WO1995009255A1

WO1995009255A1 - Method of protecting solderable copper and copper-alloy surfaces from corrosion

Info

Publication number: WO1995009255A1
Application number: PCT/EP1994/003140
Authority: WO
Inventors: Ulrich Schröder; Norbert Wagner; Harald Keller; Jürgen DEMBOWSKI; Veronika Kohl
Original assignee: Basf Aktiengesellschaft
Priority date: 1993-09-29
Filing date: 1994-09-20
Publication date: 1995-04-06
Also published as: DE4333127A1

Abstract

Proposed is a method of protecting solderable copper and copper-alloy surfaces from atmospheric corrosion, the method calling for the surface of the metal to be contacted with a solution of one or more sulphur compounds of the formula (I): R?2-A1-S-R1¿, in which R1 is hydrogen or a group of the formula: -S-A?2-R3, A1 and A2¿ are straight-chain or branched-chain C¿4?-C30 alkyl groups and R?2 and R3¿ are hydrogen atoms or hydroxyl groups, the surface then being rinsed if necessary and finally dried.

Description

Process for protecting solderable copper and copper alloy surfaces from corrosion

description

The present invention relates to an improved method for protecting solderable copper and copper alloy surfaces, the solderability of which is maintained or even improved after the implementation of this method and after storage under the influence of the atmosphere.

Storage under the influence of the atmosphere affects the appearance of copper objects and the solderability of the copper, which is a major problem in the field of electronics and here in particular in the case of printed circuit boards.

Exposed copper surfaces on printed circuit boards are usually protected mainly by hot tinning or by galvanic deposition of tin or a lead / tin alloy and are thus kept solderable. Following the galvanic deposition, however, a remelting process is generally necessary.

In order to avoid these technically complex and therefore expensive hot tinning and deposition processes, simpler methods have been proposed which serve to temporarily maintain the solderability of exposed copper surfaces on printed circuit boards. These procedures involve treating copper surfaces with solutions that contain certain protective agents.

For example, DE-A 20 03 175 (1) relates to the use of 2-alkylimidazoles with a long straight-chain alkyl group having 5 to 21 carbon atoms in the 2-position and hydrogen or a lower alkyl group in the 4-position or the like Acid addition salts as protective agents for copper and copper alloy surfaces to be protected from the effects of the atmosphere.

EP-A 178 864 (2) relates to a process for the production of printed circuit boards using aqueous solutions of acid addition salts of 2-alkylimidazoles, as described in (1).

The disadvantage of using such 2-alkylimidazoles is the low temperature stability of the protective layers produced therewith. If a circuit board equipped with such a protective device for curing the adhesive of surface-mountable If components are heated or if two solder layers are to be fitted, the protective layer is destroyed, the products of thermal degradation impairing or preventing subsequent soldering.

PE Laibinis and GM Whitesides describe in J. Am. Chem. Soc. 1992, Vol. 114, pp. 9022-9028 (3) the slowdown in the oxidation of copper by adsorption of n-alkanethiols in a monomolecular layer on the metal surface. Cβ- to C ₂₂ -n-alkanethiols are mentioned as suitable sulfur compounds. The sulfur compounds are applied to the copper surfaces as a solution, for example in isooctane.

Y. Yamamoto, H. Nishihara and K. Aramaki describe in

J. Electrochem. Soc, Vol. 140, No. 2 (1993), pp. 436-443 (4) also the corrosion protection effect of C ₆ - to Cia-alkanethiols and of ω-hydroxyalkanethiols for copper surfaces by forming monomolecular layers. 1-Hexanethiol, 1-dodecanethiol, 1-octadecanethiol, 1-docosanethiol and 11-mercapto-1-undecanol were tested as such sulfur compounds. The sulfur compounds were applied to the copper surfaces as a solution, for example in ethanol, acetonitrile or water.

The object of the present invention was to provide a method in which protection for copper and copper alloy surfaces is produced by treatment with the solution of a suitable active ingredient, the protective layer produced being temperature-stable and, after thermal stress, a Soldering on such a surface is prevented as little as possible.

Accordingly, a process has been found for protecting solderable copper and copper alloy surfaces from corrosion under the influence of the atmosphere, which is characterized in that the metal surfaces are treated with a solution of one or more sulfur compounds of the general formula I

R -A'-SR ¹ (I)

in the

R ¹ denotes hydrogen or a radical of the formula -SA ² -R ³ , A ¹ and A ^{2 are} straight-chain or branched C - to C ₃ o-alkyl groups and

R ² and R ^{3 represent} hydrogen or hydroxyl groups,

brings in contact, rinsed if necessary and then dries.

Due to the negative experiences with the 2-alkylimidazoles according to (1) or (2), it was surprising that the process according to the invention can achieve a considerably greater corrosion protection effect and a considerably better preservation of the solderability. In some cases, solderability can not only be maintained, but even improved.

It was particularly surprising here that the solderability is not impaired by the organic molecules or their degradation products covalently bonded as described in (3) and (4) after treatment of the coated copper and copper alloy surfaces at high temperatures.

