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(11) | EP 0 091 498 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Tin or tin-lead alloy electroplating bath |
| (57) Tin or tin-lead alloy electroplating bath which comprises (i) a stannous salt or
a mixture of a stannous salt and a lead salt, (ii) at least one compound selected
from the group consisting of gluconic acid, glucoheptonic acid, alkali metal salts
of gluconic acid and glucoheptonic acid, gluconic lactone and glucoheptonic lactone,
and (iii) a surface active agent. |
BACKGROUND OF THE INVENTION:
Field of the Invention:
Prior Art:
[0002] In the past, a sulfate or borofluoride bath has been used as a tin plating bath and a borofluoride bath has been used as a tin-lead alloy plating bath.
[0003] However, borofluorides are compounds which form wasteproducts, thereby causing pollution problems unless the waste liquids are pre-treated before they are discharged. However, it is required a lot of expenses for the running cost and installation investment for processing the waste liquids discharged from the boro- x fluoride-containing electrolyte and for processing the rinsing water used to rinse the plated products. Further, difficulties are encountered in maintenance of the installation or equipment, and additionally the materials for the equipment are limiteds because borofluorides have intense corrosive actions. Furthermore, the use of borofluorides is not preferred from the environmental and health point of view because harmful gas is generated during the processing.
[0004] On the other hand, a variety of electroplating baths containing no borofluorides such as a bath contain-5 ing a sulfonate, a pyrophosphate or a sulfamate as the primary component have been proposed. However, these known electroplating baths are not satisfactory when used as the bath in which tin or a tin-lead alloy is deposited, because they have poor uniform electrodeposition properties as regards uniform deposition and hardly give a smooth and dense deposited coating.
SUMMARY OF THE INVENTION:
[0005] It is, therefore, a primary object of this invention to provide a composition for use in an electroplating liquid for depositing tin or a tin-lead alloy to form an electroplated coating having excellent properties.
[0006] Another object of this invention is to provide the composition as aforementioned which does not cause any pollution or health problem.
[0007] The composition provided by this invention comprises (1) a stannous salt or a mixture of a stannous salt and a lead salt, (ii) at least one compound selected from the group consisting of gluconic acid, glucoheptonic acid, alkali metal salts of gluconic acid and glucoheptonic acid, gluconic lactone and glucoheptonic lactone, and (iii) a surface active agent. According to a further advantageous feature of this invention, hydrazine or a salt thereof is added to the aforementioned composition.
DESCRIPTION OF THE INVENTION:
[0008] The tin and lead salts used in the present invention are not specifically limited, and are those which are generally used in a tin or a lead electroplating bath. Representative examples of the stannous salt include stannous chloride, stannous sulfate and stannous acetate; and representative examples of the lead salt include lead nitrate, lead acetate and lead carbonate.
[0009] The amount of said metal salts used in the electroplating liquid may be in the range of from 1 to 40 g/1, preferably 5 to 20 g/l, of metallic tin and 0.5 to 40 g/ls preferably 1 to 15 g/1, of metallic lead. Preferably,the total amount of metallic tin and lead may be not more than 50 g/l. If the content of the metal salts exceeds the aforementioned range, the viscosity of the liquid is increased, which results in increase in the volume of the entrained liquid, which is uneconomical.
[0010] Examples of the alkali salts of gluconic acid and glucoheptonic acid used in this invention include sodium gluconate, potassium gluconate, sodium glucoheptonate and potassium glucoheptonate. The term
[0011] "lactone" used in this specification and appended claims means a compound having an ester functional group in the ring, and it is classified into β-lactones, γ-lactones, 6-lactones and the like according to the number of the ring constituting atoms. A compound having an intramolecular hydroxy group and an intramolecular carboxylic group, such as gluconic acid or glucoheptonic acid, readily forms an intramolecular ester or a lactone by heating or other means. In an aqueous solution, free gluconic acid, γ- and δ-lactones can exist only in the form of an equilibrium mixture of the three compounds. The composition of the equilibrium mixture in an aqueous solution varies depending on solvent, pH, concentration,temperature and so on. A similar phenomenon is observed with glucoheptonic acid.
[0012] A suitable amount of gluconic acid, glucoheptonic acid, salts thereof or mixtures thereof ranges from 50 to 300 g/1, preferably 100 to 200 g/l. If the amount is less than 50 g/1, the applicable range of the current density for obtaining a dense plated coating becomes narrower; whereas it is uneconomical to increase the amount to more than 300 g/l because no further improvement in the properties of plated coating is obtained.
[0013] According to the present invention, a surface active agent is used to act effectively to deposit finely divided particles to form a denser coating, whereby the formation of branch-shaped or sponge-like coating and the formation of pits or pin-holes are suppressed. Specific examples of the anionic surface active agents used in the present invention are preferably sulfates of polyoxyethylenealkyl ethers having 2 to 4 moles of added ethylene oxide per molecule. Suitable non-ionic surface active agents include polyoxyethylenealcohol ethers, polyoxyethylene- alkylphenol ethers, esters of polyoxyethylenealiphatic acids and polyoxyethylenepolypropylene-block polymers, the preferable molar number of the added ethylene oxide being not less than 8. Examples of preferred caticnic surface active agent are polyoxyethylene adducts of aliphatic amine and polyoxyethylene adducts of amides, each having not less than 8 moles of the added ethylene oxide.
