Bridge wire initiator for explosives and method for making such an initiator

Abstract: The exothermic instrument for firing an explosive comprises a circuit board 1 having a heat resist layer 6 on a substrate 11 and an exothermic resistance 4, which connects to a pair of through hole conductive electrodes 13 passing through the substrate 11, contacting with the explosive 19 on the heat resist layer 6, a pair of electrode pins 8, each one thereof is inserted into each one of the through hole conductive electrodes, and an insulator for holding the circuit board 1 through which the electrode pins 8 pass.

Description: The present invention relates to an exothermic instrument for firing an explosive used to fire a squib loaded, for example, with powder and a circuit board used for such an exothermic instrument as well as to a manufacturing method of the circuit board. Some gas generators have been used as a driving source for such devices requiring large instantaneous motive power as air bags and seat belts for automobiles. The gas generator has an squib and gas a generating agent, for example, the mixture of sodium azide and metal oxide with nitrate or perchlorate and reducing agent. The squib incorporates an exothermic instrument comprising an exothermic resistance and electrode pins communicating therewith, and an explosive. When an electric current is made to flow into the exothermic resistance from the electrode pins in the squib, the exothermic resistance generates heat, activating the squib to ignite the explosive. The combustion of the explosive in the squib thus induces the activation of the gas generating agent, which generates a large amount of gas within an extremely short time to inflate an air bag. The gas generating power also drives instantaneously any device that is linked thereto. Japanese Patent Provisional Publication No. 5-133699 discloses a conventional squib. The squib connects a pair of electrodes formed on a surface of an insulating sheet with an exothermic resistance made of a metal film. The exothermic resistance of the metal film is made to contact directly the ceramic substrate of the insulating sheet. Additionally, Patent application No. WO 96 24024 A , which forms a basis for the preamble of claim 1, discloses a method of fabricating an electroexplosive device which utilises a semiconductor bridge as an ignition element. This semiconductor bridge consists of a circuit board, which contains a substrate with a pair of holes passing through, a heat resist layer thereon and an exothermic resistance on the heat resist layer. The heat resist layer on the semiconductor bridge connects to one electrode of the pair of electrodes passing through the pair of holes in the substrate. The second electrode of the bridge is connected via wire bonds to one or two conducting pins which penetrate a metal header and are insulated by surrounding glass. Another method and apparatus for firing explosives is disclosed in Patent Application No. US 4 944 225 A, in which the method and apparatus is used in well perforating systems and for firing slapper on exploding foil initiator over relatively lengthly firing lines. Specially designed detonators or initiators with a plated through hole conductive electrode are used to connect the electrode pins for the electrical power supply with the exploding metal foil, serving as the exothermic resistance. It is an object of the present invention to provide an improved circuit board for being installed into an exothermic instrument according to the preamble of claim 1, a manufacturing method for the circuit board and an exothermic instrument for firing an explosive in which such a circuit board is installed. Further, it should be provided an exothermic instrument for an explosive ignition that acts assuredly with a constant electric current. This object is achieved with respect to the circuit board by the features of claim 1, with respect to the manufacturing method for the circuit board by the features of claim 5 and with respect to the exothermic instrument by the features of claim 3. Advantageous further developments are set out in the dependent claims. FIGURE 1 is a plane view of an embodiment of a midway-to-manufacturing of the circuit board for being installed into the exothermic instrument of the present invention. FIGURE 2 is a perspective view of another embodiment of a midway-to-manufacturing the circuit board for being installed into the exothermic instrument of the present invention. FIGURE 3 is a cross sectional view of an embodiment of an exothermic instrument for firing explosive of the present invention. FIGURE 4 is a cross sectional view of another embodiment of an exothermic instrument for firing explosive of the present invention. FIGURE 5 is a plane view of the exothermic instrument for firing explosive as shown in FIGURE 4. Embodiments would be described, which are preferable ones of the present invention, referring to figures and, however, is to be used to understand the present invention but should not be used for the present invention to be limited. In the circuit board for an exothermic instrument for firing explosive as seen FIGURE 1, a punching operation forms simultaneously the grid-like grooves 17 that isolate horizontally and vertically the surface of an alumina green sheet, the raw material of the ceramic substrate 11, into grids, and the round holes 14 centered on the grooves 17. Baking of this green sheet will give the ceramic substrate 11. A heat resist layer 6 made of glass ceramics is printed a on the ceramic substrate 11 by