The method according to the invention can even be used on unpurified, i.e. not pre-cleaned copper and copper alloy surfaces with good results.

The method according to the invention is preferably used for the corrosion protection of printed circuit boards.

Suitable sulfur compounds I are:

Alkanthiols (R ¹ = R ² = hydrogen)

ω-hydroxyalkanthiols (R ¹ = hydrogen, R ² = hydroxyl)

- alkyl disulfides (R ^{1 →} → SA ² -R ³ , R ² = R ³ = hydrogen)

ω-hydroxyalkyl disulfides (R ^{1 =} -SA ² -R ³ , R ² = hydroxyl, R ³ = hydrogen)

- ω, ω'-dihydroxyalkyl disulfides (R ^{1 =} -SA ² -R ³ , R ² = R ³ = hydroxyl).

Of the groups listed above, the ω-hydroxyalkanethiols of the formula HO-A ¹ -SH and the symmetrical ω, ω'-dihydroxyalkyl disulfides of the formula HO-A ¹ -SSA ¹ -OH are particularly preferred. The alkylene groups A ¹ and A ² are usually only slightly branched or preferably straight-chain alkyl groups with 4 to 30, preferably 6 to 24, in particular 7 to 18, especially 8 to 12 carbon atoms. With branched alkylene groups there are usually up to three methyl or ethyl side chains.

Particularly preferred for A ¹ and A ^{2 are} polymethylene groups of the formula - (CH) _n -, i where n is 6 to 24, in particular 7 to 18, especially 8 to 12.

The alkylene groups A ¹ and A ² can be the same or different, but preference is given to the same groups A ¹ and A ² because the corresponding compounds I are easier to prepare.

Suitable solvents for the compounds I are, above all, alcohols, e.g. Methanol, ethanol, iso-propanol or ethylene glycol, ether, e.g. Tetrahydrofuran, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether (butyl glycol), dipropylene glycol monomethyl ether or dipropylene glycol monoethyl ether, ketones, e.g. Acetone, butanone or N-methylpyrrolidone, carbonic acid esters, e.g. Carbonic acid propylene glycol esters, polyethylene glycols, carboxylic acid esters, e.g. Ethyl acetate or butyl acetate, or mixtures thereof. Solvent systems with low vapor pressure, in particular dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether or ethylene glycol monobutyl ether (butyl glycol) are preferred.

The solutions are generally applied by setting these treatment solutions to a temperature of 15 to 60 ° C., preferably 15 to 55 ° C., in particular 20 to 40 ° C., and thereby the copper and copper alloy surfaces for a time of at least 5 sec, preferably 10 to 60 sec, in particular 15 to 40 sec, in contact. The easiest way to bring them into contact is to immerse the object to be protected in the solution, but it is also possible to spray or brush the object with the solution.

To avoid an excess of active substance, the surface of the article can be rinsed after the treatment with the other suitable solvent or water used, but preferably with the solvent or water used. Rinsing is not absolutely necessary. Rinsing takes place, for example, by immersing the object in the solvent or water, or by transferring the solvent or water over the surface of the object. Then the surface of the object is Licher dried, for example at room temperature in air or in a drying cabinet at about 40 to 100 ° C.

The content of the compounds I in the solutions is usually 0.01 to 10% by weight, preferably 0.1 to 5% by weight, in particular 0.5 to 3% by weight. If the contents are less than 0.01% by weight, no protective effect can be demonstrated, and if the contents are greater than 10% by weight, an increased impairment of the solderability of the metal surfaces is observed after thermal stress.

The solutions described are suitable for the effective protection of all metal surfaces, which consist primarily of pure copper, but also of copper alloys, for example also of brass surfaces. In the process according to the invention, it is also possible to use relatively low-concentration treatment solutions, as a result of which active substance can be saved. The method according to the invention is of particular importance for printed circuit board technology. The protective layers produced on the objects with copper-containing surfaces are temperature stable and do not hinder the soldering processes even after thermal stress.

Examples

Solutions of the test substances were made. The chemical structure of the test substances, the solvent used in each case and the concentration of the test substances in the solution are given in the tables below.

Drilled, copper-clad PCB base material pieces with a size of approx. 1 x 10 cm were 20 sec at 30 ° C in a solution consisting of 25 ml of conc. Sulfuric acid, 5 ml of 30% hydrogen peroxide, 1 drop of 5% by weight aqueous copper sulfate solution and 75 ml of water, etched, rinsed with water and compressed air dried.

An etched circuit board base material piece was immersed in the solution of the test substance at 30 ° C. for 20 seconds and, after dripping, was dried in air at room temperature.

The PCB base material pieces treated in this way were subjected to the following tests: Test A (steam test)

The dipped and dried base material piece was in a test tube with a diameter of about 3 cm and a volume of 100 ml, which contained about 5 ml of water and a boiling stone and immersed in a heating bath at a temperature of 110 ° C, using a Thread and a glass rod hung for 45 minutes without being immersed in the water.

Test B (heat test)

The base piece of material was stored in the drying cabinet at 200 ° C. for 15 minutes after test A.

