Process of tin plating by immersion

Claim 2

A process for depositing a smooth, even tin coating on a metallized surface, said process comprising immersing into a tin plating bath comprising a soluble stannous salt, a sulfur component which comprises a mixture of alkali metal polysulfides and at least one other sulfur-containing compound, a mineral acid, and a wetting agent, wherein said alkali metal polysulfides are present in sufficient amount to produce said smooth, even coating, an article having a metallized surface capable of chemically displacing tin from the tin plating bath, wherein the article is immersed in the bath until tin forms in a continuous coating on said metallized surface.

Claim 3

The process as defined in claim 1 wherein the stannous salt is selected from the group consisting of stannous salts of halides, sulfate, fluoborate, nitrate and acetate; the wetting agent is selected from the group consisting of organic anionic, non-ionic and cationic surfactants; and said mineral acid is selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid.

Claim 4

The process as defined in claim 1 wherein said stannous salt is stannous chloride, said sulfur component contains thiourea and alkali metal polysulfides, said mineral acid is hydrochloric acid and said wetting agent is a fluorinated carboxylic acid.

Claim 5

The process of claim 3 wherein the alkali metal polysulfides are present at a concentration of 0.005 to 0.2 g/l of the bath composition.

Claim 6

The process as defined in claim 3 wherein the ingredients of said tin plating bath are present in the following concentrations: ______________________________________ Stannous chloride 21 g/l Thiourea (chemically pure) 90 g/l Alkali metal polysulfide 0.1 g/l Hydrochloric acid 36 mg/l Fluorinated carboxylic acid 0.5 g/l Water Balance ______________________________________

Claim 7

The process as defined in claim 3 wherein said fluorinated carboxylic acid is present in a concentration of from about 0.1 to about 5 g/l.

Claim 8

The process as defined in claim 1 wherein said bath is operated at a temperature of 50°-80° C.

Claim 9

The process as defined in claim 1 wherein said sulfur-containing compounds are selected from organic and inorganic sulfur compounds, said organic sulfur compounds selected from the group consisting of aliphatic sulfur-nitrogen compounds, 5- and 6-membered heterocyclic compounds containing S-N in the ring, dithiols, thio derivatives of alkyl glycols, thioamine acids; and said inorganic sulfur compounds selected from the group consisting of alkali metal sulfides, alkali metal thiocyanates, and alkali metal dithionates.

Claim 10

The process as defined in claim 1 wherein said tin plating bath comprises the components in the following concentrations: ______________________________________ Soluble stannous salt 15 - 30 g/l Sulfur component 15 - 120 g/l Mineral acid 25 - 50 ml/l Wetting agent 0.1 - 10 g/l Water Balance ______________________________________

Claim 11

The process as defined in claim 8 wherein said sulfur component is present in an amount of 50-120 g/l.

Claim 12

The process as defined in claim 1 wherein said sulfur component contains thiourea and alkali metal polysulfides and wherein said alkali metal polysulfides are present in the range of 0.005 to 0.2 g/l of the bath composition.

Claim 13

A process for the manufacture of printed circuit boards having a smooth, even tin coating over areas of clean copper circuitry comprising the steps of: (1) immersing said circuit boards into an agitated immersion tin plating bath comprising a stannous salt, a sulfur component which comprises a mixture of alkali metal polysulfides and at least one other sulfur-containing compound, a mineral acid, and a wetting agent, wherein said alkali metal polysulfides are present in sufficient amount to produce said smooth, even coating, until tin forms in a continuous coating on said copper circuitry; (2) rinsing said boards; and (3) drying said boards.

Claim 14

A process for the manufacture of printed circuit boards having a smooth, even tin coating over areas of clean copper circuitry comprising the steps of: (1) immersing said circuit boards into an agitated immersion tin plating bath comprising a stannous salt, a sulfur component which comprises a mixture of alkali metal polysulfides and at least one other sulfur-containing compound, a mineral acid, and a wetting agent, wherein said alkali metal polysulfides are present in sufficient amount to produce said smooth, even coating, until tin forms in a continuous coating on said copper circuitry; (2) rinsing said boards; and (3) drying said boards.

