EP1716385B1

EP1716385B1 - An electropyrotechnic initiator with heat dissipation

Info

Publication number: EP1716385B1
Application number: EP05707343A
Authority: EP
Inventors: Yann Le Gallic; Rémi PADEL
Original assignee: Autoliv Development AB
Current assignee: Autoliv Development AB
Priority date: 2004-02-11
Filing date: 2005-02-09
Publication date: 2010-09-29
Anticipated expiration: 2025-02-09
Also published as: DE602005023835D1; FR2866106A1; FR2866106B1; JP2007522428A; ATE483149T1; WO2005078377A1; EP1716385A1; JP4800975B2

Abstract

An initiator, comprising a pyrotechnic ignition charge (19), a resistive heating element (17) covered by the charge, fed with electricity by two electrodes and set on a support (16), incorporates a heat dissipator (31) between the heating element and support, and electrical insulators (32) that insulate the heating element from the dissipator and allow heat to be transmitted from one to the other. The heat dissipator can be of metal or ceramic, the metal selected from copper, aluminum and their alloys or oxides.

Description

The invention relates to an electropyrotechnic initiator.
The field of application of the invention relates to initiators for firing the pyrotechnic charges of gas generators for activating devices for protecting the occupants of motor vehicles, such as, for example, air bags.
Usually, such an initiator comprises a pyrotechnic ignition charge and a heater resistor element covered by the pyrotechnic ignition charge to ignite it by the Joule effect. Two electrodes are provided for being fed with electricity.
The operation of electropyrotechnic initiators is influenced by the pair constituted by the resistor element and the ignition pyrotechnic charge, and by the sensitivity imparted to such initiators by the pair used.
Thus, the all-fire threshold corresponds to the limit value for electrical current passing via the electrodes, above which it is certain that the initiator will function, i.e. that the ignition pyrotechnic charge will ignite.
The no-fire threshold, lower than the all-fire threshold, corresponds to the limiting value for electrical current passing via the electrodes, below which it is certain that the initiator will not function, i.e. that the ignition pyrotechnic charge will not ignite.
Between the no-fire threshold and the all-fire threshold, there exists a transient zone in which it is not certain whether the initiator will or will not function.
Document US-A-6 272 965 concerns an electro-explosive device ("EED") having resistors fabricated on a thermally conductive substrate and interconnected by a central bridge element. The resistance of the bridge element is lower than that of the resistors, which have a larger surface area to volume ratio. A layer of zirconium is placed on the bridge element and explodes into a plasma along with the bridge element in order to ignite a pyrotechnic compound. The substrate using integrated circuit fabrication techniques and the conductive bridge of the EED is overcoated with a composite overcoat comprising a metal and an oxidizer, which produces a chemical explosion upon plasma vaporization of the conductive bridge. In the second embodiment of this document, the EED comprises a silicon wafer, or a thermally conductive but electrically insulating substrate, such as alumina, which has layers of silicon dioxide grown on the front and back surfaces. The silicon dioxide layers electrically insulate the substrate while providing a low thermal path of resistance across the front and back surfaces of the substrate. Preferably, the substrate has a nominal low resistivity. A layer of titanium is vapor deposited onto the front surface followed by a layer of zirconium. The zirconium/titanium layer is then selectively etched away to form a bowtie pattern having a central bridge portion. A layer of titanium/nickel/gold is deposited over the back layer of silicon dioxide and Ti/Ni/Au layers are also deposited over the ends of the bowtie shaped zirconium layer to form contact pads. The resistance of the EED is comprised of three resistors in series, with Rland being the resistance through the Ti/Ni/Au layers to either end of the bowtie-shaped zirconium layer and Rbow being the resistance of the bowtie-shaped zirconium layer. With low levels of input signals, the resistances Rland have a much larger surface to volume ratio than the resistance Rbow. Thus, at these levels, the resistances Rland receive most of the energy from the input signals and convert the energy into heat. The Ti/Ni/Au contacts present a large surface area for the conduction of heat through the top silicon dioxide layer, through the thermally conductive substrate and to the header. As a result, at low levels of input signal, the zirconium-shaped bowtie dissipates only a fraction of the heat and remains relatively cool. Thus, the EED can remain insensitive to any RF power or ESD which is coupled to the EED. The EED is ignited by supplying a firing signal which has a relatively high intensity. The resistances Rland comprise metal-oxide variable resistances which are formed between the titanium layer in contacts and an oxide-phase layer formed on the zirconium layer. The metal-oxide variable resistances Rland have a relatively high resistance at lower voltages. With higher intensity signals, the metal-oxide resistances Rland decrease substantially and become small in comparison to the resistance Rbow. As a result, with a high intensity firing signal, the resistance Rbow will become the largest resistance and will accordingly receive most of the energy from the firing signal until the zirconium layer vaporizes in a plasma.
The invention seeks to obtain an electropyrotechnic initiator serving to reduce the range of uncertainty concerning the functioning of electropyrotechnic initiators between the no-fire threshold and the all-fire threshold.
To this end, in a first aspect, the invention provides an electropyrotechnic initiator according to claim 1.
By means of the invention, a fraction of the heat produced by the heater resistor element in operation of the initiator is dissipated towards the support. The no-fire threshold value is thus raised towards the all-fire threshold, thereby reducing the transient zone between them. The value of the no-fire threshold is raised, while the value of the all-fire threshold remains substantially constant. Consequently, the initiator of the invention presents greater safety in operation and is more robust against stray electrical currents. Thus, the invention makes it possible to satisfy better the requirements laid down by motor vehicle manufacturers for the no-fire and all-fire thresholds.
According to claims 2 to 9 depending on claim 1:

