SOLDER PASTE

Abstract

Solder paste consisting of 85 to 92% by weight of a tin-based solder and 8 to 15% by weight of a flux, wherein the flux comprises i) 30 to 50% by weight, based on its total weight, of a combination of at least two optionally modified natural resins, ii) 5 to 20% by weight, based on its total weight, of at least one low-molecular carboxylic acid; and iii) 0.4 to 10% by weight, based on its total weight, of at least one amine, and wherein the combination of the optionally modified natural resins has an integral molecular weight distribution of 45 to 70% by area in the molecular weight range of 150 to 550 and of 30 to 55% by area in the molecular weight range of >550 to 10,000 in the combined GPC.

Claims

1-15. (canceled)

16. A solder paste consisting of 85 to 92% by weight of a tin-based solder and 8 to 15% by weight of a flux, wherein the flux comprises: i) 30 to 50% by weight, based on its total weight, of a combination of at least two optionally modified natural resins; ii) 5 to 20% by weight, based on its total weight, of at least one low-molecular carboxylic acid; and, iii) 0.4 to 10% by weight, based on its total weight, of at least one amine, and, wherein the combination of the optionally modified natural resins has an integral molecular weight distribution of 45 to 70% by area in the molecular weight range of 150 to 550 and of 30 to 55% by area in the molecular weight range of >550 to 10,000 in the combined GPC.

17. The solder paste according to claim 16, wherein the solder is a soldering alloy comprising at least 80% by weight of tin.

18. The solder paste according to claim 16, wherein the soldering alloy has a liquidus temperature in a range of 200 to 250° C.

19. The solder paste according to claim 16, wherein the optionally modified natural resins are unmodified natural resins or natural resins modified by hydrogenation, dimerization, and/or esterification of their carboxyl groups.

20. The solder paste according to claim 16, wherein the combined GPC below a molecular weight of 150 and above a molecular weight of 10,000 has no signals for the at least two optionally modified natural resins.

21. The solder paste according to claim 16, wherein the at least two optionally modified natural resins differ with regard to their average molecular weight M.

22. The solder paste according to claim 21, wherein it is a combination of at least one optionally modified natural resin with an M.sub.w in the range of 150 to 550 and at least one optionally modified natural resin with an M.sub.w in the range of >550 to 10,000.

23. The solder paste according to claim 16, wherein the at least two optionally modified natural resins differ with regard to their acid number.

24. The solder paste according to claim 23, wherein the optionally modified natural resin(s) with the higher M.sub.w have an acid number in the range of 1 to 50 mg KOH/g, and the optionally modified natural resin(s) with the lower M.sub.w have an acid number in the range of 180 to 280 mg KOH/g.

25. The solder paste according to claim 16, wherein the at least one low-molecular carboxylic acid is selected from the group consisting of 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.

26. The solder paste according to claim 16, wherein the flux has an acid number in the range of 85 to 145 mg KOH/g, based on the total flux.

27. The solder paste according to claim 16, wherein the at least one amine is selected from the group consisting of 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 alkylamine, hydrogenated (tallow alkyl)dimethylamine, and hydrogenated bis(tallow alkyl)methylamine.

28. The solder paste according to claim 16, wherein the flux comprises 1 to 5% by weight of one or more thickening agents and/or 32 to 46% by weight of one or more organic solvents and/or 0.1 to 3% by weight of one or more halogen-containing compounds.

29. A use of a solder paste according to claim 16 for connecting electronic components to substrates or for producing solder deposits on substrates.

30. A method for attaching an electronic component to a substrate using the solder paste according to claim 16, comprising the steps of: a) providing an electronic component having a contact surface, b) providing a substrate having a contact surface, c) providing the contact surface of the electronic component and/or the contact surface of the substrate with the solder paste according to the invention, d) contacting the contact surface of the electronic component with the contact surface of the substrate via the solder paste; and, e) heating the solder paste above the liquidus temperature of the solder and subsequently allowing the solder to cool and solidify while forming a solid connection between the electronic component and the substrate.

Description

EXAMPLES

[0052] Test methods used:

[0053] 1. Wetting ability:

[0054] The wetting properties of solder pastes were assessed using the melting test according to IPC-TM-650 (1/95) test method 2.4.45. For this purpose, the solder pastes to be tested were applied to copper sheets (20 mm x 20 mm x 0.5 mm). If the copper sheets had an oxide layer on the surface, they were polished to the bare metal with P600 grade sandpaper and cleaned with alcohol. Copper sheets that had a bright and pure surface were cleaned only with alcohol. The prepared copper sheets were printed with the aid of a stencil. For this purpose, the stencil was pressed firmly onto the copper sheet so that the openings in the stencil were in the middle of the copper sheet. The solder paste to be tested was placed on a Japanese spatula and spread first lightly, and then with somewhat more pressure, over the openings in the stencil until there was no more solder paste left on the stencil. The stencil was then carefully removed to obtain the pattern given by the stencil. The printed copper sheet was placed for 2 minutes onto a first heating plate set at 200° C. to a temperature below the liquidus temperature of the solder and was then immediately placed onto a second heating plate with a temperature that was approx. 50° C. above the liquidus temperature of the solder (peak temperature). After the solder paste or solder had melted, the copper sheet was left on the heating plate for another 5 seconds and then removed and cooled.

