JPS6277481A - Method for preventing growth of tin whisker - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Whisker-Resistant Tin Coatings This invention relates to coating metal or metal alloy substrate materials to which tin or tin-containing materials have been added. The present invention has unique utility in applying tin coatings to copper and copper-based alloy substrates such as circuit boards for use in electrical and electronic applications.

Coating components that include tin or tin-containing materials have become increasingly important when manufacturing electronic circuits, electronic devices, and electrical connectors because they offer a number of desirable advantages.

For example, tin coated metal and metal alloy components help protect the components from corrosion. The tin coating also helps provide a chemically stable surface for soldering and helps maintain good electrical contact between the surfaces.

Tin coatings can be applied to metal substrates using a variety of plating solutions and methods. For example, tin coatings can be made by dipping the article to be coated into a tin-containing solution. Alternatively, the tin coating can be made by electrodepositing a tin layer onto the substrate or by electroless plating the tin coating onto the substrate. Viner (
Hyner), et al. U.S. Patent No. 3.966.5
No. 64 store and Horrissey
U.S. Patent No. 4,049,508, Japanese Patent Application No. 1983
-113894, USSR Patent No. 187゜746, and G. Doubleu Kavanaugh (G, H, c
``Tin-Lead Electrodeposition: Effects of Impurities in the Solution (Tin-Lend EI
eCtrOdeEIO3itiOnS: The Ef
feCtOfImpur;ttes in the
5olution) J, Brating (Pla
tino), April 1970, pp. 369-371, tin coating with gold l1SXl.
It explains some techniques for making one body. Rescue (L
U.S. Pat. No. 3,663.384 to Rosenberg et al., U.S. Pat.
(Honaco), other U.S. Pat. No. 4.029.
No. 556 Akira! [1i13, Wilson
, and other U.S. Pat. No. 4,162,205;
Igarashi, U.S. Pat. No. 4,168,223N, Hsu, U.S. Pat. No. 4,207,148, 111m, and Te1chian et al, U.S. Pat. Special 1
No. 1154-128947 Mei IQ book and patent application 1982-
No. 65993-4 describes various solutions for making tin-containing coatings.

Regardless of the coating technique used, to minimize porosity, a smooth,
Preferably, a flat precipitate is produced. It is also preferred to create a coating that is relatively constant in thickness. Relatively constant coating thickness is important in reducing etching problems, especially when fine tolerances are involved.

However, there are some negative aspects that typically must be overcome in order to effectively use tin coatings, particularly coatings applied to copper and copper-based alloy substrates. Tin suffers from interdiffusion and copper-tin compound formation in platforms overlying copper-based substrates. These copper-tin compounds can be brittle and can impair the usefulness of the tin corticine-fluid components. The presence of copper-tin compounds can also have a detrimental effect on subsequent soldering operations. High purity tin is often used as a coating material to overcome these problems. The solution is not without its own problems. Metallic wires, known as tin whiskers, sometimes grow naturally from these tin coatings. Because of this, these filaments can create both electrical shorts and electrical bridges across the insulation space between the conductors.

The mechanics of tin whiskers are not clearly understood.

Striae may begin to grow within days of application of the coating, or even years later. In the literature, whiskers are assumed to grow from stress concentration sites such as those created by many electrodeposition techniques, such as tin extrusion, which are resinous in nature. It is also assumed that drought and humidity affect whisker growth. S, C, Br1tton
``Simultaneous Growth of Whiskers on Tin Coatings: 20 Years of Observations'' by
owth of1 bolt) Old 5kars on Tin
Coatit+os 220 Years o
fObservation) J, Transactions of the Institute Op Metal Finishing (Transactions of the Institution)
In5titude of Metal Finish
ing), Vol. 52, 1974, pp. 95-102, discusses the problem of tin whisker growth and proposes various recommendations for reducing the risk of whisker formation.

One way to deal with the tin whisker problem is to
Short storage time for coated materials! It was to stipulate the time. However, this approach does not adequately address the problem, or necessarily avoids it.

Another approach was to gently strengthen the tin matrix to prevent whisker protrusion. Although the formation of intermetallic compounds and the diffusion of solute steel into the tin plating served this purpose, the operational cost of the final product was significant.

