FR2842698A1 - Electronic circuit overvoltage protection having substrate with electrical conductor track each end and central section connected. - Google Patents

Abstract

The protection mechanism has a substrate (10) with an electrical conductor track (15) at each end (16,17). The track has a central section (18) connected.

Description

I The present invention relates to a method and a protection device

  against electrical overcurrents. It applies, in particular, to the protection of electronic circuits. To protect electronic components against electrical overcurrents, it is known to use removable fuse components placed on fuse supports welded to the electronic circuit. These protections have many drawbacks: on the one hand, the volume and the surface occupied are large and,

  on the other hand, their manufacturing and installation cost is high.

  It is also already known to produce the fuse directly on the printed circuit. However, this type of fuse malfunctions and does not make a clean blast of the fuse track. In fact, the heat dissipated in the fuse track tends to melt the support of the track first, then the track. To avoid this inconvenience, it is

  known to laminate a thickness of substrate under the copper layer of the printed circuit.

  However, this technique is expensive because it must take place before the deposition of the copper layer. In addition, the frank breakdown of the track is still not guaranteed because the support tends to deteriorate under the effect of the heat created by the overcurrent.

passing through the fuse track.

  The present invention aims to remedy these drawbacks and, in particular, to integrate a fuse element into a printed circuit while retaining the standardized properties of the fuse element, that is to say in particular a frank breakdown at a

given overcurrent.

  To this end, the present invention relates to a device for protection against electric overcurrents comprising a non-conductive substrate, a track of electrical conductor adhering, at each of its ends to the substrate, characterized in that the track of electrical conductor adheres substantially less. to the substrate, in its part

central only at its ends.

  Thanks to these provisions, during an overcurrent, the electrical conductor track elongates under the effect of heat and detaches from the substrate, thus avoiding both the dissipation of heat in the substrate which risks preventing the melting of the electrical conductor track and the ignition of the substrate under the effect of the dissipated heat

  by the electrical conductor track.

  According to particular characteristics, the track is made in serigraphy

  by exposure of a substrate initially covered with a layer of conductive material.

  According to particular characteristics, an area of the substrate surrounding the

  central part of the electrical conductor track is chemically treated.

  According to particular characteristics, said chemical treatment is carried out

after exposure of the substrate.

  According to particular characteristics, said chemical treatment is a deposit

solvent.

  According to particular characteristics, said solvent is acetone deposited

  as a liquid jet, gel or patch.

  According to particular characteristics, the solvent deposition is carried out by

a robotic arm.

  Thanks to each of these arrangements, the production of the device which is the subject of the present invention easily fits into a circuit manufacturing process.

  electronic and has a low cost.

  The present invention also relates to a method for producing the device succinctly described above, characterized in that it comprises: - a screen printing step by exposure of a substrate initially covered with a layer of conductive material, to produce a track of electrical conductor, - a step of chemical treatment of a central part of said track of electrical conductor so that the track of electrical conductor adheres

  substantially less to the substrate, in its central part than to its ends.

  The advantages, aims and particular characteristics of said method being identical to those of the device as succinctly set out above, they are not

recalled here.

  Other advantages, aims and characteristics of the present invention

  will emerge from the description which follows, with reference to the appended drawing in which:

  - Figure 1 shows, in top view, a first embodiment of a device object of the present invention, - Figure 2 shows, in section along the line 11-II of Figure 3, a second embodiment of a device which is the subject of the present invention, and

  - Figure 3 shows, in top view, the device illustrated in Figure 2.

  In FIG. 1, we observe a substrate 10 carrying an electrical conductor track 15 having ends 16 and 17 and a central part 18 on a zone 20

of substrate 10.

  The substrate 10 is, for example, epoxy known in the manufacture of printed circuits. The electrical conductor track 15 is, for example, produced in screen printing by exposure of a substrate initially entirely covered with a layer of conductive material, for example copper. A printed circuit (not shown) produced on the substrate 10 is adapted to be protected from electrical overcurrents by melting of the track 15. For this purpose, the central part 18 of the electrical conductor track has a width less than that of the ends 16 and 17. In this way, during an electrical overcurrent, it is the central part 18 which heats up the most, melts and becomes

  cut, thus playing its role of fuse and interrupting the flow of current.

  According to the present invention, the electrical conductor track 15 adheres substantially less to the substrate 10 in its central part 18 than to its ends 16 and 17. For this purpose, the area 20 of the substrate 10 which surrounds the central part 18 of the electrical conductor track 15 is chemically treated. Preferably, said chemical treatment is carried out after exposure of the substrate, by depositing a solvent, for example acetone deposited in the form of a liquid jet, gel or patch. The purpose of this treatment is to detach (or facilitate detachment from) the central part 18. In fact, during the heating of the central part 18, the latter expands and detaches from the support. A thermally insulating layer of air then appears, between this central part 18 and the

area 20 of substrate 10.

