GB2072709A - Immersion tin plating compositions - Google Patents

Abstract

An immersion tin plating solution, for plating over a base metal such as copper, by displacement, comprises a source of stannous ions (e.g. stannous chloride), thiourea as a complexing agent for the stannous ions, a mineral acid, and an inhibitor to inhibit etching of the base metal. The inhibitor is preferably an organo sulphonic acid, e.g. a phenol sulphonic acid, a salt thereof and permits the use of relatively high thiourea: stannous ion molar ratios, e.g. 10:1 or more, thereby making it possible to obtain increase yields of tin from the plating solution. The solution preferably also contains a hypophosphite reducing agent. The invention also comprises a method of tin plating a base metal by displacement by contacting a solution of the invention with the base metal.

Description

SPECIFICATION Immersion tin plating compositions This invention relates to acidic aqueous composi tions for immersion coating tin over metal surfaces displaceable by tin and processes for using the same.

Immersion tin plating compositions capable of providing tin plate over a copper surface are known and disclosed, for example, in United States Patents Nos. 2,282,511; 2,369,620; 2,891,871; 3,303,029 and 3,917,486. Such compositions typically comprise a tin salt, a complexing agent for the tin, typically a sulphur complexing agent, especially thiourea, an acid and, frequently, a reducing agent such as a hypophosphite compound.

It is known that immersion tin plating composi tions plate by displacement whereby theoretically one mole of stannous tin displaces 2 moles of cop per thereby introducing 2 moles of copper into solu tion for each mole of tin plated as a consequence of the plating reaction. As plating continues, the copper over which the tin deposits becomes coated with tin and is thereby less available for displacement.

Therefore, as plating proceeds, the plating rate decreases until plating substantially stops with little or no further build up of tin regardless of the amount of tin remaining in the plating composition. Conse quently, such compositions are characterized by a relatively low yield of plateable tin from solution.

It is known by applicant that when thiourea is used as a complexing agent, it would be desirable to use a high molar ratio of thiourea to tin and a relatively high tin concentration - i.e., 0.2 moles per litre. This combination increases the "throwing power" of the solution. However, it is also known by applicant that under such conditions, as the ratio of thiourea to tin increases, the deposit formed becomes discoloured and non-adherent and is aptly described as "black tin".

The tin plating compositions of the invention are of the type disclosed in U.S. Patent Nos. 3,303,029 and 3,917,486 and comprise a base aqueousformu- lation of a stannous salt, an acid capable of dissolv ing solution components, thiourea as a complexing agent and, preferably, a reducing agent such as a hypophosphite. In accordance with the invention, the base formulation also contains an inhibitor, pref erably a solution soluble organic sulphonic acid or a salt thereof, which results in improved plating and permits the use of a higher ratio of thiourea to tin at a relatively high tin concentration without formation of a black tin deposit. Hence, in accordance with this invention, tin plating compositions are provided that deposit good quality tin in a higher yield than heretofore believed to be possible.

Though we do not wish to be bound by theory, it is believed that as tin deposits over a cuprous metal surface from an immersion tin plating solution, two reactions take place that cause copperto entersolution. The first reaction comprises displacement of two moles of copper for each mole of tin deposited.

The second reaction is believed to comprise an etching reaction whereby copper is etched by the plating solution. This latter reaction is evidenced by discoloration of the plating solution from pale yellow to pale green and buy a higher concentration of cop per in a depleted tin plating solution than can be attributed to the copper entering solution solely by displacement. It is believed that the etching reaction becomes more vigorous as the molar ratio of thiourea to tin increases resulting in the formation of loosely adherent tin, referred to herein as black tin.

Therefore, though a high thiourea to tin molar ratio at a relatively high tin concentration may be desired, the tin obtained underthese conditions is not of a commercially acceptable form. Based upon the theory set forth herein, it has been found that the addition to a tin plating solution of a compound that inhibits etching of copper by the plating solution permits use of a higher thiourea to tin ratio at the higher concentration with a concomitant increase in tin yield without formation of black tin.

As noted above, the compositions of this invention comprise a stannous salt, thiourea, an acid, preferably hypophosphite as a reducing agent, and an inhibitor.

Stannous chloride is the preferred source of stannous tin in the tin plating composition though other known acid soluble stannous salts can be used provided the an ion of the salt is not detrimental to the performance of the solution. The concentration of the stannous ion is not critical since small amounts provide some deposition but substantial amounts are preferred. A preferred composition comprises from 0.01 moles (expressed as metallic tin) litre to the solubility limit of the compound, and more preferably from 0.05 to 0.2 moles per litre.