Test C (solderability test)

The solderability of the copper-clad piece of base material was checked by tinning in a vertical hot-tinning machine (soldering temperature: 240 ° C; immersion time: 4 sec; flux: 85% by weight of a commercially available mixture of several alkoxylates / 0.45% by weight). -% glutamic acid hydrochloride / water ad 100 wt .-%).

Table 1 shows the results obtained.

Table 1

Maintaining the solderability of copper-clad printed circuit board base material

E.g. substance conc. Solvent tests grade no. [% By weight]

1 HO- (CH ₂ ) ιo-SH 5 butyl glycol A + C 1

2 HO- (CH ₂ ) ιo-SH 0.1 dipropylene glycol A + C 1-2 monoethyl ether

3 HO- (CH ₂ ) ιo-SH 1 Dipropylenglykolmo¬ A + C 1 noethyl ether

4 HO- (CH _? ) ₈ -SH 5 butyl glycol A + C 1-2

5 [HO- ⁽ CH ₂ ⁾ ιo-S] ₂ 1 butyl glycol A + C 1-2

6 as example no.1 A + B + C 2

7 as example no.2 A + B + C 1-2

8 as example no.3 A + B + C 1-2

9 HO- (CH ₂ ) i ₂ -SH 1 butyl glycol A + B + C 2

10 HO- (CH ₂ ) ₆ -SH 5 butyl glycol A + C 2-3

For comparison:

A 2-Undecylimidazole 2 H ₂ 0 / HOAc A + C 3

B as for example A A + B + C 4

H ₂ 0 / HOAc: approx. 0.2 wt. -% aqueous acetic acid solution (pH 4 to 4, 5)

Grading:

Grade 1 = perfect tinning

Grade 2 = slight errors (low dewetting)

Grade 3 = moderate defects (dewetting, very low non-wetting) Grade 4 = very strong defects (almost no wetting with solder).

The effect of the method according to the invention can also be shown on printed circuit boards themselves. For this purpose, plated-through, tin-free printed circuit boards were used instead of copper-clad printed circuit board base material. As described above, these were etched, immersed and subjected to the tests mentioned. The circuit boards were then sent through a wave soldering machine (test D) without assembly (soldering temperature: 240 ° C; transport speed: 0.9 m / min). A 2% by weight solution of adipic acid in isopropanol was used as the activator.

The results are shown in Table 2.

Table 2

Maintaining the solderability of non-tinned circuit boards

E.g. substance tests note no.

11 as example no. 1 D 1

12 as example no.2 B + D 1

13 as Example No. 3 B + D 1-2

14 as example no.2 A + B + D 1

15 as example no.9 A + B + D 1

For comparison:

C like Ex. AA + B + D 3-4 Grading:

Grade 1 = perfect soldering; Solder rises in the pads; SMD surfaces (contact surfaces for surface-mountable components) completely wetted Note 2 = soldering with slight defects; SMD areas with less

Dewetting grade 3 = soldering with defects; Solder does not rise in the solder eyes; SMD surfaces with clear dewetting

Grade 4 = badly disturbed soldering; SMD surfaces and pads only partially wetted with solder.

Claims

claims

1. Process for the protection of solderable copper and copper alloy surfaces against corrosion under the influence of

Atmosphere, characterized in that the metal surfaces with a solution of one or more sulfur compounds of the general formula I

R ² -A ¹ -SR ¹ (I)

in the

R ¹ denotes hydrogen or a radical of the formula -SA ² -R ³ ,

A ¹ and A ^{2 are} straight-chain or branched C - to C o ~ alkyl groups and

R ² and R ^{3 represent} hydrogen or hydroxyl groups,

brings in contact, rinsed if necessary and then dries.

2. The method according to claim 1, characterized in that it is used for the corrosion protection of printed circuit boards.

3. The method according to claim 1 or 2, characterized in that the sulfur compounds I used are those in which A ¹ and A ² polymethylene groups of the formula - (CH ₂ ) _n ~, i ⁿ of n is 6 to 24, mean.

4. Process according to claims 1 to 3, characterized in that the sulfur compounds I ω-hydroxyalkanthiols of the formula HO-A ¹ -SH or symmetrical ω, ω'-dihydroxyalkyl disulfides of the formula HO-A ¹ -SSA ^1st -OH sets in.

5. Process according to claims 1 to 4, characterized gekennzeich¬ net that as a solvent for the compounds I see organic solvents or mixtures of organic solvents or mixtures of organic solvents with water.

6. The method according to claims 1 to 5, characterized gekennzeich¬ net that the solutions of the compounds I at a temperature of 15 to 60 ° C and hereby brings the Metall¬ surfaces in contact for a time of at least 5 sec.

7. The method according to claims 1 to 6, characterized gekennzeich¬ net that solutions of the compounds I are used with a content of 0.01 to 10 wt .-%.

Use of solutions of one or more sulfur compounds I according to claim 1, 3 or 4 for protecting solderable copper and copper alloy surfaces from corrosion under the influence of the atmosphere.