Abstract:
Improved fuse elements used with the circuit board to provide more reliable sequential firing in a flash lamp array. The circuit board includes a lamp firing circuitry along with radiation switches connected in said circuitry and more reliable fuse elements cooperate with said radiation switches in the circuit operation. The improved fuse elements are constructed as an integral part of the lamp firing circuit pattern deposited on the circuit board at circuit locations wherein the underlying circuit board substrate has a thickness less than the substrate thickness elsewhere to enhance melting or thermal decomposition of the circuit board substrate.

Description:
BACKGROUND OF THE INVENTION 
     Radiation sensitive fuse elements for use with a circuit board to provide more reliable sequential firing in the flash lamp array are already known. For example, U.S. Pat. No. 4,133,631, assigned to the assignee of the present invention, describes a circuit board configuration which includes such fuse elements being deposited at circuit locations wherein the underlying substrate has a thickness less than the substrate thickness elsewhere to produce holes in the circuit board by melting or thermal decomposition when activated by radiation being emitted from adjacent flash lamps. Said fuse elements are deposited on the underlying circuit board substrate and comprise at least 20% or more of a radiation absorbent solid particulate such as carbon black mixed with conventional liquid adhesive which hardens to a solid at ordinary temperatures. When this fuse material composition is deposited directly on the circuit pattern and allowed to harden at locations on the circuit board that have been recessed to enhance localized melting or decomposition of the underlying substrate, the fuse actuation produces a hole through the entire composite medium. Interruption of the circuit path is obtained in this manner to the activating flash lamp which can be disposed in a branch circuit along with a fuse element and thereby disrupt electrical connection to the already fired flash lamp. 
     Another type known radiation-sensitive fuse element construction for use in the same general manner is described in U.S. Pat. No. 4,154,569. This type fuse element construction utilizes a strip-like element which overlies a reduced thickness area of the circuit board member and which can be constructed from a polymer film deposited on the circuit board substrate which is thereafter perforated to help isolate the absorbed thermal energy in said strip element for more reliable thermal decomposition of the entire fuse. The circuit board pattern on which said strip elements are deposited is said to be formed of metal, for example silver, nickel, tin, copper or other readily conducting material, for example graphite. The circuit board support for the electrical circuit is also said to be pigmented in order to improve the heat absorbing capacity of the overall construction and soluble dyes can also be incorporated with the thermoplastic synthetic resin composition of said dielectric support for this purpose. Additionally, a dye is recommended for incorporation in the polymer material forming the strip-like fuse element to further improve radiation absorption in the overall construction. 
     Still other issued U.S. Pat. Nos. 3,968,056 and 3,988,647, which are also assigned to the present assignee, describe a circuit board construction which can be used with a photoflash array wherein a particular type electrically conductive ink provides the circuit pattern. Specifically, said conductive ink comprises an organic resin matrix having sufficient particulated electrically conductive material dispersed therein to form point by point electrical contact when the deposited liquid ink is cured to the solid state. In a preferred embodiment, a liquid ink composition is radiation curable to provide a solid coating having a resistivity of less than 10 ohm-centimeters when cured. The particulated electrically conductive material providing the electrical conductivity in said circuit pattern can be selected from a particulated electrically conductive metal and/or a particulated electrically conductive metal containing material which can further contain up to approximately 15% by weight of said particulated electrically conductive material in flake-like form with an aspect ratio of diameter to thickness greater than 20. The content of particulated electrically conductive material in said preferred embodiment is at least about 40% and not more than about 90% by weight with said particulated electrically conductive material preferably being in the form of metal-coated glass spheres. 