In order to maintain the stannous salt soluble in solution, thiourea is used as a complexing agent for the stannous salt, preferably alone but also in combination with other known complexing agents for tin if desired. Othercomplexing agents that are useful are known in the art and described in the above mentioned United States Patents.

The minimum concentration of thiourea in the compositions of the prior art is generally an amount in excess of that amount necessary to complex the stannous ion initially in solution, i.e. in excess of at least 2 moles of complexing agent per mole of stannous ion. For the purpose of this invention, for the reasons above, it has been found that higher concentrations are desired and a preferred mole ratio of complexing agent to stannous tin is at least 10:1, more preferably at least 12 1 and most preferably, between 18:1 and 30: It has been found that as the said ratio exceeds about 30 1, dependent upon the conditions of use and other components in solution, the yield of the plating solution drops and the formation of black tin is again evidenced.

Though the higher ratios of thiourea to tin expressed above are preferred, it has been found that benefits are obtained by the addition of an inhibitor to the plating solution even at the lower ratios in that the solution is capable of greater latitude in opera tion.Therefore, the invention comprises the addition of the inhibitor and in the preferred embodiment, the use of the inhibitor in a solution containing an increased molar ratio of thiourea to tin.

An acid is used in the tin composition to dissolve solution components. Typically the acid used is a non-oxidizing mineral acid in an amount sufficient to reduce the pH of the composition to 1 or less, hydrochloric and sulphuric acids being preferred.

The plating composition preferably contains a hypophosphite salt to promote solution stabiiity as is known in the art.

Tin deposits from solution by displacement with or without the hypophosphite but in its absence, within a relatively short time, typically within several hours, the solution will become turbid and the deposit properties will be adversely affected.

Alkalimetal hypophosphites, such as sodium hypophosphite, are preferred.

The concentration of the hypophosphite, when used, can vary from about 0.01 to about 2.5 moles per litre, but preferably is lower than in the prior art as represented, for example, in U.S. Patent No.

3,303,029. Consequently, the preferred concentration varies from about 0.05 to 1.0 moles per litre, and more preferably from about 0.05 to 0.25 moles per litre.

The tin plating compositions of the invention are characterized by the addition of a compound that inhibits etching of copper as explained above. The preferred inhibitors are organo sulphonic acids and salts thereof, preferably alkali metal salts thereof.

Typical organo sulphonic acids and salts that may be employed in accordance with this invention comprise, for example, phenol sulphonic acid, toluene sulphonic acid, 1 - phenol - 2 - amino - sulphonic acid, 2-amino benzene4 suiphonic acid, xylene sulphonic acid, sulphosalicyclic acid and salts such as alkaii metal salts of each of the foregoing. The preferred sulphonic acids are aromatic sulphonic acids having one aryl nucleus; phenol sulphonic acids and salts thereof being most preferred.

The concentration of inhibitor in the tin solution is difficult to define with precision as it is dependent upon the specific inhibitor used. Some inhibitors can be used within wide concentration ranges whereas others can be used only in narrower concentration ranges. Hence, for some inhibitors, minor amounts of a slow as 0.1 grams per litre provide some improvement and as the concentration increases, greater improvements are obtained up to an optimum amount. Beyond the optimum amount, tin yield drops off probably because the inhibitor acts to prevent both copper etching and the displacement reaction. For other inhibitors, the upper limit is of lesser importance. Based upon the above, it should be apparent that any concentration range set forth herein might not be accurate for all inhibitors and the preferred concentration is that which permits a high yield of good quality tin.However, for purposes of a guideline only, the inhibitor concentration pref erablyvaries between 0.01 and 12 grams per litre and more preferably, between 0.1 and 5 grams per litre.

