EP1590415A1

EP1590415A1 - Thermo-activated adhesive material for fpcb agglutinations

Info

Publication number: EP1590415A1
Application number: EP04705391A
Authority: EP
Inventors: Marc Husemann; Christian Ring; Dieter Zimmermann
Original assignee: Tesa SE
Current assignee: Tesa SE
Priority date: 2003-01-29
Filing date: 2004-01-27
Publication date: 2005-11-02
Also published as: KR20050094048A; WO2004067664A1; WO2004067664A8; JP2006517604A; US20060121272A1

Abstract

Disclosed is an adhesive film comprising (i) at least one thermoplastic polymer and/or a thermoplastic elastomer, (ii) at least one resin, and (iii) at least one organically modified layered silicate and/or betonite.

Description

description

Heat-activatable adhesive for FPCB bonds

The invention relates to a heat-activatable adhesive with low flow at high temperatures for bonding flexible printed circuit board laminates.

Adhesive tapes are widespread processing aids in many technical fields. Adhesive tapes are subject to very high requirements, particularly for use in the electronic industry.

There is currently a trend in the electronics industry towards ever narrower, lighter and faster components. In order to achieve these requirements, ever greater demands are placed on the manufacturing process. This also applies to the so-called flexible printed circuit boards (FPCBs), which are very often used for the electrical contacting of IC chips or conventional printed circuit boards.

Flexible printed circuit boards are used in a variety of electronic devices such as Cell phones, car radios, computers, etc. represented. FPCBs consist of layers of copper and polyimide, where appropriate polyimide is glued to the copper foil.

For the use of the FPCBs, they are glued to substrates or to one another. In the latter case, polyimide films are bonded to one another.

For the bonding of FPCBs, heat-activated adhesives are generally used that do not release any volatile components and can also be used in a high temperature range.

Furthermore, the heat-activatable adhesive must be self-crosslinking after temperature activation, since the bonded FPCBs generally also have to be solder bath-resistant. Pure thermoplastics, which are used as heat-activated adhesives for a number of bonds, become soft again at high temperatures and therefore have no resistance to solder baths. Pure thermoplastics are therefore unsuitable as the basis for the adhesive compositions for the abovementioned application. Thermoplastic adhesives would, however, be preferred for the bonding process as such, since they can be activated in a few seconds and the adhesive connection could accordingly be set up quickly.

Further heat-activatable adhesive tapes, such as the block copolymers based on epoxidized styrene-butadiene or styrene-isoprene described in US Pat. No. 5,478,885, have the disadvantage that they require very long curing times for full curing and thus significantly slow down the processing process. This also applies to other epoxy-based systems, e.g. are described in WO 96/33248.

Phenolic resin-based heat-activated adhesive tapes are generally excluded because they release volatile components during curing and thus lead to the formation of bubbles.

Another general disadvantage of the known adhesive systems described above is that the fluidity is too great at elevated temperature. The FPCB's are glued at temperatures of approx. 200 ° C and under high pressure. The adhesive must not run during the curing of the bond. For certain applications, holes and millings are also made in the component and thus also in the adhesive film before the pressing and curing. The modifications must also be retained in the adhesive so that the compound must not run during the process. An undesirable consequence would otherwise be that later contacts via the holes e.g. would not work with solder or only to a limited extent.

The object of the invention was therefore to meet the need for a heat-activatable adhesive system which is self-crosslinking and solder bath-resistant, has a low flow behavior at temperatures above 120 ° C. and has good adhesion to polyimide. This object is surprisingly achieved by an adhesive film, as is characterized in the main claim. The subclaims relate to advantageous developments of the subject matter of the invention.

Accordingly, the invention relates to an adhesive film comprising (i) at least one thermoplastic polymer or a thermoplastic elastomer, (ii) at least one (tackifying) resin and (iii) at least one organically modified layered silicate and / or concreteite.