     SUMMARY OF THE INVENTION 
     Lower intensity flash lamps are now being used for cost and energy saving considerations which create a need for fuse elements in the above type photoflash array that interrupt the circuit path reliably with a lesser amount of radiation being emitted by such flask lamps. Accordingly, it is a primary object of the present invention to provide an improved fuse element construction in a circuit board of this type having both radiation sensitive switching elements and radiation sensitive fuse elements as part of the circuit pattern for cooperation therebetween when the associated lamps are flashed. Another object of the present invention is to provide fuse means which more reliably interrupts the circuit path when exposed to radiation from a cooperating flash lamp to thermally melt or decompose the underlying substrate and create a hole or opening for permanent disruption of the electrical connection. In accordance with said objects, the present improvement is accomplished with a circuit board pattern comprising an organic resin matrix having particulated electrically conductive material and a radiation absorbing material dispersed therein with said circuit pattern further including radiation sensitive fuse elements located where said dielectric substrate of the underlying circuit board member has a reduced thickness relative to the remaining thickness of said dielectric substrate and with said fuse elements being formed by providing a more narrow width of said circuit pattern at the fuse locations than the remaining width of said circuit pattern. The preferred radiation absorbing material dispersed in the circuit pattern is an organic dye dissolved in the thermoplastic polymer material. As previously indicated, the particulated electrically conductive material incorporated in said circuit pattern comprises metal coated glass spheres and the particulated electrically conductive material can further contain up to 15% by weight of said particulated electrically conductive material being metal flakes. 
     The particular circuit pattern material employed in the present invention, which is fully described in the aforementioned U.S. Pat. Nos. 3,968,056 and 3,988,647 patents hence need only be illustrated herein, serves an important role in accomplishing more reliable interruption of the circuit path when the present fused elements forming part of said circuit pattern are actuated. Electrical conductivity in said circuit pattern is solely attributable to physical contact between the electrically conductive filler particles so that physical separation therebetween effectively interrupts the circuit path. It is thereby not required that the fuse element portions of said circuit pattern be completely decomposed to form an opening entirely across the width of the circuit pattern since a lesser physical separation such as multiple cracks in the circuit pattern at the fuse locations raises the dielectric breakdown voltage of the opened fuse elements sufficiently to block a subsequently applied firing pulse. Such fuse operation represents an improvement compared to that taking place in the aforementioned U.S. Pat. No. 4,154,569 patent wherein the circuit pattern can be entirely metal thereby requiring thermal decomposition of the entire circuit pattern along with the underlying dielectric substrate at the fuse locations. In further explanation, the present fuse elements operate effectively to interrupt the circuit path when the underlying substrate supporting the circuit pattern is not totally destroyed so long as sufficient thermal decomposition of the circuit pattern takes place at the fuse locations to block a subsequently applied firing pulse. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of an otherwise known multiple flash lamp circuit board having the lamp firing circuit pattern deposited on the dielectric substrate and radiation sensitive switching devices connected in said circuitry for sequential firing of a plurality of flash lamps connected thereto which has been modified in accordance with the present invention; and 
     FIG. 2 is an electrical schematic diagram illustrating the circuit path for one group of four flash lamps being sequentially fired by the circuit board configuration in FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the preferred circuit board embodiment depicted in FIG. 1, both radiation sensitive switches and radiation sensitive fuse elements are deposited on the circuit pattern adjacent flash lamp locations for simultaneous actuation of each adjoining switch and fuse element by radiation being emitted from an adjacent flash lamp. Accordingly, the circuit arrangement of FIG. 1 is generally the same as disclosed in the aforementioned U.S. Pat. No. 4,133,631 patent which includes a circuit board member 10, in the form of a dielectric substrate constructed with a thermoplastic organic polymer, having connector tabs 12 and 14 located at each end for insertion of said circuit board member into an associated camera socket (not shown). The camera socket has contacts for conductor line 16, 18 and 20 of the lamp firing circuit pattern to connect four flash lamps in parallel between conductive lines 16 and 18 when connector tab 12 has been inserted in the camera socket. Correspondingly, a second group of four lamps is connected between conductor lines 18 and 20 when connector tab 14 has been inserted in the camera socket. While an entire illustrative circuit board is shown, there is need to discuss only a single group of four flash lamps since the operation of the second group of four flash lamps is essentially the mirror image of the first group. Conductor lines 16 and 20 are interrupted by radiation switches 22 to prevent the flash lamps in a group from all being fired together on release of the camera shutter. As can be noted, the switches 22 are each arranged in a region of a flash lamp termination 