WO1983001153A1

WO1983001153A1 - Integrated protection device against overvoltages in an electronic circuit and electronic circuit protected by such device

Info

Publication number: WO1983001153A1
Application number: PCT/FR1982/000134
Authority: WO
Inventors: 15 Interfaces
Original assignee: Carreras, Michelle
Priority date: 1981-09-14
Filing date: 1982-08-13
Publication date: 1983-03-31
Also published as: FR2513032B1; FR2513032A1; EP0088081A1

Abstract

The device is intended for the protection of circuits and semiconductors against static overvoltages or dynamic voltage overloads. The device according to the invention comprises the deposition on the support (3) and on the external access connections (4, 5) to the circuit of a non-linear resistance or varistance (6), of which the composition and the thickness of the deposition are selected so that the threshold voltage (Vo?) of the varistance is lower than the critical overvoltage for the circuit. According to a preferred embodiment of the invention, the varistance (6) is deposited by serigraphy as a ring covering the external access connections (4, 5), and metallization (7) deposited on the varistance (6) provides for the grounding in case of overvoltage. According to another embodiment, the grounding is provided by the access connections themselves. Application to MOS and FET circuits in ceramic micro-packages or to hybrid circuits.

Description

INTEGRATED PROTECTION AGAINST OVERVOLTAGES OF AN ELECTRONIC CIRCUIT, AND PROTECTED ELECTRONIC CIRCUIT

BY THIS DEVICE

The present invention relates to an integrated protection device for electronic circuits and semiconductors against electrical overvoltages of static or dynamic origin. It applies to the 5 circuits and semiconductors known to be particularly fragile with overvoltages, such as for example MOS or field effect transistors, these being further encapsulated in housings made of insulating material such as ceramics. 0 The protective device according to the invention is applicable to any existing insulating package, such as, for example, discrete semiconductors in TO 220 ^* or TO 126 type plastics, which often replace, for reasons of economy, the 5 metal boxes of type TO 3. It is also applicable to mounted integrated circuits _. in housings ^' of the DIL type or in micro-housings commonly called chip-carriers, ceramics, finally with hybri¬ circuits on ceramic substrates or enameled sheet. In 0 in the case of plastic encapsulation boxes, the object of the invention must be realized before the plastic encapsulation.

More generally, the device according to the invention applies to any discrete, integrated or hybrid electronic component mounted in a housing the surfaces of which, internal or external, poorly flow electrical charges.

It is known that electronic components are calculated to operate at relatively low voltages, such as for example + 5 V for TTL circuits, 15 V for MOS circuits, some hundreds of volts for power transistors, without this being limiting. These same components are of course produced with a safety coefficient which allows a certain margin on the voltages, of a few tens of volts depending on the types. This does not, however, protect these components from static electricity discharges or accidental voltage overloads, in service. Indeed, especially with regard to static electricity, the developed voltages are incommensurate with the admissible voltages. Thus, for example, an operator seated at a work station creates static voltages between 500 and 3,000 volts, while walking on an insulating mat, he creates static voltages compri¬ * between 12,000 and 40,000 volts. By way of comparison, the MOS and FET circuits are sensitive to electrostatic discharges from 100 to 200 volts and the bipolar transistors to electrostatic discharges from 380 to 7000 volts.

It is therefore necessary to protect the components, e ^* t especially the most vulnerable, such as integrated circuits and MOS against overvoltage static or dynamic origin to prevent their destruction. The currently known solutions are solu¬ tions - which involve temporary bypass external connections ^hangs' ant storage, handling or welding on a complex circuit. This is how, for example, the components encapsulated in. boxes fitted with plug-in external connections, such as DIL type boxes, or supported by substrates also fitted with connections such as hybrid circuits, are pricked for storage and transport, in organic foams made electrically conductive or covered with a metallic foil which short-circuits the connections. During their handling, these same cases are taken up by special clamps which short-circuit the connections between them and ground them.

• It is no longer the same with housings without plug-in connections such as chip-carriers. On the one hand, since the connections are metal strips deposited on and under a ceramic plate, it is no longer possible to prick them in a conductive material. On the other hand, the connections being very generally on, the sides of the plate located at the bottom of a protective groove, it is difficult to take the plate with a clamp which short-circuits all the connections.

