Patent Document

The present invention is related to the field of electrical components, and more particularly is related to the field of shunts for circuit panels. 
     BACKGROUND OF THE INVENTION 
     Shunts for circuit panels are known which have a plurality of pairs of contacts for completing circuit paths of a circuit panel, where it is desired to program the circuit panel by bringing at least a selected one of the circuit paths into a closed condition by a corresponding electrically joined pair of contacts. Programming of the shunt to program the circuit panel by opening a selected circuit path is accomplished by physical breaking of the strap electrically connecting the particular pair of contacts in the shunt corresponding to that path, as known from the DIP shunt sold by AMP Incorporated of Harrisburg, Pa. under Part No. 435704-5. Such a shunt cannot be sealed, because the aperture must remain open to receive the strap-breaking tool. Also and more importantly close spacing of the contact pairs is not feasible due to the need for adequate lateral spacing to receive the tool for assured breaking of only the selected strap and not adjoining ones. An improved method of creating a discontinuity between a selected pair of contacts is desired not requiring tool insertion. 
     SUMMARY OF THE INVENTION 
     The electrical component of the present invention has pairs of contacts having outer contact sections electrically engageable with contact sections of circuit paths of a circuit panel such as a circuit board or a flexible panel. Inner contact sections are in spaced pairs in respective cavities of the housing and are bridged by a fuse element secured thereto in electrical engagement therewith. The fuse element has a selected in-service current-carrying capability corresponding with that of the circuit path whose circuit it completes upon the components being secured to the circuit panel, and also has characteristics selected to cause opening of the fuse upon application of a designed electrical programming current by melting due to heat buildup from high resistance due to the small diameter. 
     According to one method of using the electrical component of the invention, the component serves as a programmable shunt for programming the circuit panel. The outer contact sections corresponding to the circuit path desired to be an open path, have applied thereto a designed programming current prior to mounting the component to the circuit panel. Programming current can also be applied after mounting the component to the panel by applying it to the circuit path desired to be opened, which may be done prior to mounting the thus-programmed circuit panel into its ultimate in-service site. Such applied programming current opens the corresponding fuse element and breaks the electrical connection between the inner contact sections. 
     According to other aspects of the electrical component of the present invention the contacts can be insert molded in the housing, the cavities of the housing containing the fused inner contact sections can be sealed, and the outer contact sections can be disposed in a dual in-line arrangement and can either comprise vertical legs for insertion into panel holes and soldered to conductive pads of the circuit paths or can have horizontal sections for surface mounting to conductive pads of the circuit paths. The fuse element can be welded, bonded or crimped to the inner contact sections and can comprise a length of fine wire such as 0.0015 inch diameter constantan or copper alloy wire, or 0.0007 inch diameter aluminum wire. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the electrical component of the invention exploded from a printed circuit board with a housing end wall portion broken away. 
     FIG. 2 is a cross-sectional view taken along lines 2--2 of FIG. 1. 
     FIG. 2A is similar to FIG. 2 representing an alternate embodiment of securing the fuse element, and of the outer contact sections. 
     FIG. 3 is a perspective view showing a segment of a flexible circuit panel having conductive pads for surface mounting of the embodiment of the electrical component of FIG. 2A. 
     FIGS. 4A-4D are perspective views illustrating sequentially one method of making the electrical component of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2 show the electrical component 10 of the present invention having a housing 12 and a plurality of pairs of contacts 14A,14B spaced therealong preferably in a dual in-line arrangement. Each contact 14A has an innner contact section 16A spaced proximate an inner contact section 16B of its corresponding contact 14B. A fuse element 30 extends between the pair of inner contact sections 16A,16B and is mechanically secured thereto at joints 32 in electrical engagement therewith. Each of the pair of contacts 14A,14B also has an outer contact section 18A,18B respectively for electrical engagement with a corresponding contact means of an elecrical circuit path on a circuit panel. Fused inner contact sections 16A,16B are preferably disposed in respective cavities 20 of housing 12. The electrical component is preferably sealed by securing a sealing member 22 to the top 24 of housing 12 and a sealing member 26 to the bottom 28 of the housing. Outer contact sections 18A,18B are vertical leg sections for insertion into holes 54 which form part of the circuit paths 52 on a rigid circuit panel such as a printed circuit board 50. The fuse element 30 is preferably secured for performance reasons to the top surfaces of inner contact sections 16A,16B and arced upwardly away from the ends of the contact sections. 
     FIG. 2A illustrates an alternate embodiment where fuse element 30 is secured by joints 32 to bottom surfaces of the inner contact sections 46A,46B of contacts 44A,4B, in housing 42 of component 40. Because the walls of the housing must have a height sufficient to enable handling by automated handling apparatus for positioning on a circuit panel, it is easier during fabrication of the fuse shunt to secure the fuse element 30 to the bottom surfaces of contact sections 46A,46B which are proximate the bottom face of the fuse shunt. Contacts 44A,44B include outer contact sections 48A,48B which comprise a pair of coplanar horizontally extending sections, for surface mounting to conductive pads 64 of the circuit paths 62 on the surface of a circuit panel, such as flexible circuit panel 60 as shown in FIG. 3. 
     The fuse element is secured to the inner contact sections such as by conventional resistance welding or wire bonding techniques to form joints 32. Another method of joining the fuse element is disclosed in U.S. patent application Ser. No. 857,209 filed Apr. 29,1986. In that method the fuse element is a wire segment first disposed in a groove skived axially along the inner contact sections and then terminated by deforming portions of the inner contact sections forming sidewalls of the groove over the top of the wire at at least one location on each inner connect section by means of a terminating tool. Various methods based on conventional techniques may be used so long as heat is not generated in sufficient amounts to inadvertently open or damage the fuse element which is fragile requiring care in handling and processing. 
     It is believed preferable to secure the fuse element to the pair of contact sections after securing the contacts in the housing, so that the housing provides mechanical stability and enhances physical protection of the fragile wire fuse element during fabrication as is shown in FIGS. 4 A to 4C. While joined to a carrier strip 70, the contacts 14A,14B are preferably placed in a mold and a dielectric housing 12 molded thereto by conventional insert molding techniques, as shown in FIG. 4B. The fuse elements 30 are then secured to respective contact sections, as in FIG. 4C. Thin, transparent sealing membranes 22,26 are then preferably adhered to the top and bottom surfaces 24,28 respectively of housing 12 completing the manufacture of the electrical component, as in FIG. 4D. The completed components can then be severed from the carrier strip and the outer contact sections 18A,18B formed into the desired configuration. The contacts 14A,14B are preferably stamped from a strip of copper alloy, and outer contact sections 18A,18B may be tin-lead plated for solderability. Housing 12 may be formed of a thermoplastic material such as glass-filled polyester resin. Sealing membranes 22,26 may be MYLAR (trademark of E. I. du Pont de Nemours and Company). 
     Fuse element 30 is preferably a wire segment of a selected very small diameter creating high resistance, and may be any of several conventional types of conductive metals such as high copper content alloy, aluminum, silver alloy, or constantan. The proper material to be used, and the actual diameter selected depend on the type of current desired to be carried by the fuse during normal in-service use and also the designed programming current for opening the fuse element. For example, a satisfactory fuse element can be a short length of aluminum wire having a diameter of 0.0007 inches if it is desired that the fuse carry an in-service current of 0.100 amperes and open upon receiving a programming current of 1.0 amperes for 100 milliseconds or less. A satisfactory fuse element can be a short length of constantan alloy having a diameter of 0.0015 inches for the same in-service and programming currents. 
     The fuse element opens by melting upon sufficient heat buildup resulting from the programming current passing through its very small diameter, and limited length for sufficient time. It is possible to estimate an appropriate small diameter for the fuse element when the following items are known: the programming current (l p ) and programming time (t), length of the fuse (L), ambient temperature (T a ), and metal alloy being used for the fuse. Characteristic properties of the metal alloy are ascertained: melting temperature T m , specific heat (Cp), latent heat of fusion (Q f ), resistivity (ρ), and specific gravity (SG). 
     The heat required to melt the fuse is related to the fuse element dimensions and properties as follows: 
     
