Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Adhesive outermost layer
Reexamination Certificate
1999-11-15
2002-09-10
Truong, Duc (Department: 1711)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Adhesive outermost layer
C428S347000, C428S3550RA, C428S3550EP, C428S3550EN, C428S3550CN, C428S402000, C428S403000, C428S406000
Reexamination Certificate
active
06447898
ABSTRACT:
The invention describes an electrically conductive, thermoplastic and heat-activatable adhesive sheet as used for the permanent connection of two articles.
Electronic components are becoming ever smaller, which makes their handling and processing ever more difficult. Especially when producing electrical contacts between the components and/or the connections, it is found that conventional soldering can no longer provide simple and cost-effective connection of the corresponding contacts. The adhesive bonding of electronic components by means of electrically conductive layers of adhesive is an alternative.
For the field of electrically conductive adhesive tapes it is known to employ conductive pigments such as carbon black, metal powders, ionic compounds and the like in adhesive compositions. In sufficient quantities, the particles contact one another and the possibility of current flow from particle to particle is provided. The current flow here is not directionally oriented (isotropic).
For specific applications, such as electronic switches, contacting of conductors, etc, there is, however, the requirement to achieve electrical conductivity only in the depth direction (z direction) through the adhesive tape, with no conductivity, however, in the two-dimensional extent (x-y plane) of the adhesive layer.
In special cases, it is also required and must be ensured that the conductive sites through the adhesive layer (in the z direction)
are distributed homogeneously, so that any sites on the adhesive tape can be used identically and lead to the same results;
have small cross-sections, so that even in the electronics sector conductor tracks lying close together can be selectively connected without the risk of short circuits; and
are insulated from one another, in that the areas in between are filled with non-conductive materials.
U.S. Pat. No. 3,475,213 describes randomly distributed spherical particles in a self-adhesive composition, which consist entirely of a conductive metal or are provided with an electrically conductive layer. The best results are obtained with particles which are only slightly smaller than the thickness of the layer of adhesive composition.
In U.S. Pat. No. 5,300,340, a special production method using a rotating drum is employed to site the electrically conductive particles in the adhesive composition, the particles being slightly larger than the thickness of the film of adhesive composition.
The solutions described above firstly reduce the bond strength and secondly distance the adhesive tape from the surface, since the particles protrude to a certain extent from the surface, which is disadvantageous to the bond strength but entirely desirable for improving the electrical conductivity.
U.S. Pat. No. 4,606,962 is concerned to improve the electrical contact. It describes how the use of soft, spherical particles instead of spherical, silverized glass beads permits permanent electrically conducting contacts through the layer of adhesive composition. The reason given for this is that the soft particles, which are no harder than pure silver at the activation temperature of the adhesive, flatten under stress against the contact faces and so increase the contact surface area.
In trials it has been possible to show that protruding particles always lead to unwanted air inclusions in the bonded joint, which reduces the strength of the bond perceptibly and results in a situation where, under mechanical stress in an elastic bonded joint, the particles lose contact, which can only be re-established by applying pressure again.
Particularly in the case of electronic devices which are small and flexible, as are used in an electronic toy or in chip cards, the electrically conducting adhesive bond is frequently not protected by a rigid casing and is required to withstand flexural stresses without losing the electrical contact.
In the processes presented above, therefore, either the bond strengths are inadequate for electrical contacts placed under mechanical stress, or there is no permanently secure electrically conducting contact.
DE 195 19 499, furthermore, discloses a thermoplastic adhesive sheet for implanting electrical modules in a card body which is provided with a recess intended to accommodate an electronic module which has on its first side a plurality of contact faces and on its second side, which is opposite to the first side, an IC chip whose terminals are connected via electrical conductors to the contact faces, the adhesive sheet being used to connect the second side of the module to the card body. The adhesive sheet has the combination of the following components:
i) a thermoplastic polymer in a proportion of from 50 to 100% by weight, plus
ii) one or more tackifying resins in a proportion of from 0 to 50% by weight, or, alternatively,
iii) epoxy resins with hardeners, in a proportion of from 0 to 40% by weight.
The object of the invention is to achieve an effective and permanent bond while simultaneously producing an electrically conductive contact in the case where a carrier element in data carriers or electronic components is bonded with the aid of a heat-activatable adhesive layer.
This object is achieved by means of an adhesive sheet as characterized further in the main claim. The subclaims relate to advantageous developments of the subject matter of the invention.
In accordance with the invention, the electrically conductive, thermoplastic and heat-activatable adhesive sheet comprises
i) a thermoplastic polymer in a proportion of at least 30% by weight,
ii) one or more tackifying resins in a proportion of from 5 to 50% by weight and/or
iii) epoxy resins with hardeners, with or without accelerators, in a proportion of from 5 to 40% by weight,
iv) metallized particles in a proportion of from 0.1 to 40% by weight, with particular preference 10% by weight,
v) non-deformable or virtually non-deformable spacer particles, in a proportion of from 1 to 10% by weight, which do not melt at the bonding temperature of the adhesive tape.
The adhesive sheet preferably has a thickness of from 10 to 500 &mgr;m.
The adhesive sheet is a mixture of reactive resins which crosslink at room temperature and form a three-dimensional polymer network of high strength, and permanently elastic elastomers, which act against embrittlement of the product and so allow the product to withstand permanent stresses (compression, extension).
The elastomer comes preferably from the group consisting of the polyolefins, polyesters, polyurethanes and polyamides or else can be a modified rubber such as, for example, nitrile rubber, or else polyvinyl butyral, polyvinyl formal, polyvinyl acetate, or carboxylated or epoxidized SEBS polymer.
The chemical crosslinking reaction (on the basis of epoxies or phenolic resin condensation) of the resins at elevated temperature brings about high strengths between the adhesive film and the surface to be bonded, and achieves a high internal strength in the product.
The addition of the reactive resin/hardener systems also leads to a reduction in the softening temperature of the abovementioned polymers, which has the advantageous effect of reducing their processing temperature and processing rate. The suitable product is a product which is self-adhesive at room temperature or slightly elevated temperature. When the product is heated, there is a short-term reduction in the viscosity, as a result of which the product is even able to wet rough surfaces.
By means of the appropriate mixture of resins/hardener systems and the polymers, a characteristic is obtained which enables the thickness of the layer of adhesive composition to be changed significantly in the course of adhesive bonding under pressure and heat. For example, it is easy to produce an approximately 45 &mgr;m thick bonded joint using an approximately 65 &mgr;m thick product. The “excess” adhesive composition in this case is displaced, which is taken into account in the construction of the adherends or in the formulation of the adhesive. In the case of chip cards, for example, the adhesive composition can
Norris & McLaughlin & Marcus
tesa AG
Truong Duc
LandOfFree
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