In all cases, the short-circuiting action of the external connections, whether with plug-in em¬ connections or by short-circuiting the metallizations of a chip-carrier, ceases when the pin releases this component. , or when the component is removed from the foam in which it was stuck. Finally, this action is only valid during assembly or manipulation of the component against static discharges: there is no longer any action against dynamic overvoltages during the operation of the semiconductor or the electronic circuit.

The device according to the invention provides a continuous, rather than a temporary, solution to the protection against overvoltages, whatever the forms of the external connections. Indeed, the protection device consists in bringing together the external connections, taken two by. two, by a non-linear resistance element with a ^• low voltage varistor whose resistance is very high at normal operating voltages of the component and becomes low or very low as soon as the so-called varistor threshold voltage, short-circuiting the connections and - protecting the component since a short-circuit is established between its external connections. According to the configuration of the micro-housing or of the substrate, the non-linear resistance element or varistor consists of a ring which covers all the output metallizations, or by a localized deposit which joins them two by two. According to a preferred form of the invention, a metallization which covers the non-linear resistance makes it possible to drain the overvoltages to ground.

More specifically, the invention relates to an integrated protection device against surten¬ sions of an electronic circuit mounted on a support made of insulating material ^* and provided with at least one external access connection secured to the support, this device , protective being - characterized in that it is constituted by a resistor with a non-linear effect, called a low-voltage varistor - of threshold (V), of which o a first terminal is in ohimic contact with a metal electrode for setting the mass, the thickness of the varistor material between the connection and the grounding electrode being such that, for any overvoltage (U) (dangerous for the electronic circuit and greater than the threshold voltage

(Vo) of the varistor), it crosses its tilting threshold and ^' establishes a weak resistive connection between the connection and the ground.

The invention will be better understood from the description of embodiments which will be made thereof, by relying on the attached figures which represent: FIG. 1: curves of current-voltage characteristics of varistors according to known art,

FIG. 2: diagram of the protection device according to the invention, in a first type, FIG. 3: diagram of the device according to the invention, in a second type,

- Figure 4: a first embodiment according to the first type on a • micro-housing or chip-carrier, - Figure 5: a second embodiment according to the first type,

FIG. 6: a third embodiment example according to the second type,

- Figure ^* 7: a fourth embodiment on a metal base housing.

Figure 1 recalls the current characteristics as a function of the voltage for the varistan¬ these in the general case of the known art.

The varistors are, quite generally, obtained from mixtures of powders based on zinc oxide, the structure of which has defects and to which various oxides of bismuth chromium, manganese, cobalt have been added. .., the oxide Bi203 being found essential for the development of the non-linearity of the varistor. The characteristic current-voltage of a varistor is therefore not of the linear form I = V / RV as ^' in the - classic resistors, but is of a non-linear form and follows a relation of the type I = Kλ with k being a constant and α * a coefficient of non-linearity depending on the microstructure of the material for manufacturing the variβtance. By different compositions of the mixture and different methods of manufacturing the varistors, which are outside the scope of the invention and belong to the known art, knows how to make varistors whose characteristics are variable, and illustrated by way of example in FIG. 1 by two curves. The tension being plotted on the abscissa and the intensity. being plotted on the ordinate we see that a varistor corresponding to curve 1 has a monotonous variation of its characteristics while another varistor corresponding to curve 2 remains very resistant up to a certain threshold voltage marked V °, voltage beyond which the varistor becomes almost conductive. It is this second type of varistor, which has a threshold voltage with ^* sudden tilting, which is of interest for the implementation of the device according to the invention.

We also know that the varistor effect is linked to an aux field effect. grain boundaries. The voltage V across a varistor can therefore be expressed as being equal to:

V = - n. V g g with n = 'number of grains in series in the g varistor and

V = the tension per grain,. this voltage per grain being between 2 and 4 volts approximately for the varistors currently known. Consequently, if it is desired to protect a circuit against a given voltage, it suffices to lay down a deposit thickness determined by the number of grains. The protective device according to the invention therefore consists of having a given thickness of varistor between an access electrode to a circuit or a semiconductor and an electrode that is grounded, this varistor being considered to have a very high resistance. high at voltages below the threshold voltage V of the varistor, o

^" BÏÏRË

, _OAÎPI while it becomes almost conductive at voltages higher than the threshold voltage, thus passing the overvoltages of static or dynamic origin to ground via the ground electrode.

FIG. 2 represents the diagram of the protection device according to the invention, in a first type of operation.

