Solder paste

Abstract

Disclosed is a solder paste consisting of 40 wt% to 60 wt% of a solder powder having an absolute particle size of powder particles in the range of 2 [mu] m to 25 [mu] m, and 40 wt% to 60 wt% of a flux.

Description

[0001] Solder pastes, in particular soft solder pastes, are used primarily in the manufacture of electronic circuits and for producing mechanical, electrical and thermal connections between electronic components and substrates, more precisely between their corresponding metal contact pads (metal contact surfaces, contact metallizations) provided for this purpose. The metal contact pads consist of bulk metal (e.g. pure metal or metal alloy), or they may have a metal surface coating; the metal itself or the metal of the metal surface coating may be selected from metals such as, for example, copper and copper-based alloys, tin and tin-based alloys, silver and silver-based alloys, gold and gold-based alloys and nickel and nickel-based alloys.

[0002] Examples of electronic components in the sense of the present disclosure include diodes, LEDs (light emitting diodes), dies, IGBTs (insulated gate bipolar transistors, bipolar transistors with insulated gate electrodes), MOSFETs (metal oxide semiconductor field effect transistors), ICs (integrated circuits), sensors, heat sinks, resistors, capacitors, coils, connecting elements (e.g., clips), base plates, antennas, etc.

[0003] In the sense of the present disclosure, examples of substrates include lead frames, PCBs (printed circuit boards), flexible electronic devices, ceramic substrates, metal ceramic substrates (such as DCB substrates (direct copper bonded substrates)), IMS (insulated metal substrates), glass substrates, etc.

[0004] The electronic components are usually contacted with or applied to the substrate via solder paste. The solder paste is heated by a reflow process to melt the solder in the paste, thereby forming contact between the corresponding metal contact pads of the electronic components and the substrate. After cooling and solidification of the solder, the electronic components and the substrate are firmly connected (attached) to each other via their corresponding metal contact pads (with the solidified solder located between them).

[0005] Solder pastes usually contain flux which serves in particular to dissolve any undesired oxide layers present on the surface of the solder powder, the surfaces of the metal contact pads of the electronic component and of the substrate and thus ensure better wettability during the soldering process.

[0006] WO 2015 / 178374 A1 discloses a solder paste for fixing solder balls, wherein the solder paste contains solder powder and 75 volume % to 93 volume % (volume percentage) of flux.

[0007] In the electronics industry, there is a continuous development towards miniaturization, leading to so-called fine pitch applications or even ultra-fine pitch applications with very small dimensions down to ≤ 100 μm. The term "pitch" stands for the distance between the center of a metal contact pad and the center of an adjacent (neighboring) metal contact pad. Such developments lead to an increase in the density of electronic packaging and place higher demands on solder pastes.

[0008] It is an object of the present invention to provide a solder paste which can be used even in fine pitch applications or ultra-fine pitch applications.

[0009] The present invention relates to a solder paste, which is composed of 40 wt % to 60 wt % (weight percentage) of solder powder and 40 wt % to 60 wt % of flux, wherein the absolute particle size of the powder particles of the solder powder is in the range of 2 μm to 25 μm. The flux itself is composed of:

[0010] i) 50 to 60% by weight of at least one optionally modified natural resin;

[0011] ii) 15 to 30 wt. % of at least one organic solvent;

[0012] iii) 5 to 15 wt. % of at least one thickener;

[0013] iv) 5 to 10 wt % of at least one activating agent; and

[0014] v) 0 to 10% by weight of at least one additive other than components i) to iv).

[0015] The weight % of the solder powder and the weight % of the flux add up to 100 weight %. The same applies to the sum of the weight % of the flux components i) to v).

[0016] The solder paste of the invention comprises 40 to 60 wt % of solder powder, in particular tin-based solder powder. The solder powder has powder particles with an absolute particle size in the range of 2 to 25 μm, preferably in the range of 2 to 15 μm.