For the purposes of this invention, the general term “adhesive film” encompasses all flat structures, such as extended films, film sections and cutouts, tapes (extended length, limited width), tape sections, labels, die-cuts and the like, with the structures can have regular or irregular shapes.

An adhesive film which has the following components has proven to be particularly advantageous: i) a thermoplastic polymer or an elastomer with a proportion of 25-70

% By weight ii) one or more tackifying phenolic resins with a proportion of 0-30% by weight iii) epoxy resins, advantageously with hardeners, optionally also with accelerators, with a proportion of 5-60% by weight iv) organically modified Layered silicates or concrete with a share of 1 - 15% by weight

The reactive film is advantageously a mixture containing reactive resins which crosslink at room temperature and form a three-dimensional, high-strength polymer network, and containing thermoplastic compounds, in particular permanently elastic elastomers, which counteract embrittlement of the product.

The elastomer can preferably originate from the group of polyolefins, polyesters, polyurethanes or polyamides or can be a modified rubber, e.g. Nitrile rubber.

The particularly preferred thermoplastic polyurethanes (TPU) are reaction products of polyester or polyether polyols and organic diisocyanates such as Diphenylmethane diisocyanate, known. They are made up of predominantly linear marrow molecules. Such products are usually commercially available in the form of elastic granules, for example from Bayer AG under the trade name "Desmocoll".

Synthetically produced nitrile rubbers can also be used as elastomers. In this case e.g. Hycar ™ types used by BF Goodrich. Suitable nitrile rubbers are also available under the trade name Nipol ™ from Nippon Zeon.

Amorphous types are particularly preferably used as polyester. Here e.g. various types offered under the trade name Griltex ™ by Emsland Chemie.

Advantageously, the adhesive film further comprises substances which, in particular under elevated pressure and / or elevated temperature, serve as hardeners for at least one of the resins present.

The softening temperature of the adhesive film can be sufficiently reduced by combining elastomers with selected compatible resins. At the same time, there is an increase in adhesion. Rosin, hydrocarbon and coumarone resins, for example, have proven to be suitable resins.

Epoxy resins are usually understood to mean both oligomeric compounds with more than one epoxy group per mole and the thermosets produced from such compounds. In the sense of the invention, the entire group of epoxy compounds should be understood. The corresponding monomers, oligomers or polymers can be used which have at least two epoxy groups. Polymeric epoxy resins can be aliphatic, cycloaliphatic, aromatic or heterocyclic in nature. The molecular weight M _{n of} the added epoxy resins is preferably chosen between 100 and 25,000 g / mol.

Epoxy resins which can advantageously be used according to the invention include, for example, glycid esters and / or the reaction products epichlorohydrin and at least one of the following compounds: Bisphenol A, the reaction product of phenol and formaldehyde (novolak resins), p-amino phenol.

Preferred commercial examples are e.g. Araldite ™ 6010, CY-281 ™, ECN ™ 1273, ECN ™ 1280, MY 720, RD-2 from Ciba Geigy, DER ™ 331, DER ™ 732, DER ™ 736, DEN ™ 432, DEN ™ 438, DEN ™ 485 from Dow Chemical, Epon ™ 812, 825, 826, 828, 830, 834, 836, 871, 872, 1001, 1004, 1031 etc. from Shell Chemical and HPT ™ 1071, HPT ™ 1079 also from Shell Chemical.

Examples of commercial aliphatic epoxy resins are e.g. Vinyl cyclohexane dioxides such as ERL-4206, ERL-4221, ERL 4201, ERL-4289 or ERL-0400 from Union Carbide Corp.

By mixing with epoxy resins in combination with the appropriate hardener, post-curing is achieved under temperature and pressure during the bonding, for example when the bonded FPCB is passed through a solder bath. Hardeners are substances that are added to crosslinkable resins (prepolymers) to cause them to harden (crosslink).

Due to the chemical cross-linking reaction of the resins, great strengths are achieved between the adhesive film and the polyimide film of the FPCB and a high internal strength of the product is achieved.