In order to ^'simplify the figure and the text it ^n' has

10 has been shown that part of a substrate supporting two electrodes., Let 3 be a ceramic substrate which in the case may be the substrate of a phip-quarry, but which could also be the base of a DIL type encapsulation box, this ceramic substrate

15 mique '3 therefore supporting the external connections 4 and 5, of which it is known that one end is used to fix the encapsulation box on a support while the other end provides connections with the semiconductor pas¬ tille. According to the invention, a band

20 of varistor effect material 6 is deposited between the connections, this strip itself being surmounted by a metal strip 7 which is connected to a ground connection. Thus the varistor effect develops between each access connection such as 4 and

25 -the grounding electrode 7 through a marked thickness of varistor-effect material

As long as the semiconductor is subjected to normal supply voltages, the voltage existing between the access electrode 4 and the release electrode

30. 7 weight is less than the threshold voltage V o selected for the varistor 6: therefore the latter behaves as a .A having a very high resistance almost as ^insulation. On the other hand, if a surge, such as for example a discharge

-ΛΓV ^

of static electricity, occurs on the same elec¬ access electrode 4 the varistor becomes conductive in an extremely short time since it is a phene¬ effect. field, and the overvoltage, instead of 8th deteriorating the semiconductor, is drained to ground. The thickness "e" of the varistor is partly determined by the diameter of the grains with which the varistors are made, and partly by the voltage beyond which the operation of the circuit protection device has been fixed. MOS, a deposit thickness of the order of three to five grain diameters can be used, that is to say, with grains whose diameter is between 10 and 20 microns, a thickness of 30 to 100 microns for varistor.

The varistor is deposited according to known art processes such as screen printing, vacuum deposition or plasma torch. The material is then annealed by a global or localized annealing treatment, with known means such as the laser, so as to constitute the restoration of the properties of the grain boundaries.

By way of example, with varistor rials maté¬ ^'currently known, it is possi- ble to achieve protection of outputs of a circuit with insulation values between conne¬ xions external access and grounding electrodes:

- V <10 Volts, R ≈ 50 to 100 k ohms - V - 5 Volts, R ~ 10 Moh s

^" - V> 10 Volts, R ~ 10 to 100 Mohms In addition, certain compositions or changes in thickness make it possible to protect at higher voltages than those mentioned, for example 25 or 50 Volts. The upper electrode 7 can be made of non-noble materials, such as for example bismuth, or aluminum or silver, or nickel, doped with oxides of bismuth, silicon or lead. According to a preferred form of the device according to the invention, the varistor 6 is deposited in the form of a ring which surrounds the circuit, soldered in the middle of the substrate 3, and realizes, in the event of overvoltage, a short table. circuit of all external access connections, acting somewhat like a Faraday cage. In this first type of embodiment of the invention, the presence of a varistor in the region 8 located between two access electrodes such as 4 and 5 plays practically no role since the spacing between the two electrodes considered .e counts rather in fraction of a millimeter and not in microns: there is therefore a transverse operating effect of the varistor 6 and this distance 8 is too high to play a role. On the other hand, it is the transverse operation between two access electrodes, which is implemented in the second type of operation of the invention, that is illustrated in FIG. 3.

As in FIG. 1, and in order to simplify the drawings and the text, only a fragment of substrate 3 has been represented as well as two access electrodes 4 and 5.

As in the previous case, a ribbon 6 of varis¬ tance is deposited on and between the external connections, by a method such as screen printing. However, the action of the varistor is transverse, that is to say that the thickness of the varistor which will cross its voltage threshold V in the event of a static or dynamic overvoltage that is called the "high speed". you correspond at the distance or at the distance 10 between the metallizations of two connections 4 and 5. This distance "-fc" determines the threshold, for switching of the varistor and it is expressed as has already been said by a thick - grain size of the varistor material. This thickness is therefore counted in a few hundred microns.

Since it is not possible to produce an external connection grid, each connection of which is separated from its neighbor by an interval 10 of the order of 100 or 200 microns, the safest method for carrying out this second type of protection against voltage overloads consists first of all in making a grid of external connections which are all short-circuited and cut from the same sheet of metal, then separating them by laser cutting or by chemical attack, preceded by photomasking . In this case having produced a continuity solution 10 between two electrodes 4 and 5, it becomes easy to deposit "a" varistor ribbon 6 which fills this inervall 10 with the varistor material over a thickness of a few grains.

