Flux and solder paste containing said flux

Abstract

To provide a flux which has good viscosity and storage stability, and has good solderability in the atmosphere, a solder paste composed of the flux and a use of the same.SOLUTION: There are provided a flux which is composed of (i) 30 to 80 wt.% of one or more acidic (meth)acryl copolymers having an acid value of 100 to 350 mgKOH / g and a weight average molecular weight Mw of 1,000 to 5,000, (ii) 10 to 60 wt.% of at least one organic solvent, (iii) 0 to 15 wt.% of one or more amines, and (iv) 0 to 20 wt.% of one or more components other than the components (i) to (iii); and a solder paste which is composed of 80 to 92 wt.% of one or more different solders, and 8 to 20 wt.% of a flux.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a flux for a solder paste, and in particular to a solder paste containing the flux for attaching electronic components to a substrate.

[0002] Solder pastes, especially soft solder pastes, are primarily used in the manufacture of electronic circuits to create mechanical, electrical and thermal connections between electronic components and substrates, or more precisely between the contact surfaces of the latter provided for this purpose. Summary of the Invention [Problem to be solved by the invention]

[0003] Examples of electronic components within the meaning of this patent application include diodes, LEDs (light emitting diodes), dies, IGBTs (insulated gate bipolar transistors), 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.

[0004] Examples of substrates within the meaning of this patent application include lead frames, PCBs (printed circuit boards), flexible electronics, ceramic substrates, metal-ceramic substrates such as DCB substrates (direct bond copper substrates), IMS (insulated metal substrates), etc.

[0005] Electronic components are typically contacted or applied to a substrate via a solder paste. The solder paste is heated, for example, by a reflow process, to melt the solder (solder metal, solder alloy) in the paste. After the solder cools and solidifies, the electronic component and the substrate are firmly joined together.

[0006] In addition to the solder powder, solder paste typically contains a flux, which, among other things, serves to dissolve the oxide layer on the surfaces of the solder powder, electronic components, and substrates, thereby improving wetting during the soldering process.

[0007] Fluxes that form part of the solder paste are typically based on natural resins, especially colophony, and furthermore organic solvents, buffer bases, such as amines, and activators, such as carboxylic acids or halogen compounds, are typically included as ingredients in such fluxes.

[0008] The object of the present invention is to provide a solder paste which has an improved viscosity and thus an improved storage stability, and preferably also an improved solderability in the presence of air, i.e., good solderability without the need to take additional special measures to exclude atmospheric oxygen, such as vacuum soldering or soldering under inert gas. High viscosity stability is likewise associated with consistent processability, for example, particularly consistent printability.

[0009] The present applicant has been able to develop a flux that solves this problem, or more precisely, a solder paste that includes a flux that solves this problem. Accordingly, the present invention provides a flux comprising: (i) 30 to 80 wt. % (weight %), preferably 35 to 70 wt. %, of an acid value in the range of 100 to 350 mg KOH / g and a weight average molecular weight M in the range of 1000 to 5000 w one or more acidic (meth)acrylic copolymers having (ii) 10 to 60% by weight, preferably 20 to 50% by weight, of at least one organic solvent; (iii) 0 to 15 wt. %, preferably 4 to 12 wt. %, of one or more amines; (iv) 0 to 20% by weight of one or more components other than components (i) to (iii); The object of the present invention is to provide a flux comprising:

[0010] "(Meth)acrylic" means "methacrylic" and / or "acrylic".

[0011] The term "acid number" as used in the present specification and examples refers to the acid number (SZ) which can be determined according to DIN EN ISO 2114 in mg KOH / g (milligrams KOH / gram).

[0012] Unless otherwise noted, all standards cited herein are the versions prevalent as of the priority date of this patent application.

[0013] The weight average molecular weight M referred to in the present specification and examples w can be determined in the usual manner known to those skilled in the art by GPC, for example according to DIN 55672-1 (March 2016; cross-linked polystyrene as stationary phase, tetrahydrofuran as liquid phase, polystyrene standards, 23° C.).

[0014] Depending on the presence of components (iii) and / or (iv), the flux may consist accordingly of components (i)+(ii), or (i)+(ii)+(iii), or (i)+(ii)+(iv), or (i)+(ii)+(iii)+(iv), where in each of these alternatives the weight percent of the respective components sums to 100 weight percent.

