DE3037341C2 - - Google Patents

Description

The invention relates to an electrical connector that mentioned in the preamble of claim 1 Art.

In particular, the invention relates to an electrical connector for the production of electrically conductive contact connections under pressure of those to be connected Contacts. In particular, the invention relates an electrical connector that is intended sandwiched under pressure between two to each other connecting contact connection fields, in particular Contact strips, is clamped.

Such electrical connectors for manufacture electrically conductive connections between two assemblies, especially circuit boards that control the electrical Establishing connections under contact pressure are in have been used numerous times recently, for example with connecting and connecting circuit boards with multiple connections. Such an electrical connector has an anisotropic electrical conductivity towards each other and that Connector clamping connecting contacts of each other electrical components to be connected, in particular Circuit cards, on. This creates an electrical Line assigned exclusively between those in the Rule contacts arranged opposite each other of the circuit cards. Across this direction the anisotropic conductivity, i.e. in the direction of a connection contact of the to be connected Contact strips to the adjacent connection contact is that electrical connector an insulator.  

This type of connector is a composite body formed from an electrically insulating Elastomer as the main body. This basic body has two flat surfaces opposite each other on as well as a crowd of electrically conductive Elements (hereinafter referred to as "conductor tracks"), which are arranged parallel to each other and from one the flat surfaces of the base body to the opposite Area. The conductor tracks are included embedded in the body so that they are electrical are isolated from each other. You connect on this way the two opposite each other flat surfaces of the main body of the electrical Connector. Such a connector will sandwiched between those to be connected Contact strips, especially those with each other connecting contact strips of two circuit boards, clamped, one contact strip each of the circuit cards with one of the flat surfaces of the Connector is in contact. Here are the Connection contacts of the contact strip of the circuit card each assigned to certain conductor tracks, so that the conductor tracks at both ends with the Contact connections of the circuit cards are in contact and connect them together (see US 42 01 423 A1).

Electrical pressure contact connectors of this type are used, for example, for the electrical connection of Display units, for example liquid crystal display units, Electroluminescent display units or electrochromic display units, with a circuit card uses a driver circuit for the display unit contains. More recently, the connecting contacts for such display components from an electrical conductive transparent film made, which consists of a metal oxide.  

The applicability of the pressure contact connectors is regarding the manufacture of the electrical Contacts used materials and of the so made electrical contacts flowing electrical Electricity restrictions. So must for example electrochromic display units or Electroluminescent display units with much larger ones electric drive currents as liquid crystal display units be charged. The well-known and common electrical connectors, at which the conductor tracks from a carbon-filled Elastomer exist, the carbon is the brings about electrical conductivity of the elastomer, cannot be used to connect display units who need high driver currents because the conductor tracks already have too high a specific one have electrical resistance. The total resistance the link will in particular then large when the terminal contact electrodes are on the circuit cards from the above described transparent Metal oxide layers exist.

As an alternative, use in the area of higher currents to the connectors described above such pressure contact connectors are known  where the conductor tracks are made of thin metal wires consist of the electrically insulating elastomer body push through. Also are connectors based on a base body made of an electrical insulating elastomer known to be a powdery Contain material that is particularly good specific has electrical conductivity, in particular an elastomer filled with silver powder.

The known connectors described above with high specific electrical conductivity for use in the area of higher currents but not to meet the claims. For example, those with the thin metal wires fail enforced connectors often when it is cyclically subjected to compression and relief be, for example when exchanging or repeated exchange of the connected or connected Components. Such a repeated burden and relief of such a constructed electrical connector easily leads to kinking or breaking the into the electrically insulating Elastomer matrix embedded thin metal wires. The filled with the electrically conductive metal powder Connectors based on an elastomer, in particular the connectors filled with silver powder, often have contact problems, namely especially when the connector is in relative humid atmosphere is used. This unwanted Appearances in the filled connectors are likely to train on local elements between those in the surface of the connector exposed metal particles and as the material of Metal oxide serving contacts.  

When the air humidity is high, the moisture condenses in the contact area between the two materials and leads to the formation of these local elements.

For a more detailed explanation of the formation of the local elements mentioned above, it is assumed that the metal powder in the elastomer is silver and the transparent metal oxide electrode is typically made of indium oxide. The following electrochemical equilibria occur in the presence of water: where E ₁ ° = -0.342 V and K ₁ = 1.8 × 10 ^-6 at 25 ° C; and where E ₂ ° = -0.344 V and K ₂ = 6.0 × 10⁵ at 25 ° C.

