GB2061632A - Electrical connector of the press-holding type - Google Patents

Abstract

A connector of the press-holding type for use sandwiched between, and electrically-connecting, two circuit boards. The connector comprises a plurality of linear conductive bodies, which are made of a conductive rubber with a metal, e.g. silver, powder dispersed in a rubbery matrix, and which are embedded in an insulating base body of a rubbery elastomer having two oppositely facing flat contacting surfaces, the ends of each of the linear conductive bodies reaching the flat surfaces of the base body. At least one of the flat surfaces of the base body is provided with a multiplicity of microconcavities eg by foaming for reducing moisture induced oxidation and migration of the silver.

Description

SPECIFICATION Electrical connector of the press-holding type The present invention relates to an electrical connector of the press-holding (press-contact) type as well as to a method for electrically connecting two circuit boards at arrays of contacting terminals by sandwiching such a connector under pressure therebetween.

In recent years, there have been widely used connectors of press-holding type for electrically connecting two, for example, printed circuit boards at the arrays of the contacting terminals thereof by sandwiching the connectors therebetween with a appropriate pressure. The connector has anisotropical electrical conductivity in the direction connecting the oppositely positioned contacting terminals on both of the circuit boards so that electrical conduction is obtained only between the oppositely positioned contacting terminal on the different circuit boards with electrical insulation between the terminals positioned in the transverse direction on the same circuit board.

For one kind of such connectors, have been used composite bodies composed of an electrically insulating rubbery elastomer base body having two oppositely facing flat surfaces and a plurality of electrically conductive linear bodies running parallel with one another, extending between the two flat surfaces and embedded in the base body so as to be electrically isolated from one other, the ends of each of the conductive linear bodies substantially reaching the opposite flat surfaces. Such a connector is used sandwiched between two circuit boards with the flat surfaces in contact with different circuit boards so as that the conductive linear bodies are contacted with the contacting terminals on the circuit boards at both ends thereof to electrically connect them (see, for example, U. S. P. 4,201,435).

These connectors of the press-holding type are used, for example, for electrically connecting display units such as liquid-crystal displays, electroluminescence displays, electrochromic displays and the like with a circuit board for driving the display unit. It is a recent trend that the electrodes or the contacting terminals of these display units are made of an electroconductive transparent film of a metal oxide such as vanadium oxide V2O3, silver oxide AgO, zinc oxide ZnO, titanium oxide TiO or TiO2, nickel oxide NiO, indium oxide In2O3 and tin oxide SnO2 as well as certain oxide-based composite materials such as indium oxide admixed with tin oxide, tin oxide admixed with cadmium or antimony, and the like.

The use of a connector of the press-holding type is of course subject to some limitation depending on the particular situation in respect of the materials involved and the electric current flowing through the contacting points. For example, electrochromic display units or electroluminescence display units are driven with a much larger electric current than that for driving a liquid-crystal display unit. Therefore, those conventional connectors of the press-holding type, in which the electroconductive linear bodies are made of an electroconductive rubber containing carbon powders as the conductivity-imparting material, cannot be used for connecting such a high-current display unit due to the relatively large electrical resistivity of the conductive linear bodies, especially, when the contacting electrode is made of a transparent metal oxide film.

Alternatively, there have been proposed connectors of the press-holding type for high-current use in which the conductive linear bodies are thin metal wires penetrating the insulating base body or elastomeric linear bodies of a rubber conta-ining powders of good conductivity such as a silver powder dispersed therein.

These connectors of high conductivity are also not free from several problems. For example, the connectors having thin metal wires as the conductive bodies are subject to frequent failure in conduction when the connector receives an excessively large compressive force or compression and releasing of the connector are repeated so many times leading to the bending or breaking of the metal wires within the insulating rubber matrix. In the latter type of the connector in which the eiectroconductive linear bodies are made of a conductive rubber with a metal, e.g. silver, powder dispersed therein, a problem of unstable contacting resistance is sometimes unavoidable, especially, when the connector is used in a highly humid atmosphere.

This undesirable phenomenon is presumably due to the formation of local cells between the metal particles exposed at the end surface of the linear conductive body and the metal oxide electrode in contact with the connector by the dew condensation on the contacting surface between them.

To discuss the formation of the above mentioned local cells, taking the case where the powdery metal in the conductive rubber is silver and the metal oxide electrode is a transparent indium oxide electrode as a typical example, there exist the following electrochemical relationships in the presence of water::

where E1 = -0.342 V and Kl = 1.8 X 10-6 at 25 "C; and

where Eo2 = -0.344 "and K2 = 6.0 x 105 at 25 C, the values of the standard potentials E , and E 2 being taken from Metals Data Book compiled by Nippon Kinzoku Gakkai and the equilibrium constants Kl and K3 being calculated from the relation log K = 16.8 E between the standard potential and the equilibrium constant.

