The present invention relates to an electrically conductive composition especially for use as a hardenable electrically conductive adhesive. Electronic devices require electrically conductive connections to active and passive components.

Electrically conductive adhesives are used in a variety of applications to attach bare dies and other electronic components such as resistors or condensators to a substrate, for example lead frames, printed circuit boards/flexible electronics, ceramic substrates (e.g. LTCC) or DCBs and the like. Such devices are for example used for communication (e.g. mobile phones, tablets, computer), power conversion (e.g. power electronic modules), lighting (e.g. LED) or the like. There are a number of requirements for an electrically conductive adhesive in these applications. It should be mechanically compliant and highly conductive. Low processing temperatures and compatibility with a wide range of substrates are required. Further requirements include elasticity and flexibility, biocompatibility, stable physical and chemical properties, good temperature stability, low price and so on,

Nevertheless, the major requirement is obviously a low resistivity of the material and good contact to the substrate and the electronic components.

Despite electrical conductivity, also thermal conductivity is of major importance in some applications, especially if it is used to bind for example heat sinks to electronic components. Although some of known conductive adhesives have a good bulk thermal conductivity, they show limited thermal conductivity when used. It is believed that at least some of these problems are related to an insufficient contact between the electrically conductive adhesive and the electronic component or the substrate.

Although a number of different electrically conductive adhesives exist, there is still a need for electrically conductive adhesives with improved properties.

It is the aim of the invention to provide electrically conductive adhesives with alternative composition showing at least some improved properties, preferably in thermal conductivity.

The inventive solution is an electrically conductive composition comprising (a) metal containing particles,

(b) at least one epoxy resin,

(c) at least one hardener for the at least one epoxy resin, and

(d) at least one lactone.

The electrically conductive composition of the invention is a hardenable (curable, crosslinkable) composition.

The metal containing particles (a) of the invention are electrically conductive particles. Examples of such particles include well-known metal particles which comprise or consist of silver, aluminum, copper, tin, nickel or alloys thereof. Examples of further suitable metal containing particles comprise inorganic particles which are metal coated with for example silver, aluminum, copper, tin, nickel or alloys thereof. Also metal coated metal particles, e.g. silver coated copper particles may be used. In accordance with the invention, different metal containing particles may be combined together and used as a mixture.

A suitable proportion of metal containing particles lies in the range of 25 to 95 wt.-% (weight-%) of the electrically conductive composition of the invention. In some embodiments, the proportion of metal containing particles is at least 30 wt.- % or at least 35 wt.-% or at least 50 wt.-% of the electrically conductive composition. In other embodiments a proportion of metal containing particles of up to 93 or up to 92 wt.-% of the electrically conductive composition is preferred. In some embodiments, a preferred proportion of metal containing particles is in the range of 70 to 93 wt.-% of the electrically conductive composition, for other types of the electrically conductive composition it is in the order of 30 to 70 wt.-%.

The metal containing particles may have any shape. Flakes and spherical parti- cles are preferred.

An average particle size (d50) of the metal containing particles in the range of 1 to 15 m is preferred. The term "average particle size" shall mean the average primary particle size (mean particle diameter, d50) determined by means of laser diffraction; d50 is the particle size where 50 wt.-% of all particles have a larger particle size and 50 wt.-% have a smaller particle size. Laser diffraction measurements can be carried out making use of a particle size analyzer, for example, a Mastersizer 3000 from Malvern. Component (b) of the electrically conductive composition is at least one epoxy resin. Preferred epoxy resins are bisphenol A epoxy resins, bisphenol A/F epoxy resins, bisphenol F epoxy resins, novolac epoxy resins, aliphatic epoxy resins, cycloaliphatic epoxy resins and cyclosiloxane epoxy resins. Examples of suitable bisphenol A epoxy resins include Araldite® GY 279 as well as Quatrex™ 1010 commercially available from Huntsman or D.E.R.™ 331 as well as D.E.R.™ 732 commercially available from Dow Chemical. Examples of suitable bisphenol A F epoxy resins include Araldite® GY 891 , Araldite® PY 302-2 and Araldite® PY 3483 commercially available from Huntsman.