The values for the standard potentials E ₁ ° and E ₂ ° are taken from a data collection (Metals Data Book by Nippon Kinzoku Gakkai). The equilibrium constants K ₁ and K ₂ are calculated from the relationship log K = 16.8 E ° between the standard potential and the equilibrium constant. Equation (I) is based on the assumption that oxygen is released in the dissociation of indium oxide In₂O₃.

The combination of equations (I) and (II) gives where E ₃ ° = 0.686 V and K ₃ = 3 × 10 ^-12 at 25 ° C.

Due to the unusually small equilibrium constant K ₃, the reaction of equation (III) practically does not proceed from left to right, so that no portions of silver oxide Ag₂O are formed on the surface of the silver particles. However, it can be confirmed experimentally that the resistance between an indium oxide electrode and an electrically conductive elastomer filled with silver powder gradually increases when the indium oxide electrode is connected as the anode and the rubber filled with the silver powder is connected as the cathode. With this polarity, the reversible electrochemical equilibrium of the reaction of equation (III) is acted upon by an electromotive force which shifts the equilibrium of the reaction from left to right, that is to say the formation of electrically insulating silver oxide Ag₂O.

The above for the material combination indium oxide and silver detailed electrochemical Correspondingly, conditions also apply to other combinations of silver with the oxides of other metals.

So if an electrical current through a contact flows on one side from an electric conductive rubber of the type described above and on the other hand consists of a metal oxide electrode, a local element at the contact interface arises, the silver particles dispersed in the elastomer quickly oxidized to silver oxide Ag₂O when the electric current the electrochemical balance is polarized opposite, so that the contact resistance increases rapidly and eventually even for an interruption of electrical contact.  

Another problem arises with elastic connectors with conductor tracks made of a powder filled with silver electrically conductive rubber that when applying an external DC voltage in humid Atmosphere a silver migration takes place. This in silver dispersed in the elastomer matrix gradually migrates across the conductor track on the surface of the electrical insulating elastic body made of borders on the conductor track so that short circuits between neighboring conductor tracks can occur.

In summary it can be said that the known elastic connectors with parallel to each other Conductor tracks transverse to the longitudinal direction of the connecting piece, where the conductor tracks are made of metal powder, specifically with silver powder, filled elastomers, still have significant defects.

In view of this prior art, the invention lies based on the task of a rubber-elastic connector for making electrical connections under mechanical pressure of the type described above to improve that the problems mentioned are not occur more, so that such connectors permanently low contact resistance with permanently more reliable Isolation of the individual conductor tracks against each other exhibit. So especially the penetration of moisture in the contact area between the two connections can be prevented in high humidity.

To solve this problem, the connector of the Invention in the characterizing part of claim 1 mentioned features.

The elastic connector of the invention for manufacturing electrical connections under mechanical pressure the contacts or contact strips therefore consists of  (a) an electrically insulating base body which (b) a plurality of substantially linear conductor tracks wearing. The electrically insulating body (a) consists of an electrically insulating rubber elastic Elastomer that is at least two planar, has opposing surfaces. The conductor tracks (b) generally run at least essentially parallel to each other and are also configured at least essentially linearly. The conductor tracks are in the electrically insulating Basic body embedded. The conductor tracks consist of an electrically conductive elastic Material, which in turn consists of an elastomer matrix exists in the production of electrical conductivity a metal powder is dispersed. Get lost the conductor tracks in such a way that one their two ends each in one of the two each other opposite flat surfaces of the electrical insulating body is freely accessible. Here are opposite to each other in at least one of these two flat surfaces of the electrically insulating Basic body numerous independent of each other very small concave cavities formed, the following are briefly referred to as "micro-concavities".

Preferably there is the electrically insulating elastic Base made of a foamed elastomer, whose structure is made up of closed cells. The cut empty cells of the foam, the exposed on the surface of the base body, form then the mutually independent micro-concavities.

