Method of manufacturing battery explosion prevention safety device

A method of fabricating a battery explosion prevention safety device includes the steps of electroforming a first metal floor layer on a conductive substrate, coating the first metal layer with a photoresist layer, overlaying the photoresist layer with a photomask having windows that define the shape of a safety valve, irradiating the photoresist layer with ultraviolet light, developing the photoresist layer, and removing unexposed portions of the photoresist layer to expose the first metal layer at those portions, electroforming a second metal layer on the first metal layer, removing exposed portions of the photoresist layer, and separating the conductive substrate from the first metal layer. This method is used to fabricate a plurality of connected explosion prevention safety devices, which can then be readily detached one at a time when needed.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a safety device for preventing a battery
 exploding and a method for manufacturing the safety device, thereby also
 preventing related damage to surrounding equipment, the safety device
 being in the form of a thin safety valve welded or otherwise attached to a
 battery casing to seal a gas vent in the casing so as to allow the release
 of gas that has risen to a certain pressure in the battery casing, such as
 a casing of a lithium battery or hydrogen-nickel battery.
 2. Discussion of the Background
 Explosion prevention safety devices for batteries include those such as the
 device disclosed by JP-A-59-79965. In that device, the battery cover is in
 the form of a thin strip of rolled stainless steel 0.4 mm thick having a
 circular groove 0.2 mm wide with a base 0.2 mm thick formed by pressing,
 to comprise a safety valve. A rise in internal pressure in the battery
 causes rupturing along the circular groove, blowing off the safety valve
 and allowing the gas to be released.
 However, based on the rolling specification the rolled stainless steel
 strip has a tolerance of .+-.3 to 5 .mu.m. This means that a groove formed
 to have a width of 0.2 mm and a floor thickness of 0.02 mm in the strip
 having a thickness of 0.4 mm.+-.3 to 5 .mu.m will have a floor thickness
 of 0.2 mm.+-.3 to 5 .mu.m, and the pressure at which the safety valve will
 blow when the thickness of the floor is 0.02 mm+5 .mu.m will not be the
 same as the pressure at which the safety valve will blow when the
 thickness of the floor is 0.02 mm-5 .mu.m. Moreover, the precision of the
 thickness produced by pressing will vary slightly depending on the
 precision of the die and how long it has been in use, the variation being
 around .+-.5 to 7 .mu.m which, when included with respect to the thickness
 of the groove floor, further increases the variation in internal battery
 pressure at which the safety valve will blow. Furthermore, the rolled
 steel material always includes impurities, and if such impurities should
 form in the floor thickness of the groove, it can weaken that portion to
 the point where a rise in internal pressure in the battery can result in
 that portion giving way and allowing the release of the gas, even if the
 pressure is not high enough to require such release. In extreme cases,
 such impurities at that portion can form pinholes, resulting in a
 defective product.
 Another arrangement comprises coating the stainless steel strip with a
 photoresist layer, overlaying the photoresist layer with a photomask that
 defines the shape of the safety valve groove, exposing and developing the
 photoresist, removing the unexposed photoresist portion defining the shape
 of the safety valve groove to expose that portion of the stainless steel
 strip, etching the exposed portion of the stainless steel strip to reduce
 the thickness of the floor of the safety valve groove, and then removing
 the remainder of the photoresist layer from the stainless steel strip.
 However, the material used is still a rolled stainless steel strip, so
 there are still the problems of the .+-.3 to 5 .mu.m thickness tolerance
 and the existence of impurities, in addition to which the precision of the
 thickness of the floor of the groove formed by etching is .+-.10 .mu.m,
 worse than the .+-.5 to 7 .mu.m precision of the thickness when the groove
 is press formed, so that the variation in the internal pressure at which
 the safety valve blows is even greater than when the groove is press
 formed.
 The basic technical point of the floor of the groove of the safety valve of
 a battery casing is to prevent an explosion by rupturing when the gas
 pressure in the casing reaches a set pressure, thereby allowing the gas to
 escape from the casing. Consequently, the narrower the range of the set
 pressure, the more reliable the safety device that can be provided.
 Realizing such a safety device, along with execution by the
 photolithography method, is an urgent task.
 SUMMARY OF THE INVENTION
 The object of the present invention is to provide a battery explosion
 prevention safety device that operates with high reliability when pressure
 within the battery casing reaches a prescribed pressure, and a method for
 manufacturing the device.