This second type of embodiment has two advantages: it makes it possible to connect all the external -connections before depositing the varistor, which facilitates the electrolytic deposits of the surface layers such as nickel, gold, silver, etc. etc, and in addition it is only necessary to make a single deposit, that of the varistor since there is no longer any metalization 7 for grounding, as in the first type.

FIG. 4 represents a first example of embodiment of the protection device according to the invention, on the base of a micro-housing of the chip-carrier type. Compared to partial figures 2 and 3,

-DRANK 1

we find in igure 4 a ceramic base 3, on which have been deposited a number of metallizations of external connections such as 4 and 5. In order not to complicate the figuration, only four metallizations have been shown on each of the four sides of the chip carrier, but it is known ^"* " that the chip carriers can reach the hundred or so external connections. On the base of the chip carrier, provided with its metallic deposits of external connections, a ring 6 of varistor has been deposited then, on top of this, an electrode 7 in the form of a ring also, but this electrode is joined together _^ to one of the external connections, by a metal bridge II, said external connection being that which corresponds to the ground potential.

FIG. 4 and this example of embodiment also remain in the field of the invention. if the metallization 7 had not been carried out over the varistor 6. In this case, the varistor acts as a sort of Faraday cage ^' short-circuiting all the connections and the electrical charges flow through the connection to ground, but this case is of course less favorable than that which provides for metallization 7. FIG. 4 shows the base of a chip-carrier: it is understood that this is only a matter of an example to explain the invention and. that the device according to the invention also applies to any other type of substrate. insulator badly draining the charges - electrical and the example of application also applies, and with the same advantages, to the pads -ceramics of hybrid circuits, or to the ceramic bases of encapsulated integrated circuits in DIL boxes for example, as well as any other known encapsulation using materials such as ceramics.

FIG. 5 represents a second exemplary embodiment according to the first type of the protection device of the invention.

The first type of protection, as previously described, acts according to the thickness of the varis¬ tance. In the example of FIG. 4, the varistor is deposited in the form of a ring, consequently having a continuous band, and this varistor band is surmounted by a continuous metallization band which drains the overvoltages to ground. In this second embodiment, the varistor is produced in the form of a stud, deposited on each external connection, and the upper metallization or ground flow electrode, is a metalli¬ tion taken out on the neighboring connection. According to this second type, all the connections act in series.

As in the cases of the preceding figures, only a fragment of substrate 3 has been represented, this fragment supporting a few external connections such as 4 and 5. On each external connection is deposited a varistor block such as 12 on connection 4 and 13 on connection 5. This stud has reduced dimensions but it is important during deposition that it partially covers at least one side ^' of the connection in order to avoid short circuits between the elect trode upper ^'16 with the connection 4, oul7 with the connection 5. This lateral deposition such as 14 and 15 has no role but a varistor insulator. When the plots 12 and 13 have been deposited, a second localized metallization such as 16 and 17 covers the varistor block 12 or 13 and is in ohmic contact with the connection close to the connection which supports the block:. thus if we consider the connection 4 The electrode 16 is in ohmic contact with the connection 5 and the electrode 17 above the connection 5 is in ohmic contact with the neighboring connection, and so on. In this type of integrated protection, the varistor therefore acts according to the thickness of the varistor layer between two metal parts which constitute the terminals of the aristor.

In FIG. 5, only three external connections have been shown, but it is understood that the protection device may concern either one or more connections which are particularly exposed to overvoltages, or all of the external connections.

Figure 6 shows a third embodiment of the protection device according to one ^• in- vention, according to the second type. To be consistent with the previous figures, the same reference signs have ^'been retained, and Figure 6 shows a fragment 3 ^• substrate supporting at least two external connections 4 and 5. ^* However, in this type of embodiment, the. external connections, ^'that they are made in a cut metal plate, as is the case for example for DIL packages, or that they are produced by screen printing of a metal-based paste on a substrate ceramic, as is the case for chip-carrier micro-housings, are all originally joined by a metal bridge 18. In order to highlight this structure at the origin, FIG. 6 is cut away and represents in its part left the structure of the connections before the start of the operations intended to set up the device for protection against overvoltages, and, in its right part, the completed device. So, originally, the connections are joined by. metal bridges 18 and these are cut by any process known to those skilled in the art, such as a chemical attack under mask or a laser beam cutting, so as to leave two arches 9 of the bridge 18, these arches or beaches of _. metallization 9 leaving between them a space 10 which corresponds to the thickness "t" of desired varistor, as a function of the composition of the material, of its characteristics and of the tension against which it is sought to protect itself. After cutting the metal areas 18, the varistor 6 is deposited either by screen printing or by spraying and it fills the intervals 10 between the metal areas, also forming incidentally but this is not essential for the operation of the device one. continuous strip as described on the occasion of FIG. 2 or of FIG. 3.