The electrically insulating elastic body, the determines the outer contour of the connector, exists preferably made of a relatively soft rubber elastomer.  As rubber elastomers for the Base materials are preferably the following materials uses: polychloroprene, styrene-butadiene copolymer, Polysulfite rubber, polybutadiene and silicone rubber. Of course, the outer dimensions are correct and the configuration of the main body of the Connector according to the requirements of the special Field of application. The connector can, for example the shape of a disc, a square or rectangular plate, a ring, a rectangular Frame or any other shape. Usually, however, the base body is elongated be and the shape of a bar or strand material with a rectangular cross-section in particular. It is also often used in the manner of a contact strip strand-like basic bodies more complicated Have cross sections that are of rectangular cross section differ significantly. The only essential thing is that the electrically insulating body of the connector two opposite flat Has surfaces that with the contact strips of the electrical assemblies to be connected, in particular Circuit cards that can be brought into contact. These contact is made under mechanical pressure must ensure that the contacts of the Contact strip of the circuit card with electrically conductive the one embedded in the body of the connector Conductors can come into electrical contact. The flat contact surfaces, better contact strip surfaces, of the main body do not necessarily need to run parallel to each other, but also can include a defined angle.

The most important feature of the connector essential to the invention is that at least one of the two is at each other  opposite flat contact surfaces of the electrically insulating base body with a variety tiny and independent concave Cavities, the "micro-concavities" is provided. A such surface with isolated micro-concavities can for example, by shaping the base body a rubber mixture can be obtained in a mold, the mold cavity surfaces are embossed. The description "independently" or "isolated" with respect The micro-concavities mean that each one of these micro-concavities a clearly defined concave Recess in the surface forms by each neighboring micro-concavity strictly demarcated, in the geometric Meaning is isolated, even then, if the connector is sandwiched between two to be connected Assemblies, for example circuit cards, is clamped and the electrically insulating base body, in which the micro-concavities are formed with the or with the surfaces of the circuit cards directly in Touch.

The cross section of the individual micro concavity is not specifically critical. The micro-concavities can have a circular, rectangular or any cross-section. If the micro-concavities have a circular cross-section and at least essentially have the shape of a hemisphere, the diameter of each individual micro-concavity is preferably in the range from 1 to 500 μm, in particular in the range from 50 to 200 μm. The distribution density of the microcavities on the surface of the base body, in particular within the scope of the dimensional ranges mentioned above, is preferably at least 100 cm ^-2 , in particular at least 400 cm ^-2 .

 As already mentioned, the electrically insulating Base body of the connector with the at least a flat surface in which the micro-concavities are formed by compression molding or injection molding an uncrosslinked elastomer mixture in one Forming tool made of metal. Alternatively, a electrically insulating base body with the above Configuration features also as a cellular foam body with a closed pore structure, in which each pore is isolated from the other is. If such a foam is cut in one plane, the cutting plane has isolated micro-concavities in the sense of the invention defined above on.

Process for producing such foams, especially foamed elastomer molded materials are in themselves known to any specialist. Usually a rubber compound, which is physical, but especially also contains chemical blowing agents when heated molded. The degree of foaming is preferably in the range from 1.1 to 4.0, especially in the Range from 1.3 to 2.5 to meet the above conditions for the dimensions of the individual micro-concavities and their density distribution on the surface of the Adjust basic body. If the foaming ratio is less than 1.1, the elastomer foam has a hardness that is not significantly less than that Is hardness of the elastic conductor tracks. Also be the dimensions of the micro concavities are too small. Is on on the other hand, the degree of foaming is greater than 4.0, so a too coarse pore structure is obtained in the foam. In addition, the foam body has insufficient Values for the compression set. From one Filler connectors made meet not the requirements placed on such connectors.  

As a material for the production of such elastomeric Foams preferably become a silicone rubber with a SHORE A hardness of 20 to 40. A such material is characterized by a very small Compression set, due to an unusually good Resistance to aging and good processability out.

If the micro-concavities are preferably provided in the Embodiment of the invention by cutting of a foam, it goes without saying does not require the entire volume of the electrically insulating elastic base body is designed as a foam. Rather is enough it completely out if a surface layer of the elastic base body with a thickness of at least about 100 µm, preferably at least about 200 µm, has the described foam structure.