[0015] As component (i), the flux according to the present invention comprises 30 to 80% by weight, preferably 35 to 70% by weight, of one or more different acidic (meth)acrylic copolymers. Preferably, there is only one acidic (meth)acrylic copolymer. The acidic (meth)acrylic copolymer has an acid number in the range of 100 to 350 mg KOH / g, preferably 150 to 300 mg KOH / g. The acidic (meth)acrylic copolymer has a weight-average molecular weight M in the range of 1000 to 5000, preferably in the range of 1000 to 3000, in particular in the range of 1500 to 2500. w It has an acid value in the range of 150 to 300 mg KOH / g and a weight average molecular weight M in the range of 1000 to 3000 or 1500 to 2500. wPreferred are acidic (meth)acrylic copolymers having the formula: These are copolymers of (meth)acrylic compounds that can be prepared by conventional methods known to those skilled in the art through radical copolymerization, and the copolymers may also contain comonomers other than (meth)acrylic-type comonomers, such as vinyl compounds and / or other olefinically unsaturated radically copolymerizable compounds, in a total weight proportion of <50% by weight, based on the total acidic (meth)acrylic copolymer. Examples of (meth)acrylic compounds that constitute >50% by weight, based on the total (meth)acrylic copolymer, are (meth)acrylic acid, (meth)acrylic acid esters, and (meth)acrylamides. Examples of vinyl compounds include compounds such as vinyl esters, vinyl ethers, and styrene.

[0016] The acidic (meth)acrylic copolymer has a glass transition temperature (T) in the range of >0°C, but generally ≦105°C. g The glass transition temperature can be determined using dynamic differential scanning calorimetry (DSC) according to DIN 51007 at a heating rate of 10 K / min.

[0017] Acidic (meth)acrylic copolymers of the type described above are commercially available, examples of which can be found under the name Indurez from Indulor Chemie GmbH and under the name Joncryl® from BASF.

[0018] As component (ii), the flux according to the present invention contains 10 to 60% by weight, preferably 20 to 50% by weight, of at least one organic solvent. Examples include diols, alcohols, ether alcohols, and ketones that are liquid at 25°C, particularly 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, as well as mixtures thereof. Examples of glycol ethers include mono-, di-, and tripropylene glycol methyl ether, mono-, di-, and tripropylene glycol n-butyl ether, mono-, di-, and triethylene 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, as well as mixtures thereof.

[0019] As component (iii), the flux according to the present invention comprises 0 to 15 wt. %, preferably 4 to 12 wt. %, of one or more amines, i.e. the flux according to the present invention may or may not comprise one or more amines, preferably one or more amines. Examples of amines include N,N,N',N'-tetramethylethylenediamine, N,N,N',N'-tetraethylethylenediamine, N,N,N',N'-tetrapropylethylenediamine, N-coco-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)dimethylamines, and hydrogenated bis(tallow alkyl)methylamines.

[0020] As component (iv), the flux according to the present invention comprises 0-20 wt. % of one or more components different from components (i)-(iii), i.e., the flux according to the present invention may comprise one or more type (iv) components, or none. Examples of type (iv) components include, in particular, thickeners, but may also include activators, defoamers, wetting aids and / or stabilizers.

[0021] Examples of thickening agents include ethyl cellulose, hydrogenated castor oil, glycerol-tris-12-hydroxystearin, and modified glycerol-tris-12-hydroxystearin.

[0022] Examples of low molecular weight carboxylic acids that can be used as activators include benzilic acid, 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. It may be advantageous for the flux according to the present invention to contain 2 to 15 wt. % of at least one low molecular weight carboxylic acid as type (iv) component. Thus, in such embodiments, up to 5 to 18 wt. % of additional type (iv) components (but different from the low molecular weight carboxylic acid) may be included in the flux according to the present invention.

[0023] Examples of halogen-containing compounds useful as activators include aniline hydrochloride, glutamic acid hydrochloride, diethanolamine hydrochloride, diethanolamine hydrobromide, triethanolamine hydrochloride, triethanolamine hydrobromide, and trans-2,3-dibromo-2-butene-1,4-diol.