  Since the electrical conductor track 15 has a melting temperature much higher than that of the substrate 10, the present invention makes it possible to thermally isolate the fusible central zone 18 from the substrate 10. This layer of air makes it possible to prevent the heat from the heating of the fusible zone disperses in the substrate 10 (which would cause the substrate to heat up and / or give off smoke and / or possibly, ignition of the substrate) and prevent frank and rapid melting of

  the fuse zone (which would take longer to heat up and therefore to blow).

  The implementation of the present invention therefore allows a frank breakdown of the fuse zone at the predetermined overcurrent and avoids local deterioration of the substrate 10. It is observed that the appearance of the layer of thermally insulating air between the central part 18 and the area 20 of the substrate 10 can be done either at the time of the chemical treatment, or at the first use of the electronic circuit (first heating of the

  track 15), or during an overcurrent (strong heating of track 15).

  To reduce the cost of implementing the present invention, the deposit of

  solvent is carried out by a robotic arm.

  Of course, the width of the fuse track 15 is determined as a function of the overcurrent necessary for the breakdown of the track. In Figure 1, different values of

  fuses are shown schematically by different track widths.

  FIGS. 2 and 3 show an embodiment of the protection device according to the invention in which the transmission of heat to the substrate is further attenuated. This attenuation is achieved by grooves or millings in the support as well as by specific shapes given to the central part of the fuse track. Thus, there is a substrate 30 carrying an electrical conductor track 35 having ends 36 and 37 and a thinned central portion 38 on an area 40 of the substrate 30. The area 40 has a thickness less than the rest of the substrate 30, by

  example following a step of milling the face of the substrate 30 opposite the track 35.

  In the area 40, two grooves 41 and 42 surround the central part 38 of the track 35. The circular shape given to the central part 38 makes it possible to balance the

  expansion forces when the central part 38 heats up under the effect of an overcurrent.

  This causes, more quickly than for a linear form, a separation between the part 38 of the track 35 and the substrate 30. The part thus detached heats up more quickly, for lack of heat dissipation in the substrate 30, reaches its temperature of fusion and slap. The heating of the part 38 is favored by the grooves 41 and 42 which limit the thermal diffusion in the substrate 30. As a variant, the grooves 41 and 42 are replaced by holes (not shown) in order to reduce, even more, the thermal diffusion in the substrate 30. The reduction in the thickness of the part 40 of the substrate 30 has the effect of reducing the release of fumes caused by a possible start of

combustion of the substrate 30.

  The substrate 30 and the track 35 are constituted as shown opposite the

figure 1.

  According to the present invention, the electrical conductor track 35 adheres substantially less to the substrate 30 in its central part 38 than at its ends 36 and 37. The area 40 of the substrate 30 which surrounds the central part 38 of the conductor track Electric 35 is chemically treated. Preferably, said chemical treatment is carried out after exposure of the substrate, by depositing a solvent, for example acetone deposited in the form of a liquid jet, gel or patch. When the central part 38 heats up, it expands and a layer of thermally insulating air appears between

  this central part 38 and the area 40 of the substrate 30.

  To make certain embodiments of the device which is the subject of the present invention, there is carried out: - a screen printing step by insolation of a substrate initially covered with a layer of conductive material, to produce an electrical conductor track, - a step of chemical treatment of a central part of said electrical conductor track so that the electrical conductor track adheres

  substantially less to the substrate, in its central zone than to its ends.

Claims (7)

  1 - Device for protection against electric overcurrents comprising a non-conductive substrate (10,), an electrical conductor track (15, 35) adhering, at each of its ends (16, 17, 36, 37) to the substrate, characterized in that the electrical conductor track adheres substantially less to the substrate, in its central part (18, 38) than to its ends. 2 - Device according to claim 1, characterized in that the electrical conductor track is produced in screen printing by exposure of a substrate initially covered   a layer of conductive material.   3 - Device according to any one of claims 1 or 2, characterized in that a   substrate area surrounding the central part of the conductor track   electric is chemically treated.   4 - Device according to claims 2 and 3, characterized in that said treatment   chemical is carried out after exposure of the substrate.   - Device according to any one of claims 3 or 4, characterized in that said   chemical treatment is a deposit of solvent.   6 - Device according to claim 5, characterized in that said solvent is acetone   deposited as a liquid jet, gel or patch.   7 - Device according to any one of claims 5 or 6, characterized in that   said central part (38) has a curved shape.   8 - Device according to any one of claims 1 to 7, characterized in that said   the substrate has a smaller thickness in at least one part (40, 41, 42) surrounding said central part than in other parts remote from said central part.   9 - Method for producing the device according to any one of claims 1 to 8,   characterized in that it comprises: - a screen printing step by insolation of a substrate initially covered with a layer of conductive material, to produce an electrical conductor track, and - a step of chemical treatment of a central part of said electrical conductor track for the electrical conductor track to adhere   substantially less to the substrate, in its central zone than to its ends.   - Method according to claim 9, characterized in that said processing step   chemical involves a step of depositing a solvent.