The elastic conductor tracks can in principle be made of any electrically conductive elastic material exist and consist in particular of one Matrix elastomer with an electrically conductive Powder, especially a metal powder, is filled. The metal powder is used to manufacture the electrical conductive elastomer used in particular silver powder. Instead of silver, you can also use a silver-plated powder Copper particles or glass particles used will. These electrically conductive powders are made in an electrically insulating plastic matrix or Dispersed elastomer matrix. As a material for the Matrix of the conductor tracks preferably serve urethane resins, Urethane elastomers, epoxy resins, polyester resins, Silicone resins or silicone elastomers.  

The electrically conductive dispersed in this matrix Powder, in particular the silver powder, preferably has a grain size of less than 50 µm, preferably in particular less than 15 µm, on. The amount or concentration of the in the dispersed electrically insulating matrix of the conductor tracks electrically conductive powder depends primarily of course on the electrical conductivity from the conductor tracks of the connector should have. It also depends on the hardness and other mechanical characteristics both the conductor track also of the entire composite body, that forms the connector. Usually is the silver powder concentration in the electrically insulating Matrix polymer preferably about 70 to 95 wt .-%, based on the weight of the polymer. The Hardness of the conductor tracks is preferably in the range from 40 to 80, measured according to SHORE A. This is due to a hardness of the electrical insulating base body of 20 to 40 units of the same Scale related. The material is basic the conductor tracks are preferably harder than the material of the electrically insulating base body.

The upper limit of the silver concentration in the polymer matrix the conductor tracks is principally increasing through that deteriorating processability of the rubber compounds and the increasing embrittlement of the networked filled conductor polymer.

The manufactured from the material described above Conductor tracks or conductor track bodies are in the specified way aligned in the electrical insulating base body embedded. The type of embedding of the conductor tracks in the electrically insulating  Base body is made according to common procedures and therefore does not need to be described here in detail to become. The conductor tracks are preferably at least essentially parallel to each other so in embedded the electrically insulating base body, that one end of each trace in one of the two opposite flat surfaces of the electrically insulating base body. It is not necessary that the front End faces of the conductor tracks coplanar with the surface of the electrically insulating base body. Rather, the contacting end faces the conductor tracks or conductor paths also slightly across the level of the surface of the electrically insulating Basic body protrude or can also slightly below this surface level of the electrical insulating body end as long as guaranteed is that this contacting face End faces of the conductor tracks or conductor paths when pressed the contact strip to be connected under electrical Contacting the connection contacts of the to be connected Component or to be connected Touch the assembly.

The conductor tracks need to be electrically insulating The basic body is of course not necessarily planar, but can according to the special requirements of the special Application area arranged and aligned be. In particular, the arrangement of the conductor tracks can according to the geometry of the connection contacts align the circuit boards and modules to be connected. The conductor tracks are commonly used for strand-shaped configuration of the electrically insulating Base body arranged in this so that the conductor tracks  essentially parallel to each other and in are arranged in a row that lie in one plane, which extend in the direction of the longitudinal axis of the strand Connection piece extends. Here are the Cross sections of the conductor tracks and the distance between two successive conductor tracks in each case Arrangement of course not critical and can vary according to the dimensions, arrangements and conditions the contacts on the contact strips the circuit assemblies to be interconnected, especially circuit boards.

The invention is based on an exemplary embodiment explained in more detail.

To produce the connector is first on a surface material suitable as a carrier required arrangement of the conductor tracks produced. The The carrier can be, for example, a plastic film, in particular a film made of polyethylene terephthalate, Polyethylene or polypropylene. The conductor tracks are as non-networked electrically conductive and with one Metal powder, preferably silver powder, filled molding compound applied in the required width and thickness. Such conductor tracks are usually made by printing, especially by screen printing, gravure printing or offset printing, printed on the carrier film in the required pattern. The printed surface of the carrier film can previously treated with a release agent.

After the conductor tracks have been applied, the printed one is printed Surface of the carrier film with a layer of uncrosslinked, blowing agent containing, electrically insulating Rubber compound that covers the material after crosslinking of the base body of the connector.  

By pressing the film onto the uncrosslinked material are printed on the surface of the carrier Conductor tracks in the uncrosslinked elastomer layer indented. Then the carrier film be pulled off so that the conductor tracks are pressed in remain in the uncrosslinked elastomer mixture. Subsequently becomes a second film from the same uncrosslinked Elastomeric material on the surface of the placed the first uncrosslinked film in which the conductor tracks lie. The carrier film is therefore against one second uncrosslinked elastomer film replaced. The from the two joined non-cross-linked elastomer films formed with the intermediate conductor tracks Structure is then networked. If the unconnected, electrically insulating elastomer compounds a blowing agent contain, foaming occurs at the same time during networking a. After foaming and cross-linking the homogeneous and one-piece is obtained Composite elastomer structure in a vertical plane to the longitudinal direction of the conductor tracks in the required Cut way.

