CA1037207A

CA1037207A - Immersion tin bath composition and process for using same

Info

Publication number: CA1037207A
Application number: CA199,110A
Authority: CA
Inventors: Frederick W. Schneble (Jr.)
Original assignee: Kollmorgen Corp
Current assignee: Kollmorgen Corp
Priority date: 1973-07-24
Filing date: 1974-04-26
Publication date: 1978-08-29
Also published as: JPS5624713B2; DK397374A; FR2238772B1; CH606499A5; AU6664174A; AT332188B; SE7409520L; JPS5033935A; ATA609174A; GB1436645A; DE2433820B2; IT1016948B; FR2238772A1; DE2433820A1; US3917486A; NL7410019A; SE411228B

Abstract

ABSTRACT OF THE DISCLOSURE

The present invention relates to a novel immersion tin plating bath composition and method of utilizing salt composition in depositing a smooth even metallic tin coating over metallic surfaces. The new plating bath composition comprises a soluble stannous salt, a mineral acid and a wetting agent and for its improved properties further contains at least two sulfur containing compound which nay be organic or non-organic.
It is particularly preferred that the sulfur component comprise a mixture of alkali metal polysulfides and at least one other sulfur containing compound.

Description

' `- 1037Z0~
Methods are well-known to plate tin over metallic ~' surfaces. The instant baths and methods are to be disti,nguished .' from the techniques based upon electrolytic deposition and ~-' electroless plating.
,-' - Electrolytic plating is the production of adherent . deposits of metals on conductive surfaces carried out by pas-, , sage of electric current through an electroplating solution.
', The plating rate is determined by the current density impressed ' on the surface being plated.
Electroless plating is a method of metal deposition ;, without the assistance of an external supply of electrons but, requiring an agent present in the processing solution capable of reducing the ions to be deposited. The process ';''.' , ~
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: 10372~7 1 ¦is further characterized by the cata1fytic nature of tne sur~ace

2 ¦which enables the metal to be plated to any thickness.

3 Typically, such solutions comprise a solvent, a supply of

4 ions of the metal to be deposited, an agent capable of ¦reducing the ions of the metal to be deposited, a complexing S ~ agent for the ions of the metal to be deposited~and a pH

8 regulator.
-Among other problems, a major difficulty is sometimes encoun-;:9 ¦ tered with depositing electroless metal on closely defined areas.
¦There is a tendency for non-sensitive areas after prolonged 11 ¦ immersion in or contact with electroless metal solutions - 12 to receive scattered or random metal deposits. In addition, the electroless metal solutions sometimes produce metal deposits 14 which contain a substantial amount of hydrogen causing the .- 15 deposits to be brittle, breaking under rough mechanical 16 handling and bending.
17 Immersion plating or "contact plating" depends, however, 18 upon a galvanic displacement reaction. The current instead of 19 being furnished from an outside source, arises from reaction of the substrate itself and the metal being plated. Because of this, metal thickness has traditionally been limited 22 to lO to 50 millionths of an inch. As the immersion process 23 depends upon the electrolytic action of ~he base metal, 24 deposition stops as soon as the base metal is entirely covered forming a very thin deposlt. f:

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; 1 1037207 ¦ Summary of the Invention ,, .

1 This invention is concerned with immerslon plating and its 2 attendant advantages which include,among others: immersion deposits 3 which are decidedly adherent; deposits with con~iderable resistance 4 to corrosion; the production of dense impervious deposits; and the ability to deposit metal on closely defined areas of metallized 6 sur~ace.
7 It has been found that up to about 300 millionths of an inch 8 of tin can be plated in accordance with the present invention to g provide surprisingly good solderability immediately after plating and particularly, after exposure to adverse conditions often re-11 quired in subsequent fabrication.
12 Additionally, the high quality of solderability provided by 13 this invention endures for a period in excess of six months of storage under normal stock room conditions.

