FR2882814A1 - HEAT RESISTIVE ELEMENT FOR PYROTECHNIC INITIATOR - Google Patents

Abstract

The present invention relates to a heating resistive element for electro-pyrotechnic initiator intended in particular to initiate automotive safety devices, comprising a substrate (2) on which is fixed a resistive layer (3) via a bonding layer (4). The substrate, comprising a support made of composite material (21), in particular of printed circuit type, based on fiber and resin, is doped with thermal charges and has, at least at its face in contact with the bonding layer , a thermal conductivity greater than or equal to 0.9 W / mK, said bonding layer providing thermal and mechanical bonding between the substrate and the resistive layer.

Description

2882814 1

  The present invention relates to a heating resistive element for electropyrotechnic initiator intended in particular to initiate automotive safety devices, in particular to control the operation of air bags, commonly known as "airbags", or seat belt tensioners. The present invention more particularly relates to a heating resistive element with a substrate with improved thermal conductivity.

  Electro-pyrotechnic initiators are generally constituted by a container containing a pyrotechnic composition which is raised to its deflagration temperature by a resistive heating element. It is known, in particular from US Pat. No. 5,544,585, a pyrotechnic initiator in which the resistive heating element is formed of a resistive layer or resistive ribbon, etched and glued on an insulating substrate consisting of a material support. composite type circuit board. A thermosensitive explosive varnish is deposited on the resistive ribbon. The resistive ribbon is connected to power supply terminals. The passage of a current results in the heating by Joule effect of the resistive ribbon and varnish. When the self-decomposition temperature of the varnish is reached, a deflagration occurs.

  This temperature can be reached by voluntary application of a short current pulse, a few milliseconds, and high amplitude, representing an energy of the order of milliJoule. In this case, we speak of an "all-fire" impulse and we search for a probability of explosion close to 1. This temperature can also be reached accidentally if parasitic currents cross the resistive ribbon. These parasitic currents are simulated by the "No Fire" pulse, of a longer duration and smaller amplitude, corresponding to an energy of a few Joules. When applying this parasitic impulse, the probability of explosion should be close to 0. Quite paradoxically the probability of explosion should be 1 when the energy applied is the lowest and 0 when it is higher. This result is obtained by the heat transfer time between the resistive heating element and the heat sink. For the "All-Fire" impulse, the heat is confined in the resistive ribbon which heats up adiabatically. For the "No Fire" impulse, the heat is diffused through the composite material support to reach the external environment, the resistive ribbon heats the less as the depth of penetration of heat is important.

  This type of resistive heating element, using a resistive layer deposited on a support of composite material, can be manufactured in a simple, accurate and reproducible manner according to well known techniques for manufacturing printed circuits. In addition, its shape can be easily adapted by simply stamping various pyrotechnic initiators. However, this type of resistive heating element is no longer suitable for certain operating reliability and safety standards, particularly those of the automotive industry, which impose intervals 2882814 3 between "Not Fire" impulse and "All Fire" impulse "more and more narrow. The thermal conductivity of the composite material support is not high enough to rapidly drain the energy of the "No Fire" pulse towards the heat sink.

  The object of the present invention is to overcome the drawbacks of the prior art cited above, by providing a heating resistive element simple design and manufacturing, which meets the requirements of the automotive industry.

  For this purpose, the present invention proposes a resistive heating element for electropyrotechnic initiator comprising a substrate on which is fixed a resistive layer via a bonding layer, said substrate comprising a support made of composite material, in particular of circuit type. printed film, based on fiber and resin, characterized in that said substrate is doped with thermal charges and has, at least at its face in contact with the bonding layer, a thermal conductivity greater than or equal to 0.9 W / mK, said bonding layer providing thermal and mechanical bonding between the substrate and the resistive layer.