Examples of suitable bisphenol F epoxy resins include Araldite® GY 281 commercially available from Huntsman and D.E.R.™ 354 commercially available from Dow Chemical.

An example of a suitable novolac epoxy resin is D.E.N™ 431 commercially available from Dow Chemical. Examples of suitable cycloaliphatic epoxy resins include JER YX8000 commercially available from Mitsubishi Chemical and EPONEX™ Resin 1510 commercially available from Momentive Specialty Chemicals.

An example of a suitable cyclosiloxane epoxy resin is CS-697 commercially available from Designer Molecules Inc..

A suitable total proportion of the at least one epoxy resin is in the range of 3 to 30 wt.-% of the electrically conductive composition of the invention with a preferred range of 5 to .15 wt.-%. In some embodiments the total proportion of the at least one epoxy resin is at least 7 wt.-% or at least 9 wt.-% or at least 11 wt.-%. In some embodiments the total proportion of the at least one epoxy resin is up to 14 wt.-% or up to 12 wt.-%. In some embodiments, a preferred total proportion of the at least one epoxy resin is in the range of 5 to 7 wt.-% of the electrically conductive composition, for other types of the electrically conductive composition it is in the range of 8 to 12 wt.-%. The electrically conductive composition of the invention comprises a component (c), i.e. at least one hardener for the at least one epoxy resin. Preferred hardeners are hardeners having at least 2 reactive groups in one molecule, preferably wherein each reactive group contains at least one heteroatom having at least one free electron pair. Preferably, the hardener comprises an unsaturated ring system and/or nitrogen heteroatoms with a free electron pair. Suitable hardeners for epoxy resins are known in the art. Examples include pyra- zine and dicyandiamide, both commercially available from Sigma Aldrich. Other examples are Curezol® 2MZ-H, 2MZ-CN, 2PZ, C17Z, 2E4MZ, 2E4MZ-CN or C2MZH from Shikoku Chemical. A suitable total proportion of the at least one hardener is in the range of 0.05 to 3 wt.-% of the electrically conductive composition of the invention with a preferred range of 0.05 to 1 wt.-%.

Component (d) of the electrically conductive composition of the invention is a lac- - tone or a mixture of lactones.

A lactone is a cyclic ester that can be seen as the condensation product of an OH group and a COOH group. Preferably the lactone ring of the at least one lactone of component (d) comprises 3 to 7 or 4 to 6 carbon atoms. In some embodiments, a lactone may be used having 4 or 5 ring carbon atoms, in other embodiments the lactone ring may have 6 or 7 carbon atoms. Lactones are usually named according to the precursor acid molecule (propio = 3, butyro = 4, valero = 5, capro = 6 carbon atoms, etc.), with a -lactone suffix and a Greek letter prefix that specifies the number of ring carbon atoms in the hetero- cycle— that is, the distance between the relevant -OH and -COOH groups along said backbone. The first carbon atom after the carbon in the -COOH group on the parent compound is labelled alpha, the second will be labeled beta, and so forth. Therefore, the prefixes also indicate the size of the lactone ring: alpha-lactone = 3-membered ring, beta-lactone = 4-membered, gamma-lactone = 5-membered, etc.

Preferably, the at least one lactone (d) is a saturated lactone, especially a gamma-lactone, a delta-lactone or an epsilon-lactone.

In a preferred embodiment, the at least one lactone (d) has a melting tempera- ture below 50 °C; most preferably it is a liquid.

Preferred lactones for use according to the invention are delta-valerolactone, del- ta-hexalactone, delta-nonalactone, delta-decalactone, delta-undecalactone, gamma-butyrolactone, gamma-hexalactone, gamma-heptalactone, gamma- octalactone, epsilon-caprolactone, epsilon-octalactone, epsilon-nonalactone and mixtures thereof.