[0017] Preferably, the solder has a liquidus temperature in the range of 200°C to 250°C, preferably in the range of 200°C to 230°C.

[0018] The term "tin-based solder" means a tin-based solder alloy comprising tin as a base and one or more other alloying elements. The tin content may be at least 80% by weight. Examples of other alloying elements include silver, copper, antimony, bismuth, indium, nickel and cobalt. Lead is an example of a possible but less preferred alloying element.

[0019] The solder paste according to the invention comprises 40% to 60% by weight of flux. The flux comprised by the solder paste according to the invention consists in each case, based on its total weight, of:

[0020] i) 50 to 60% by weight of at least one optionally modified natural resin;

[0021] ii) 15 to 30 wt. % of at least one organic solvent;

[0022] iii) 5 to 15 wt. % of at least one thickener;

[0023] iv) 5 to 10 wt % of at least one activating agent; and

[0024] v) 0 to 10% by weight of at least one additive other than components i) to iv).

[0025] As component i), the flux comprises 50 to 60 wt. %, preferably 50 to 55 wt. %, of at least one optionally modified natural resin. For the person skilled in the art, it is not necessary to explain that "modified" stands for chemical modification. The at least one optionally modified natural resin can be an unmodified natural resin or a modified natural resin. Modified natural resins are understood to mean natural resins modified by hydrogenation, dimerization and / or esterification of their carboxyl groups. In particular, the natural resins are natural resins of the rosin type (rosin resin type), i.e. unmodified or modified rosins (rosins modified by hydrogenation, dimerization and / or esterification of their carboxyl groups).

[0026] As component ii), the flux contains 15 to 30 wt. %, preferably 20 to 25 wt. %, of at least one organic solvent. Examples include diols, alcohols, ether alcohols and ketones which are liquid at 25° C., in particular trimethylpropanol, 1,2-octanediol, 1,8-octanediol, 2,5-dimethyl-2,5-hexanediol, isobornylcyclohexanol, glycol ethers, 2-ethyl-1,3-hexanediol, n-decyl alcohol, 2-methyl-2,4-pentanediol, terpineol and isopropanol and mixtures thereof. Glycol ethers represent preferred examples. The glycol ethers may be partially or completely etherified; particular examples include monopropylene glycol methyl ether, dipropylene glycol methyl ether, tripropylene glycol methyl ether, monopropylene glycol n-butyl ether, dipropylene glycol n-butyl ether, tripropylene glycol n-butyl ether, ethylene glycol dimethyl ether, triethylene glycol methyl ether, diethylene glycol dibutyl ether, tetraethylene glycol dimethyl ether and diethylene glycol monohexyl ether and mixtures thereof. It may be preferred that the at least one organic solvent may comprise or consist of one or more glycol ethers.

[0027] As component iii), the flux comprises 5 to 15 wt. %, preferably 8 to 12 wt. % of at least one thickener. Examples include ethylcellulose, castor oil, hydrogenated castor oil, tri-12-hydroxystearin, modified tri-12-hydroxystearin, fatty acid amides and polyamides. Castor oil and polyamides represent preferred examples. It may be preferred that the at least one thickener comprises or consists of castor oil and / or one or more polyamide thickeners.

[0028] As component iv), the flux comprises 5 to 10 wt. % of at least one activator. The at least one activator serves to remove surface oxides of the solder powder that are finally present. Examples include carboxylic acids, preferably dicarboxylic acids, such as oxalic acid, adipic acid, malonic acid, succinic acid, glutaric acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid and tridecanedioic acid. The carboxylic acid may be combined with at least one amine. Examples of amines include imidazoles, N,N,N',N'-tetramethylethylenediamine, N,N,N',N'-tetraethylethylenediamine, N,N,N',N'-tetrapropylethylenediamine, N-cocoyl-1,3-diaminopropane, 1,6-diaminohexane, 1,7-diaminoheptane, 1,8-diaminooctane, 1,9-diaminononane and 1,10-diaminodecane, bis(2-ethylhexyl)amine, bis(2-methylhexyl)amine, diethylamine, triethylamine, cyclohexylamine, diethanolamine, triethanolamine, hydrogenated tallow alkylamines, hydrogenated (tallow alkyl)dimethylamine and hydrogenated bis(tallow alkyl)methylamine. Other examples of the activator include halogen-containing compounds such as aniline hydrochloride, glutamic acid hydrochloride, diethanolamine hydrochloride, diethanolamine hydrobromide, triethanolamine hydrochloride, triethanolamine hydrobromide, and trans-2,3-dibromo-2-butene-1,4-diol.