The tin compositions of this invention are used in a manner similar to those of the prior art,though the pretreatment of the copper surface prior to tin plating should be performed with greater care. A copper substrate such as a copper clad epoxy printed circuit board base material is preferably first pre-dipped in a solution of a wetting agent such as non-ionic, cationic or anionic surfactant. Next, the part is rinsed and etched with a mild etchantthat provides a smooth surface on the copper rather than a coarse granular surface. Light mechanical abrasion preceded by a surfactant pre-dip, though less preferred, can be used.After pretreatment including water rinsing, the copper surface may be immersed in a predip having the same formulation as the tin plating solution minus the tin or may be immersed directly in the tin solution for a time sufficient to deposit the tin to a desired thickness, preferably in excess of 60 millionths of an inch (1.524 micrometers). The temperature of the plating solution can vary from about room temperature to as high as 200"F (93.3"C) but the composition is preferably used at a temperature of from 130 and I 600F (54.4-71.1 C) or higher - i.e., 185 to 2000F (85 to 93.3"C) for heavier deposits.The tin deposits are used for essentially the same purposes as in the prior art and the preparation of parts for deposition and the use of tin coated surfaces are fully described in the aforesaid United States Patents Nos. 3,303,029 and 3,917,486.

The tin plating compositions of this invention are particularly usefui for the manufacture of printed circuit boards. Procedures for the formation of printed circuit boards are described in numerous publications, for example, in Coombs, PRINTED CIRCUITS HANDBOOK, McGraw-Hill Publishing Company, New York, 1967, Chapter 5.

In a typical process for the manufacture of printed circuit boards, a suitable base material, such as a copper clad epoxy, is selected. Holes are drilled at appropriate locations on the board and the walls of the holes are metallized, such as with copper, to provide electrical contact between the two surfaces of the base material. Methods for metallization are known and include the steps of cleaning, catalyzing and electroless copper deposition.

Following metallization of the holes, utilizing a process known as pattern plating, a conductor pattern is formed on the copper by application of an organic resist material which may be either a photoresist or a screen resist dependent upon design and definition. The resits coats the copper that is not part of the conductor pattern and leaves the copper bare in a conductor pattern. The thickness of the conductor pattern is then increased during electroless andlor electrolytic copper plating procedures. Following the steps of copper plating, tin from a plating solution of the invention is applied over the copper in the conductor pattern to protect the same from subsequently applied etchants, the organic resist is removed exposing the unwanted copper (not part of the conductor pattern), and the unwanted copper is dissolved with a suitable etchantthat does not attack tin such as those disclosed in United States Patent Application Serial No. 822,003 of August 1977, assigned to the same assignee as the subject invention and incorporated herein by reference.

Such etchants comprise sulphuric acid activated with hydrogen peroxide or the synergistic combination of hydrogen peroxide and molybdenum and characterized by a source of phosphate ions as an inhibitor against attack on tin. During etching, the copper coated with tin is protected from the etchant so that following etching, there is provided copper in a conductor pattern coated with tin. Thereafter, electrical connections can be joined directly to the tin if desired. Preferably, the tin is reflowed by heating the circuit to a temperature above the melting point of the tin prior to joining electrical connections thereto.

In order that the invention may be well understood the following examples are given by way of illustration only.

EXAMPLES 1-7 Seven tin plating formulations were prepared and seven clean copper foils, each having a surface area of 36 square inches, were pretreated by immersion in a peroxide-sulphuric acid etch (746 Etch of Shipley Company Inc.) and rinsed with water. One foil was then immersed in each solution. Plating was permitted to continue until it could be visually observed .that tin of inferior quality was beginning to deposit or the solution was no longer capable of depositing tin. This was in part evidenced by a change in colour of the solution from pale yellow to pale green, indicative of copper etching. At this point, the foil was removed and the amount of tin plated from solution determined.

Each of the plating formulations was made up from a base formulation having the following composition: Stannous chloride 20 grams Hydrochloric acid (37%) 25 ml Sulphuric acid (50%) 50 ml Sodium hypophosphite 16 gm Thiourea 200 gm Water to one litre.

The formulation contained phenol sulphonic acid in the amount indicated in Table 1 below in which the condition of plating and the results obtained are also given.

TABLE 1 Examples 1 2 3 4 5 6 7 Phenolsulphonicacid 0 0.1 0.25 0.5 1.0 25 50 (gmllitre) Water to 1 litre Temperature (C) 71.1 72.2 71.1 73.1 71.1 71.1 71.1 Results: (1) %tin plated 21 21 35 52 48 45 48 The above table illustrates that when the concentration of phenol sulphonic acid reaches and exceeds 0.5 grams per litre for the formulation given, the yield of tin from solution is consistently twice that of the yield from the same solution free of the phenol sulphonic acid used and in all cases, in excess of forty percent of the initial tin concentration.

Moreover, in all such cases, the deposit appearance is satisfactory.