[0014] Any one of the aforementioned surface active agents may be used singly, or two or more of them may be used in combination. The suitable content thereof ranges between 0.05 and 20 g/1, preferably between 1 and 10 g/l. If the content is less than 0.05 g/1, the effect for obtaining a dense plated coating attained by the addition of surface active agent does not reach a satisfactory level; whereas, in case of more than 20 g/1, no further improvement in the properties of the resultant coating can be realized with the attendant disadvantage that the foaming tendency of the bath is exceedingly increased.
[0015] In a further preferred embodiment of this invention, hydrazine or a salt thereof is added to prevent divalent tin from being oxidiziug to tetravalent tin with the concurrent advantage that the formation of a branch-shaped coating at high current density portion can be eliminated whereby a dense plated coating can be obtained. A further advantageous effect obtainable by the addition of surface active agents is that the anode or positive electrode made of tin or an tin-lead alloy is prevented from being converted into a passive state, and that the solubility of the anode and the conductivity of the electroplating liquid are improved. The suitable content of hydrazine or of a salt thereof ranges from 2 to 100 g/l, preferably from 10 to 50 g/l. The advantageous effects are not obtained satisfactorily if the content is less than 2 g/l, whereas no additional improvement in the properties of the formed coating is recognized even if the content exceeds 100 g/l. The suitable pH value of the electroplating liquid according to this invention ranges from 2.0 to 10.0, preferably 3.0 to 7.0. If the pH value.is lower than 2.0 or higher than 10, it becomes difficult to obtain a dense coating. Any known alkaline compounds or acids, such as sodium hydroxide, potassium hydroxide, aqueous ammonia or hydrochloric acid, sulfuric acid, nitric acid, may be used to adjust the pH value if desired. The suitable cathode current density ranges from 0.1 to 6 A/dm2, preferably from 1 to 3 A/dm2. The suitable temperature of the-bath ranges from 20 to 60°C, preferably 30 to 55°C. It is preferred that the bath is agitated by fluidization agitation means using, for instance, a cathode rocker. Air agitation is not possible since the salt of tin (II) is oxidized to the corresponding salt of tin (IV).
[0016] An anode made of tin is used in the tin electroplating and an anode made of an alloy of tin and lead which has the same composition as that of the deposited coating to be formed is used in the tin-lead alloy electroplating.
[0017] The tin or tin-lead alloy electroplating liquid according to this invention has the advantage that, as compared to the conventional fluoroborate bath, it requires easier operations for treating the waste liquid and has no corrosive property.
[0018] The electroplating liquid of this invention is also improved in that the plated coating formed is superior in uniformity, and that the resultant deposited coating forms a semigloss surface which is smooth and dense and has no pits or pin-holes. In the tin-lead alloy electroplating bath of this invention, the composition of the resultant deposited coating can be easily controlled by simply varying the ratio of the metallic components, i.e. the ratio of tin to lead, in the electroplating liquid, with an additional advantage that the fluctuation in composition of the plated alloy due to the change in current density is minimized.
[0019] The present invention will now be described in more detail by referring to the following Examples.
Example 1
[0021] The bath above was used to plate a copper plate at 40°C at a current density of 1 A/dm2 for 10 minutes. A white and semigloss plated coating having a smooth and dense surface was obtained.
Example 2
[0023] The bath above was used to plate a steel plate at 50°C at a current density of 3 A/dm2 for 5 minutes. A white and semigloss plated coating having a smooth and dense surface was obtained.
Example 3
[0025] The bath above was used to plate a copper plate at 50°C at a current density of 1 A/dm2 for 20 minutes. A semigloss coating having a smooth and dense surface was obtained. The content of lead in the coating was 40%.
Example 4
[0027] The bath above was used to plate a steel plate at 40°0 at a current density of 3 A/dm2 for 10 minutes. A semigloss coating having a smooth and dense surface was obtained. The content of lead in the coating was 28%.
Example 5
[0029] The bath above was used to plate a brass plating previously plated with copper at 55°C at a current density of 4 A/dm2 for 15 minutes with cathode rocker agitation. A semigloss plated coating having a smooth and dense surface was formed. The content of lead in the resultant coating was 38%.
Example 6
[0031] The bath above was used to plate a copper plate at 30°C at a current density of 1 A/dm2 for 15 minutes. A semigloss coating having a smooth and dense surface was obtained. The content of lead in the coating was 15%.
Example 7
l. Tin or tin-lead alloy electroplating bath comprising:
(i) a stannous salt or a mixture of a stannous salt and a lead salt;
(ii) at least one compound selected from the group consisting of gluconic acid, glucoheptonic acid, alkali metal salts of gluconic acid and glucoheptonic acid, gluconic lactone and glucoheptonic lactone; and
(iii) a surface active agent.
2. The bath as defined in claim 1, further comprising hydrazine or a salt thereof.