A suitable total proportion of the at least one lactone is in the range of 0.5 to 10 wt.-% of the electrically conductive composition of the invention, with a proportion of 0.5 to 5 wt.-% being preferred. In some embodiments, the total proportion of the at least one lactone is at least 1 wt.-% or at least 2 wt.-% or at least 3 wt.-%. In some embodiments the upper limit of the at least one lactone is 7 wt.-% or 5 wt.-% or 3 wt.-%. Surprisingly, it has been found that the use of lactone improves the electrical and the thermal conductivity of the electrically conductive composition after hardening. It is believed that in practice this is related to an improved contact of the electrically conductive composition to the substrate and/or to the electronic compo- nent attached to each other via the electrically conductive composition in its function as adhesive.

It is understood by a person skilled in the art that the electrically conductive composition of the invention may comprise one or more further components (e) in a total proportion of, for example, 0 to 10 wt.-% of the electrically conductive composition of the invention. The further components (e) may be selected from the group consisting of diluents, electrically non-conductive particles and other additives. Examples of other additives include Theological modifiers and surface- active agents.

As diluents compounds are considered which may be readily but not necessarily volatile at room temperature; preferred diluents are ethers, esters, carboxylic acids and ketones. Preferably, the proportion of diluents is less than 10 wt.-%, preferably less than 5 wt.-% and more preferred less than 3 wt.-% or less than 1 wt.- % of the electrically conductive composition of the invention.

Inorganic particles are preferably used as electrically non-conductive particles. Such electrically non-conductive inorganic particles or fillers may be selected for example from silica, aluminosilicates, aluminum oxide, zirconia, silicon dioxide, titanium dioxide and the like. Preferably, the proportion of the electrically non- conductive particles is less than 10 wt.-%, preferably less than 5 wt.-% and more preferred less than 3 wt.-% of the electrically conductive composition of the invention. Preferably, the proportion of additives like rheological modifiers and surface- active agents is less than 5 wt.-%, preferably less than 3 wt.-% of the electrically conductive composition of the invention, In preferred embodiments, the electrically conductive composition of the invention does not comprise glass frit or at least not more than 1 wt.-% of glass frit.

Preferably, the viscosity of the electrically conductive composition of the invention is in the range of 4 to 45 mPa-s, most preferably 8 to 35 mPa-s, measured in accordance with DIN 53018 (at 23 °C, CSR-measurement, cone-plate system, shear rate of 50 rounds per second).

The electrically conductive composition of the invention can be produced by mix- ing components (a) to (d) and, optionally, one or more components (e).

A further embodiment of the invention is an assembly comprising a substrate, the electrically conductive composition of the invention and an electronic component, i.e. the electrically conductive composition of the invention is located between the substrate and the electronic component. Examples of substrates include those mentioned above, while examples of electronic components include the above mentioned bare dies, resistors and condensators.

According to the invention, the composition is hardened by heating. Hardening conditions may be for example 5 to 60 minutes at an object temperature in the range of for example 60 to 200 °C, preferably 80 to 80 °C or 100 to 160 °C. It may be expedient to let said hardening precede by a heating-up phase of for example 5 to 60 minutes until reaching the object temperature. A further embodiment of the invention is a hardened composition obtainable by hardening the hardenable electrically conductive composition of the present invention. Therefore, also an assembly, wherein the electrically conductive composition of the present invention is in a hardened state is part of the invention. In other words, a further embodiment of the invention is an assembly comprising a substrate, the hardened electrically conductive composition of the invention and an electronic component. The hardened electrically conductive composition of the invention serves as an electrically conductive adhesive and is therefore located between the substrate and the electronic component.

A further embodiment of the invention is the use of the electrically conductive composition of the invention as a hardenable electrically conductive adhesive, in particular in electronic applications. The embodiments and advantages of the invention are further explained by the following non-limiting examples.