[0029] As component v), the flux includes 0 to 10 wt % of at least one additive other than components i) to iv). Examples of such additives include surfactants, antioxidants and surface modifiers.

[0030] A further subject matter of the present invention is a method for producing the solder paste according to the invention.

[0031] The method for producing the solder paste of the present invention comprises the following steps:

[0032] - the composition of the mixed flux, and

[0033] -Add solder powder as described above.

[0034] The addition of solder powder is preferably done in portions while being stirred into the current mixture of flux, usually without heating.

[0035] The solder paste of the present invention can be used to connect (attach) an electronic component to a substrate. When the electronic component is connected to the substrate, the metal contact pad of the substrate and the metal contact pad of the electronic component are contacted via the solder paste of the present invention. Therefore, the present invention also relates to a method for connecting an electronic component to a substrate, the method comprising the following steps:

[0036] a) providing an electronic component having at least one metal contact pad,

[0037] b) providing a substrate having at least one metal contact pad corresponding to the at least one metal contact pad of the electronic component (= corresponding to the one single metal contact pad or those two or more metal contact pads of the electronic component),

[0038] c) providing the at least one metal contact pad of the electronic component and / or the corresponding at least one metal contact pad of the substrate with the solder paste of the invention,

[0039] d) contacting the at least one metal contact pad of the electronic component with the corresponding at least one metal contact pad of the substrate via the solder paste; and

[0040] e) heating the solder paste to above the liquidus temperature of the solder, and then allowing the solder to cool and solidify while forming a tight connection between the electronic component and the substrate.

[0041] Steps a) and b) are self-explanatory for the skilled person and do not require further explanation. However, in order to prevent misunderstandings, the skilled person will understand that both the electronic component and the substrate may have one, two or more of said metal contact pads. The term "corresponding" means that the metal contact pads of the electronic component are similar or even identical to (=correspond to) the metal contact pads of the substrate in size, shape and arrangement, and that both the metal contact pads of the electronic component and the metal contact pads of the substrate will be connected to each other.

[0042] In step c), at least one metal contact pad of the electronic component and / or at least one corresponding metal contact pad of the substrate is provided with the solder paste of the present invention, i.e. the solder paste of the present invention can be applied to at least one metal contact pad of the electronic component and / or at least one corresponding metal contact pad of the substrate, in particular to at least one corresponding metal contact pad of the substrate. The application can be carried out by conventional methods known to those skilled in the art, for example by printing, dispensing, spraying or transferring. Printing methods such as screen printing or stencil printing are preferred. Advantageously, the solder paste of the present invention meets the challenges of stencil printing using stencils with openings as small as hundreds of μm in size.

[0043] The applicant has discovered an unexpected and advantageous feature of the solder paste of the invention; the solder paste of the invention exhibits a so-called self-aligning property, i.e. the solder paste of the invention that extends beyond the non-metallic surrounding of the metal contact pads is able to be pulled back between the metal contact pads of the electronic component and the corresponding metal contact pads of the substrate when heated during the final process step e). In other words, when heated during step e), it can be pulled back from the non-metallic surface portion and gathered between the metal contact pads of the electronic component and the corresponding metal contact pads of the substrate. Normally, the solder paste should be applied to the metal contact pads and no solder paste should be allowed to diffuse into the non-metallic and electrically insulating surrounding of the metal contact pads, as this would mean a high risk of generating an electrical short circuit after the end of step e); however, said pulling back behavior of the solder paste of the invention allows some inaccuracies when it is applied to the metal contact pads. In other words, in step c), the solder paste of the invention allows to be applied to the metal contact pads (to a certain small extent beyond those metal contact pads), for example, to diffuse it into the non-metallic surrounding of the metal contact pads, for example, up to 50 μm or preferably only up to 25 μm.