Moreover, the chemical resistance of the tin plate of the 16 present invention is surprisingly excellent. The tin plate remains 17 solderable after exposure to normal printed circuit processing 18 chemicals i.e., chromic acid, dilute hydrochloric acid, etc., ~}~ and will remain bright after cleaning with trichloroethylene, E'reon, isopropyl alcohol and other normal flux-removing solvents.
21 It has additionally been discovered that optimt~ solderabilit r 22 is achieved by the present invention with plating thicknesses of on Y
23 between 50-lO0 millionths inch. Plating of greater thickness unde:
24 , the present invention is now possible by merely extending the im-25 ¦ ersion time of the plating process. However, it has been found 26 ¦ hat such greater thicknesses do not improve the solderability 27 ¦ characteristics to any appreciable degree. The tenacious tin plate ¦ achieved by the present invention achieves much greater solderabili y ._ .. - - \

` ~.037Z~7 - characteristics than even thicker tin plates formed by other processes, The much improved solderability with relatively thin plate thickness, therefore, proves to be a great economic ~, saving.
~,~- Accordingly, it is an object of the present invention to ~....
,: . ., provide a novel immersion tin plating bath.
~;; In one broad aspect the present invention provides an -immersion tin plating bath composition for depositing a smooth, - even tin coating on metallized surfaces capable of chemically displacing tin from a tin plating bath, said composition com-prising a soluble stannous salt, a sulfur component comprising ,, .
' ;- at least two sulfur-containing compounds, a mineral acid, a i;~ wetting agent and water.
,;~. In another broad aspect the present invention provides , a process for depositing a smooth, even tin coating on a i metallized surface, said process comprising immersing into a .~ ,, .
;~ tin plating bath comprising a soluble stannous salt, a sulfur ~

, ~ component which comprises at least two sulfur containing ~i ,j, ~.
1 compounds, a mineral acid, a wetting agent and water, 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. :

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':'' :, , :; 4 i jc/ ~, ' ` 11 1037~7 her objects and advantages Or the invention will~ ¦
be set forth in part hereinafter and in part will be obvious 3 herefrom, or may be learned by practice with the invention.

Detailed Description of the Preferred Embodiments .: . , . , ,.. .
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According to the present invention, there are 11 provided immersion tin plating bath compositions for 12 depositing a smooth, even tin coating on metallized surfaces, 13 comprising a soluble stannous salt, a sulfur component, a mineral acid, and a wetting agent. The metal of the sub-16 strate surface must have an electronegativity greater than tin in order that it be capable of chemically displacing tin from the tin bath.

A further preferred feature of the invention is to provide a process for depositing a smooth, even tin coating on 21 a metallized surface, said process comprising immersing into a tin plating bath comprising a soluble stannous salt, a 22 sulur component which comprises at least two sulfur containing 24 compounds, a mineral acid, and a wetting agent, an article having a metallized surace capable of chemically displacing tin 26 rom the tin plating bath, wherein the article is immersed 27 in the bath until tin forms in a continuous coating on said '::' .
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~ ¦¦ metalliz surface k 2 A still further preferred embodiment of the invention , 3 is in a process for the manufacture of printed circuit boards - 4 having a smooth, even tin coating over areas of clean copper ~ 5 circuitry having grease-free and oxide-free copper surfaces, ,~ 6 comprising the steps of:
i 7 (1) immersing said circuit boards into an agitated s~ immersion tin plating bath comprising a soluble stannous 9 salt, a sulfur component,a mineral acid, and a wetting ! 10 agent for such time until a continuous coating of tin f',' ~ forms on said copper surfaces;
;12 (2) rinsing said boards, and . (3) drying said boards.
14 .
The Bath -~

