The present invention relates to a metal sintering preparation, and to a method for the connecting of components, in which said metal sintering preparation is used. In power and consumer electronics, the connecting of components, such as LEDs or very thin silicon chips that are highly pressure and temperature sensitive, is particularly challenging.

For this reason, said pressure- and temperature-sensitive components are often con- nected to each other by means of gluing. However, adhesive technology is associated with a disadvantage in that it produces contact sites between the components that comprise only insufficient heat conductivity and/or electrical conductivity.

In order to solve this problem, the components to be connected are often subjected to sintering. Sintering technology is a very simple method for the connecting of components in stable manner.

It is known in power electronics to use metal sintering preparations in a sintering process to connect components. For example, WO2011/026623 A1 discloses a metal sin- tering paste containing 75 to 90 % by weight (percent by weight) of at least one metal that is present in the form of particles that comprise a coating that contains at least one organic compound, 0 to 12 % by weight of at least one metal precursor, 6 to 20 % by weight of at least one solvent, and 0.1 to 15 % by weight of at least one sintering aid, as well as the use of said metal sintering preparation to connect components by means of a sintering method.

The term, "coating of particles", used herein shall be understood to refer to a firmly adhering layer on the surface of particles. It is the object of the invention to provide a sintering method for the connecting of components in stable manner. The method is to be used to produce contact sites of low po- rosity and high electrical and thermal conductivity between the components to be connected. In particular, the sintering method is to be well-suited for stably connecting components having metal contact surfaces made of non-precious metal, for example made of aluminium or, in particular, of copper and copper-rich alloys (copper content > 94 % by weight). It is another object of the present invention to provide a metal sintering preparation that is well-suited for implementing said sintering method.

The invention relates to a method for the connecting of components, in which (a) a sandwich arrangement is provided, which comprises at least (a1 ) a component 1 , (a2) a component 2, and (a3) a metal sintering preparation that is situated between component 1 and component 2,' and in which (b) the sandwich arrangement is being sintered, whereby the metal sintering preparation comprises (A) 70 to <84 % by weight of at least one metal that is present in the form of particles that comprise a coating, and (B) 6 to 30 % by weight of organic solvent, characterised in that the total amount of the coat- ing is 0.4 to 2 % by weight, with respect to the weight of the coated metal particles, and in that 95 to 100 % by weight of the coating consist of a combination of (i) 15 to 85 parts by weight of at least one fatty acid compound having 8 to 14, preferably 8, carbon atoms in the non-branched fatty acid residue and (ii) 85 to 15 parts by weight of at least one fatty acid compound having 16 to 22, preferably 18, carbon atoms in the non- branched fatty acid residue, whereby the parts by weight of components (i) and (ii) add up to 100 parts by weight, and whereby the metal sintering preparation comprises less than 3 % by weight of NH ₄ BF ₄ .

Moreover, the invention relates to a metal sintering preparation that comprises (A) 70 to <84 % by weight of at least one metal that is present in the form of particles that comprise a coating, and (B) 6 to 30 % by weight of organic solvent, characterised in that the total amount of the coating is 0.4 to 2 % by weight, with respect to the weight of the coated metal particles, and in that 95 to 00 % by weight of the coating consist of a combination of (i) 15 to 85 parts by weight of at least one fatty acid compound having 8 to 14, preferably 8, carbon atoms in the non-branched fatty acid residue and (ii) 85 to 1 5 parts by weight of at least one fatty acid compound having 16 to 22, preferably 18, carbon atoms in the non-branched fatty acid residue, whereby the parts by weight of com- ponents (i) and (ii) add up to 100 parts by weight, and whereby the metal sintering preparation comprises less than 3 % by weight of NH ₄ BF ₄ .

The metal sintering preparation according to the invention comprises less than 3 % by weight of NH ₄ BF ₄ or it is even free of NH ₄ BF ₄ . In case of the NH ₄ BF ₄ -free embodiment of the metal sintering preparation according to the inventuion, NH ₄ BF ₄ is used neither during its production nor is it added as an ingredient.