The addition of these reactive resin / hardener systems advantageously also leads to a lowering of the softening temperature of the abovementioned polymers, which lowers their processing temperature and speed.

According to the invention, all of the hardeners known to the person skilled in the art, which lead to a reaction with the corresponding resins, can advantageously be used as hardening systems for epoxy resins and / or phenolic resins and / or other added resins in the adhesive film. All formaldehyde donors, such as hexamethylenetretraamine, fall into this category. Acid anhydrides, cationic crosslinking agents, guanidines such as dicyandiamide or peroxides can also be used. Combinations of these crosslinkers can also be used. If appropriate, accelerators can also be used, such as imidazoles. Suitable accelerators are, for example, imidazoles, commercially available from 2M7, 2E4MN, 2PZ-CN, 2PZ-CNS, P0505, L07N from Shikoku Chem. Corp. or Curezol 2MZ from Air Products. Furthermore, amines, especially tertiary amines, can also be used for acceleration.

Another component of the inventive PSA are organically modified layered silicates or concrete. Silicates which are available under the trade name Bentone ™ (company Elementis Specialties) are particularly preferred for use. Bentone are preferably used ™ types, low outgassing at 200 ^C C - preferably less than 200 micrograms volatiles at a temperature of 200 ° C over a period of 1 h - as well as a low chloride content - preferably less than 0, 2% by weight, particularly preferably less than 0.1% by weight

High levels of chloride can negatively affect the electrical conductivity of the copper conductor tracks. High outgassing components, on the other hand, lead to deformation of the FPCB laminates after curing and reduce the resistance to the solder bath.

Furthermore, phenolic resins, such as e.g. Use YP 50 (Toto Kasei), PKHC (Union Carbide Corp.) and / or BKR 2620 (Showa Union Gosei Corp.).

Polyisocyanates such as e.g. Coronate ™ L (from Nippon Polyurethane Ind.), Desmodur ™ N3300 or Mondur ™ 489 (from Bayer) can be used in addition or as an alternative to the phenolic resins.

The composition for the adhesive film can be varied within a wide range by changing the type and proportion of raw material. Likewise, further product properties such as color, thermal or electrical conductivity can be achieved through the targeted addition of dyes, fillers and / or carbon or metal powders. The adhesive film preferably has a thickness of 5 to 100 μm, preferably 10 and 50 μm.

To produce the adhesive film, the mass forming the film is coated as a solution or from the melt onto a flexible substrate (release film, release paper) and countered. if necessary dried so that the mass can be easily removed from the substrate. After appropriate assembly, die cuts, sections of a roll or other shaped bodies can be glued from this adhesive film to the substrate (polyimide) to be bonded at room temperature or at a slightly elevated temperature.

In a further variant, the adhesive is coated on a polyimide carrier. Such adhesive films can then be used to cover copper interconnects for FPCBs.

The admixed reactive resins should preferably not yet undergo any chemical reaction at the slightly elevated temperature. It is not necessary for the adhesive to be carried out as a one-step process, but for the sake of simplicity, as in the case of bonding with commercially available pressure-sensitive adhesive tapes, the adhesive film can first be tacked onto one of the two substrates by laminating in the heat. During the actual hot glue process with the second substrate (second polyimide film of the second FPCB), the resin cures in whole or in part and the adhesive joint achieves the high bond strength, which is significantly higher than that of known pressure sensitive adhesive systems. Accordingly, the adhesive film is particularly suitable for a hot pressing process at temperatures above 80 ° C., preferably above 100 ° C., particularly preferably above 120 ° C.

Unlike other adhesive films, which mostly consist of pure epoxy resins, the adhesive film according to the invention has a high elastic component due to the high elastomer component (rubber component). Due to this tough elastic behavior, the flexible movements of the FPCB's can be compensated particularly well, so that even high loads and peeling movements can be withstood well.