; Immersion tin baths are not new and have been used 18 for many years, particularly in decorative plating. The 19 combination of a stannous salt and HCl has been known, but such a bath proves inadequate in the plating of tin over 21 metal circuitry. For one thing, the tin plated surface 22 was found to be porous and crystalline on the copper sub-` 23 strate. It has been now discovered that by adding a wetting 24 agent to this composition, a beautiful, smooth plate can be achieved which yields exceptionally improved tin thickness.
26 It has also been found that the addition of a sulfur component 27 aids in removal of impurities and secondary reaction products '~''.: . .
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-; ( 6 3 ~ ` I 1037Z07 1 and generally enhances the stability of the bath. The tin bath 2 of the present invention is capable of forming a tin plate up to about 300 millionths of an inch being so non-porous it can act as an etch resist, The result is improved plating and a more effici-. - ;.
ent bath.
S Among the stannous salts found operable in the present inven-tion include ~soluble organic and inorganic acid sal~s of tin.
8 While applicant does not limit himsel to any specific stannous 9 salt, illustxative of those contemplated within this invention are stannous salts of halides, nitrates, acetate, boron-1uoride 11 complexes, and sulfates.
12 Organic anioni.e, non-ionic and cationic surface active agents 13 have been found useful as the wetting agents in the present inven-14 tion. Preferred wetting agents include fluorinated carboxylic acid~
~ such as FC-98, manufactured by the Minnesota Mining and Manufactur-16 ing Company and the Triton- ~series of wetting agents manufactured 17 by the Rohm and Haas Company.
18 The acids e~fective in the present invention are strong inor-19 ganic and organic acids. The preferred inorganic acids are the minexal acids such as hydrochloric acid, sulfuric acid, nitric acic 21 and phosphoric acid. Useful organic acids incl~de, for example, 22 acetic acid and formic acid. `
23 The sulfur component useful in the present invention includes 24, organic and inorganic sulur-containing compounds. The sulfur com-2; ponent comprises 8t least two sulfur-containing compounds. Example 26 of organic compounds include aliphatic sulfur-nitrogen compounds, .
27 such as thiocarbamates, eOg. thiourea; 5-membered heterocyclics containing S-N in the S-membered ring, such as, thiazoles and ;`" ':
( 7 ) _ _ .

~ 1037Z0~7 isothiaz es; dithiols, e.g., 1,2-ethanedithiol 6-membered 2 heterocyclics containing S-N in the ring such as thiazines, e.g., , 1,2-benzisothiazine, benzothiazine; thioamine acids such as 4 methionine, cystine, cysteine; and, thio derivatives of alkyl --.5 glycols. Examplesof inorganic compounds include alkali metal . sulfides, alkali metal thiocyanates and alkali metal dithionates.
: 7 It is important however, that no matter what gulfur compound 8 are used, alkali metal polysulfides should be present within a 9 certain limited range of concen~rations, preferably between about 0.005 to about 0.2 g/l of the total bath composition.
11 Commercial imported thiourea is often used to prepare the 12 plating bath of the present invention. Commercial imported 13 thiourea is distinguished from, for example, reagent grade 14 thiourea as the commercial grade has a greater concentration of sulfides present. An example of such a commercial grade thiourea 16 is that manuactured by the DeGussa Company.
It has been found that when a fresh bath has been prepared 18 and a commercial imported thiourea has been used as the sulfur 19 component, a greenish-brown precipitate is formed. Articles in the plating bath plate unacceptably for about the first hour. It 21 has been found that when this precipitate is eliminated as by 22 filtration, plating can be accomplished, but an unacceptable 23 crystalline tin deposit forms. It has been found that if after t e 24 precipitate is removed there is add~d an alkali me~al polysulfide of the general formula, M2SX, wherein x is a number from 2 to 5, 26 in amounts of about .005-.2 g/l of the bath composition, the tin 27 bath composition is capable of depositing a smooth, even tin 28 coating.