The connector of the connection is relative to one large number of isolated micro-concavities provided at least one of its contact surfaces. If such isolated micro-concavities in its Connector having contact surface in one humid atmosphere is used in which a condensation from the atmosphere is inevitable on the contacting surface of the connector and on the connection electrodes of the module to be connected inevitably a layer of condensation moisture on. In the connector of the invention this layer of moisture is interrupted and trapped in the individual on the surface of the Connector trained micro-concavities that  each hermetically sealed from the neighboring micro-concavities are isolated. By breaking up the Moisture condensation layer in isolated areas the formation of a coherent layer of condensation moisture prevented in the contact area and thereby becomes the silver migration in the contact interface and electrochemical oxidation of silver suppressed in the formation of local elements. This mechanism works for the electrical Connector also when operating in a humid atmosphere Long-term unusually constant characteristic data. This tearing open of the moisture layer is special effective when the connector is sandwiched inserted between two circuit boards under a compression set of approximately 5 to 30%, preferably 5 to 15%. Under these conditions act the surrounding each micro-concavity Boundary walls as partitions in the moisture layer, as it were as seals that seal the cavity any micro-concavity against the surface of the Seal the circuit board. The condensation moisture is therefore hermetic in each of the micro-concavities sealed included.

Comparison test

A connector according to the invention is made from a electrically insulating base body made of a cellular silicone rubber, and linear Conductor body made from a silicone rubber filled with silver powder.

The connector has a strand-like configuration, is 50 mm long, 3.5 mm high and 3.0 mm wide and has a section perpendicular to the longitudinal axis rectangular cross-section.  

The electrically insulating base body made of cellular silicone rubber is produced from a mixture consisting of a commercially available silicone rubber, a crosslinking agent and azobisisobutyronitrile as a blowing agent. The molding compound is cross-linked with expansion and foaming. The foaming ratio is 1.6. A surface cut from this foamed elastomer has a micro concavity distribution density of 500 cm ^-2 . The average diameter of the concavities is 150 µm. The diameter is determined microscopically. The hardness of the elastomer foam corresponds to a value of 30 according to SHORE A.

The conductor tracks are 3.5 mm long, 0.20 mm wide and 0.050 mm thick. They are made from a commercially available silicone rubber that is crosslinkable at low temperatures and filled with 80% by weight of silver powder. The average particle diameter of the silver powder is approximately 0.3 µm. The specific electrical resistance of the conductor tracks is 1.5 × 10 ^-3 Ohm · cm. The hardness of the conductor tracks is 60 according to SHORE A.

These conductor tracks or conductor track bodies are in the Way in the electrically insulating body of the Connector incorporated that they are in the central plane of the connector, in which also the central Longitudinal axis of the connector is. The distance of the individual conductor tracks from each other 0.4 mm. The individual conductor tracks run in parallel to each other. The end faces of each individual tracks just reach the top or the lower surface of the electrically insulating Basic body. The wide side of the rectangular lies  End faces of the conductor tracks in the direction the longitudinal axis of the strand-shaped connecting piece. The connector thus formed is as follows referred to as "connector A".

For comparison, a "connector B" with the same Dimensions made like connector A, however instead of the closed-cell foamed silicone rubber another elastomer as a material for the electrically insulating base body is used. The conductor tracks in connector B are the same as in connector A.

The material for the electrically insulating base body in connector B is a non-foamed silicone rubber with smooth surfaces that have no micro-concavities. The hardness of the silicone rubber is 50 according to SHORE A.

In addition, two other connectors, "Connector C" and "Connector D", are manufactured. Connector C has the features of the invention, while connector D is for comparison purposes. The structure and configuration of these two connectors correspond to the connector according to FIG. 2 of US 42 01 435 A1. The connectors are therefore made up of alternating layers of an electrically conductive elastomer with a hardness of 60 and an electrically insulating elastomer with a hardness of 50 according to SHORE A. Both the electrically conductive and the electrically insulating elastomer are silicone elastomers. These layered composite structures are provided with protective elements on the end faces.