[0044] Step d) is a so-called pick and place step. In step d), at least one metal contact pad of the electronic component and at least one corresponding metal contact pad of the substrate can be contacted with each other via the solder paste of the invention applied in step c). In other words, a sandwich arrangement can be created from the electronic component and the substrate, with the solder paste of the invention interposed therebetween, i.e. between the metal contact pad of the electronic component and the corresponding metal contact pad of the substrate.

[0045] Step e) is the so-called solder reflow step. In step e), the sandwich arrangement can be soldered by heating the solder paste to a temperature above the liquidus temperature of the solder so as to produce a strong connection between the electronic component and the substrate via the solder paste after subsequent cooling and solidification of the solder. The sandwich arrangement or the solder paste is preferably heated to a temperature of 5°C to 60°C, preferably 10°C to 50°C, above the liquidus temperature of the solder. The skilled person will understand that during step e), the flux is removed, for example by evaporation and / or decomposition. The skilled person, having read the description of step c), will also understand that during the heating during step e), the solder paste is pulled back from the non-metallic surface portion and accumulates between the metal contact pads of the electronic component and the corresponding metal contact pads of the substrate.

[0046] The self-alignment properties of the solder paste of the present invention as well as the solder properties can be checked by visual inspection of the soldered sandwich arrangement after step e) is completed. Good self-alignment properties mean that there are no solder beads (solder material masses) on the non-metallic surroundings of the metal contact pads, i.e. no solder beads are retained on the non-metallic surroundings of the metal contact pads. Good solder properties mean that there are no solder bridges that are electrical short circuits between the metal contact pads of the substrate or, respectively, between the metal contact pads of the electronic component. If visual inspection by the human eye is not possible directly, an X-ray image of the soldered sandwich arrangement can be obtained, and the absence of the solder beads and the absence of the solder bridges can be checked.

[0047] The pull-back behavior or self-alignment properties of the solder paste of the present invention make the latter particularly useful in the fine pitch application or ultra-fine pitch application described. This means that the solder paste of the present invention can be used to connect an electronic component having at least one pair (= positive and negative) of metal contact pads to a substrate having a corresponding pair or more pairs of corresponding metal contact pads, wherein each of the metal contact pads of a single pair of metal contact pads is spaced apart from each other by not less than 50 μm and not more than 100 μm; in other words, there is a narrow gap of 50 μm to 100 μm between the metal contact pads of a single pair of metal contact pads. Here, in the case of fine pitch application or ultra-fine pitch application, the method for connecting an electronic component to a substrate is a special embodiment of the method disclosed above comprising steps a) to e). Here, the present invention relates to a method for connecting an electronic component to a substrate, the method comprising the following steps:

[0048] a′) providing an electronic component having at least one pair of metal contact pads with a gap of 50 μm to 100 μm between the metal contact pads,

[0049] b′) providing a substrate having at least one pair of metal contact pads corresponding to the at least one pair of metal contact pads of the electronic component (=corresponding to the one single pair or those two or more pairs of metal contact pads of the electronic component),

[0050] c') providing the solder paste of the present invention for at least one pair of metal contact pads of the substrate and the narrow gap of 50 μm to 100 μm therebetween,

[0051] d′) contacting the at least one pair of metal contact pads of the electronic component with the corresponding at least one pair of metal contact pads of the substrate via the solder paste; and

[0052] e') heating the solder paste to above the liquidus temperature of the solder, and then allowing the solder to cool and solidify while forming a tight connection between the electronic component and the substrate.