Other approaches to dealing with this problem have generally included whisker inhibiting additives to the tin coating solution. One such process involves immersion plating into a tin bath composition,
Includes the incorporation of soluble leaden salts and sulfur-containing complexing agents such as thiourea or thiourea-type derivatives. Davis's United States viC'i No. 4,09
No. 3,466, and No. 4.1'! 14,91
This method is explained in No. 3/1u. Another method uses lactones, lactams, cyclic sulfates,
Producing a coating by electroplating tin, lead, or an alloy of tin and lead from an aqueous acidic bath containing one or more additives selected from the class of organic compounds including cyclic imides and cyclic oxasilinones. Contains. Kohl U.S. Pat. No. 4.263,1
No. 06 describes this approach to the problem. Yet another approach involves the use of acidic electroplating baths containing tin ions and chelate salts, including acidic bismuth sulfate gluconate. Wilson, US Pat. No. 4,331,518, describes an approach to this problem.

Two other approaches to dealing with the tin whisker problem are described in British Patent No. 1,167.138 by M. Arnold (S, H, Herrmouth 01d) in Varnish, and in the article "Repressing Tin Whisker Growth". theG
row of Tin Whiskers ) J
, Brating, January 1966, page 96~
It is explained on page 99. British Patent No. 1.167
．． No. 138 suggests that tin whisker growth can be inhibited by reducing the amount of hydrogen absorbed or occluded into the structure of the plated metal. Ultrasonic agitation of the plating solution and/or polarization of the electrodes during plating is used to reduce the amount of hydrogen. Arnold, during p-ting, 1
Several metals, including antimony, cobalt, copper, germanium, gold, lead, and nickel, have been identified to weaken tin whisker growth if present in concentrations ranging from 2% to 2%.

Although these techniques have been useful in the past, there are still techniques for which it is believed that an effective solution to the tin whisker problem is needed.

Therefore, tin on a tin-coated substrate
It is an object of the present invention to provide an effective technique for preventing whisker growth.

tin from tin and tin-containing coatings without adversely affecting the weldability or backflow properties of the coating.
It is another object of the present invention to provide such an approach for preventing whisker growth.

All metal additions are made to the tin coating solution to
It is a further object of the present invention to provide a technique as described above for preventing the formation of whiskers.

These and other objects and advantages will become more apparent from the description below.

According to the invention, the growth of these tin whiskers in tin or tin-containing coatings is prevented by adding gold B salts to the tin-containing solution from which the coating is made. One of the advantages of the present invention is that it can be used with a wide variety of standard tin coating solutions. Another advantage is that tin whisker growth can be prevented without reducing the quality of the coating.

Indeed, high quality coatings can be made using the solutions of the invention. Metal salt additives that have been found to be of unique utility include palladium salts, such as palladium chloride, and silver salts, such as silver sulfate. Other salts that can be used include nickel, cadmium, cobalt, iron, platinum, gold, indium, and ruthenium salts.

In a first embodiment of the invention, the tin coating solution giving rise to the Heuss 7J-generated resistance is first prepared by adding one of the metal salts mentioned above to the initial, i.e. base, tin-containing solution. be able to. As already mentioned,
There are many standard tin coating solutions that can be used as base solutions. In this first approach, the metal salt is added to the base solution at a concentration above the saturation point of the solution. This creates a saturated tin-containing solution of the metal salt. This saturated tin-containing solution is then diluted by adding it to the base solution of the second bond. This dilute solution becomes the tin coating solution used to apply the tin or tin-containing coating to the substrate material.

In a second embodiment of the invention, tin coating solutions are prepared by adding each set of hydrochloric acid to a tin-containing base solution.

The metal salt to be added to the base solution is also first added to the multi-R hydrochloric acid.The metal salt-hydrochloric acid solution is then gradually added to the base solution to remove the tin to be applied to the substrate!1. Make a containing solution.

If a palladium salt additive is used to provide whisker resistance, the palladium salt may be added to the tin coating solution from about 5 ppm to about 10,000 ppm, preferably from about 20 ppm to about 11,000 ppm. should be present at a concentration within

Some useful coating solutions contain palladium in concentrations ranging from about 50 ppm to about 11,000 ppm. If a silver salt additive is used to provide whisker resistance, approximately 50% of the silver salt is added to the tin coating WJVI.
0 ppm to approx. 50. It should be present at a concentration up to OOOppm, preferably within the range of about 500 ppm to about 5000 ppm.

