a 0.0003 mm thick outer termination  37  (shown in FIG. 3 only). The outer termination is typically provided by a precious metal silver or silver palladium alloy. The palladium is typically added to prevent the leaching of the silver from the outer terminal during a subsequent soldering attaching operation. The end caps can vary in height, typical values are 0.35 to 0.6 mm. The first surface device will typically have a {fraction (1/16)} to 1.0 watt power rating with a 0.1 to 22 Kohms resistance range. The first surface mounted device  24  is attached with the first conductive membrane  16  by solder  40 . 
     Spaced generally parallel away from the first substrate  10  is a second flexible generally non-conductive substrate  44 . The second substrate  44  can be generally similar or identical to the first substrate  10  and in certain applications may be formed on one and the same sheet of substrate and then be folded to the position as shown in FIG.  1 . Additionally, the first and second substrates can be held in position by a frame or housing, not shown. Connected on a surface  46  of the second substrate  44  is a second conductive membrane  50 . The second conductive membrane  50  has a gap  54 . Bridging the gap  54  is a surface mounted device  60 . The surface mounted device  60  provides a thick film capacitor. The thick film capacitor  64  is supported on an alumina base  70 . In a similar manner, afore-described in regards to the first surface mounted device  24 , the surface mounted device  60  has end caps  72  and  74 . End caps  72  and  74  are connected with the second substrate  50  by solderings  76 . 
     The end caps provide contacts for the circuit shown in FIG.  2 . The substrates  10 ,  44  can be displaced with respect to one another by mechanical contact with surfaces  80  or  82  or by utilization of an electromotive force which acts upon ferritic members positioned adjacent to the first and second substrates  10  and  44 . Upon displacement of the substrates  10 ,  44  the end caps  36  and  72  are allowed to make contact with one another to complete the circuit. It is apparent to those skilled in the art that alternatively, one of the substrates can be rigid allowing the opposite substrate to be flexible allowing its respective surface mounted device to be pushed towards the opposite surface mounted device. A significant advantage of the flexible circuit arrangement  7  is that the spaced-apart positions of the substrates  10  and  44  for one another can be restored by the natural rigidity of the polymeric substrates  10 ,  44 . Additionally, the end cap surfaces  86  and  84  can make adequate electrical contact without perfect alignment with one another. Still another advantage of the present invention is that the contacts  36  and  72  provided by the end caps takes advantage of the fact that the end caps are typically coated with silver or a silver palladium alloy or other precious metals which already provide low contact resistance without a special preparation as required in other electrical device contacting members. 
     Referring to FIG. 4, a flex circuit relay  107 , according to the present invention is provided. The relay  107  includes a first substrate  102 . The properties of the substrate  102  are as those previously described in regards to substrate  10 . The substrate  102  has connected thereon a first conductive membrane  106 . Conductive membrane  106  has properties essentially the same as those of previously mentioned conductive membrane  16 . Conductive membrane  106  has three gaps  108 ,  110  and  112  respectively. Merging over the gaps  108 ,  110  and  112  are surface mounted devices  114 ,  116  and  118 . Surface mounted devices  114  and  116  are both larger in the height dimension then surface mounted device  118 . Spaced from the first substrate  102  is a second polyester flexible substrate  122 . The substrate  122  has connected thereto on a side  124  facing the first substrate  102 , a conductive membrane  130 . The conductive membrane  130  has a gap  132 . 
     The substrate  102  has a slot  140 . The substrate  122  has a corresponding generally aligned slot  144 . Inserted within the slots  144  and  142  is a ferritic member  150 . The ferritic member  150  is shaped generally as a hair pin having a closed end  152  with a first extending arm  154  and a second extending arm  160 . The second arm  160  has a laterally bent portion  164  which is encircled by a spool  170 . The spool  170  is encircled by coil windings  174 . The coil windings  174  can be selectively energized by a power source (not shown). The substrate  122  is supported away from the first substrate by the end caps  178  and  180  of the surface mounted devices  116  and  114 . Excitation of the coils  174  causes an electromagnetic field to be generated within the ferritic member  150  causing the arms  160  and  154  to seek to close together to complete the electromagnetic field causing the second conductive member  130  to come into contact with an end cap  188  of the first surface mounted device to complete an electric circuit. Accordingly, the arrangement  107  provides a flexible circuit relay. A significant advantage of a flexible circuit relay  107  is that the relay ferritic member and coil windings  174  can be added to the circuit subsequent to the soldering of the surface mounted devices  114 ,  116  and  118  to the first conductive membrane  106 . Therefore, the windings  174  can be made as small as possible and are not exposed to the heat in the soldering operation. Additionally, separate contacts need not be made and the electric connection can be made directly between the conductive membrane  130  and the end cap  188 . 
     While the invention has been described in connection with a preferred embodiments, it will be understood that it is not intended to limit the invention to those particular embodiments. On the contrary it is endeavored to cover all alternatives, modifications and equivalent as may be included within the spirit and scope of the invention as encompassed by the description and as defined by the appended claims.