The metal sintering preparation according to the invention contains 70 to≤84 % by weight, for example, 77 to 83 % by weight or 78 to 82 % by weight of at least one metal that is present in the form of particles. The metal particles comprise a coating. The weights given presently for the metal particles include the weight of the coating on the particles. The total amount of the coating is 0.4 to 2 % by weight with respect to the weight of the coated metal particles. A total of 95 to 100 % by weight of the coating con- sist of a combination of (i) 15 to 85 parts by weight of at least one fatty acid compound having 8 to 14, preferably 8, carbon atoms in the non-branched fatty acid residue and (ii) 85 to 15 parts by weight of at least one fatty acid compound having 16 to 22, preferably 18, carbon atoms in the non-branched fatty acid residue, whereby the parts by weight of components (i) and (ii) add up to 100 parts by weight.

The term, "metal", used in the context of coated metal particles shall include both pure metals and metal alloys.

In the scope of the invention, the term, "metal", refers to elements in the periodic system of the elements that are in the same period as boron, but to the left of boron, in the same period as silicon, but to the left of silicon, in the same period as germanium, but to the left of germanium, and in the same period as antimony, but to the left of antimony, as well as all elements having an atomic number of more than 55. In the scope of the invention, pure metals shall be understood to be metals containing a metal at a purity of at least 95 % by weight, preferably at least 98 % by weight, more preferably at least 99 % by weight, and even more preferably at least 99.9 % by weight. According to a preferred embodiment, the metal is copper, silver, gold, nickel, palladium, platinum or aluminium, in particular silver. Metal alloys shall be understood to be metallic mixtures of at least two components of which at least one is a metal.

According to a preferred embodiment, an alloy containing copper, aluminium, nickel and/or precious metals is used as metal alloy. The metal alloy preferably comprises at least one metal selected from the group consisting of copper, silver, gold, nickel, palladium, platinum, and aluminium. Particularly preferred metal alloys contain at least two metals selected from the group consisting of copper, silver, gold, nickel, palladium, platinum, and aluminium. Moreover, it can be preferred that the fraction of metals selected from the group consisting of copper, silver, gold, nickel, palladium, platinum, and alu- minium accounts for at least 90 % by weight, more preferably at least 95 % by weight, and even more preferably at least 99 % by weight of the metal alloy. The alloy can, for example, be an alloy that contains copper and silver, copper, silver and gold, copper and gold, silver and gold, silver and palladium, platinum and palladium or nickel and palladium.

The metal sintering preparation according to the invention can contain, as metal, a pure metal, multiple types of pure metal, a type of metal alloy, multiple types of metal alloys or mixtures thereof. The metal is present in the metal sintering preparation in the form of particles.

The metal particles can differ in shape. The metal particles can be present, for example, in the form of flakes or be of a spherical (ball-like) shape. According to a particularly preferred embodiment, the metal particles take the shape of flakes. However, this does not exclude a minor fraction of the particles employed being of different shape. However, preferably at least 70 % by weight, more preferably at least 80% by weight, even more preferably at least 90 % by weight or 100 % by weight, of the particles are present in the form of flakes.

As mentioned above, the metal particles comprise a coating, whereby 95 to 100 % by weight, preferably 100 % by weight, of the coating consist of a combination of (i) 15 to 85 parts by weight of at least one fatty acid compound having 8 to 14, preferably 8, carbon atoms in the non-branched fatty acid residue and (ii) 85 to 15 parts by weight of at least one fatty acid compound having 16 to 22, preferably 18, carbon atoms in the non- branched fatty acid residue, whereby the parts by weight of components (i) and (ii) add up to 100 parts by weight. Preferably, the weight ratio adding up to 100 parts by weight is 60 to 40 parts by weight of (i) and 40 to 60 parts by weight of (ii).

Preferably, the fatty acid compounds without branching in the fatty acid residue are corresponding free fatty acids, fatty acid salts or fatty acid esters with the free fatty acids being particularly preferred. Preferably, the free fatty acids, fatty acid salts or fatty acid esters are saturated compounds.

Fatty acid salts that are preferred in the scope of the invention include the ammonium, monoalkylammonium, dialkylammonium, trialkylammonium, aluminium, copper, lithium, sodium, and potassium salts.