Another advantage of the present invention is that the whisker-resistant tin coating can be created on a substrate material using any suitable application technique known in the art, including electroless plating, electrodeposition, and infiltration. .

Yet another advantage is the ability to create relatively fine, relatively smooth, high quality coatings with excellent weldability properties.

As previously described, the present invention generally provides that the formation of tin whiskers from tin and U-containing coatings can be prevented by metal salt additions to the bath solution from which the coating is made. It's about discovery. In practice, there are many types of tin coating solutions that can be used as base solutions for adding metal salts. Suitable base solutions may contain sodium stannate, tin fluoroborate, tin pyrophosphate, tin sulfate and/or tin chloride. It goes without saying that the concentration of tin salts in these base solutions can be varied over a wide range. Depending on your wishes,
The base solution may contain a desired concentration of soluble lead compounds or silver compounds. When such compounds are added to a tin solution, a tin-lead or tin-silver coating is created on the substrate. It has been discovered that for applications requiring soldering or backflow of coatings, the base solution as well as the coating solution should be completely free of zinc.

Particularly effective tin-containing base plating solutions have been found to include those sold by ShipleV, designated TIN-PO3IT and LT-26. LT-26 is stannous chloride 20g/l, sodium hypochlorite 169#
! , thiourea 759/1 and a wetting agent 1 g/l. Up to 75 td/1 hydrochloric acid can be added to LT-26 to create a base plating solution. The base solution can be made in any desired manner.

To prepare the coating solution of the invention, a metal salt selected from the group consisting of palladium, silver, nickel, cadmium, cobalt, iron, platinum, gold, indium and ruthenium salts is added to a suitable tin-containing base solution. . In one embodiment of the invention, sufficient m metal salt is added to the tin-containing solution to exceed the saturation point of the base solution. This creates a saturated tin-containing solution of the metal salt. The presence of the added metal salt at the bottom of the base solution indicates that the saturation point of the base solution has been passed. Preferably, a palladium salt, such as palladium chloride, or a silver salt, such as silver sulfate, is added to the base solution to create a saturated solution. When using palladium chloride as an additive,
Palladium chloride approximately 19/j! would be sufficient to exceed the saturation point. When using silver salt as an additive,
It can be prepared by first dissolving and saturating water.

In this first approach, the solution from which the coating is to be made is prepared by diluting a saturated base solution with another base solution. When diluting the saturated solution, one must keep in mind that a certain metal salt level is required in the final coating solution to produce the desired resistance to tin whisker formation in the coating. If a palladium salt additive is used, the palladium concentration in the final coating solution will range from about 5 ppm+ to about 10
Up to ゜o o o ppm, preferably about 20 Elf
) III to about 1 o o o ppm. Some useful coating solutions contain palladium in concentrations from about 50 ppm to about 1000+)Dlll.

If a silver salt additive is used, the silver concentration in the final coating solution will range from about 50 ppm to about 50,000 ppm.
+n, preferably from about 500 ppm to about 50o
o ppm. If other metal salts listed above are used, the specific metal in the salt should be added to the final coating solution from about 10D11m to about 50.0D11m.
000+)Ill, preferably at a concentration within the range of about 50 ppm to about 5.000 ppm.

In another embodiment of the invention, the tin coating solution is prepared by adding hydrochloric acid to a tin-containing base solution, such as a solution designated LT-26 manufactured by Sibley. LT
-26 as the base solution, the base solution 1j! Up to 75d t=m can be added per unit. Approximately 5% by volume of the metal salt that is to be added to the solution to create whisker resistance in the coating is initially added to the solution.
Dissolved in an aqueous hydrochloric acid solution containing up to about 30% by volume of hydrochloric acid. To make the palladium addition, the dissolved palladium salt is added to the tin-containing base solution such that the palladium is added at a rate of less than about 10 μ/min per solution 81211. If palladium salt addition is carried out, the palladium concentration in the final coating solution should be approximately 51) E
) Ill to about 10.000 ppm. For solutions such as LT-26, the palladium concentration in the final coating solution is preferably within the range of about 2 oppm to about 309 EII. If silver salts, or one of the other metal salts mentioned above, are added, they should be added to the tin-containing base solution in sufficient quantities to reach the above moisture range in the final coating solution. be.

