No-clean low-residue solder paste for semiconductor device applications

Abstract

A no-clean low residue solder paste is described for die-attach dispensing or fine pitch printing of semiconductor devices. The solder paste has a homogeneous consistency with no tendency to paste separation. The ultimately remaining residue is clear and crystal like and is compatible with further processing steps without prior cleaning with a solvent to remove flux residue. The solder paste comprises a relatively low amount of rosin in combination with a viscous solvent system, thixotropic agents, activators, additives and optionally plasticizers.

Description

EXAMPLE 1

In this example a solder paste according to the invention was prepared which can be applied to a workpiece by dispensing.

For preparing the solder paste 85 parts by weight of the solder alloy Sn5Pb92.5Ag2.5 (−325 to +500 mesh) were mixed with 15 parts by weight of a flux.

The flux consisted of 30 wt.-% of isobornyl cyclohexanol, 5 wt.-% of 1,2-octanediol (very viscous solvent and solid solvent), 13 wt.-% of butyl carbitol (ethylene glycol ether), 14 wt.-% of 2-ethyl-1,3-hexanediol, 4 wt.-% of isopropanol (alcohol), 16 wt.-% of Foral AX (hydrogenated rosin, trade name of company Hercules), 3 wt.-% of stearamide (thixotropic agent), 5 wt.-% of 4-n-butylbenzoic acid, 5 wt.-% of salicylic acid (activator), 4 wt.-% of diethanolamine (activator), and 1 wt.-% of Polyrad 1110 (additive, trade name of company Hercules).

Each component of the flux was added, heated, and dissolved in a glass beaker using a mechanical stirring blade to make a solution. After heating and mixing all the different solvents became a homogenous mixture. The paste was made by mixing the solder powder uniformly with the flux. Brookfield viscosity of the flux was 30 to 100 kcps, and 200 to 300 kcps for the paste.

The no-clean paste made by this flux shows very good dispensability, solderability and wetting characteristic, also it showed very good paste stability (no separation occurs during application) with about 2% of residue remaining.

EXAMPLE 2

In this example a solder paste according to the invention was prepared which can be applied to a workpiece by fine pitch printing.

For preparing the solder paste 88 parts by weight of the solder alloy Sn96.5/Ag3.5 (−325 to +500 mesh) were mixed with 12 parts by weight of a flux.

The vehicle consisted of 20 wt.-% of isobomyl cyclohexanol, 6 w.-t % of trimethylopropane (very viscous solvent and solid solvent, 27 wt.-% of terpineol, 5 wt.-% of 2-ethyl-1,3-hexanediol, 5 wt.-% of isopropanol (alcohol), 16 wt.-% of Foral AX (hydrogenated rosin), 5 wt.-% of stearamide (thixotropic agent), 5 wt.-% of 4-n-butylbenzoic acid, 5 wt.-% of benzoic acid, 4 wt.-% of diethanolamine (activator), 1 wt.-% of Polyrad 1110 (additive), and 1 wt.-% of dimethylphthalate (plasticizer).

Each component of the flux was added, heated, and dissolved in a glass beaker using a mechanical stirring blade to make a solution. After heating and mixing all the different solvents became a homogenous mixture. The paste was made by mixing solder powder uniformly with flux. Brookfield viscosity of the flux was 20 to 80 kcps, and 600 to 800 kcps for the paste.

The no-clean paste made by this flux shows very good printability, solderability and wetting characteristic, also it shows very good paste stability (no separation occurs during application) with about 2% of residue remaining.

While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to essential features hereinbefore set forth and as follows in the scope of the appended claims.

Claims

1. Solder paste for die-attachment of semiconductor devices comprising a solder powder dispersed in a flux, the flux comprising a homogeneous mixture of a solvent system, at least one thixotropic agent, at least one activator, and a rosin, wherein the solvent system comprises a highly viscous solvent and/or solid solvent at room temperature and the concentration of the solvent system amounts to 55 to 75 wt.-% and the concentration of the rosin amounts to 10 to 25 wt.-%, each relative to the total weight of the flux.

2. Solder paste according to claim 1, wherein the solvent system further comprises one or more glycol ether, and one or more alcohols.

3. Solder paste according to claim 2, wherein the highly viscous solvent and/or solid solvent are selected from the group consisting of trimethylopropane, 1,2-octanediol, 1,8-octanediol, 2,5-dimethyl-2,5-hexanediol, isobornyl cyclohexanol or a combination thereof.