Furthermore, the flow behavior under high temperatures is minimized by the special layered silicates.

Furthermore, the adhesive film has an advantage over other heat-activated materials due to the high viscoelastic content. Holes are often drilled through the adhesive film for contacting. One problem here is that existing heat-activated adhesives flow into the holes and thus interfere with the contact. With the inventive use of the adhesive films described above, this problem does not occur at all or only to a greatly reduced extent. In addition to the bonding of FPCBs based on polyimide, FPCBs based on polyethylene naphthalate (PEN)) and polyethylene terephthalate (PET) can also be bonded. In these cases, too, the adhesive film achieves a high bond strength.

experiments

The invention is described below, without wishing to be restricted unnecessarily by the choice of examples.

The following test methods were used.

Test Methods

Production of the thermally activated adhesive film

Example 1 :

A mixture of 50 g nitrile rubber (Breon® 41, Zeon), 50 g epoxy resin (Rüapox ™ 166, Bakelite AG), 5 g organic layered silicate (Bentone 38®, Elementis Specialties) and 3.4 g of dicyandiamide is dissolved in methyl ethyl ketone and coated from solution onto a release paper siliconized with 1.5 g / m ² and dried at 90 ° C. for 10 minutes at this temperature. The thickness of the adhesive layer was 25 μm.

Example 2:

A mixture of 60 g nitrile rubber (Breon® 41, Zeon), 40 g epoxy resin (Rüapox ™ 166, Bakelite AG), 5 g organic layered silicate (Bentone 38®, Elementis Specialties) and 3 g of dicyandiamide is dissolved in methyl ethyl ketone and coated from solution onto a release paper siliconized with 1.5 g / m ² and dried at 90 ° C. for 10 minutes at this temperature. The thickness of the adhesive layer was 25 μm.

Example 3:

A mixture of 100 g polyester (Griltex®, Emsland Chemie), 130 g epoxy resin

(Rütapox ™ 164, Bakelite AG), 24 g organic layered silicate (Bentone 38®, Fa. Elementis Specialties) and 9 g dicyandiamide is dissolved in methyl ethyl ketone and coated from solution onto a release paper siliconized with 1.5 g / m ² and dried at 90 ° C for 10 minutes at this temperature. The thickness of the adhesive layer was 25 μm.

Example 4:

A mixture of 100 g polyester (Griltex®, Emsland Chemie), 130 g epoxy resin (Rütapox ™ 164, Bakelite AG), 24 g organic layered silicate (Bentone 38®, Elementis Specialties) is mixed in a Z-kneader. and 9 g of dicyandiamide kneaded for 4 h and then coated via an extrusion die on a release paper siliconized with 1.5 g / m ² . The thickness of the adhesive layer was 25 μm.

As a reference example (R), a commercially available adhesive film, namely Pyralux® LF001 from DuPont, with 25 μm film thickness was used in the comparative tests.

Bonding of FPCB's with the adhesive film

Two FPCB's were each bonded with the adhesive films produced according to Examples 1 to 4 and with the reference film (R) (Pyralux® LF001, DuPont). For this purpose, the adhesive film was laminated onto the polyimide film of the FPCB laminate made of polyimide / copper film / polyimide at 100 ° C. This process was then repeated with a second polyimide film from a further FPCB, and an adhesive joint was thus produced between two polyimide / copper film / polyimide laminates, the polyimide films in each case being bonded to one another. For curing, the composite was pressed in a heated press from Bürkle at 170 ° C. for 30 minutes at a pressure of 50 N / cm ^z .

The connections produced in this way had the structure shown in FIG. 1, (a) denoting a polyimide layer, (b) denoting a copper layer and (c) the adhesive film. A composite (a-b-a) of a copper layer (b) with a polyimide layer (a) on each side represents an FPCB unit. Test methods

The properties of the adhesive films produced according to the above examples were investigated using the following test methods. A, I: Pee! Tesj: .with FPCB

Using a tensile testing machine from Zwick, the composites made of FPCB / adhesive film / FPCB (FIG.) Produced using the method described above were pulled apart at a speed of 50 mm / min at a 180 ° drawing angle and the force measured in N / cm. The measurements were carried out at 20 ° C under 50% humidity. Each measurement was determined in triplicate and averaged.