Meanwhile, it is assumed in the equation (I) that dissociation of indium oxide In203 readily takes place to liberate oxygen.

Combination of the above given equations (I) and (II0 leads to a following equation

where E 3 = -0.686 V and K3 = 3 X 10-12 at 25 C.

In view of the extremely small value of the equilibrium constant K3, there is very little chance that the reaction of the equatidn (III) proceeds from the left to the right and substantial amounts of silver oxide Ag2O are formed on the surface of the silver particles. It is, however, an -.

experimental fact that the resistance between an indium oxide electrode and silver powders, containing conductive rubber gradually increases when the former is the anode and the latter is the cathode. This means that an electromotive force is applied in the reverse direction t5.the reversible electrnchernical reaction of the equation (III) with a shift of the reaction équilibrium from the left to the right producing insulating silver oxide Ag2O The above given analysis is for the combination of indium oxide and silver but similar analyses can be made for different c6rnbinations of silver and other metal oxides.

When an electric current is passed between a connector of the above described type and a metal oxide electrode with local cells forrned at the contacting interface, the surface of the silver particles dispersed in the conductive rubber is readily converted into silver oxide Ag20 when the direction of the electric current is in the adverse direction leading to a.remarkable increase in the contacting resistance between them with eventual failure of conduction.

Another problem in the connectors with linear conductive bodies made of a silver powdercontaining conductive rubber is the well known phenomenon of migration of silver with application of a DC v'oltäge in a highly humid atmosphere That is. the silver in the conductive rubber gradually migrates on to the surface of the insulating base body adjacent to the conductive rubber so that the indulation between the neighboring conductive bodies is enventualiy destroyed in the long run.

Accordingly, bno satisfactory connectors of the press-holding type have not yet been developec in which the linear conductive bodies embedded in the indulating rubbery matrix are made of a conductive rubber imparted with electroconductivity by dispersing a metal powder or, in particular, sil\te'r powder therein.

It is therefore an object of the present invention to provide a novel and improved connector of the press-holding type free from the above described problems in the prior at connectors of the similar types, in which the conductive linear bodies embedded in the rubbery insulating base body are made of a conductive rubber imparted with electroconductivity by dispersing 9 metal powder, in particular, silver poider in a rubbery matrix.. -, Tge connector of the press-holding type according to the present invention comprises (a) an insulating base body made of an electrically insulating rubbery élastomer and havint two oppositely facing flat surfaces, and (b) a plurality of linear conductive bodies running substantially in parallel with each other embedded in the insulating abase body and made of an electroconductive elastic material imparted with electroconductivity by dispersing a metal powder in a rubbery matrix, the ends of each of the linear conductive bodies substantially .reaching the oppositely facing fiat surfaces of the insulating base body, wherein at least one of the oppositely facing flat surfaces of the: insulating base body is provided with a multiplicity of independant microconcavities.

In particular, the insulating base body may be shaped from 9 foamed rubber having a.closed cell strocture so as that the part cells of the foamed rubber appearing 6n the surface forme the above mentioned independent microconcavities.

The insulating base body which defines the outer shape of the connector may be made' f å relatively soft rubbery elastomer. Suitable rubbery elastomers are exemplified by polychloroprene rubbers, styrene-butadiene copolymeric rubbers, polysulfide rubbers, polybutadienes, silicone rubbers and the like. The form of the insulating base body is of course determined in accordance with the particular application of the connector including disc-like, square plate-like, annular, square frame-like or any other forms. Most typically, it is in a form of an extende rod like form having a rectangular cross section.At any rate, it is an essential n or Bn c'ori'Jition' that the insulating base body has two oppositely facing flat surfaces where the connector is contacted with two different circuit boards so that the linear conductive bodies embedded in the insulating base body are contacted with the contacting terminals on the circuit boards. These flat surfaces are not necessarily in parallel with each other but may make a definite angle.

The most characteristic feature of the inventive connector is that at least one of the oppositely facing flat surfaces of the above mentioned insulating base body is provided with a multiplicity of tiny independent concavities, which are called microconcavities hereinafter. Such a surface provided with microconcavities can be obtained by moulding a rubber stock by use of a metal mould having an embossed moulding surface. The word "independent" means that each of the microconcavities forms a void space isolated from the neighboring ones when the connector is sandwiched between two circuit boards and the surface of the insulating base body is contacted with the surface of the circuit board.