[0024] Preferably, the flux according to the invention, i.e., the entire flux consisting of components (i)+(ii) or (i)+(ii)+(iii) or (i)+(ii)+(iv) or (i)+(ii)+(iii)+(iv), has a flux acid number (FMSZ) in the range of 100-250 mg KOH / g, preferably in the range of 120-220 mg KOH / g. The flux acid number referred to in the specification and examples can be determined in accordance with IPC TM-650 2.3.13 (Amendment A dated 06 / 2004). The flux acid number is formed at least substantially or entirely from the carboxyl groups of component (i) and optionally from the carboxyl groups provided by component (iv).

[0025] The present invention further comprises providing a solder paste comprising 80-92% by weight of one or more different solders and 8-20% by weight of a flux according to the present invention, i.e., a flux according to the present invention in one of the embodiments described above, wherein the sum of the weight percentage of the solder according to the present invention and the weight percentage of the flux is 100% by weight.

[0026] As mentioned above, the solder paste according to the invention comprises 80-92% by weight of one or more different solders, in particular tin-based solders (solder alloys containing at least 80% by weight, preferably at least 83% by weight, in particular 90-99.5% by weight, of tin) or bismuth / tin-based solders (solder alloys containing 50-60% by weight of bismuth and 40-50% by weight of tin).

[0027] The solder preferably has a liquidus temperature in the range of 150 to 350°C, preferably in the range of 180 to 300°C.

[0028] The solder is present in the solder paste according to the invention as solder powder, as is usual for solder pastes. The ball size of the solder balls constituting the solder powder can correspond to any of the classifications according to the IPC J-STD-005A standard, i.e. the solder paste according to the invention can have solder balls of any type and ball size within the type range T1 to T7.

[0029] Preferably, the solder paste according to the present invention has a viscosity of 50 to 250 Pa·s. The viscosity referred to in the present specification and examples is measured at 25°C and 10 s using a plate-plate rheometer (e.g., Anton-Paar Physica MCR 150 plate-plate rheometer) with a plate diameter of 50 mm and a measuring gap of 400 μm. -1 can be determined using a shear rate of

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

[0031] The method for producing a solder paste according to the present invention comprises the steps of: mixing the components of the flux according to the invention; and b. adding solder powder as described above.

[0032] The solder powder is preferably added in multiple batches, with stirring, to the prepared mixture of flux components according to the present invention, generally without heating.

[0033] The solder paste according to the present invention can be used to connect electronic components to a substrate, and can also be used to produce solder deposits on a substrate.

[0034] When connecting an electronic component to a substrate, the contact surface of the substrate and the contact surface of the electronic component are joined via the solder paste of the present invention.

[0035] A method for attaching an electronic component to a substrate using a solder paste according to the present invention comprises the steps of: a) providing an electronic component having a contact surface; b) providing a substrate having a contact surface; c) applying solder paste to contact surfaces of electronic components and / or contact surfaces of a substrate; d) joining contact surfaces of the electronic component to contact surfaces of the substrate using solder paste; e) heating the solder paste above the liquidus temperature of the solder, and then cooling and solidifying the solder to form a strong bond between the electronic component and the substrate.

[0036] Steps a) and b) are self-explanatory and do not require further explanation.

[0037] In step c), the solder paste according to the invention can be applied to one or both contact surfaces by conventional methods known to those skilled in the art, such as screen or stencil printing, or dispensing or jetting.

[0038] In step d), the contact surfaces of the electronic component and the substrate can be joined together using solder paste, i.e. a sandwich arrangement can be created from the electronic component and the substrate with solder paste between their contact surfaces.

[0039] In step e), the sandwich assembly is soldered by heating the solder paste above the liquidus temperature of the solder, so that after cooling and solidification of the solder, a robust connection is formed between the electronic component and the board via the solder paste. The solder paste is preferably heated so that the solder transitions to a liquid phase but does not damage the electronic component and / or the board. The sandwich assembly, or more precisely the solder paste, is preferably heated to a temperature 5-60°C above the liquidus temperature of the solder, preferably 10-50°C above.