[0053] In step a'), an electronic component having at least one pair of metal contact pads having a narrow gap of 50 μm to 100 μm between the metal contact pads is provided. As already mentioned, a single pair of metal contact pads has a narrow gap of 50 μm to 100 μm between the metal contact pads. Although the metal contact pads represent a conductive metal surface portion, the narrow gap portion represents a non-conductive (electrically insulating) non-metal surface portion, i.e. a non-metallic gap. Examples of electronic components of this type include small LEDs and micro-LEDs equipped with a pair of metal contact pads, which are separated from each other by a narrow gap of 50 μm to 100 μm. In a preferred embodiment, the metal contact pads of the electronic component, in particular the metal contact pads of the small LED or micro-LED, may present a surface of tin or a tin-based alloy, for example in the form of a tin or tin-based alloy surface plating.

[0054] In step b'), a substrate having at least one pair of metal contact pads corresponding to at least one pair of metal contact pads of the electronic component provided in step a') is provided. The term "corresponding" means that the at least one pair of metal contact pads of the electronic component is similar to or even identical to (= corresponding to) the at least one pair of metal contact pads of the substrate in terms of size, shape, arrangement and gap width, and both the one or more pairs of metal contact pads of the electronic component and the one or more pairs of metal contact pads of the substrate will be connected to each other. For example, in the case of the small-sized LED or micro-LED, the substrate has a pair of metal contact pads corresponding to the metal contact pads of the small-sized LED or micro-LED; in other words, the term "corresponding" here means that the paired metal contact pads of the small-sized LED or micro-LED are similar to or even identical to (= corresponding to) the paired metal contact pads of the substrate in terms of size, shape, arrangement and gap width, and both the paired metal contact pads of the small-sized LED or micro-LED and the paired metal contact pads of the substrate will be connected to each other, wherein the connection will be made so as to match the narrow gap, or more precisely, such connection will be made so as to position the narrow gap as uniformly as possible.

[0055] In step c'), the solder paste of the present invention is applied, i.e., the solder paste of the present invention is provided to at least one pair of metal contact pads of the substrate and a narrow gap of 50 μm to 100 μm therebetween. The application can be carried out by conventional methods known to those skilled in the art, for example by printing, dispensing, spraying or transferring. Printing methods such as screen printing or stencil printing are preferred. However, due to the existing tiny structures in fine pitch application or ultra-fine pitch application, the application tool (dispenser, printing screen, printing stencil, etc.) does not allow the solder paste to be applied absolutely accurately only on at least one pair of metal contact pads of the substrate without some diffusion on the non-metallic gap forming area therebetween. However, in the embodiment of fine pitch application or ultra-fine pitch application here, applying the solder paste of the present invention to this narrow non-metallic surface portion is not just intentionally accepted or tolerated; it is explicitly allowed. Therefore, in this step c'), the solder paste of the present invention is applied to at least one pair of metal contact pads of the substrate and the narrow gap of 50μm to 100μm therebetween; that is, the solder paste is applied to the metal surface portion of the contact pads of the substrate and the entire narrow non-metallic surface portion of 50μm to 100μm therebetween that forms the gap, that is, the narrow non-metallic surface portion of 50μm to 100μm that forms the gap does not leave an uncovered portion, but is filled with solder paste.

[0056] The solder paste of the present invention exhibits the above-mentioned self-alignment property, which is very advantageous. When heated during step e'), the solder paste of the present invention in the non-metal gap area is pulled back and gathered between at least one pair of metal contact pads of the electronic component and at least one pair of corresponding metal contact pads of the substrate.