The cross sectional form of each of the microconcavities may take various shapes, including circular, square and any other desired forms. Typically, when each of the microconcavities has a circular cross section as in the case where each of the microconcavities is in the form of a semisphere, the diameter of the microconcavity is in the range from 1 to 500 ,um or, preferably, from 50 to 200 pm and the density of the microconcavities on the surface is desirably at least 100 or, preferably, at least 400, per square centimeter of the surface.

As is mentioned above, the insulating base body having a flat surface provided with the microconcavities can be obtained by moulding a rubber with a corresponding metal mould. An alternative convenient way of obtaing such an insulating base body is the use of a cellular foamed body having a closed cell structure in which each of the void cells is isolated from the neighboring ones so that, when such a cellular foamed body is cut in a plane, the surface as cut is provided with independent microconcavities in the meaning defined above.

The method for the preparation of such a cellular foamed body of a rubber is well known in the art and readily obtained by moulding a rubber compound containing a foaming agent which releases a gaseous decomposition product on heating to expand the rubber body with the cellular foams. When a cellular foamed rubber is used as the insulating base body, the ratio of expansion by foaming is desirably in the range from 1.1 to 4.0 or, preferably, from 1.3 to 2.5 so as that the above described desirable conditions of the dimension of each microconcavity and the distribution density of the microconcavities are satisfied.When the ratio of expansion is srnaller than 1.1, the cellular foamed rubber has a hardness not definitely smaller than the linear conductive bodies and the dimension of the-microconcavities-is too small-while--aratio-of--- expansion larger than 4.0 results in too coarse cells as well as in poor permanent compression set of the foamed rubber so that the performance of the connector prepared therewith is unsatisfactory. Among the above named kinds of synthetic rubbers, silicone rubbers having a hardness of 20 to 40 by the JIS scale are particularly preferable owing to their small permanent compression set, excellent anti-aging resistance and good workability.Needless to say, it is not essential that whole body of the insulating base body is made of such a cellular foamed rubber but at least the surface layer of a thickness of 100 ym or larger or, preferably, 200 jtm or larger is formd of the cellular foamed rubber of the closed cell structure.

Next follows a description of the linear conductive bodies. These conductive bodies may be prepared with any kind of known electroconductive elastic materials prepared with a conductive powdery material such as a metal powder, e.g. silver powder, or an equivalent powdery material such as silver-plated copper particles or glass beads dispersed in an insulating matrix such as resinous or rubber polymers including urethane resins and urethane rubbers, epoxy resins, polyester resins, silicone resins and silicone rubbers and the like.The above mentioned conductive dispersant, e.g. silver powder, has desirably a particle diameter not exceeding 50 ,um or, preferably, 1 5 calm. The amount of the conductive dispersant in the insulating polymeric matrix is naturally determined in accordance with desired conductivity of the linear conductive bodies as well as the hardness and other mechanical properties of the bodies. For example, a silver powder is formulated in an amount of 70 to 95 % by weight based on the insulating polyrneric matrix. The hardness of the linear conductive bodies is desirably in the range from 40 to 8C in the JIS scale and larger than the hardness of the insulating base body by 20 to 40.An excessive loading with a silver powder is naturally detrimental in the poor workability of the rubber compound and the brittleness of the cured rubber.

The linear conductive bodies prepared with the above described materials are embedded in the insulating base body in alignment. The manner in which these linear conductive bodies are embedded in the insulating base body is rather conventional and need not be described in detail here. They are aligned substantially in parallel with each other within-the insulating base body and both ends of each of the linear conductive bodies reach the oppositely facing flat surfaces of the insulating base body. It should be noted that it is not essential that the end surface of the linear conductive body is coplanar with the surface of the insulating base body but the end of each of the linear conductive bodies is a little protruded out of or recessed below the surface of the insulating base body.

Needless to say, the-arrangement-of the linear conductive bodies in the insulating base body may not be plantar but they are arranged according to the intended application of the connector in relation to the arrangement of the array of-the contacting terminals on the circuit boards to be electrically connected with the connector. It is, however, the most conventional -or basic arrangement of the linear conductive bodies when the connector has an extended rod-like configuration that the linear conductive bodies are arranged in a row within a plane extending in the longitudinal direction. The cross sectional dimensions of the linear conductive bodies and the pitch of their arrangement are of course not limitative and should be determined according to particular need in relation to the contacting terminals on the circuit boards.

The method for preparing the inventive connectors with the above described materials includes a variety of modifications and is not limited to a particular process. Following is an exemplary process for the preparation thereof.