24 so that the heat and light released when the flash lamp is fired operates to convert the respective switch from a non-conductive to an electrically conductive state, thus establishing a current path to the next lamp to be flashed. Accordingly, when the particular flash lamp in the circuit is flashed, then the adjacent radiation switch is actuated providing a current path to the next lamp to be flashed. Each of the radiation switches is serially connected between the branch circuits connecting the lamps in a parallel circuit relationship as better shown by the schematic diagram in FIG. 2. Each of said radiation switches employ a material exhibiting an open circuit or high resistance condition and with said resistance thereafter becoming zero or a low value when said material absorbs radiation and/or heat from the adjoining flash lamp. Individual fuse elements 26 are connected serially to each of the flash lamps in the branch circuits at locations proximate to the flash lamp terminations 24 so as to also be actuated by the heat and light released when the flash lamp is flashed. Circular shaped indentations 28 (shown in phantom) surrounding each of said fuse elements 26 have been formed on the opposite side of the circuit board member to provide a lesser thickness than exists elsewhere on the circuit board and thereby assist interruption of the circuit path from conductor 18 to the already fired flash lamp. As can be noted, each of said fuse elements simply comprises a more narrow width of the circuit pattern at the fuse locations than the remaining width of said current pattern elsewhere. When the particular flash lamp adjacent to a given fuse element is flashed, the radiation and/or heat created thereby is absorbed by the circuit pattern at the fuse location whereupon the more narrow fuse element portions of said circuit pattern become sufficiently decomposed to disrupt the electrical connection to the already fired flash lamp. As previously indicated, such interruption of a circuit path is accomplished by physical separation of the electrically conductive filler particles in the circuit pattern which can simply produce a series of cracks in the circuit pattern itself at the fuse locations. There can also be additional thermal decomposition whereby the underlying reduced thickness portion of the circuit board member either thermally melts or decomposes to create an actual hole or opening at each fuse location. 
     A more detailed description of the sequential firing operation for the circuitry of a four-lamp group in the above described circuit board can be provided in connection with the schematic drawing shown in FIG. 2. Accordingly, in FIG. 2 there is shown flash lamps 24 electrically connected in branch circuits to assure that the branch circuit of each lamp will become an open circuit upon flashing of the lamp in that circuit. The desired mode of operation is carried out starting with the leftmost branch circuit when the lamp in said circuit is flashed and causes fuse element 26 in the same branch circuit to physically interrupt the circuit path to said lamp while simultaneously opening the circuit path to the flash lamp in the adjacent branch circuit by actuating the radiation sensitive switch 22 serially connected between said branch circuits to the conductive state. This cooperative action is repeated between successive adjacent branch circuits until all four lamps in a group have been fired from simultaneous actuation of adjoining switch and fuse elements with radiation being emitted from an adjacent flash lamp. 
     As previously indicated, a fuse element for operation in the above described lamp firing circuitry can simply comprise a more narrow width of the circuit pattern comprising an organic resin matrix having particulated electrically conductive material and a radiation absorbing material dispersed therein than the width of a current pattern elsewhere in the electrical circuit and which is located where said dielectric substrate has a reduced thickness relative to the remaining thickness of the dielectric substrate of the circuit board member. Suitable fuse elements constructed in this manner are thereby modifications of the conductive ink compositions disclosed in the aforementioned 3,968,056 and 3,988,647 patents wherein a radiation absorbing material has been dispersed in the liquid ink to enhance thermal decomposition when actuated by radiation being emitted from the operatively associated flash lamps. Accordingly, an illustrative circuit ink composition can have in proportions by weight 0.5 parts hydrocarbon soluble dye, 32.5 parts of an ultraviolet curable polyester resin, and 67.0 parts silver coated glass spheres. When cured to the solid state at ordinary temperatures the more narrow width fuse element portions of said circuit board pattern effectively interrupts the circuit path when actuated by an adjoining flash lamp to produce physical separation of the incorporated electrically conductive filler particles. The indentations or reduced thickness portions of the circuit board members at fuse locations serve to reduce dissipation of the absorbed thermal and/or light energy when the fuse elements are actuated in this manner as well as enhance localized melting or decomposition of the circuit board member at the fuse locations causing an actual opening in said underlying substrate. 
     While preferred embodiments of the invention have been shown and described, various other embodiments and modifications thereof will become apparent to persons skilled in the art. For example, the desired circuit interruption can also be achieved in other firing circuit configurations such as one wherein the switch and fuse elements are both disposed appropriately in branch circuits with the flash lamps to provide the desired firing sequence. It is therefore intended to limit the present invention only by the scope of the following claims.