EXAMPLES 8-12 Using the base formulation of Examples 1-6 and the precedure set forth therein, an alkyl naphthalene sulphonic acid (identified as NEKAL NF by General Aniline and Film Company) was used in varying amounts in place of the phenol sulphonic acid.

The results obtained are set forth in Table 2 below.

TABLE2 Results Example Alkyl naphthalene No. suiphonic acid (gmll) tin yield { /O) Appearance 8 0 21 satisfactory 9 0.1 40 satisfactory 10 0.5 75 satisfactory 11 2.5 7 satisfactory 12 10 4 satisfactory Erom the above, it is evident that the sulphonic acid used is effective forthe purposes herein, but it can only be used in the base formulation within pre scribed limits. Hence, in the narrow concentration range of from 0.1 to 0.5 grams per litre of the sulphonic acid, satisfactory deposits were obtained with yields of forty percent or better.By comparison, the phenol sulphonic acid could be used in substantial quantities without stopping or retarding tin plate-out.

EXAMPLES 13-17 The procedure of Examples 8-12 was repeated, but this time, aminomethane sulphonic acid was used in place of the phenol sulphonic acid with the results as shown in Table 3.

TABLE 3 Results Example Aminomethane No. Suiphonic acid (gm/I) Tin yield f MO) Appearance 13 0 21 satisfactory 14 0.1 62 satisfactory 15 0.25 57 satisfactory 16 0.5 47 satisfactory 17 1.0 15 satisfactory As in the case of the alkyl naphthalene suiphonic acid, the higher concentrations of the aminomethane suiphonic acid inhibited both the etching and the displacement reactions, but within the range of from 0.1 to 0.5 grams of the sulphonic acid, satisfactory deposits with yields in excess of forty percent were obtained.

EXAMPLES 18-33 The following examples illustrate the effect of the thiourea to tin ratio with and without the use of phenol sulphonic acid as a stabilizer. The base formutation used was as follows: Stannous chloride variable Hydrochloric acid (37%) 25 ml Sulphuric acid (50%) 50 ml Sodium hypophosphite 16 gm Thiourea variable Dhenol sulphonic aciå variable Water to one litre The concer.traliGn of the components used in vary- ing quantities and the results obtained using the plating procedure of Examples 5-6 are shown in Table 4 below.

RESULTS Stannous Phenol Mole Ratio Example Thiourea Chloride Sulphonic Thiourea No. (Gll) (ugli) acid (gll) toxin Yield Appearance 18 50 20 0 6.4:1 13.6 satisfactory 19 50 15 0 8.6:1 7.4 satisfactory 20 50 10 0 12.9:1 4.3 satisfactory 21 50 5 0 25.8:1 12.9 satisfactory 22 200 20 0 25.8:1 20.3 black 23 200 15 0 34.6:1 10.2 black 24 200 10 0 51.6:1 10.9 black 25 200 5 0 103.2:1 10.5 black 26 50 20 3 6.4:1 15.5 satisfactory 27 50 15 3 8.6:1 11.2 satisfactory 28 50 10 3 12.9 :1 8.1 satisfactory 29 50 5 3 25.8:1 54.5 satisfactory 30 200 20 3 25.8::1 47.4 satisfactory 31 200 15 3 34.6:1 24.0 satisfactory 32 200 10 3 51.6:1 28.5 satisfactory 33 200 5 3 51.6:1 24.2 black The above table illustrates that in the absence of an inhibitor, thiourea : tin ratios of up to about 25:1 can be tolerated without formation of black tin at the relatively low tin concentrations (grams per litre).

However, with higher concentrations of tin, even at a thiourea : tin ratio of 25:1, black tin forms. By comparison, in those examples where an inhibitor is used, higherthiourea : tin ratios at higher tin concentrations can be used, and moreover, substantially greater yields of tin are obtained, particularly with a molar ratio of thiourea to tin of about 25 1. Therefore, for the formulation used in the above examples, atin yield in excess of forty percent can only be obtained without the formation of black tin when both the thiourea and tin are present in high concentration at a ratio of about 25:1 and the inhibitor is present in the formulation.

EXAMPLES 34-37 The procedures of Examples 30 to 33 where repeated except that the concetration of the phenol sulphonic acid was used to 9 gramsilitre and 0.6 gramellitre of a surfactant was added to the fornDula- tion to give the resuits shown in Table 5 below.