In the case of connector C, these lateral protective flanks consist of a closed-cell foamed silicone rubber, the hardness of which corresponds approximately to 30 on the scale of the Japanese industrial standard. The contact strip surface of the connector C has a distribution density of the micro concavities of approximately 800 cm ^-2 . The average diameter of the micro concavities is approximately 120 µm.

In the case of the connector D these side protection elements consist of one non-foamed silicone rubber with a hardness of 20 SHORE A, so the contact surfaces of the connector are smooth and none Have micro-cavities.

Each of the connectors A, B, C and D thus produced is then sandwiched between performance checks the indium oxide electrodes of an electronic display assembly and gold-plated copper contacts clamped by etching on a printed Circuit board that is the driver for the display assembly bears, are manufactured. In all cases this is Compression of the between the display unit and the driver card clamped connector 15%. The test circuit is switched so that the indium oxide electrode is poled as the anode and the gold-plated Connection contacts are poled as a cathode. About the connectors a current of 100 mA flows. At the beginning of the comparison tests the total resistance is between the electrodes 15 ohms in all cases.

The functional tests of the connectors are carried out under three different climatic conditions,  namely (a) at 60 ° C in a relative humidity of 17%, (b) at 25 ° C in a relative humidity of 60% and (c) at 60 ° C in a relative 95% humidity. The attempts in the individual Atmospheres are carried out up to 1000 h. The Test attempts are stopped earlier if the total resistance a value of between the electrodes greater than 100 kOhm, so practically no more usable value.

The under these conditions for each connector Test results obtained are in Table 1 summarized.

Table 1

The connectors A through D continue to migrate to silver examined. For this purpose, two gold-plated rectangular electrodes on one and the same Contact surface of the connector at a distance of 0.6 mm from each other. Between a direct voltage of 12 V or 250 V applied. The tests are carried out at 60 ° C in one Atmosphere with a relative humidity of 95% carried out.

In all test cases, the leakage current lies over the contact area the connector between the created Electrodes immediately at the start of the tests of the order of a few microamps. In the course of some Time after the start of the experiment occurs in particular with the Comparative connectors with continuously attached DC voltage between the electrodes a rapid increase of the leakage current, the values in the milliampere range reached. This increase in leakage current will for silver migration on the surface of the connector returned. The time to relative steep rise in the leakage current passes as Measure of resistance to silver migration rated. The test results obtained are in the Table 2 compiled.

Table 2

Claims (5)

1. Electrical connector for making electrical connections under contact pressure, the electrical connector made of

• (a) an electrically insulating base body consists of an electrically insulating rubber elastomer with two opposing flat surfaces, in which
• (b) numerous electrically conductive bodies ("conductor tracks") are included, which are arranged at least substantially parallel to one another embedded in the electrically insulating base body and consist of an electrically conductive elastic material, the electrical conductivity of this conductor material by dispersing a metal powder in one Rubber matrix is brought about, and wherein the ends of each of the conductor tracks at least substantially each extend up to the mutually opposite flat surfaces of the electrically insulating base body, i.e. extend in the manner from one of these two surfaces to the other surface and on these surfaces for producing electrical connections be available,

characterized in that at least one of the two opposing flat surfaces of the electrically insulating base body is provided with a large number of micro-concavities which are delimited and insulated from one another. 2. Connection piece according to claim 1, characterized in that the flat surface of the electrically insulating base body has the micro-concavities in a distribution density of at least 100 cm ^-2 and that the micro-concavities have an average opening diameter on the surface of the base body in the range of 1 to 500 microns . 3. Connector according to one of claims 1 or 2, characterized, that the electrically insulating and made of a rubber elastomer existing basic body a hardness in the area from 20 to 40, measured on the scale of SHORE A, and the electrically conductive elastic material of the conductor track body a hardness in the range of 40 to 80, too measured on the scale of SHORE A and that the material of the electrically conductive elastic material of the conductor track body 20 to 40 harder units on the SHORE A scale is the hardness of the electrically insulating body. 4. Electrical connector according to one of claims 1 to 3, characterized, that the electrically insulating base body from one cellular foamed elastomer foam with closed-pore  Structure in which the micro-concavities pass through cut cells appear on the surface and the expansion ratio caused by the foaming, namely the degree of foaming, a value in Range from 1.1 to 4.0.