After preparing the tin coating solution, any suitable method known in the art can be used to create the tin or tin-containing coating on the substrate material. One of the major advantages of the present invention is that the coating solution thus made can be used in three widely used methods for coating substrate materials with tin or tin-containing materials: (1) electrodeless plating; (2) (3) infiltration; and (3) infiltration. The choice of which coating method to use is a function of the type of substrate to be coated and the relative advantages and disadvantages between the methods. Coatings made in accordance with the present invention generally consist essentially of about 0.1% to about 2011% palladium by weight, with the balance consisting essentially of tin. The coating is essentially about 0 palladium
．． Preferably, from 5% to about 10%, with the remainder being woody and tin. Most preferably, the coating consists essentially of about 1% to about 5% by weight palladium, with the balance consisting essentially of tin. When applying a tin coating to a copper-based substrate, the coating may contain copper and/or a copper-tin intermetallic compound. Similarly, for platforms where the base solution and coating solution contain lead or silver, the coating may also contain lead or silver.

It has been found that coatings made with the solutions of the present invention are characterized by a relatively fine grain structure and a relatively smooth surface. Such coatings are highly preferred from the standpoint of avoiding the problems associated with porous type coatings. Furthermore, coatings made from the solutions of the present invention do not adversely affect weldability or backflow properties and are suitable for electronic and electrical applications where the coated article is to be welded or bonded to some other component. This is highly preferred if you plan to use this.

The coating solution of the present invention has unique utility in creating weldable or reflowable tin coatings on copper or copper-based alloy substrates, such as circuit foils, after completing the desired circuit structure. . When coatings are to be applied to copper or copper-based alloy substrates for use in electrical and electronic applications, tin coatings applied by the techniques of the present invention and thus tin coating solutions contain substantially no zinc. Preferably not. The coating solution of the present invention can also be used to coat circuits on non-conductive flexible plusnacks substrates. Yet another advantage of the present invention is that there is no need to maintain the coating solution at a particular concentration during the coating process. However, the solution can be kept between room temperature and about 65°C.
It has been found preferable to maintain the concentration between .

The specific coating technique used to coat the substrate material is not critical from the standpoint of the present invention, but the relative rate of mixing, known as the mixing rate, between the liquid solution being applied and the substrate being coated is not critical. It has been found that it is preferable to set the flow rate. In the absence of such relative flow rates, the supply of available salt additives to the substrate surface can be quickly overwhelmed. Obviously, if no additives are applied to the surface coating,
Substantially no whisker resistance can be created in the coating. It has been found that effective mixing rates are in the range of about 2 CIJ/sec to about 100 aR/sec. Preferred mixing speeds are from about 501/sec to about 30
The range is up to aR/sec.

It will be appreciated that the particular mechanism for setting the mixing rate will depend on the particular technique being used to coat the substrate material. Any suitable mechanism known in the art can be used to set the mixing rate for a particular coating technique.

It is preferable, but not essential, to clean the substrate material before applying the coating solution described above. It will be appreciated that the type of cleaning process used is a function of the composition of the particular substrate and the type of contaminant being removed. After cleaning, the substrate material can be rinsed to remove any residual chemicals and/or unimmobilized particles on the substrate. After rinsing,
If desired, the substrate may be dried. Rinse and remove the substrate material using any suitable technique known in the art.
or) may be dried.

After applying the tin or tin-containing coating to a substrate material, the coated substrate may be rinsed again to remove any residual chemicals and unfixed particles, and/or dried.

Once again, any suitable rinsing and/or drying technique known in the art can be used.

For example, the substrate may be rinsed hot followed by a cold water rinse.

The following examples were performed to demonstrate the invention.

Example 1 Teinboshito 50#ll sold by Shipley! PdCJ
! , powder 0.079 T-do-1. Even if it's 50"C,
There was still significant undissolved material in the solution. After the undissolved matter has settled, 10d of the above solution is diluted by adding fresh tinpolyte solution to 60 ai! I made it. This solution was then used to deposit tin onto samples made of copper alloy 011000.