Firstly, an array of the linear-conductive bodies is formed on a suitable carrier sheet made of a synthetic resin such as polyethylene terephthalate, polyethylene, polypropylene and the like in a stripe-wise arrangement, each of the stripes having a desired width and thickness, by Gse of an uncured electroconductive compounded mixture containing a metal, e.g. silver, powder. A printing technique such as screen printing, gravure printing or offset printing is useful for forming such a stripe-wise pattern on the carrier sheet. The carrier sheet may be treated in advance with a mould-releasing agent.

Thereafter, the carrier sheet provided with the conductive stripe patfern is overlaid and integrated with a sheet of the insulating rubbery material before curing formulated with a foaming agent, if desired, so as that the striped pattern of the conductive material is transferred on to the insulating sheet with removal of the carrier sheet by peeliiig. Further, another sheet of the same insulating rubber as above is laid in place of the removed carrier sheet and buzzed integrally, with foaming 'in the case where the insulating rubbery sheets are formulated With a foaming agent The last step is slicing 'f the above obtained composite body in a plane perpendicular to the longitudinal direction of the stripes made of the conductive material.

As is understood from the above description, the inventive connector is provided with a large number of independent microconcavities on at least one of the contacting surfaces so that. even when the connector is used in a highly humid atmosphere where dew condensation is unavOidable, the moisture layer which may be formed on the contacting surface of the connector and the electrode is presumably partitioned and confined within each of the microconcavities isolated--from the neighboring ones and no continuous layer is formed contributing to preventing the migration of silver and the electrochemical oxidation of silver by the formation of local celles otherwise unavoidable by the dew condensation resulting in the very stable performance of the inventive connector over a long period of time.The above described partitiobning effect of the moisture layer is particularly prominent when the connector is used as sandwiched between tgwo circult boards with à suiable, såy 5 to 30 % or, preferably, 5 to j 15 %, compression so as that the walls around leech of the microconcavities may serve as the partitioning walls bf the moisture layer into the areas confin'ed within each of the microconcavities.

Following is an example to illustrate the preparation and performance of the inventive connector in comparison with conventional connectors but not to limit the scope of the invention.

Example A connector according to the present invention was prepared with the insulating base body made of a cellular foamed silicone rubber dhd the linear conductive bodies made of a silicone rubber filied with a silver powder.

The connector had a rod-like Outer configuration of 50 mm length, 3.5 mm height and 3.0 mm width having a rectangular cross section as cut in a plane perpendicular to the longitudinal direction.

The insulating base body ways made of a cellular foamed silicone rubber obtained by adinix'in'g" a silicone rubber compound (KE 151 U, name of a product by Shin-Etsu Chemical Co., Japan) with a recommended curing agent and azobisisobutyronitrilé as a foaming agent followed by curing with simultaneous expansion by foaming. The ratio of expansion was 1.6 and a surface of this foamed rubble as dtit had about 500 microconcavities per cm having an average. diameter of about 1 50 ,um as determined by a microscópic examination. The hardness of this foamed rubber as a whole was 30 in the' JIS scale.

The linear conductive bodies each having a.dimensions of 3.5 mm length, 0.20 mm width and 0.050 mm thickness were prepared with a low temperature curable silicone rubber compound (KE 106LTV'; name of a product by Shin-Etsu Chemical Co., supra) with admixture of 80 % by weight of a silver powder having an average particle diameter of ab'6ut 0.3 Sem. The volume resistivity of these linear conductive bodies was 1.5 X ff)-3 ohm-cm and the hardness thereof was 60 in the JIS scale.

These linear conductive bodies were embedded in the insulating base body as aligned on the centre plane in the direction of the length of the rod-like body with a regular pitch of 0.4 mm in such a manner that the linear bodies lie in parallel with each other and the ends of each of the linear bodies just reach the top and the bottom surfaces of the insulating base body, the direction of the width of each of the linear bodies being in the longitudinal direction of the rodlike body. This connector is called Connector A hereinafter.

For comparison, Connector B was prepared with the same dimensions as Connector A above but with different rubbery material for the insulating base body in place of the cellular foamed silicone rubber. The material for the linear conductive bodies was the same as in Connector A.

The material for the insulating base body in Connector B was a solid silicone rubber cured without the addition of a foaming agent so that the contacting surface of the connector was smooth and had no microconcavities. The hardness of this silicone rubber was 50 in the JIS scale.

Further, two connectors C and D were prepared, C according to the present invention and D being for comparative purpose. These connectors had a similar structure to that depicted in Fig.