a second layer of heat-conducting and electrically-insulating adhesive is interposed between the heat dissipator and the support;
or the heat dissipator is in direct contact with the support;
the heat dissipator is formed by a heat-conducting layer;
the heat dissipator is made of a material selected from metal materials and ceramic materials;
the heat dissipator is made of a metal selected from copper, aluminum, alloys thereof, or oxides thereof;
the heat dissipator is located beneath the heater resistor element; and
the heater resistor element is covered by two electrical contacts that are separate from each other and in contact with respective ones of the two electrodes.

The invention will be better understood on reading the following description given purely by way of nonlimiting example and with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic axial vertical section of an initiator according to the invention;
Figure 2 is a diagrammatic axial vertical section showing the disposition of the heater resistor element on the support in a first embodiment of the initiator according to Figure 1;
Figure 3 is a diagrammatic axial vertical section showing the disposition of the heater resistor element on the support in a second embodiment of the initiator according to Figure 1; and
Figure 4 is a diagrammatic axial vertical section showing the disposition of the heater resistor element on the support in a third embodiment of the initiator according to Figure 1.

An electropyrotechnic initiator 1 according to the invention is shown in Figure 1. The initiator 1 comprises a cylindrical container 2 that is circular about the vertical axis of the initiator, and that is also fragmentable and open at one of its ends. A solid circular cylindrical body 3 closes the open end of the container 2. The side wall 4 of the body 3 presents an outside shoulder 5 against which the open end of the container 2 comes to bear. The container 2 and the body 3 are encased in overmolding 6 that holds them together. The container 2 is thus in the form of a cylindrical cap presenting a side wall 7 and a top wall 8 which is plane, closed, and solid. By way of example, the container 2 also contains a metal tube 20 reinforcing the side wall 7. The container 2 is made of thin light metal, e.g. aluminum, and its plane wall 8 is advantageously weakened so as to open easily under the effect of an increase in the pressure that exists inside the container. The overmolding 6 is preferably made of a thermoplastic resin such as polyethylene terephthalate, for example. The body 3 is made out of a dense metal such as steel, for example.
The body 3 presents a plane top face 9 and a likewise plane bottom face 15, and over its full height h, it encases an electrically-insulating structure through which there pass two electrically-conducting electrodes 12 and 13. This structure for passing the two electrodes 12 and 13 is vitreous, for example. The two electrodes 12 and 13 are in the form of pins, for example. In the embodiment shown, this structure for passing the two electrodes 12 and 13 is formed by two hollow electrically-insulating tubes 10 and 11, e.g. made of glass, themselves encasing respective ones of the two electrodes 12 and 13. In a variant, the structure for passing the two electrodes 12 and 13 could comprise a single tube 10 or 11 encasing the corresponding electrode 12 or 13. In another variant, the body 3 could be electrically-insulating, and could itself encase the electrodes 12 and 13 directly without using tubes 10 and 11, with the body then being made of glass, for example. In an embodiment that is not shown, one of the electrodes 12, 13 surrounded by an electrically-insulating tube 10, 11 may be of the coaxial type, i.e. being electrically connected to the body 3 via a connection situated beneath it.
The plane top face 9 of the body 3 is fixed to a support 16, e.g. by means of adhesive. In the embodiments shown as examples in the figures, the support 16 is isolated from the electrodes 12 and 13, is made of an electrically-insulating material, and is constituted, for example, by a board made of a glass/resin mixture, such as a composite polymer of the epoxy type filled with glass fibers. Two through cylindrical channels 22 and 23 are provided in the support 16 for passing the electrodes 12 and 13. Each electrode 12, 13 presents a respective top end 12b, 13b which projects beyond the plane top face 9 of the body 3 and that of the support 16, and a second end 12c, 13c which projects beyond the bottom face 14 of the overmolding 6.
The support 16 carries an electrical circuit 18 which includes a heater resistor element 17 or heating resistive element 17 and which is electrically connected to the electrodes 12 and 13.
The resistor element 17 is photoetched in conventional manner. The resistor element 17 presents a top surface 24 which is covered by an ignition pyrotechnic charge contained in the container 2, or in the embodiment shown, in the container 2 and in the metal tube 20. This ignition pyrotechnic charge comprises, for example, an initiation pyrotechnic composition 19 which is in contact with the heater resistor element 17 and which is based, for example, on lead trinitroresorcinate, and an ignition composition or powder 21 which covers the initiation pyrotechnic composition 19 and which is constituted, for example, by powder based on nitrocellulose or by a mixture of boron and of potassium nitrate. By way of example, the heater resistor element 17 is flat. As for the initiators in the table below, the initiation pyrotechnic composition 19 could also be a mixture of zirconium and of potassium perchlorate, and the ignition powder 21 could be a mixture of titanium hydride and potassium perchlorate.