 To attain the above object, the present invention provides a battery
 explosion prevention safety device comprising a first metal layer
 electroformed on a conductive substrate that is finally separated from the
 first metal layer, to have a safety valve, and a second metal layer
 electroformed on the first metal layer at a portion other than the safety
 valve, and a method of manufacturing a battery explosion prevention safety
 device, comprising electroforming on a conductive substrate a first metal
 layer constituting a floor layer, coating the first metal layer with a
 photoresist layer, overlaying the photoresist layer with a photomask
 having a transparent portion defining an outline of a safety valve,
 irradiating the photoresist layer with ultraviolet light, developing the
 photoresist layer, and removing unexposed portions of the photoresist
 layer to expose the first metal layer at those portions, electroforming a
 second metal layer on the first metal layer, removing exposed portions of
 the photoresist layer, and separating the conductive substrate from the
 first metal layer.
 In accordance with the arrangement described above, since the first metal
 layer having a safety valve is electroformed on a substrate, the thickness
 thereof can be controlled with a precision of 0.1 .mu.m. Since, moreover,
 the first metal layer is electroformed, the layer is free of impurities,
 so there is no formation of highly weakened portions or pinholes.
 As a result, in accordance with this invention, a battery explosion
 prevention safety device can be provided in which the safety valve
 operates at high reliability when the internal pressure in a battery
 casing reaches a prescribed pressure.
 Further features of the invention, its nature and various advantages will
 be more apparent from the accompanying drawings and following detailed
 description of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 FIGS. 1A and 1B are a plan view and cross-sectional view, respectively, of
 battery explosion prevention safety devices according to the present
 invention. Each explosion prevention safety device 1 is in the shape of a
 square measuring, for example, 9.0 mm along each side, electroformed of
 nickel or nickel alloy 50 .mu.m thick and having in the center a safety
 valve annular groove 2 that is 0.35 mm wide and has an inside diameter of
 2.8 mm and an outside diameter of 3.5 mm. The thickness of the floor 3 of
 the groove 2 is determined by the internal pressure at which the safety
 valve is to blow (i.e. be operable). If the groove floor has a thickness
 of 10 .mu.m, the groove will blow when the pressure in the battery casing
 reaches 13.4 kg/cm.sup.2, producing a hole 3.5 mm in diameter through
 which the gas in the casing discharges. If the groove floor thickness is
 20 .mu.m, the groove will blow when the internal pressure reaches 22.5
 kg/cm.sup.2, producing a hole 3.5 mm in diameter through which the gas in
 the casing discharges. Reference numeral 6 denotes the gap between the
 individual explosion prevention safety devices.
 The fabrication of the explosion prevention safety devices 1 will now be
 described. With reference to FIG. 2, electroforming is used to form a
 first metal layer 11 of a prescribed thickness on a conductive substrate
 10. The thickness of the first metal layer 11 is determined according to
 the internal pressure at which the safety valve is to blow. The use of
 electroforming allows the thickness to be controlled with a precision
 within 0.1 .mu.m. If the first metal layer 11 is to be formed of nickel or
 nickel alloy, the conductive substrate 10 may be of any metal other than
 nickel, such as, for example, stainless steel, brass or titanium in order
 to facilitate its subsequent separation from the first metal layer 11. For
 the reasons explained below, prior to forming the first metal layer 11 it
 is desirable to treat the conductive substrate with a separating agent.
 Next, the first metal layer 11 is coated with a photosensitive resin to
 form a photoresist layer 12 on the first metal layer 11. If the explosion
 prevention safety device 1 is 50 .mu.m thick and the floor 3 of the groove
 defining the safety valve is 10 .mu.m thick, the thickness of the
 photoresist layer 12 is 50 .mu.m-10 .mu.m=40 .mu.m. If the floor 3 of the
 groove defining the safety valve is 20 .mu.m thick, the thickness of the
 photoresist layer 12 will be 50 .mu.m-20 .mu.m=30 .mu.m.
 After the photoresist layer 12 has dried, a photomask 20 is superposed on
 the photoresist layer 12. As shown in FIG. 3, the photomask 20 has a
 portion 21 corresponding to the line of the safety valve groove 2 that
 transmits ultraviolet light, and a portion 22 making up the rest of the
 photomask 20 that does not transmit ultraviolet light. The photoresist
 layer 12 is then irradiated with ultraviolet light (FIG. 2A), and the
 photomask is then removed and the photoresist layer developed. The
 photoresist layer thus developed is negative. When a positive photoresist
 layer is used, a photomask with its white and black patterns reversed is
 used.
 The development process hardens the safety valve groove portions 12a of the
 photoresist layer exposed to the ultraviolet light. Photoresist layer
 portions 12b are the portions that were not exposed to the ultraviolet
 light. Next, the portions of the first metal layer 11 covered by the
 unexposed photoresist layer portions 12b are exposed by using alcohol or
 the like to remove the portions 12b (FIG. 2B).