Claim 15

The process as defined in claim 12 wherein the plating of tin in said tin plating bath is continued to a thickness of between about 50 and 300 millionths of an inch.

Claim 16

The process as defined in claim 12 wherein said tin plating bath is operated at a temperature of 50°-80° C.

Claim 17

The process as defined in claim 12 including a pre-cleaning cycle which is a mild etching process comprising the steps of (1) soaking the circuit boards in an alkaline cleaner, (2) rinsing the boards in water (3) dipping the boards in a 10% sulfuric acid solution and thereafter (4) rinsing the boards again in water.

Claim 18

The process as defined in claim 12 including a pre-cleaning cycle which is a strong etching process comprising the steps of (1) immersing said circuit boards into an etchant dip wherein said etchant is ammonium persulfate, (2) rinsing with water, (3) dipping into a 10% sulfuric acid solution and (4) rinsing said board in water.

Claim 19

The process as defined in claim 12 wherein said rinsing step comprises rinsing said circuit boards in warm water at a temperature of about 100° F.-120° F.

Claim 20

The process as defined in claim 12 wherein said drying step comprises a warm oven bake operated at a temperature between about 200° F.-250° F.

Claim 21

The process as defined in claim 12 wherein after the drying step said boards are wire brushed.

Claim 22

The process as defined in claim 12 wherein before said pre-cleaning step a solder mask is applied to the circuit boards and said circuit boards are then cured sufficiently to prevent blistering during the tin plating step.

Claim 23

The process as defined in claim 12 wherein said sulfur-containing compounds are selected from organic and inorganic sulfur compounds, said organic sulfur compounds selected frpom the group consisting of aliphatic sulfur-nitrogen compounds, 5- and 6-membered heterocyclic compounds containing S-N in the ring, dithiols, thio derivatives of alkyl glycols, thioamine acids; and said inorganic sulfur compounds selected from the group consisting of alkali metal sulfides, alkali metal thiocyanates, and alkali metal dithionates.

Claim 24

The process as defined in claim 12 wherein said sulfur component contains thiourea and alkali metal polysulfides and wherein said alkali metal polysulfides are present in the range of 0.005 to 0.2 g/l of the bath composition.

Claim 25

The process as defined in claim 12 wherein in the tin plating bath the stannous salt is selected from the group consisting of stannous salts of halides, sulfate, fluoborate, nitrate, and acetate; the wetting agent is selected from the group consisting of organic anionic, nonionic and cationic surfactants; and said mineral acid is selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid.

Claim 26

The process of claim 23 wherein the alkali metal polysulfides are present at a concentration of 0.0005 to 0.2 g/l of the bath composition.

Claim 27

The process as defined in claim 23 wherein said tin plating bath comprises the components in the following concentrations: ______________________________________ Soluble stannous salt 15 - 30 g/l Sulfur component 15 - 120 g/l Mineral acid 25 - 50 ml/l Wetting agent 0.1 - 10 g/l Water Balance ______________________________________ and wherein the alkali metal polysulfides of the sulfur component is present at a concentration of 0.005 to 0.2 g/l of the bath composition.

Claim 28

The process as defined in claim 12 wherein said tin plating bath comprises stannous chloride, thiourea, alkali metal polysulfides, hydrochloric acid and fluorocarboxylic acid wetting agent.

Claim 29

The process as defined in claim 26 wherein said fluorocarboxylic acid is present in a concentration of from about 0.1 to about 5 g/l.

Claim 30

The process as defined in claim 26 wherein the ingredients of said tin plating bath are present in the following concentrations: ______________________________________ Stannous chloride 21 g/l Thiourea (chemically pure) 90 g/l Alkali metal polysulfides 0.1 g/l Hydrochloric acid 36 mg/l Fluorocarboxylic acid 0.5 g/l Water Balance ______________________________________

Claim 31

The process as defined in claim 12 wherein said tin plating bath comprises the components in the following concentrations: ______________________________________ Soluble stannous salt 15 - 30 g/l Sulfur component 15 - 120 g/l Mineral acid 25 - 50 ml/l Wetting agent 0.1 - 10 g/l Water Balance ______________________________________

Claim 32

The process as defined in claim 29 wherein said sulfur component is present in an amount of 50-120 g/l.