[0057] Step d') is a so-called pick and place step, during which at least one pair of metal contact pads of the electronic component and at least one corresponding pair of metal contact pads of the substrate can be brought into contact with each other via the solder paste of the invention. In other words, a sandwich arrangement can be created of the electronic component and the substrate, with the solder paste of the invention interposed therebetween, i.e. between the metal contact pads of the at least one pair of metal contact pads of the electronic component and the metal contact pads of the at least one corresponding pair of metal contact pads of the substrate.

[0058] Step e') is the so-called solder reflow step. In step e'), the sandwich arrangement can be soldered by heating the solder paste of the present invention to a temperature above the liquidus temperature of the solder so as to produce a strong connection between the electronic component and the substrate via the solder paste after subsequent cooling and solidification of the solder. The sandwich arrangement or the solder paste is preferably heated to a temperature of 5°C to 60°C, preferably 10°C to 50°C, above the liquidus temperature of the solder. During the heating process, the solder paste of the present invention undergoes the above-mentioned advantageous pull-back process. As a result, the non-metallic gaps are cleared and the solder of the solder paste is only found between the metal contact pads of at least one pair of metal contact pads of the substrate and the metal contact pads of the corresponding at least one pair of metal contact pads of the electronic component. The skilled person will understand that during step e'), the flux is removed, for example by evaporation and / or decomposition. A technician who has read the description of step c') will also understand that during the heating during step e'), the solder paste is pulled back from the non-metallic surface portions (including the non-metallic surface portions of the gaps) and gathered between the metal contact pads of at least one pair of metal contact pads of the substrate and the metal contact pads of the corresponding at least one pair of metal contact pads of the electronic component.

[0059] The pull-back behavior or self-alignment properties of the solder paste of the invention and the solder properties can be checked by taking an X-ray image of the soldered sandwich arrangement after step e') has ended. The X-ray image can then be visually checked. Good self-alignment properties mean that there are no solder beads on the non-metallic surround of the metal contact pads or in the gap area, i.e. no solder beads remain there. Good solder properties mean that no solder bridges that are electrical short circuits occur between the metal contact pads of the substrate or, respectively, between the metal contact pads of the electronic component.

[0060] Processing Example

[0061] After 52 pbw (parts by weight) of hydrogenated rosin (hydrogenated rosin resin) having an acid value of 240 mg KOH / g was melted at 170° C., 25 pbw of ethylene glycol dimethyl ether, 8 pbw of a polyamide thickener, 4 pbw of succinic acid, 3 pbw of N,N,N',N'-tetramethylethylenediamine, 6 pbw of a surfactant and 2 pbw of an antioxidant were added at 140° C. to prepare a flux.

[0062] A solder paste was formed by mixing 50 pbw of the flux thus prepared with 50 pbw of solder powder (SnAgCu alloy consisting of 96.5 wt % Sn, 3.0 wt % Ag and 0.5 wt % Cu, type 6 powder according to IPC-TM-6502.2.14.2).

[0063] The solder paste stencil thus prepared is printed on a glass-reinforced epoxy laminate FR4 PCB substrate presenting pairs of rectangular copper contact pads measuring 55μm×80μm. The copper contact pads have an ENIG (electroless nickel immersion gold) surface finish. The copper contact pads in each pair of copper contact pads are spaced apart from each other by a narrow gap of 50μm. The opening in the stencil is a rectangular shape measuring 70μm×130μm. The solder paste is applied to the copper contact pads of the substrate and to the 50μm narrow gap of the non-metallic surface portion between the copper contact pads. After the solder paste stencil printing step, a small LED die measuring 100μm×150μm is placed on top of each pair of rectangular patterned solder paste. The back of each small LED die consists of two tin alloy-plated contact pads, each of which is 30μm×50μm; there is a narrow gap of 50μm between the two contact pads. The placement of the LED die was done so that the narrow 50 μm gaps between the tin alloy plated contact pads of the LED die overlapped precisely with the narrow 50 μm gaps between the copper contact pads of the substrate and were filled with solder paste that had been applied by stencil printing.