a screen printing method and then is baked. A silver palladium, the raw material of the conductive electrodes 13 is printed, also by a screen process, in the circumferential periphery of the round holes 14 in the ceramic substrate 11. At the same time, the silver palladium is absorbed from the reverse side of the ceramic substrate 11 into the inner wall of the round holes 14 to form the through hole conductive electrodes 13. The conductive electrode 13 has a convex portion 15 to which will be attached a resistance value measuring terminal when trimming the exothermic resistance 4. The ceramic substrate 11 with the silver palladium printed will then be baked. Printed also by the screen process on the heat resist layer 6 of the ceramic substrate 11 will be the exothermic resistance 4 made of ruthenium oxide (RuO 2 ). After the simultaneous printing of the mark 16, the substrate 11 will be baked again with the mark 16. With the position recognized with the mark 16, the exothermic resistance 4 will be trimmed, by laser beam, into predetermined resistance value. Then, manual splitting and isolation of the ceramic substrate 11 all along the grid-like grooves 17 will allow to get the circuit board for explosive igniting exothermic instrument with concave portions formed from a pair of round holes 14. In another embodiment of circuit board 1 for the explosive firing exothermic instrument as seen FIGURE 2, a punching operation forms simultaneously the grid-like grooves 17 and round holes 5 on an alumina green sheet, the raw materials of the ceramic substrate 11, to divide the sheet into a hundred 5 mm x 5 mm square grids. This green sheet is baked into the ceramic substrate 11. Printed, by the screen process, and baked on the ceramic substrate 11 will be a heat resist layer 6 made of boron-silicate glass. A silver palladium, the raw material of the conductive electrodes 13 is printed, also by screen process, in the circumferential periphery of the round holes 5 in the ceramic substrate 11. At the same time, the silver palladium is absorbed from the reverse side of the ceramic substrate 11 into the inner wall of the round holes 5 to form the through hole conductive electrodes 13. Printed at the same time on the grooves 17 will be triangular metal marks 9, which will serve as tick marks when dividing the ceramic substrate 11. The ceramic substrate 11 thus printed will then be baked. Printed also by the screen process on the heat resist layer 6 of the ceramic substrate 11 will be the exothermic resistance 4 made of ruthenium oxide (RuO 2 ). The substrate 11 will be baked again. Then, manual splitting and isolation of the ceramic substrate 11 along the grid-like grooves 17 will allow to have the circuit board 1 for explosive firing exothermic instrument. After baking the alumina green sheet, the ceramic substrate 11 may have the round holes 5 opened and grooves 17 formed, both by laser beam. FIGURE 3 shows an exemplary exothermic instrument for explosive firing that uses the circuit board 1 thus manufactured. As shown in this figure, the exothermic instrument is housed in the insulating container 18. A pair of electrode pins 8 pass through the bottom of the insulating container 18 to be inserted into the round hole of the circuit board 1 for explosive igniting exothermic instrument. The explosive 19 is loaded into the insulating container 18. The explosive igniting exothermic instrument acts as follows. When the electrode pins 8 are connected to a power supply to flow an electric current, the exothermic resistance 4 of the circuit board 1 for explosive igniting exothermic instrument gets heated to ignite and combust the explosive 19. The heat emanating from the exothermic resistance 4 is kept from dissipating over the ceramic substrate 11 due to the existence of the heat resist layer 6, and is therefore transferred efficiently to the explosive 19, which will thus be ignited assuredly. FIGURES 4 and 5 are the cross sectional and plan views, respectively, of another exemplary exothermic instrument for explosive firing to which this invention applies. As shown in these figures, two electrode pins 8 in the exothermic instrument pass through the insulator 9 packed into the metal cylinder 10. Laminated on the insulator 9 are the base substrate 11 made of ceramics and the heat resist layer 6 made of glass or glass ceramics. The tip of the electrode pin 8 is connected with the though hole silver-based electric conductor 7 loaded into the holes opened in the base substrate 11, while the conductor 7 is connected to the through hole silver-based electric conductor 12 loaded into the holes opened in the heat resist layer 6. The tip of the conductor 12 is connected with the terminal electrode 13 fixed on the heat resist layer 6, while the terminal electrode 13 is bridged by the exothermic resistance 4 with its neck-formed central part. The explosive 19 is applied on the exothermic resistance 4 in such a fashion that the former envelopes the latter. The electrode pins 8 are connected to a power supply (not shown). The exothermic instrument for firing an explosive comprises a circuit board 1 having a heat resist layer 6 on a substrate 11 and an exothermic resistance 4, which connects to a pair of through hole conductive electrodes 13 passing through the substrate 11, contacting with the explosive 19 on the heat resist layer 6, a pair of electrode pins 8, each one thereof is inserted into each one of the through hole conductive electrodes, and an insulator for holding the circuit board 1 through which the electrode pins 8 pass.