Alkyl esters, in particular methyl esters, ethyl esters, propyl esters, and butyl esters, are preferred fatty acid esters according to the scope of the invention. Preferred fatty acid compounds having 8 to 14 carbon atoms in the non-branched fatty acid residue include caprylic acid (octanoic acid), caprinic acid (decanoic acid), lauric acid (dodecanoic acid), myristic acid (tetradecanoic acid) as well as the esters and salts thereof, but in particular the free fatty acids as such. Preferred fatty acid compounds having 16 to 22 carbon atoms in the non-branched fatty acid residue include palmitic acid (hexadecanoic acid), margaric acid (heptadecanoic acid), stearic acid (octadecanoic acid), arachidic acid (eicosanoic acid/icosanoic acid), behenic acid (docosanoic acid) as well as the esters and salts thereof, but in particular the free fatty acids as such.

The fatty acid compounds can be applied to the surface of the metal particles by means of conventional methods that are known from the prior art. It is feasible, for example, to slurry the fatty acid compounds in solvents and to triturate the slurried fatty acid compounds together with the metal particles in ball mills. After trituration, the metal particles coated with the fatty acid compounds can be dried and any dust can be removed, if applicable.

The degree of coating, defined as the ratio of the mass of fatty acid compounds and the surface of the metal particles, preferably is 0.00005 to 0.03 g, more preferably 0.0001 to 0.02 g of fatty acid compounds (i) plus (ii) per square metre (m ² ) of surface area of the metal particles.

The metal sintering preparation according to the invention contains 6 to 30 % by weight, for example, 7 to 25 % by weight or 8 to 20 % by weight or 10 to 15 % by weight of organic solvent (B), i.e. an organic solvent or a mixture of at least two organic solvents.

The organic solvent or solvents is/are organic solvent(s) that is/are used commonly for metal sintering preparations. Examples include terpineols, N-methyl-2-pyrrolidone, ethylene glycol, dimethylacetamide, 1 -tridecanol, 2-tridecanol, 3-tridecanol, 4-tridecanol, 5- tridecanol, 6-tridecanol, isotridecanol, with the exception of a methyl subsitution on the penultimate C-atom, unsubstituted 1 -hydroxy-C16-C20-alkanes such as 16- methylheptadecan-1-ol, dibasic esters (preferably dimethylesters of glutaric, adipic or succinic acid or mixtures thereof), glycerol, diethylene glycol, triethylene glycol, and aliphatic hydrocarbons, in particular saturated aliphatic hydrocarbons, having 5 to 32 C- atoms, more preferably 10 to 25 C-atoms, and even more preferably 16 to 20 C-atoms. Said aliphatic hydrocarbons are being marketed for example by Exxon Mobil by the brand name Exxsol™ D120 or by the brand name Isopar M™. The metal sintering preparation according to the invention can contain 0 to 12 % by weight, preferably 0.1 to 12 % by weight, more preferably 1 to 10 % by weight, and even more preferably 2 to 8 % by weight of at least one metal precursor (C). In the scope of the invention, a metal precursor shall be understood to mean a compound that contains at least one metal. Preferably, said compound decomposes at temperatures below 200 °C while releasing a metal. Accordingly, the use of a metal precursor in the sintering process is preferably associated with the in situ production of a metal. It is easy to determine whether a compound is a metal precursor. For example, a paste containing a compound to be tested can be deposited on a substrate having a silver surface followed by heating to 200 °C, and maintaining this temperature for 20 minutes. Then, it is tested whether or not the compound to be tested decomposed under these conditions. For this purpose, for example, the content of the metal-containing paste components can be weighed before the test to calculate the theoretical mass of metal. After the test, the mass of the material deposited on the substrate is determined by gravimetric methods. If the mass of the material deposited on the substrate is equal to the theoretical mass of metal, taking into account the usual measuring inaccuracy, the tested compound is a metal precursor. According to a preferred embodiment, the metal precursor is a metal precursor that can be decomposed endothermically. A metal precursor that can be decomposed endo- thermically shall be understood to be a metal precursor whose thermal decomposition, preferably in a protective gas atmosphere, is an endothermic process. Said thermal decomposition is to be associated with the release of metal from the metal precursor.

According to another preferred embodiment, the metal precursor comprises a metal that is also present in the particulate metal (A).