[0064] The sandwich arrangement thus prepared consisting of the small LED die and the FR4 PCB substrate with the solder paste in between was soldered by heating in a reflow oven at a peak temperature of 250° C. in a nitrogen atmosphere. After the solder paste melted, the sandwich arrangement was taken out and cooled.

[0065] After the sandwich arrangement had cooled, the self-alignment properties of the solder paste were evaluated. To this end, an X-ray image of the soldered sandwich arrangement was acquired from above using a Nordson X-ray (Model: Q5) machine and any solder beads remaining on the non-metallic gaps between the copper contact pads of the FR4 PCB substrate were visually inspected. No solder beads were detected, which means that the solder paste exhibited very good self-alignment properties.

[0066] The solder performance was also evaluated by visually inspecting the same X-ray images to check whether solder bridging had occurred between the two tin-plated alloy contact pads of the LED die. No solder bridging occurred, which means that the contact pads of the LED die did not have an undesired electrical short circuit.

Claims

1. A solder paste comprising 40 to 60 wt% of solder powder and 40 to 60 wt% of flux, wherein the absolute particle size of powder particles of the solder powder is in the range of 2 μm to 25 μm.

2. The solder paste according to claim 1, wherein the flux consists of: i) 50 to 60% by weight of at least one optionally modified natural resin; ii) 15 to 30 wt. % of at least one organic solvent; iii) 5 to 15 wt. % of at least one thickener; iv) 5 to 10 wt % of at least one activating agent; and v) 0 to 10% by weight of at least one additive other than components i) to iv). The solder paste according to claim 1 or 2, wherein the solder powder is a tin-based solder powder.

4. The solder paste according to any one of the preceding claims, wherein the solder has a liquidus temperature in the range of 200°C to 250°C. 5 . The solder paste according to claim 2 , wherein the at least one organic solvent comprises or consists of one or more glycol ethers. 6 . The solder paste according to claim 2 , wherein the at least one thickener comprises or consists of castor oil and / or one or more polyamide thickeners. 7 .

7. A method for connecting an electronic component to a substrate, the method comprising the steps of: a) providing an electronic component having at least one metal contact pad, b) providing a substrate having at least one metal contact pad corresponding to the at least one metal contact pad of the electronic component, c) providing the at least one metal contact pad of the electronic component and / or the corresponding at least one metal contact pad of the substrate with a solder paste according to any one of claims 1 to 6, d) contacting the at least one metal contact pad of the electronic component with a corresponding at least one metal contact pad of the substrate via the solder paste; as well as e) heating the solder paste to above the liquidus temperature of the solder, and subsequently cooling and solidifying the solder while forming a tight connection between the electronic component and the substrate.

8. The method according to claim 7, wherein in step c), the solder paste is applied to the at least one metal contact pad by spreading the solder paste up to 50 μm onto the non-metallic surrounding of the at least one metal contact pad and to a small extent beyond the at least one metal contact pad.

9. A method for connecting an electronic component to a substrate, the method comprising the steps of: a′) providing an electronic component, wherein the electronic component has at least one pair of metal contact pads, wherein a gap of 50 μm to 100 μm is provided between the at least one pair of metal contact pads, b′) providing a substrate having at least one pair of metal contact pads corresponding to the at least one pair of metal contact pads of the electronic component, c′) providing the solder paste according to any one of claims 1 to 6 for at least one pair of metal contact pads of the substrate and a narrow gap of 50 μm to 100 μm between the at least one pair of metal contact pads, d′) contacting the at least one pair of metal contact pads of the electronic component with the corresponding at least one pair of metal contact pads of the substrate via the solder paste; as well as e') heating the solder paste to above the liquidus temperature of the solder, and subsequently cooling and solidifying the solder while forming a tight connection between the electronic component and the substrate. 10 . The method according to claim 9 , wherein the electronic component is a small LED or a micro LED provided with a pair of metal contact pads spaced apart from each other by a narrow gap of 50 μm to 100 μm.