  According to the invention, the heating resistive element comprises a substrate with improved thermal conductivity, at least on the surface, by adding thermal charges. The substrate is doped with thermal charges, at least at its face in contact with the bonding layer, so as to have a thermal conductivity greater than or equal to 0.9 W / m.K at least at said face. By "heat load" is meant a heat conducting charge. The substrate allows rapid drainage of energy from the "No Fire" pulse to the outside. The resistive element according to the invention makes it possible to meet the demanding conditions of the automotive industry, in particular an "all-fire" pulse with a duration close to 1 millisecond (ms) and an amplitude close to 1 Ampere (A ), corresponding to an energy of 2 milliJoules (mJ), and a "non-fire" pulse with a duration close to 10 s, and an amplitude close to 0.5 A, corresponding to an energy of 5 J. Advantageously the substrate is an electrical insulator and does not include any metal parts. Said thermal charges are non-metallic, and more generally non-electrically conductive. According to one feature, the thermal charges consist of fine particles of heat-conducting ceramic, such as fine particles of boron nitride and / or alumina.

  Advantageously, the substrate has a thermal conductivity greater than or equal to 1 W / m.K, preferably less than 5 W / m.K, more preferably between 1 and 2 W / m.K.

  According to a first embodiment, the substrate is thermally doped by volume. The support made of composite material based on resin and fibers is doped with thermal charges, said thermal charges being dispersed in the resin of the composite material support, the latter having a thermal conductivity greater than or equal to 0.9 W / mK, of preferably greater than or equal to 1 W / mK

  According to a second embodiment, the substrate is thermally doped on the surface. The substrate comprises a support of rigid composite material of printed circuit type, based on fiber and resin, and at least a first heat-doped coating layer covering a first main face of the support made of composite material, so that said layer doped coating is interposed between said bonding layer and said composite material support, said doped coating layer having a thermal conductivity greater than or equal to 0.9W / mK, preferably greater than or equal to 1W / m. K. In this case, the substrate may comprise a support of standard composite material, undoped with thermal charges. In a variant, the substrate may comprise at the same time a support made of composite material doped with thermal charges, at least the first face of which vis-à-vis the bonding layer is covered with a coating layer doped with thermal charges.

  Said thermal charge-doped coating layer may be formed from a self-supported resin film, a glass fabric pre-impregnated with resin and / or a resin impregnated glass felt, preferably from from a glass cloth pre-impregnated with resin, type "pre-preg".

  In order to reinforce the substrate and in particular to avoid curvature of the substrate by bimetallic effect, the second main face of the support made of composite material, opposite to the resistive layer, is advantageously covered with a second coating layer 2882814 6 similar to the first layer of coating, possibly doped with thermal loads. The substrate thus consists of a support made of composite material covered on both sides with a coating layer.

  The composite material support is advantageously of the epoxy glass or polyimide glass type.

  The bonding layer may consist of a self-supporting resin film, a resin-impregnated fiber fabric, in particular a fiberglass fabric, or a resin-impregnated fiber felt, in particular a fiber felt. fiberglass or aramid fiber, said bonding layer preferably having a thermal conductivity of between 0.1 and 0.3 W / mK, and a thickness of between 10 and 30 μm. The bonding layer must be thick enough to ensure good thermal insulation of the resistive ribbon during the "all-fire" pulse. It must also be thin enough so as not to impede the flow of Non-Fire impulse energy. The bonding layer, of substantially constant thickness, rests continuously flat on the substrate, substantially without differences in level, or steps, which would be conducive to the occurrence of electrical non-continuity defects, especially during thermal cycling. The structure of the resistive element according to the invention is homogeneous and therefore insensitive to the thermomechanical stresses induced by the welding operations.

  The tie layer is preferably made of glass fiber felt or aramid fibers impregnated with resin. Such a felt, in which the fibers are entangled in a random manner, makes it possible to obtain a substantially constant layer thickness and a substantially homogeneous thermal conductivity.

  According to one feature, the resistive layer consists of an etched resistive alloy thin sheet, such as a photo-etched nickel-chromium sheet.

  According to another feature, the resistive heating element further comprises a conductive layer partially covering the resistive layer, intended to ensure the arrival of the electric current.