4. Solder paste according to claim 3, wherein the glycol ether is selected from the group consisting of mono-, di- or tri-propylene glycol methyl ether, mono-, di-, or tri-propylene glycol n-butyl ether, mono-, di-, or tri-ethylene glycol n-butyl ether, ethylene glycol methyl ether, tri-ethylene glycol methyl ether, di-ethylene glycol di-butyl ether, tetra-ethylene glycol di-methyl ether, or a combination thereof.

5. Solder paste according to claim 4, wherein the alcohols are selected from the group consisting of 2-ethyl-1,3-hexanediol, n-decyl alcohol, 2-methyl-2,4-pentanediol, terpineol and isopropanol or mixtures thereof.

6. Solder paste according to claim 5, wherein the rosin is selected from the group consisting of tall oil rosin, hydrogenated rosin, partially hydrogenated rosin, dehydrogenated rosin or a combination thereof.

7. Solder paste according to claim 6, wherein the thixotropic agent is selected from the group consisting of glyceryl tris-12-hydroxy stearate, modified glyceryl tris-12-hydroxy stearate, polyamide, stearamide or a combination thereof.

8. Solder paste according to claim 7, wherein the activator comprises organic acids selected from the group consisting of caproic acid, phenyl-acetic acid, benzoic acid, salicylic acid, aminobenzoic acid, 4-n-butylbenzoic acid, 4-t-butylbenzoic acid, 3,4-dimethoxybenzoic acid, oxalic acid, succinic acid, maleic acid, malic acid, adipic acid, malonic acid and mixtures thereof and the activator further comprises an amine selected from the group consisting of monoethanolamine, diethanolamine, triethanolamine, isopropanolamine or a combination thereof.

9. Solder paste according to claim 8, wherein the paste further comprises an additive selected from the group consisting of ethoxylated amine rosin, amine rosin, methyl ester of rosin, n-oleylsarcosine and oleyl imidazoline or mixtures thereof.

10. Solder paste according to claim 9, wherein the flux comprises 1 to 6 wt.-% of thixotropic agent, 10 to 20 wt.-% of activators and 0.5 to 3 wt.-% of additives each relative to the total amount of flux.

11. Solder paste according to claim 10, wherein the solvent system comprises 30 to 50 wt.-% of a very viscous solvent and/or solid solvent at room temperature, 10 to 30 wt.-% of glycol ether and 10 to 20 wt.-% of alcohols, and the activators comprise 5 to 15 wt.-% of an acid and 1 to 10 wt.-% of an amine, each relative to the total amount of flux.

12. Solder paste according to claim 11, wherein the solder powder is selected from lead containing solder alloys or lead free solder alloys.

13. Solder paste according to claim 12, wherein the lead containing solder alloy is selected from PbSn—, PbSnAg—, PbAg— or PbInAg-alloys with at least 85 wt.-% of lead.

14. Solder paste according to claim 9, wherein the paste further comprises a plasticizer selected from the group consisting of dimethyl phthalate, dibutyl phthalate, diisodecyl phthalate, butyl oleate, diisobutyl adipate, dioctyl adipate, dipropylene glycol dibenzoate, dioctyl sebacate and mixtures thereof.

15. Solder paste according to claim 14, wherein the flux comprises 1 to 6 wt.-% of thixotropic agent, 10 to 20 wt.-% of activators and 0.5 to 3 wt.-% of additives, and 0.1 to 2 wt.-% of plasticizers, each relative to the total amount of flux.

16. Solder paste according to claim 15, wherein the solvent system comprises 20 to 40 wt.-% of a very viscous liquid and/or solid solvent at room temperature, 30 to 50 wt.-% of alcohols, and the activators comprise 5 to 15 wt.-% of an acid and 1 to 10 wt.-% of an amine, each relative to the total amount of flux.

17. Solder paste according to claim 16, wherein the solder powder is prepared from a lead containing or a lead free solder alloy.

18. Solder paste according to claim 17, wherein the lead containing solder alloy is selected from SnPb—, SnPbAg—, SnPbBi—, and PbSnSb-alloys.

19. Solder paste according to claim 17, wherein the lead free solder alloy is selected from SnAgCu—, SnAg—, SnCu—, SnCuIn—, SnAgCuSb—, SnAgSb—, SnSb— or BiSn-alloys.

20. Solder paste for die-attachment of semiconductor devices according to claim 1, wherein the highly viscous solvent and/or solid solvent of the solvent system are vaporized during reflow soldering process.