         HEAT=MASS×[(T.sub.m -T.sub.a)Cp+Q.sub.f ]            (1) 
    
     where the mass of the fuse element is ##EQU1## 
     The power generated by the current through the fuse is 
     
         POWER=l.sub.p.sup.2 R                                      (3) 
    
     where 
     
         R=ρ4L/πD.sup.2                                      (4) 
    
     and the power and heat to melt are related to each other as: 
     
         HEAT=POWER×t                                         (5) 
    
     EXAMPLES 
     Where two metal alloys are considered, constantan and aluminum, their characteristic values are as follows: 
     
                       TABLE 1______________________________________        Constantan Aluminum______________________________________SG (specific gravity)          0.323 lb/ft.sup.3                       0.0975 lb/ft.sup.3T.sub.m (melting temperature)          2210° F.                       660° F.Cp (specific heat)          0.098 Btu/lb/°F.                       0.215 Btu/lb/°F.Q.sub.f (latent heat of fusion)          100 Btu/lb   170 Btu/lbρ (resistivity)          374 Ω mil.sup.2 /ft                       20.37 Ω mil.sup.2 /ft______________________________________ 
    
     Typical values for the remaining variables, relevant to the present invention and its purpose and typical environment, are: 
     
                       TABLE 2______________________________________I.sub.p (programming current) =                 1.0 amperest (programming time) =                 100 millisecondsL (fuse length) =     0.10 inchesT.sub.a (ambient temperature) =                 75° F.______________________________________ 
    
     Because adjacent contact structure at the terminations 32 of the ends of fuse element 30 is at theoretical ambient temperature, the contacts act as heat sinks and absorb some of the heat from the fuse element during programming. Other matters affecting programming are dimensional variations in the diameter of the fuse elements from fabrication thereof, surface contamination thereon, and the fuse length between terminations. 
     It is believed that successful programming of a fuse shunt may be accomplished using fuse elements of the metals given above at the stated length and programming current and time at the following diameters: 
     
         D constantan=0.0015 inches 
    
     
         D aluminum=0.0007 inches 
    
     Unprogrammed (unopened) fuse elements at such diameters are believed will successfully carry in-service currents of 100 milliamperes. 
     To avoid interfering with the opening of the fuse elements, the element 30 should be preferably spaced away from the ends of the inner contact sections 16A,16B and also from any of the structure of the housing 12 or seals 22,26 which would act to dissipate heat otherwise needed to melt the fuse. The seals 22,26 serve to physically protect the fragile fuse elements 30, and contain any vapors give off during the fuse melting and avoid possible contamination of nearby circuitry or components outside of component 10.

Technology Category: 4