The metal precursor preferably comprises, as metal, at least one element selected from the group consisting of copper, silver, gold, nickel, palladium, and platinum. It can be preferred to use, as metal precursor, endothermically decomposable carbonates, lactates, formates, citrates, oxides or fatty acid salts, preferably fatty acid salts having 6 to 24 carbon atoms, of the metals specified above. In specific embodiments, silver carbonate, silver(l) lactate, silver(ll) formate, silver citrate, silver oxide (for example AgO or Ag ₂ O), copper(ll) lactate, copper stearate, copper oxides (for example Cu ₂ O or CuO) or gold oxides (for example Au ₂ O or AuO) can be used as metal precursor. According to a particularly preferred embodiment, silver carbonate, silver(l) oxide or sil- ver(ll) oxide is used as metal precursor.

The metal precursor, if present in the metal sintering preparation, is preferably present in the form of particles. j

The metal precursor particles can take the shape of flakes or a spherical (ball-like) shape. Preferably, the metal precursor particles are present in the form of flakes.

Moreover, the metal sintering preparation according to the invention can contain 0 to 10 % by weight, preferably 0 to 8 % by weight, of at least one sintering aid (D). Examples of sintering aids include organic peroxides, inorganic peroxides, and inorganic acids, such as are described, for example, in WO201 1 /026623 A1.

In addition to ingredients (A) to (D), the metal sintering preparation according to the in- vention can contain 0 to 15 % by weight, preferably 0 to 12 % by weight, more preferably 0.1 to 10 % by weight, of one or more further ingredients (E). Said further ingredients can preferably be ingredients that are used commonly in metal sintering preparations. The metal sintering preparation can contain, for example, as further ingredients, dispersion agents, surfactants, de-foaming agents, binding agents, polymers such as cellulose derivatives, for example methylcellulose, ethylcellulose, ethylmethylcellulose, carboxycellulose, hydroxypropylcellulose, hydroxyethylcellulose, hydroxymethylcellu- lose and/or viscosity-controlling (rheological) agents. The % by weight fractions specified for ingredients (A) to (E) can add up, for example, to 100 % by weight with respect to the metal sintering preparation according to the invention, i.e. prior to the application thereof. Accordingly, the metal sintering preparation according to the invention can be produced by mixing ingredients (A) to (E). Devices known to a person skilled in the art, such as stirrers and three-roller mills, can be used in this context.

The metal sintering preparation according to the invention can be used in a sintering process. Sintering shall be understood to mean the connecting of two or more components by heating without the metal particles (A) reaching the liquid phase.

The sintering method implemented through the use of the metal sintering preparation according to the invention can be implemented while applying pressure or without pres- sure. Being able to implement the sintering method without pressure means that a sufficiently firm connection of components is attained despite foregoing the application of pressure. Being able to implement the sintering process without pressure allows pressure-sensitive, for example fragile components or components with a mechanically sensitive micro-structure, to be used in the sintering method. Electronic components that have a mechanically sensitive micro-structure suffer electrical malfunction when exposed to inadmissible pressure.

Connecting at least two components shall be understood to mean attaching a first component on a second component. In this context, "on" simply means that a surface of the first component is being connected to a surface of the second component regardless of the relative disposition of the two components or of the arrangement containing the at least two components.

In the scope of the invention, the term "component" is to comprise single parts. Prefera- bly, said single parts cannot be disassembled further. According to specific embodiments, the term "components" refers to parts that are used in electronics.

Accordingly, components can, for example, be diodes, LEDs (light-emitting diodes, lichtemittierende Dioden), DCB (direct copper bonded) substrates, lead frames, dies, IGBTs (insulated-gate bipolar transistors), ICs (integrated circuits), sensors, heat sink elements (preferably aluminium heat sink elements or copper heat sink elements) or other passive components (such as resistors, capacitors or coils). The components to be connected can be identical or different components.

Embodiments of the invention relate to connections of LED to lead frame, of LED to ceramic substrate, of dies, diodes, IGBTs or ICs to lead frames, ceramic substrates or DCB substrates, of sensor to lead frame or ceramic substrate. The present invention facilitates not only the connecting of precious metal contact surfaces, but also, in particular, the connecting of one non-precious to one precious or even of two non-precious metal contact surfaces in a manner that is reliable at room temperature and at elevated temperatures. The connection can involve aluminium, copper or silver contact surfaces of the electronics components to aluminium, copper or silver contact surfaces of the substrates, i.e. for example aluminium-copper, aluminium-silver, aluminium-aluminium, copper-silver, copper-copper or silver-silver connections can be formed. The terms, "aluminium, copper, and silver contact surfaces", include contact surfaces made of aluminium alloys, copper alloys, and silver alloys; for just one example the surface of a lead frame made of copper or a copper alloy acting as contact surface may be mentioned in this context.