By way of example, the heater resistor element 17 is made of metal and is formed, for example, by the middle portion of an electrically-conducting layer 25, and for example a nickel/chromium alloy or some other alloy, e.g. a PtW alloy. Apart from its middle portion forming the heater resistor element 17, the layer 25 is covered by two separate electrical contacts 26 and 27 making contact with the two electrodes 12 and 13 respectively. The contacts 26 and 27 are covered by the ignition pyrotechnic charge, co-operate with the layer 25 to form the electrical circuit 18 and comprise, for example, a layer 28 of copper on the layer 25. This layer of copper is covered by another layer of metallization 29, for example. The layer 25 and the contacts 26 and 27 outside the heater resistor element 17 have the electrodes 12 and 13 passing therethrough.
In Figures 2 and 3, a heat dissipator 31 is provided between the support 16 and the heater resistor element 17. This heat dissipator 31 is electrically insulated from the electrodes 12 and 13 and from the resistor element 17 by any appropriate means that also enables the heat produced by the heater resistor element 17 to be conducted towards the heat dissipator 31. In Figures 2 and 3, the heat dissipator 31 does not touch the heater resistor element 17. The heat dissipator 31 conducts heat and serves to dissipate the heat produced by the heater resistor element 17 towards the support 16. By way of example, the heat dissipator presents thermal conductivity greater than or equal to 100 milliwatts per centimeter per Kelvin (mW/cm.K) (which applies to Constantan, being the least thermally conductive of copper-based alloys), and is preferably greater than or equal to 200 mW/cm.K.
The heat dissipator 31 is either situated beneath the heater resistor element 17, e.g. in register therewith as shown by dashed lines in Figures 2 and 3, or else it projects beyond said position beneath the contacts 26 and 27, as shown by continuous lines in Figures 2 and 3. The figures are not to scale nor are ratios between dimensions complied with, and the heat dissipator 31 may extend beneath the resistor element by as much as the length of the heater resistor element 17 between the contacts 26 and 27, or even more, for example. By way of example, the dissipator 31 may surround the channels 22, 23 and the pins 12, 13, without touching them.
The heat dissipator 31 is constituted, for example, by a heat-conducting layer 31 of determined and continuous thickness, as shown in Figures 2 and 3. By way of example, the heat dissipator 31 is electrically conductive, e.g. being made of metal, such as copper or aluminum. Naturally, any other heat conducting material could be used for the heat dissipator 31.
In the embodiments shown in Figures 2 and 3, in order to insulate the heat dissipator 31 electrically from the heater resistor element 17, a first layer 32 of heat-conducting and electrically-insulating adhesive is interposed between the heater resistor element 17 and the heat dissipator 31. The layer 32 of adhesive provides physical contact between the heater element 17 and the dissipator 31. The layer 32 of adhesive is of thickness between the heater resistor element 17 and the heat dissipator 31 that is less than half the total thickness of the layer 32 of adhesive between the circuit 18 and the support 16.
In the embodiment shown in Figure 2, a second layer 33 of heat-conducting and electrically-insulating adhesive is interposed between the heat dissipator 31 and the support 16. The layer 33 of adhesive provides physical contact between the dissipator 31 and the support 16. By way of example, the first and second layers 32 and 33 surround all sides of the heat dissipator 31 between the support 16, the heater resistor element 17, and the channels 22, 23 for passing the electrodes 12, 13, meeting each other between each channel 22, 23 and the thickness of the heat dissipator 31, so that the heat dissipator 31 is then embedded in the first and second layers 32 and 33. The first and second layers 32 and 33 meet around the channels 22 and 23 where they provide support for the electrically-conducting layer 25.
In the embodiment shown in Figure 3, the second layer 33 of adhesive is omitted beneath the heat dissipator 31, and the heat dissipator 31 is in direct contact with the support 16. To do this, the heat dissipator 31 is positioned in the desired location on the support 16 which has a plane top surface, and the layer 32 of adhesives followed by the circuit 18 are then deposited on the heat dissipator 31 and the support 16. The first layer 32 of adhesive surrounds the top and the sides of the heat dissipator 31 between the support 16, the heater resistor element 17, and the channels 22 and 23 for passing the electrodes 12 and 13, and meets between each channel 22, 23 and the heat dissipator 31 the support 16. The first layer 32 surrounds the channels 22 and 23 where it acts as a support for the electrically-conducting layer 25. The dissipator 31 may be inserted in part in the thickness direction in a corresponding recess in the support 16, so that its bottom surface 34 and a fraction of its flanks 35 come into contact with the recess in the support 16, as shown in Figure 4.
When electricity is caused to flow via the electrodes from an external source of a circuit for controlling operation of the initiator (not shown), heat is produced by the heater resistor element 17. A fraction of this heat is diffused through the first layer 32 in the heat dissipator 31, passing from the dissipator through the second layer 33 into the support 16 or directly from the heat dissipator 31 into the support 16. Because of its structure and/or the material out of which it is made, i.e. in the embodiment shown in the figures, a polymer composite of the fiberglass-filled epoxy type, the support 16 dumps the heat coming from the heater resistor element 17. The presence of the heat dissipator 31 enables the value of the no-fire threshold to be set and raised so as to reduce the difference between the no-fire threshold and the all-fire threshold, as shown in the table below, where the no-fire current and the all-fire current are given with reliability of 99.9%, the element 17 being made of NiCr, and where the no-fire current is raised by more than 44% by the presence of the dissipator. This makes the operation of the initiator more reliable.