Copper alloy sample r1 was immersed in the solution for 10 minutes. Prior to immersion in the solution, the sample was immersed in a 10 volume Q% aqueous sulfuric acid solution to activate the surface to be coated and then cleaned. A mixing speed of 30 rJR/sec was set while the sample was immersed in the two-coating solution. After removal from the coating solution, the samples were rinsed first in hot water for 2 minutes and then in cold water for 2 minutes.

A grayish precipitate with a thickness of about 0.5μ formed on the sample. Precipitates are very fine (1μ or smaller)
The particle structure was shown. The precipitate was also darker in color than the undoped tin precipitate. After infiltrating the solution, 3
No whisker growth was observed on the samples for weeks.

Example 2 Solution 10a used to coat the sample in Example 1!
! was further diluted to 60d by adding fresh Tein-Bosito solution. A copper alloy C11000 sample, cleaned with aqueous sulfuric acid and then rinsed, was immersed in the coating solution for 10 minutes. Keep the coating solution at 50°C,
And again the mixing speed was set at 30 CIR/sec. After removing from the coating solution, the sample was placed in boiling water for 2 minutes.
It was then rinsed in cold water for 2 minutes.

The tin precipitate obtained with this solution was much lighter in color by visual observation, but still showed a fine grain structure. Again, three weeks after immersion in the solution, the tin-coated copper alloy C11000 sample showed no evidence of tin whisker growth.

Example 3 Saturated with silver sulfate at room temperature (Ag50/100g water
40.7 g) 10 - of water was added to 60 d of Tinposite solution. This mixture was used for tin precipitation at 50°C.

Once again, a sample of copper alloy C11000, which had been cleaned and rinsed in an aqueous sulfuric acid solution, was immersed in the mixture and a mixing speed of 30 aR/sec was set. A precipitate was obtained after 12 minutes. After removal from the solution, the samples were rinsed again in hot water for 2 minutes and then in cold water for 2 minutes.

The precipitate obtained using this solution was similar in color to the tin precipitate, but had a much finer grain structure and greater crystallinity. After 4 weeks, these precipitates became tin
It showed no evidence of whisker growth.

All of the samples treated according to the above examples exhibited solder wettability comparable to that obtained with flat tin deposits.

Example 4 For comparison, a sample of copper alloy 011000 was immersed in an undoped Teinbosite solution at 50° C. for 10 minutes. As above, the copper sample was cleaned in an aqueous sulfuric acid solution,
I rinsed it off. After removal from the undoped solution, the copper samples were rinsed in hot water for 2 minutes and then in cold water for 2 minutes. A tin precipitate approximately 0.5 microns thick formed on each sample.

During the 16 hour precipitation process, tin whiskers ranging in length from 6μ to 8μ were observed on the samples.

Example 5 A piece of 1 inch by 1.5 inch electrodeposited flat copper foil was bent into a semi-circle and made into a LT-
It was carried out for 3 minutes on a soaked tin named No. 26. After plating was completed, the samples were cleaned, unbent, flattened, and taped to a glass slide. Due to this mechanism, compressive stress is introduced into the plating layer,
Caused the growth of tin whiskers.

For comparison, similar samples were tested under the same conditions, but
Plated in LT-26 solution containing 25 ppm palladium. Palladium is 20 volume B% salt M10ae
41.7111 g of palladium chloride was dissolved and added thereto. The palladium chloride-hydrochloric acid solution was added to the LT-26 solution bath at an average rate of 1 d/min.

Twelve samples of F were stored in air. After 5 days, whiskers were visible on the samples plated in a palladium-free solution (the length of the whiskers per mm2 of surface area was approximately 220 μ).

Samples plated in solutions containing palladium did not produce any whiskers. After 55 days, the whiskers on samples plated in solutions containing no palladium were approximately 940 μ/mm, whereas the whiskers on samples plated in solutions containing palladium were approximately 940 μ/mm2. It was 18 μ/lll12.

Although tin coatings can be made from saturated solutions diluted in base solutions, similar high quality coatings can be made by applying tin-containing saturated solutions to a substrate. Again, any suitable coating method can be used to apply the saturated solution to the substrate.

It is apparent that a small isker resistant coating has been provided by the present invention which fully satisfies the objects, methods and advantages set forth above. Although the invention has been described in conjunction with specific embodiments, many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore intended to cover all alternatives, modifications and variations falling within the spirit and broader scope of the claims.