Claims: A circuit board for being installed into an exothermic instrument comprising: a substrate (11) made of a ceramic material, a pair of through holes (5, 14) passing through the substrate, a heat resist layer (6) provided on at least a part of the substrate (11), and an exothermic resistance (4) on the heat resist layer (6), characterized in that the inner wall and a circumferential periphery of each of the through holes is covered by a conductive material for forming respectively a conductive electrode (13), the exothermic resistance is arranged for connecting to the pair of conductive electrodes, and the heat resist layer (6) is made of glass or a glass ceramic material and contains a boron-silicate-lead glass. The circuit board according to claim 1, wherein the heat resist layer (6) contains a mixture including alumina ceramic powder and boron-silicate-lead glass. An exothermic instrument for firing an explosive (19) comprising: a circuit board (1) according to claim 1 or 2 contacting with the explosive (19) on the heat resist layer (6), a pair of electrode pins (8), each one thereof is inserted into each one of the through holes of the circuit board (1), and an insulator (9, 18) for holding the circuit board (1) through which the electrode pins (8) pass. The exothermic instrument according to claim 3, wherein the circuit board (1) and the insulator (9, 18) are housed in a cylindrical body (10) stuffing the explosive (19). A manufacturing method of a circuit board (1) according to claim 1 or 2 for being installed into an exothermic instrument comprising: a step for making the substrate (11) as large as a plurality of the objective circuit boards (1) having the pairs of through holes (5, 14) passing through by each of the objective circuit boards (1), a step for forming the heat resist layer (6) on a middle portion of each of the pairs of through holes (5, 14) at a pitch of the objective circuit board (1) on the substrate (11), a step for forming the pair of conductive electrodes (13) on the inner walls and the circumferential peripheries of each of the pairs of the holes (5, 14), a step for forming the exothermic resistance (4) on each of the heat resist layers (6), and a step for separating plural circuit boards (1) into each one thereof. The manufacturing method according to claim 5, wherein grid-like grooves (17) are formed in the ceramic material of the substrate (11) for separating the plural circuit boards alongs the grooves (17). The manufacturing method according to claim 5, wherein each hole of said pairs of holes (5, 14) is located on a groove (17) for separating the plural circuit boards (1).

Inventors: ARAI EIJI, MIYAKOSHI MOTOHARU, MIZUSHIMA KIYOSHI, MOURI MAMORU, NAKAMURA SATOSHI, SATO HIROSHI, TSUJI SHINZO, WATANABE MASASHI, ARAI, EIJI, MIYAKOSHI, MOTOHARU, MIZUSHIMA, KIYOSHI, MOURI, MAMORU, NAKAMURA, SATOSHI, SATO, HIROSHI, TSUJI, SHINZO, WATANABE, MASASHI
Applicants: NICHU GIKEN KOGYO KK, NIKKO COMPANY, NICHIYU GIKEN KOGYO CO., LTD.
Application year: 1998
Main classification: F42B3
IPC codes: F42B3, F23Q13