[0040] Advantageously, a solder paste comprising a flux according to the invention allows soldering according to step e) in air, regardless of the ball size of the solder powder used in the solder paste. [Example]

[0041] Flux and solder paste preparation The flux ingredients were combined according to Table 1 and homogenized.

[0042] To prepare the solder paste, 11 parts by weight of the flux was mixed with 89 parts by weight of solder powder (SnAgCu: Sn 96.5 wt%, Ag 3.0 wt%, Cu 0.5 wt%, Type 4 according to IPC J-STD-005A standard) to form the solder paste.

[0043] Wetting ability under normal atmospheric conditions The wetting properties of the solder pastes were evaluated using a melting test according to test method 2.4.45 of IPC-TM-650 (1 / 95) under normal atmospheric conditions. For this purpose, the solder paste to be tested was applied to a copper plate (20 mm x 20 mm x 0.5 mm). If the copper plate had an oxide film on its surface, it was polished with P600-grade sandpaper to a bright metal finish and cleaned with alcohol. If the copper plate had a bright, clean surface, it was cleaned with alcohol only.

[0044] The prepared copper plate was printed using a stencil. To do this, the template was pressed firmly against the copper plate, with the template opening centered on the plate. The solder paste to be tested was placed on a spatula and spread over the stencil opening, first lightly and then with slightly stronger pressure, until no solder paste remained on the stencil. The stencil was then carefully peeled off, preserving the pattern defined by the stencil. The printed copper plate was placed on a first 200°C high-temperature heating plate, set below the liquidus temperature of the solder, for 2 minutes, and then immediately placed on a second heating plate, set 50°C above the liquidus temperature of the solder. After the solder paste or solder melted, the copper plate was left on the second heating plate for an additional 5 seconds, removed, and allowed to cool.

[0045] After the solder paste cooled, it was evaluated whether it melted into a spot corresponding to the size of the stencil opening or into multiple small spots, whether the solder paste had sharp edges after melting, and whether the surface was glossy or matte.

[0046] Solder pastes were classified into four classes. Class 1: The remelted area was larger than the area previously printed with solder paste. Class 2: The remelted surface corresponded to a surface previously printed with solder paste. Class 3: The remelted area was smaller than the area previously printed with solder paste (slight solder repelling was observed). Class 4: The solder paste formed one or more solder balls and did not wet the copper plate or melt completely.

[0047] Viscosity stability test at room temperature First, the initial viscosity of the freshly prepared solder paste was measured using a plate-plate rheometer (Physica MCR 150 from Anton-Paar; plate diameter 50 mm, measurement gap 400 μm) at 25 °C and 10 s-1 The viscosity was determined at a shear rate of 100 psi. The solder pastes were then stored in sealed containers at room temperature (23°C) and measured again after 7 days as described above. Before measuring the viscosity after 7 days of storage, each solder paste was briefly mixed homogeneously by hand using a spatula. The difference between the first measurement recorded immediately after preparation of the corresponding solder paste and the second measurement recorded after storing the corresponding solder paste for 7 days was evaluated as follows: ++:<5Pa·s +:5Pa·s~<10Pa·s -:>10 Pa·s [Table 1]

Claims

1. A flux, (i) 30 to 80% by weight of an acid number in the range of 100 to 350 mg KOH / g and a weight average molecular weight M in the range of 1000 to 5000 ｗ one or more acidic (meth)acrylic copolymers having the formula: (ii) 10 to 60 wt. % of at least one organic solvent; (iii) 0 to 15 wt. % of one or more amines; (iv) 0 to 20% by weight of one or more components other than components (i) to (iii); The flux has a flux acid number (FMSZ) in the range of 120 to 220 mg KOH / g.

2. The acid value of the acidic (meth)acrylic copolymer is in the range of 150 to 300 mg KOH / g, and the weight average molecular weight M ｗ 2. The flux according to claim 1, wherein the .DELTA..times ...

3. The acidic (meth)acrylic copolymer has a glass transition temperature (T ｇ 3. The flux according to claim 1 or 2, comprising:

4. A solder paste comprising 80-92% by weight of one or more solders and 8-20% by weight of the flux of claim 1.

5. 5. Use of the solder paste according to claim 4 for connecting electronic components to a substrate or for producing a solder deposit on a substrate.

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