Initiator No-fire current in mA
(10 s/+90°C) All-fire current in mA
(2 ms/-40°C)

With heat dissipator (17 micrometer thick copper layer) 551 1005

Without heat dissipator 381 998

Claims

An electropyrotechnic initiator comprising:
- an ignition pyrotechnic charge (19, 21);

- a heater resistor element (17) covered by the ignition pyrotechnic charge (19, 21);

- two electrodes (12, 13) for feeding electricity to the heater resistor element (16); and

- a support (16) for the heater resistor elements; and
between the support (16) and the heater resistor element (17), and beneath at least the heater resistor element (17):
- a heat dissipator (31) for dissipating heat produced by the heater resistor element (17) towards the support (16) ; the initiator being characterized in that it further comprises

- a first layer (32) of heat-conducting and electrically-insulating adhesive interposed between the heater resistor element (17) and the heat dissipator (31).
An electropyrotechnic initiator according to claim 1, characterized in that the heat dissipator (31) has a thermal conductivity greater than or equal to 100 mW/cm.K.
An electropyrotechnic initiator according to claim 1 or 2, characterized in that a second layer (33) of heat-conducting and electrically-insulating adhesive is interposed between the heat dissipator (31) and the support (16).
An electropyrotechnic initiator according to claim 1 or 2, characterized in that the heat dissipator (31) is in direct contact with the support (16).
An electropyrotechnic initiator according to any preceding claim, characterized in that the heat dissipator (31) is formed by a heat-conducting layer (31).
An electropyrotechnic initiator according to any preceding claim, characterized in that the heat dissipator (31) is made of a material selected from metal materials and ceramic materials.
An electropyrotechnic initiator according to any preceding claim, characterized in that the heat dissipator (31) is made of a metal selected from copper, aluminum, alloys thereof, or oxides thereof.
An electropyrotechnic initiator according to any preceding claim, characterized in that the heat dissipator (31) is located beneath the heater resistor element (17).
An electropyrotechnic initiator according to any preceding claim, characterized in that the heater resistor element (17) is covered by two electrical contacts (26, 27) that are separate from each other and in contact with respective ones of the two electrodes (12, 13).