B - .. lemp e rat u r] consistency at one end of the FPCB composites produced according to the method described above, a weight of 1 kg was attached and the composite was suspended at the other end. The test is passed if the composite holds the weight in the drying cabinet at a temperature of 70 ° C for more than 8 hours.

C, _Lötbadbesjäπdigkeit

The FPCB composites (FIG.) Bonded by the process described above were completely immersed in a 288 ° C. hot solder bath for 10 seconds. The bond was rated as resistant to the solder bath if no air bubbles were formed which caused the FPCB's polyimide film to inflate. The test was assessed as failed if there was already a slight blistering.

P.-.Ni?..k

The bond strength was measured in accordance with DIN EN 1465. The measured values were given in N / mm ² .

Results:

To assess the adhesive technology of the above examples, the T-peel test was first carried out with FPCB material. The corresponding measured values are listed in Table 1.

Table 1 shows that with Examples 1 to 4 very high bond strengths were achieved after just 30 minutes of curing. Reference example R shows lower adhesive strengths.

Another important component of the inventive adhesive is the minimized flow behavior at elevated temperatures. Test method B was therefore carried out with all the examples. The results are summarized in Table 2.

Table 2 shows that Examples 1 to 4 mixed with Bentone ™ have only a very low flow behavior and can therefore withstand high loads at high temperatures.

Another criterion for the use of adhesive films for the bonding of FPCB's is the resistance to the solder bath (test method C).

Table 3 lists the results of the solder bath resistance.

The results show that all examples are solder bath-resistant and therefore meet the requirements of the FPCB industry.

The bond strengths were also measured to investigate the shear strength of the adhesive films. The corresponding values are listed in Table 4.

Table 4 shows that the adhesive films 1 to 4 according to the invention have a significantly higher bond strength than the reference example R.

Claims

Expectations

1. Comprehensive adhesive film

(i) at least one thermoplastic polymer and / or a thermoplastic elastomer,

(ii) at least one resin and

(iii) at least one organically modified layered silicate and / or concrete.

2. Adhesive film according to claim 1, comprising (i) a thermoplastic polymer or elastomer with a mass fraction of 25 to

70 wt .-%, (ii) epoxy resins, with a mass fraction of 5 to 60 wt .-%, (iii) one or more organically modified layered silicates and / or concrete with a total mass fraction of 1 to 15 wt .-%, (iv ) optionally one or more phenolic resins with a mass fraction of up to

30% by weight.

3. Adhesive film according to one of claims 1 or 2, further comprising substances which serve as hardeners of the epoxy resins and / or phenolic resins under increased pressure and / or elevated temperature.

4. Adhesive film according to one of the preceding claims, characterized in that layered silicates and / or concrete are used which have a low outgassing behavior at a temperature of 200 ° C, in particular having less than 200 μg of volatile constituents at a temperature of 200 ° C over a period of 1 h.

5. Adhesive film according to one of the preceding claims, characterized in that layered silicates and / or concrete are used which have a low chloride content, in particular less than 0.2 wt .-%, particularly preferably less than 0.1 wt. .-%.

6. Adhesive film according to one of the preceding claims, further comprising accelerators, dyes, fillers, carbon powder and / or metal powder.

7. Process for bonding plastic parts, in which a thermally activatable adhesive film with

(i) at least one thermoplastic polymer or a modified one

Rubber, (ii) at least one resin and

(iii) at least one organically modified layered silicate and / or concrete is used.

8. The method according to claim 7 for the adhesive bonding of flexible printed circuit boards (flexible printed circuit boards).

9. The method according to any one of claims 7 or 8 for bonding to polyimide.