2 of the above mentioned U.S.P. 4,201,435. That is, a stratified body of a conductive rubber having a hardness of 60 and an insulating rubber having a hardness of 50 in the JIS scale, each made of a silicone rubber, was provided on each of the lateral side surfaces with a protecting side member.

The protective side members in Connector C were made of a cellular foamed silicone rubber having a closed cell structure. The hardness of this foamed silicone rubber was 30 in the JIS scale and the contacting surface of the connector had about 800 microconcavities per cm2 with an average diameter of about 1 20 jtm.

On the other hand, the protecting side members of the comparative Connector D were made of a solid silicone rubber having a hardness of 20 so as that the contacting surfaces of the connector were smooth without minroconcavities.

Each of the above prepared Connectors A, B, C and D was subjected to the performance test by sandwiching between an indium oxide electrode of an electronic display unit and gold-plated contacting terminals of copper foil formed by the techinque of etching on a printed circuit board for driving the display unit. The compression of the connector was about 1 5 % in each case.

The testing circuit was so constructed that the indium oxide electrode was the anode and the gold-plated contacting terminals form the cathode with an electric current of 100 mA passing through the connector. The overall resistance between the electrodes was I ohm at the start in each of the tests.

The ambient conditions were varied in three, i.e. (a) at 60 "C with a relative humidity of 1 7 %; (b) at 25 "C with a relative humidity of 60 %; and (c) at 60 "C with a relative humidity of 95 % and the test in each of the above ambient atmospheres was continued for 1000 hours at the longest although the test was discontinued when an impractical increase in excess of 100 kilo-ohm was found in the value of the overall resistance between the electrodes. The results of the overall resistance are summarized in Table 1 below.

Table 1 Ambient conditions Connector (a) (b) (c) Stable within Stable within Stable within A 168 hours 1000 hours 1000 hours B Stable within Larger than Larger than 168 hours 100 kilo-ohm 100 kilo-ohm after 40 minutes after 2 minutes C Stable within Stable within Stable within 168 hours 1000 hours 1000 hours D Stable within Larger than Larger than 168 hours 100 kilo-ohm 100 kilo-ohm after 504 hours after 22 hours Further, each of the above connectors A to D was subjected to the test for the silver migration. Thus, two gold-plated rectangular electrodes were contacted with the connector on the same contacting surface thereof with a gap of 0.6 mm and a DC voltage of 1 2 V or 250 V was applied between the electrodes in an atmosphere of 95 % relative humidity at 60 "C.

The leak current across the contacting surface of the connector between the electrodes was in the order of microamperes in each case at the start of the test while, with the continued application of the DC voltage between the electrodes, a rapid increase of the leak current was observed to reach an order of milliamperés at a moment several hours after the start of the test due to the silver migration. The time to the rapid increase of the leak current was taken as a measure of the resistance against the silver migration and the results are shown in Table 2 below.

Table 2 Voltage applied Connector 12 volts 250 volts A Longer than Longer than 72 hours 24 hours B Less than Less than 24 hours 30 minutes C Longer than Longer than 72 hours 24 hours D Less than Less than 24 hours 30 minutes

Claims (6)

1. A connector of the press-holding type which comprises (a) an insulating base body made of an electrically insulating rubbery elastomer and two oppositely facing flat surfaces, and (b) a plurality of linear conductive bodies running substantially in parallel with each other embedded in the insulating base body and made of an electroconductive elastic material imparted with electroconductivity by dispersing a metal powder in a rubbery matrix, the ends of each of the linear conductive bodies substantially reaching the oppositely facing flat surface of the insulating base body, wherein at least one of the oppositely facing flat surfaces of the insulating base body is provided with a multiplicity of independent microconcavities.

2. A connector according to claim 1, wherein the flat surface sf the insulating base body is provided at least 100 independent microconcavities per cm2 area and the microconcavities have an average diameter in the range from 1 to 500 ,um.

3. A connector according to claim 1 or claim 2, wherein the electrically insulating rubbery elastomer for the insulating base body has a hardness in the range from 20 to 40 and the electroconductive elastic material for the linear conductive bodies has a hardness in the range from 40 to 80 and greater than the hardness of the electrically insulating rubbery elastomer for the insulating base body by 20 to 40, the hardness values all being in the JIS scale.

4. A connector according to any one of the preceding claims, wherein the insulating base body is made of a cellular foamed rubber having a closed cell structure, the microconcavities being formed by part cells appearing at the surface.

5. A connector according to claim 4, -wherein the cellular foamed rubber has a ratio of expansion of foaming in the range from 1.1 to 4.0.

6. A connector according to claim 1, substantially as described in the example.