The components, for example at least one of components 1 and 2 can - in as far as they do not consist of metal anyway - comprise at least one metal contact surface, for example in the form of a metallisation layer, in particular a metal contact surface made of a non-precious metal such as copper or aluminium, by means of which the previously mentioned sandwich arrangement is effected in the scope of the method according to the invention. Said metallisation layer preferably is part of the component. Preferably, said metallisation layer is situated at at least one surface of the component.

Preferably, the connecting of the components by means of the metal sintering prepara- tion according to the invention is effected by means of said metallisation layer or layers.

The metallisation layer can comprise pure metal. Accordingly, it can be preferred for the metallisation layer to comprise at least 50 % by weight, more preferably at least 70 % by weight, even more preferably at least 90% by weight or 100 % by weight of pure metal. The pure metal is selected, for example, from the group consisting of aluminium, copper, silver, gold, palladium, and platinum. It is a particular advantage of the present invention that the pure metal can be a non-precious metal such as aluminium or copper.

On the other hand, the metallisation layer can just as well comprise an alloy. The alloy of the metallisation layer preferably contains at least one metal selected from the group consisting of aluminium, silver, copper, gold, nickel, palladium, and platinum.

The metallisation layer can just as well have a multi-layer structure. Accordingly, it can be preferred that at least one surface of the components to be connected comprises a metallisation layer made of multiple layers that comprise the pure metals and/or alloys specified above.

In the method according to the invention, at least two components are being connected to each other through sintering.

For this purpose, the two components are first made to contact each other. The contacting is effected by means of the metal sintering preparation according to the invention. For this purpose, an arrangement is provided, in which metal sintering preparation according to the invention is situated between each two of the at least two components.

Accordingly, if two components, i.e. component 1 and component 2, are to be connected to each other, the metal sintering preparation according to the invention is situated between component 1 and component 2 before the sintering process. On the other hand, it is conceivable to connect more than two components to each other. For example three components, i.e. component 1 , component 2, and component 3, can be connected to each other in appropriate manner such that component 2 is situated between component 1 and component 3. In this case, the metal sintering preparation according to the invention is situated between both component 1 and component 2 as well as between component 2 and component 3.

The individual components are present in a sandwich arrangement and are being con- nected to each other. Sandwich arrangement shall be understood to mean an arrangement, in which two components are situated one above the other with the two components being arranged essentially parallel with respect to each other.

The arrangement of at least two components and metal sintering preparation according to the invention, whereby the metal sintering preparation is situated between two components of said arrangement, can be produced according to any method known according to the prior art.

Preferably, firstly, at least one surface of a component 1 is provided with the metal sin- tering preparation according to the invention. Then, another component 2 is placed by one of its surfaces on the metal sintering preparation that has been applied to the surface of component 1 .

The metal sintering preparation according to the invention can be applied onto the sur- face of a component by means of conventional methods, such as by means of dispensing technique or printing methods such as screen printing or stencil printing or, just as well, by means of other application techniques such as spray application, pin transfer or dipping. Following the application of the metal sintering preparation according to the invention, it is preferable to contact the surface of said component that has been provided with the metal sintering preparation to a surface of the component to be connected thereto by means of the metal sintering preparation. Accordingly, a layer of the metal sintering preparation according to the invention is situated between the components to be connected. Preferably, the thickness of the wet layer between the components to be connected is in the range of 20 to 100 pm. In this context, thickness of the wet layer shall be understood to mean the distance between the opposite surfaces of the components to be connected prior to drying, if any, and prior to sintering. The preferred thickness of the wet layer depends on the method selected for applying the metal sintering preparation. If the metal sintering preparation is applied, for example, by means of a screen printing method, the thickness of the wet layer can preferably be 20 to 50 pm. If the metal sintering preparation is applied by means of stencil printing, the preferred thickness of the wet layer can be in the range of 20 to 100 pm. The preferred thickness of the wet layer in the dispensing technique can be in the range of 20 to 00 pm.