  The present invention also relates to an electropyrotechnic initiator, characterized in that it comprises a resistive heating element as defined above.

  The invention will be better understood, and other objects, details, features and advantages will become more clearly apparent from the following detailed explanatory description of two presently preferred particular embodiments of the invention, with reference to the attached schematic drawing. on which.

  - Figure 1 is a schematic cross-sectional view of a heating resistive element according to a first embodiment of the invention; and, Figure 2 is a schematic cross-sectional view of a heating resistive element according to a second embodiment.

  FIG. 1 schematically represents a resistive element 1 for an electro-pyrotechnic initiator comprising a substrate 2 consisting of a support made of composite material 21, on which a resistive layer 3 is fixed by means of a tie layer 4. The resistive layer 3 is partially covered by a conductive layer 5 of tinned copper or copper, forming two pads for making electrical contact with two electric power supply terminals.

  The resistive layer 3 is constituted by a thin layer formed of a NiCr alloy sheet having a thickness of between 3 and 5 micrometers (μm). The heat dissipated by the joule effect by the resistive layer will be transmitted by thermal conduction to a pyrotechnic composition deposited on the resistive layer. Alternatively, the resistive layer is formed of a sheet of a resistive alloy Ta2N, CrSi or PtW.

  The substrate 2 consists of a rigid composite material support with improved thermal conductivity 21. This support made of composite material is formed of a fabric of glass and epoxy resin in which are dispersed in a substantially homogeneous manner fine particles of nitride. boron or alumina. The glass fabric comprises woven glass fibers 10 to 12 μm in diameter. The support made of composite material has a thickness of 0.3 to 0.5 mm, a glass transition temperature (Tg) greater than 2882814 9 170 C, and a thermal conductivity greater than 1 W / m.K.

  Alternatively, for higher temperature suits, the composite material carrier is formed from a polyimide resin doped with boron nitride or alumina particles.

  The tie layer 4 is constituted by a glass felt impregnated with high temperature epoxy resin having a Tg greater than 150 C. The pre-impregnated felt has a thermal conductivity of the order of 0.25 W / m. K. The glass felt consists of glass microfibers, for example borosilicate glass, 3 to 5 μm in diameter, assembled without weaving.

  For their assembly, the resistive layer, the glass felt impregnated with unpolymerized epoxy resin, and the support made of composite material are stacked and then subjected to firing, preferably under pressure. The thickness of the pre-impregnated felt is of the order of 25 to 50 μm before and after firing, its density is of the order of 0.180 g / cm 3.

  Alternatively, the tie layer is formed of a self-supporting adhesive film, for example an acrylic glue or phenol butyral film, 25 to 50 μm thick before firing, and 15 to 25 μm thick after baking. , having a thermal conductivity of the order of 0.25 W / m. K. The glue film is handled on a sheet of paper, the latter being removed during stacking.

  The tie layer may also be formed from prepreg resin-like glass fiber fabric of the "prepreg" type. Preferably, the prepreg comprises a resin of the same nature as that of the support made of composite material, for example an epoxy resin, of the FR5 type, for example having a thickness close to 50 μm after firing, or a polyimide or Teflon resin.

  FIG. 2 represents a second embodiment of a resistive heating element 11 according to the invention, comprising a substrate 12, with improved surface thermal conductivity, on which a resistive layer 13 is fixed by means of a connecting layer 14 , said resistive layer being partially covered by a conductive layer 15. This resistive heating element 11 differs from that described with reference to FIG. 1 in that its substrate 12 consists of a support of standard rigid composite material 121, conventionally used in the printed circuit industry, the opposite main faces are each covered with a coating layer with improved thermal conductivity, a first coating layer referenced 122a, and a second coating layer referenced 122b.

  The support made of composite material consists of woven glass fibers, for example 10 to 12 μm in diameter, and epoxy resin, having a thermal conductivity of the order of 0.35 W / m.K. The support made of composite material 21 is, for example, of the FR4 family, with an epoxy resin having a Tg of 140 ° C. In a variant, the support is of the FR5 family, with an epoxy resin having a Tg greater than 160 ° C., for 2882814 11 better temperature resistance, especially for the transition to lead-free solders, or the polyimide G family, for even higher temperatures.