[0055] After the solder paste had cooled down, an assessment was made as to whether it had melted into spots corresponding to the size of the openings in the stencil or into several small spots, and whether the solder paste had sharp edges after melting. It was assessed, furthermore, whether the surface was glossy or matte.

[0056] The solder pastes were divided into four classes, wherein classes 1 and 2 were classified as satisfactory results, while classes 3 and 4 represented unsatisfactory results:

[0057] Class 1: The re-melted area was greater than the area previously printed with solder paste.

[0058] Class 2: The re-melted area corresponded to the area previously printed with solder paste.

[0059] Class 3: The re-melted area was smaller than the area previously printed with solder paste (slight dewetting can be seen).

[0060] Class 4: The solder paste had formed one or more solder balls and had not wetted the copper sheet or was not fully melted. 2. Surface insulation resistance (SIR) test, corrosion behavior of solder joints Test method according to J-STD-004B: 2011, with the following changes: [0061] Printed circuit board according to IPC-B-24 with 200 μm line width and 200 μm distance between the lines, [0062] Ambient conditions: 65° C./93% relative humidity, [0063] 100 V corrosion voltage, 100 V measurement voltage, measurement frequency 60 minutes [0064] Test duration 1,000 h

[0065] Two criteria were used for the evaluation of the SIR tests:

[0066] Criterion 1 (visually detectable dendrite formation): When viewed under an optical microscope with a magnification of 25x, a visual check was made as to whether or not dendrites had formed in the region of the soldered SIR conductor tracks.

[0067] Criterion 2 (hourly check for undesired shortfall of a resistance, representing a stability criterion, of >100 MO): Resistance measurements were carried out hourly over a total period of time of 1,000 h. The result of the SIR test was assessed as stable when all resistances measured over the period of time were >100 MO. In contrast, the result of the SIR test was assessed as unstable when one or more of the 1,000 measured values were 00 MO.

[0068] Table 1 shows the flux composition of comparative solder pastes 1 to 6 and solder pastes 7 to 13 according to the invention. The figures in each case refer to % by weight. In each case, 11 parts by weight of these fluxes were mixed in each case with 89 parts by weight of solder powder (SnAgCu: Sn 96.5% by weight, Ag 3.0% by weight, Cu 0.5% by weight, type 3 according to standard IPC J-STD-006) in order to form solder pastes. The properties of the solder pastes were determined by the methods described above and are also summarized in Table 1.

TABLE-US-00001 TABLE 1 Flux 1 2 3 4 5 6 Modified rosin resin A, 40 40 5 35 M.sub.w = 375, AN = 240 mg KOH/g Modified rosin resin B, 40 M.sub.w = 741, AN = 6 mg KOH/g Modified rosin resin C, 40 M.sub.w = 372, AN = 10 mg KOH/g Tripropylene glycol n-butyl 39.8 29.8 39.8 39.8 39.8 39.8 ether Sebacic acid 12 22 12 12 12 12 N,N,N′,N′- 2.1 2.1 2.1 2.1 2.1 2.1 tetramethylethylenediamine N-coco-1,3-diaminopropane 2.1 2.1 2.1 2.1 2.1 2.1 Hydrogenated castor oil 4 4 4 4 4 4 Total AN of the flux (mg 162 217 70 71 81 150 KOH/g) GPC area ratio*.sup.) 95:5 95:5 10:90 88:12 21:79 84:16 Wetting ability 4 2 3 3 3 2 (class) SIR (dendrite formation) Yes Yes No No No Yes SIR (resistance > 100 MΩ) Stable Unstable Unstable Unstable Unstable Stable Flux 7 8 9 10 11 12 13 Modified rosin resin A, 18 20 27 22 18 21.5 18 M.sub.w = 375, AN = 240 mg KOH/g Modified rosin resin B, 22 20 13 22 18 21.5 18 M.sub.w = 741, AN = 6 mg KOH/g Tripropylene glycol n-butyl 39.8 39.8 39.8 39.8 39.8 39.5 40 ether Sebacic acid 12 12 12 8 16 12 12 N,N,N′,N′- 2.1 2.1 2.1 2.1 2.1 0.75 4 tetramethylethylenediamine N-coco-1,3-diaminopropane 2.1 2.1 2.1 2.1 2.1 0.75 4 Hydrogenated castor oil 4 4 4 4 4 4 4 Total AN of the flux (mg 111 116 131 99 133 119 111 KOH/g) GPC area ratio*.sup.) 48:52 53:47 67:33 53:47 53:47 53:47 53:47 Wetting ability 2 2 2 3 2 2 2 (class) SIR (dendrite formation) No No No No Yes No No SIR (resistance > 100 MΩ) Stable Stable Stable Stable Unstable Stable Stable *.sup.)Area ratio of the molecular weight ranges 150 to 550/551 to 10,000 in the GPC of the rosin resin/in the combined GPC of the rosin resins