As an option, a drying step is introduced prior to the sintering, i.e. organic solvent is removed from the applied metal sintering preparation. According to a preferred embodiment, the fraction of organic solvent in the metal sintering preparation after drying is, for example, 0 to 5 % by weight with respect to the original fraction of organic solvent in the metal sintering preparation according to the invention, i.e. in the metal sintering preparation ready for application. In other words, according to said preferred embodiment, for example 95 to 100 % by weight of the organic solvent that is originally present in the metal sintering preparation according to the invention are removed during drying. If drying takes place in a sintering process without pressure, the drying can proceed after producing the arrangement, i.e. after contacting the components to be connected. If drying takes place in a sintering process involving the application of pressure, the drying can just as well proceed after application of the metal sintering preparation onto the at least one surface of the component and before contacting to the component to be connected.

Preferably, the drying temperature is in the range of 100 to 150 °C. Obviously, the drying time depends on the composition of the metal sintering preparation according to the invention and on the size of the connecting surface of the arrangement to be sintered. Common drying times are in the range of 5 to 45 minutes.

The arrangement consisting of the at least two components and metal sintering preparation situated between the components is finally subjected to a sintering process.

The actual sintering proceeds are a temperature of, for example, 200 to 280 °C in a process either with or without pressure.

The process pressure in pressure sintering is preferably less than 30 MPa and more preferably less than 5 MPa. For example, the process pressure is in the range of 1 to 30 MPa and more preferably is in the range of 1 to 5 MPa.

The sintering time is, for example, in the range of 2 to 60 minutes, for example in the range of 2 to 5 minutes in pressure sintering and for example in the range of 30 to 60 minutes in sintering without pressure. The sintering process can take place in an atmosphere that is not subject to any specific limitations. Accordingly, on the one hand, the sintering can take place in an atmosphere that contains oxygen. On the other hand, it is feasible just as well that the sintering takes place in an oxygen-free atmosphere. In the scope of the invention, an oxygen-free atmosphere shall be understood to mean an atmosphere whose oxygen content is no more than 10 ppm, preferably no more than 1 ppm, and even more preferably no more than 0.1 ppm.

The sintering takes place in a conventional suitable apparatus for sintering, in which the above-mentioned process parameters can be set.

The invention is illustrated through examples in the following, though these may not be construed such as to limit the invention in any way or form. Examples:

1 . Production of metal sintering preparations:

Firstly, metal sintering preparations 1 to 5 according to the invention and reference pastes 6 to 7 were produced by mixing the individual ingredients according to the following table at comparable rheological behaviour. All amounts given are in units of % by weight.

^>} Silver flakes having a mean particle diameter (d50) of 4.50 μνη

Coating of 0.7 % by weight octanoic acid/stearic acid (weight ratio 1 :1 )

Coating of 0.7 % by weight lauric acid/stearic acid (weight ratio 1 :1 )

Coating of 0.7 % by weight octanoic acid/stearic acid (weight ratio 6:4)

Coating of 0.7 % by weight aluminium stearate

Coating of 0.7 % by weight octanoic acid

2. Application and pressure-free sintering of the metal sintering preparations: The respective metal sintering preparation was applied by means of dispensing onto the copper surface of a lead frame made of a copper-rich copper/iron alloy (96 % by weight copper, 4 % by weight Fe) to produce a wet layer of 50 μιτι in thickness. Then, the applied metal sintering preration was contacted without previous drying to a silicon chip having a silver contact surface (4 ^■ 6 mm ² ). The subsequent pressUre-free sintering took place according to the following heating profile in a nitrogen atmosphere containing max. 50 ppm of oxygen: The contact site was heated steadily to 200 °C over the course of 60 minutes and then maintained at 200 °C for 30 minutes. Subsequently, the temperature was raised steadily to 230 °C over the course of 5 minutes and then maintained at this level for 30 minutes. Then, this was cooled steadily to 30 °C over the course of 50 minutes.

After the sintering, the bonding was determined by testing the shear strength. In this context, the components were sheared off with a shearing chisel at a rate of 0.3 mm/s at 260 °C. The force was measured by means of a load cell (DAGE 2000 device made by DAGE, Germany). Table 2 shows the results obtained with metal sintering preparations 1 to 7.

Table 2:

Paste 1 2 3 4 5 6 7

Shear strength at 24.3 20.6 19.8 21.3 21 .8 9.8 8.9

260°C (N/mm ² )