  Each coating layer 122a, 122b consists of a fiberglass fabric pre-impregnated with an epoxy resin doped with fine particles of boron nitride or alumina, having a Tg greater than 170 ° C., and a thermal conductivity. greater than 1W / m K.

  Each coating layer preferably has a thickness of between 75 and 125 μm.

  The bonding layer 14 provides the thermal and mechanical connection between the resistive layer and the first coating layer 122a. This first coating layer 122a has the main function of draining the energy of the "Non-Fire" pulse and a secondary function of mechanical reinforcement. The second coating layer 122b, on the face of the support of composite material opposite to the bonding layer and the resistive layer, has only a mechanical reinforcing function to avoid the curvature of the substrate by bimetal effect during its manufacture or its use.

  Although the invention has been described in connection with two particular embodiments, it is obvious that it is not limited thereto and that it comprises all the technical equivalents of the means described and their combinations if they are within the scope of the invention.

Claims (2)

12 CLAIMS   A heating resistive element for an electro-pyrotechnic initiator comprising a substrate on which a resistive layer is attached by means of a bonding layer, said substrate comprising a support made of composite material, characterized in that said substrate (2, 12 ) is doped with thermal charges and has, at least at its face in contact with the bonding layer, a thermal conductivity greater than or equal to 0.9 W / mK, said bonding layer (4, 14) providing the bond thermal and mechanical between the substrate and the resistive layer (3, 13).   2. Resistive heating element (1, 11) according to claim 1, characterized in that the thermal charges consist of fine particles of heat-conducting ceramic, such as fine particles of boron nitride and / or alumina.   3. resistive heating element (1, 11) according to claim 1 or 2, characterized in that the substrate (2, 12) has a thermal conductivity greater than or equal to 1 W / m.K, preferably less than 5 W / m. K, more preferably between 1 and 2 W / m.K.   4. Resistive heating element (1) according to one of claims 1 to 3, characterized in that the support of composite material (2) based on fiber and resin is doped with thermal charges, said thermal charges being dispersed in the resin of the support made of composite material.   5. Resistive heating element (11) according to any one of claims 1 to 4, characterized in that the substrate (12) comprises a support of rigid composite material (121) of printed circuit board, based on fiber and of resin, and at least a first thermal charge doped coating layer (122a) covering a first major face of the composite material support, such that said doped coating layer is interposed between said bonding layer (14) and said support made of composite material.   Heating resistive element according to claim 5, characterized in that said thermal charge-doped coating layer (122a) is formed from a self-supported resin film, a glass cloth pre-impregnated with resin and / or a glass felt impregnated with a resin, preferably from a pre-preg type pre-impregnated glass fabric.   7. heating resistive element (11) according to claim 5, characterized in that the second main face of the composite material support, opposite to the resistive layer, is covered with a second coating layer (122b), optionally doped with fillers. thermal.   8. Resistive heating element (1, 11) according to any one of claims 1 to 7, characterized in that the support of composite material is of the epoxy glass or polyimide glass type.   Heating resistive element (1, 11) according to any one of claims 1 to 5, characterized in that the bonding layer (4, 14) is a self-supported resin film 2882814 14, an impregnated fiber fabric resin or resin-impregnated fiber felt, preferably fiberglass felt or aramid fiber impregnated with resin.   Heating resistive element (1, 11) according to any one of claims 1 to 9, characterized in that the resistive layer (3, 13) consists of an etched resistive alloy foil.   Heating resistive element according to any one of claims 1 to 10, characterized in that it comprises a conductive layer (5, 15) partially covering the resistive layer (3, 13) intended to ensure the arrival of the electric current. .   12. Electro-pyrotechnic initiator, characterized in that it comprises a resistive heating element (1, 11) according to one of claims 1 to 11.