TABLE5 MoleRafio Results Example Thiourea to Stannous No. Chloride Yield Appearance 34 25.8:1 44.6 satisfactory 35 34.6:1 55.8 satisfactory 36 51.6:1 8.9 black 37 103.2:1 9.2 black The results obtained are quite similar to those obtained in the preceding four examples though the yield was reduced at the higher thiourea : tin ratios.

EXAMPLE 38-41 Using the base formulation and plating procedure of Example 1, the example was repeated adding various inhibitors, each in an amount of 3 grams per litre, with the results shown in Table 6.

TABLE 6 Example No. Inhibitor Yield Appearance 38 Methane sulphonic acid 50.1 satisfactory 39 Naphthalene disulphonic acid 46.9 satisfactory 40 Benzene disulphonic acid 47.4 satisfactory 41 Nitro phenol 52.6 satisfactory EXAMPLE 42 A printed circuit board was prepared from a copper clad G-10 substrate using the following sequence of steps: (a) Clean the substrate and drill holes at appropriate locations.

(b) Metallize the walls of the holes by electroless plating including the steps of catalysts with a catalyst such as Catalyst 6F of Shipley Company Inc., accelerate with a mild acid solution and electrolessly plate copper such as with copper mix CP-74 of Shipley Company Inc.

(c) Apply a positive working photo resist such as AZ-119, expose and develop.

(d) Electroplate copper to full desired thickness.

(e) Apply the immersion tin composition of Example 5 over the exposed copper.

(f) Remove the photo resist by dissolution in a suitable solvent.

(g) Remove exposed copper by immersion of the epoxy coated board in an etchant comprising 200 ml of 50% sulphuric acid, 100 ml of 35% hydrogen peroxide, 10 grams of sodium molybdate, 16 gm of phenol sulphonic acid, 50 ml of 85% of phosphoric acid and water to 1 litre. The etchant is maintained at 120"F and the time of immersion is suffient to remove all of the copper not coated with tin.

(h) Heat circuit board to above melting point of the tin to reflow the same.

Claims (19)

1. An acid immersion tin plating solution, for plating tin over a base metal by displacement, comprising a source of stannous ions and thiourea as a complexing agent for said stannous ions and also containing an inhibitor in an amount sufficient to inhibit etching of the base metal.

2. An immersion tin plating solution, for plating tin over a base metal by displacement, comprising stannous ions, thiourea as a complexing agent to said stannous ions, and a mineral acid in an amount sufficient to reduce the pH of the solution to below 1, and also containing an inhibitor in solution in an amount sufficient to prevent the formation of a loosely adherent, discoloured deposit.

3. Atin plating solution as claimed in claim 1 or claim 2 containing at least 0.01 moles of stannous ion per litre.

4. Atin plating solution as claimed in claim 3 containing at least 0.05 moles per litre of stannous ion.

5. Atin plating solution as claimed in claim 4 containing from 0.05 to 0.2 moles per litre of stannous ion.

6. Atin plating solution as claimed in any one of the preceding claims in which the mole ratio of thiourea to stannous ion is at least 10:1.

7. Atin plating solution as claimed in claim 6 in which the said molar ratio is at least 12 1.

8. Atin plating solution as claimed in claim 7 in which the said molar ratio is from 18:1 to 30 1.

9. Atin plating solution as claimed in any one of the preceding claims also containing a hypophosphite reducing agent.

10. Atin plating solution as claimed in claim 9 containing from 0.05 to 1.0 moles per litre of hypophosphite reducing agent.

11. Atin plating solution as claimed in claim 10 containing from 0.05 to 0.25 moles per litre of hypophosphite reducing agent.

12. Atin plating solution as claimed in any one of the preceding claims in which the inhibitor is an organic sulphonic acid or salt thereof.

13. Atin plating solution as claimed in claim 12 in which the sulphonic acid is an aromatic mononuclear sulphonic acid.

14. Atin plating solution as claimed in claim 13 in which the sulphonic acid is a phenol sulphonic acid.

15. Atin plating solution as claimed in any one of the preceding claims in which the inhibitor is present in an amount such that the yield of tin from the plating solution is at least 40% of the initial quanity of tin in solution.

16. Atin plating solution as claimed in claim 1 substantially as hereinbefore described with reference to the Examples.

17. A method of plating a base metal substrate with tin which comprises contacting the substrate with a tin plating solution as claimed in any one of the preceding claims.

18. A method as claimed in claim 17 in which the metal substrate is copper.

19. A method as claimed in claim 17 substantially as hereinbefore described with reference to the examples.