In this second type of overvoltage protection, the varistor therefore acts transversely between two connections. As in the previous cases, the protection device can be applied between a connection particularly subjected to external overvoltages, between several or between the all-. connections, at least one of which is grounded.

FIG. 7 represents a fourth embodiment of the device for protection against overvoltages, in the case of a metal case, such as for example the boxes T03 for encapsulation of discrete semiconductor.

- WATER BU In this FIG. 7 is shown in 19 a fragment of the metal base of a housing: the external access connection 4 is immobilized and supported on this base by means of a glass bead 3, which fulfills the function of the insulating substrate described on the occasion of. previous examples. If the semiconductor encapsulated in this metal case is sensitive and can be destroyed by overvoltages, the invention provides for depositing a film or a drop of varistor 20 at the foot of the connection 4, and on the base 19.- In this case , the action of the varis¬ tance is exerted according to the distance 21 separating the per¬ turn of the connection 4 at the edge of the hole in the base 19 which supports the glass bead 3. The overvoltage protection therefore acts according to the second type, that is to say transversely. Given, on the one hand, that this distance 21 is greater than that which separates a connection from its grounding electrode in the examples described above, but that on the other hand the switching voltage of the vaistor 20 is a function of the intergranular tension, it suffices to increase the particle size of the starting material, so as to provide protection through a length 21 of varistor corresponding to a precise number of grains of material.

The invention has been described, based on a few examples of embodiments, but it covers all the variants that a person skilled in the art will find it obvious to make thereon on ceramic or plastic housings or substrates, the cooking of the ^" varistor being, in the latter case, carried out before the plastic coating.

It is specified by the claims below.

- UREA

Claims

CLAIM

1. Integrated protection device against overvoltages of an electronic circuit mounted on a support (3) made of insulating material, and provided with at least one connection (4) of external access secured to the support (3), protection device being caracté¬ ized in that it is constituted by a resistor with non-linear effect (6), called varistor, with low threshold voltage (V), a first terminal of which is in ohmic contact with the connection (4) for external access and the other terminal of which is in ohmic contact with one. metal grounding electrode (7), the thickness of the ^* varistor material (6) between the connection (4) and the grounding electrode (7) being such that ^" for any overvoltage (U ) dangerous for the electronic circuit and higher than the threshold voltage (Vo) of the varistor (6), the latter crosses its tilting threshold and establishes a somewhat resistive connection between the connection (4) and earth (7).

2. Protection device according to claim 1, characterized in that, the circuit being mounted on a support (3) provided with a plurality of connections (4, 5) for external access, the varistor (6) is placed on the support (3) and on the external access connections (4, 5) in the form of a ring ribbon which joins together all the external connections. _*

3. Protection device according to claim 2, characterized in that the metal electrode (7) for grounding is constituted by a metallization deposited on the ^" free ^" face of the varis¬ tance ring (6), this metallization being electrically combined

(11) at least one external earth connection.

4. Protection device according to claim 1, characterized in that, the circuit being mounted on a support (3) provided with a plurality of connections, (4, 5) of external access, these are provided with pads (9) distant from each other by an interval (10) within which the varistor (6) is deposited on the support (3), the grounding of a surge on a first connection (4) of external access being ensured by at least one second connection (5) of external access.

5. ^' Protection device according to claim 1, characterized in that the varistor is deposited on a first connection (4) of external access in the form of a localized deposit (12), which covers' at (14) at at least one side of said connection (4) and in that the grounding metallization (16) is deposed on the varistor (12 and 14) and on the support (3) and is electrically joined at one second external access connection (5).

6. Protective device according to claim 1, characterized in that the varistor (6 or 12) and the metal electrode (7 or 16) are deposited in the form of pastes for screen printing.

7. Protective device according to claim 1, characterized in that the varistor (6 or 12) and the metal electrode (7 or 16) for grounding are deposited under. solid form, by sputtering or plasma spraying.

8. Protective device according to claim

1, characterized in that the composition and the particle size of the varistor material (6) are chosen so that said varistor has a threshold voltage

(Vo) a few volts higher than the normal operating voltage of the electronic circuit, and has a sudden tilting threshold.