Examples 1. Materials

1 .1. Metal Containing Particles

Flake-shaped Ag particles with a d50 value of 5 pm

1.2. Epoxy Resins

Araldite® PY 302-2 (Huntsman)

D.E.N™ 431 (Dow Chemical)

D.E.R™ 331 (Dow Chemcial) rdeners

Curezol® 2E4MZ (Shikoku Chemicals Corporation)

Curezol® 2MZ-H (Shikoku Chemical Corporation) dicyandiamide (Sigma Aldrich)

pyrazine (Sigma Aldrich) ctones

delta-valerolactone, tech. (Sigma Aldrich)

gamma-butyrolactone, .=99 wt.-% (Sigma Aldrich)

epsilon-caprolactone, 97 wt.-% (Sigma Aldrich) luents

methyl butyrate, >98 wt.-% (Sigma Aldrich)

bisphenol A diglycidyl ether (Sigma Aldrich)

neopentyl glycol diglycidyl ether, tech. (Sigma Aldrich) oleic acid (Sigma Aldrich)

undecylenic acid, 98 wt.-% (Sigma Aldrich)

cresol monoglycidyl ether (Sigma Aldrich)

octanoic acid, 98 wt.-% (Sigma Aldrich)

linoleic acid, approx. 95 wt.-% (Sigma Aldrich)

1 ,4-cyclohexanedimethanol vinyl ether, tech. (Sigma Aldrich)

C12-C14-glycidyi ether Araldite® DY-E (Huntsman)

2. Test Methods

2.1. Resistivity (mOhm cm)

The electrical resistivity was measured according to the four point probe measurement (F.M.Smits, "Measurement of Sheet Resistivities with the Four-Point Probe", published in THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1958, pages 711 -718, or the web-site: "http://en.wikipedia.org/wiki/Sheet_resistance")

2.2. Thermal diffusivity (m ² /s)

The thermal diffusivity was measured by the laser flash measurement (LFA) ac- cording to DIN/EN 821/2.

3. Preparation and application of electrically conductive compositions, general procedure:

The required amounts of epoxy resin and hardener were transfered to a beaker. and mixed with a spatula. Lactones and diluents were added and mixed with the spatula. Metal containing particles were added and mixed carefully with the spatula, followed by mixing with a paddle mixer at 300 to 400 U/min for 5 min.

Thereafter, it was milled twice in a triple roll mill at 21 °C, followed by evacuation at less than 10 mbar under stirring with a paddle mixer for 20 min. The so pro- duced mixture was screen printed in 100 pm layer thickness on an alumina ceramic substrate and then hardened in a box oven with gas circulation for 10 minutes at an object temperature of 150 °C.

The following tables 4 to 6 show proportions in wt.-% and test results for exam- pies according to the invention as well as comparative examples. 4. Examples (according to the invention)

5. Comparative Examples

C1 C2 C3 C4

Ag particles 89.000 89.000 84.200 89.000

Araldite® PY 302-2 6.875 6.875 9.000 3.500

D.E.N™ 431 3.500

C2E4MZ 0.458 0.458 0.500 0.458 methyl butyrate 1.292 1.292

bisphenol A diglycidyl 1.167 ether

neopentylglycol 2.100 2.100 diglycidyl ether

oleic acid 0.300

undecylenic acid 0.275 0.275 cresol monoglycidyl 2.100

ether

octanoic acid 0.275

1.4- 0.500

cyclohexanedimethanol

vinyl ether

Araldite® DY-E 5.500

∑ 100.000 100.000 100.000 100.000

6. Results

Resistivity classification:

++ is classified with < 0.03 mOhm-cm

+ is classified with 0.03 to < 0.1 mOhm-cm

- is classified with 0.1 to < 0.3 mOhm-cm

- is classified with >0.3 mOhm-cm

Thermal diffusivity classification:

++ is classified with > 10 m ² /s

+ is classified with > 5 to 10 m ² /s

- is classified with > 1 to 5 m ² /s

- is classified with <1 m ² /s