 Next, with reference to FIG. 2C, a second metal layer 13 is electroformed
 on the exposed first metal layer 11. The metal of the second metal layer
 13 is not necessarily nickel or nickel alloy, but can be any metal other
 than nickel or nickel alloy. The nickel alloy includes NiPd, NiCo and NiP,
 for example. Since the safety valve groove 2 portion is covered by a
 photoresist layer exposed portion 12a that has been hardened by the
 development process, the second metal layer 13 is formed on the part of
 the first metal layer 11 from which the unexposed photoresist layer
 portions 12b were removed, integrally with the first metal layer 11. If
 the thickness of the explosion prevention safety device 1 is 50 .mu.m and
 the thickness of the floor 3 of the safety valve groove is 10 .mu.m, a
 thickness of 40 .mu.m is used for the second metal layer 13, while if the
 thickness of the groove floor 3 is 20 .mu.m, the thickness of the second
 metal layer 13 will be 30 .mu.m.
 To increase the strength of the bond between the first metal layer 11 and
 second metal layer 13, prior to electroforming of the second metal layer
 13 it is desirable to use a 1:1 solution of hydrochloric acid to sensitize
 the surface of the first metal layer 11 exposed by the removal of the
 unexposed portion of the photoresist layer.
 When the second metal layer 13 has thus been formed to the prescribed
 thickness, a solvent such as acetone is used to dissolve away the hardened
 photoresist portions 12a over each safety valve groove 2, and the
 conductive substrate 10 is peeled off, to thereby obtain an explosion
 prevention safety device having the desired safety valve (FIG. 2D).
 Separation of the conductive substrate 10 is facilitated by treating the
 surface of the conductive substrate 10 with a separating agent prior to
 forming the first metal layer 11, as mentioned above.
 The explosion prevention safety device 1 can be used to seal a gas vent 5
 such as by welding the explosion prevention safety device 1 to the battery
 casing 4, as shown in FIG. 4. Thus, when the pressure in the battery
 casing rises to the prescribed level, the safety valve functions by
 blowing along the floor of the groove which, being thinner, is weakened,
 releasing the gas in the casing preventing the battery from exploding and
 thereby also preventing collateral damage to surrounding equipment from
 such an explosion.
 The shape of the groove 2 forming the thin floor 3 in the safety valve of
 explosion prevention safety device is not limited to the circular shape
 shown in FIG. 1, or to an oval, polygonal or other such closed
 configuration. Instead, the groove 2 may also have a non-closed
 configuration such as the shapes shown in FIG. 5.
 The explosion prevention safety device of the present invention is small,
 and is preferably formed as a multiplicity of contiguous devices, as shown
 in FIG. 1A, with individual devices then being cut off as required. For
 this, a substrate is used for the conductive substrate 10 that has a large
 enough area to form multiple devices thereon, and electroforming is used
 to form the first metal layer 11 over the entire surface of the substrate
 10. The photomask 20 provided on the first metal layer 11 is large enough
 to be superposed over the whole of the first metal layer 11. The photomask
 20 is provided with a transparent portion 21 to form the contour of the
 safety valve groove for one explosion prevention safety device 1, and a
 transparent portion 23 to form the gap between adjacent explosion
 prevention safety devices. When a multiplicity of the explosion prevention
 safety devices has been formed, photoresist layer portions 12c hardened by
 exposure via the transparent portion 23 is removed and the substrate is
 separated, leaving adjacent devices connected by just the thin first metal
 layer 11 having a predetermined thickness, which can therefore be readily
 cut to detach individual devices.
 As described in the foregoing, in accordance with the present invention,
 the thin floor of the groove that defines the safety valve provided in the
 explosion prevention safety device is comprised of a first metal layer on
 the conductive substrate. Since this first metal layer is electroformed on
 the substrate, the first metal layer is formed uniformly, allowing the
 floor of the groove to be formed to within .+-.1 .mu.m of a desired
 thickness. Moreover, the width of the groove formed by the transparent
 portion in the photomask does not vary by more than .+-.2 .mu.m. In
 addition, because the first metal layer is electroformed, it does not
 include impurities, so the floor of the groove has no very weak spots or
 pinholes.
 Thus, while in the prior art it has only been possible to fabricate safety
 valves that function when the pressure in a battery casing rises to 15
 kg/cm.sup.2.+-.5 kg/cm.sup.2, the present invention is able to provide a
 highly reliable explosion prevention safety device with a safety valve
 that functions at a pressure of 15 kg/cm.sup.2.+-.1 kg/cm.sup.2, that is,
 at a pressure of 14 to 16 kg/cm.sup.2.
 Obviously, numerous modifications and variations of the present invention
 are possible in light of the above teachings. It is therefore to be
 understood that within the scope of the appended claims, the invention may
 be practiced otherwise than as specifically described herein.