Claims (18)

[Claims] (1) Palladium salts, silver salts, nickel salts, iron salts, cadmium salts, platinum salts,
A tin coating solution is prepared containing a metal salt selected from the group consisting of gold salts, indium salts, ruthenium salts, and cobalt salts, the preparation step comprising providing an initial tin-containing solution and adding the metal to the initial solution. adding a salt at a concentration higher than the saturation point of the initial solution to form a metal salt saturated solution, and applying the tin coating solution to the substrate material;
A method characterized by: (2) Item (1) characterized in that the metal salt saturated solution is further diluted by adding the metal salt saturated solution to an additional amount of the initial solution, and the diluted solution is made into a tin coating solution. The method described in section. (3) Additionally, the salt addition step includes adding a sufficient amount of palladium salt to the initial solution to reduce the tin coating solution from about 5 ppm to about 1
A method according to paragraph (2), characterized in that it comprises containing a concentration of palladium in the range up to 0,000 ppm. (4) The salt addition step further includes adding a sufficient amount of palladium salt to the initial solution such that the tin coating solution contains palladium at a concentration ranging from about 20 ppm to about 1000 ppm. The first one (which is characterized by
The method described in section 2). (5) Additionally, the metal salt addition step includes adding silver salt to the initial solution for a sufficient period of time to reduce the tin coating solution from about 50 ppm to about 50 ppm.
A method according to paragraph (2), characterized in that it comprises containing a concentration of silver in the range of up to ,000 ppm. (6) Furthermore, the application step includes applying the coating solution to the substrate material at a thickness of about 2 cm/cm.
A method according to paragraph (1), characterized in that it comprises applying at a mixing speed in the range from seconds to about 100 cm/second. (7) tin whisker formation on a substrate material coated with a tin-containing material characterized by being substantially free of zinc and containing palladium at a concentration ranging from about 5 ppm to about 10,000 ppm; Tin coating solution that creates resistance. (8) Furthermore, the palladium concentration ranges from about 20 ppm to about 10 ppm.
No. (7) characterized in that the range is up to 00 ppm.
Tin coating solution as described in Section. (9) Furthermore, the palladium concentration ranges from about 20 ppm to about 30 ppm.
Tin coating solution according to paragraph (7), characterized in that the tin coating solution is in the range of up to ppm. (10) A tin coating solution for producing tin whisker formation resistance on a substrate coated with a tin-containing material, characterized by a silver sulfate concentration ranging from about 50 ppm to about 50,000 ppm. (11) A tin-containing solution for producing tin whisker formation resistance on a substrate material coated with a tin-containing material, characterized by a silver salt concentration ranging from about 500 ppm to about 5000 ppm. (12) a substrate, and a substantial whisker-resistant tin coating on at least one surface of the substrate, the tin coating comprising from about 0.1% by weight to about 0.1% by weight to inhibit tin whisker growth; Composites containing up to 20% palladium by weight. (13) Additionally, a coating containing from about 0.5% to about 10% by weight palladium.
2) Composite as described in item 2). (14) The composite of item (12) further characterized by a coating containing from about 1% to about 5% by weight palladium. (15) The composite according to item (12), further characterized in that the substrate is made of copper or a copper-based alloy. (16) To inhibit the growth of tin whiskers on a substrate material having a tin-containing coating, preparing an aqueous tin-containing solution and dissolving a palladium salt in a hydrochloric acid solution, the dissolving step further comprising: dissolving an amount of the palladium salt in the hydrochloric acid solution,
Tin coating solution is about 5 ppm to about 10,000 ppm
A method comprising: containing up to 0 ppm palladium; and adding the palladium salt and a hydrochloric acid solution to the base solution to form a tin coating solution. (17) The method according to item (16), wherein the step of preparing the base solution further includes adding a large amount of hydrochloric acid to the tin-containing aqueous solution. (18) Furthermore, the dissolving step includes dissolving the palladium salt in a solution containing from about 5% to about 30% by volume of hydrochloric acid, and the palladium salt and hydrochloric acid addition step includes dissolving the palladium salt and hydrochloric acid. solution to the base solution, about 1 liter per liter of the base solution.
17. The method of claim 16, comprising adding at a rate slower than 0 ml/min and applying the tin coating solution to the substrate material.