( 8 ) __ _ _ _ __. ~ _ _ , 1 ¦ Where reagent grade or chemlcally pure thiourea is usAed, no A 2~ ¦ precipitate 1~ formed but a granular tin plating occurs which is 3 1 very porous and unacceptable. While no filtering is necessary, the addition of an alkali metal polysulfide of the general formula M2SX, where x is from 2 to 5 in an amount between .005-,2 g/l will cause the tin bath composition to plate a smooth, even tin coating.
7 If too much of the polysulfide compound is used, a dull brownish 8 plate will be formed instead of the normal semi-matte silver-white 9 coating. This dullish brown plate is easily cleaned, however, with a solution of potassium chlorideO
11 ~t is therefore seen that the addition of a specified amount 12 of an alkali metal polysulfide to the tin plating bath causes 13 unusually and unexpectedly smooth, lustrous plating. Where 14 commercial grade thiou-ea is used, which actually contains appre-ciable and ~ i f amounts of sulfide~, such sulfides must be 16 eliminated first to allow the addition of a known quantity of 17 polysulfides. Where a reagent grade thiourea is used which does 18 not contain appreciable amounts of sulfides, no filtering step 19 is necessary and the specified amount of alkali metal polysulfides may be added directly.
21 The immersion tin plating bath may be for~ulated in either 22 of two procedures. First, de-ionized or distilled water in an 23 amount equivalent to about 70~ of the required final bath volume is heated to the bath operating temperature of 50-80C.
The chemicals as discussed above are added while stirring. After 26 the chemicals are added, the remainder of water is added ~o 27 bring the bath to full volume~
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The Platin~ Process 1 With respect to the procéss, in order for successful tin 2 plating to be accomplished, the copper or other metallized surface 3 on a panel or board must be free from grease and oxide films. The 4 industry generally uses many types of cleaning cycles. The treat-ment afforded the surface to be plated depends upon the cleanli-6 ness of the material to be treated and associate factors. Scrub-bing with conventional alkaline cleaners is used to remove heavy soils. Oxides may be removed from the metal surfaces by applica-9 tion thereto of a dilute acid solution such as dilute sulfuric hydrochloric acid, or a light etching solution such as a 25%
11 solution of ammonium persulfate in watcr. Often both of these 12 solutions may be employed, separated with a water rinse step.
13 The treatment period and temperature of this cleaning cycle are 14 significant, in that elevated temperatures and extended periods of time may result in removal not only of the oxide materials but 16 of the metal itself. The panel or board containing the metal 17 surface is rinsed thoroughly after this cleaning step with water 18 to remove all residue of etching compounds. Care should be taken 19 to avoid the formation of further oxide film during rinsing as a ¦result of air oxidation.
21 If the condition of the materials permit, a sanding opera-22 tion with a fine abrasive can also be used to remove oxides.
23 The boards or panels containing the metal surfaces are usually transported from process to process on racks. In view of the nature of the immersion tin baths special precaution must be 26 taken as to the choice o the material of these racks. Polypropy-27 lene or coated stainless steel racks are recommended. Uncoated ' .
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~ ' ~ 1037Z0'7 ~¦stainless teeL racks can be used for short runs, but as the - bath contains a sulfur compound, caution should be ~aken to 3 prevent contamination of the rack and fouling of the bath. Racks 4 made of iron and other metals easily at~acked by corrosive acids ; 5 such as hydrochloric acid should also be avoided.
6 The immersion tin bath must be agitated when in use to pre-7 vent localized starved spots. Air agi~ation should not be used but, rack agitation proves ~uite effective. Mild agitation for 9 a minute upon entering the bath solution ensures unifonm coverage.
Also found very effective is the use of a propellor mixer, suf-11 ficient to circulate solution through a rack without introducing 12 air.
13 The tin plating bath of the present invention is generally ;
14 operated at a temperature of 50-80C. Storing the bath composi-tion at temperatures of 50C or higher tends to accelerate the 16 decomposition of thiourea. However, it should be noted that at temperatures below about 50C, the chemicals begin to salt out of 18 the solution.
19 Upon ~ormulation and heating, the bath should be a pale gree color. The color will gradually turn to a coffee color, usually 21 after two to three hours. During this transitional period of 22 coffee color, parts should not be plated.
23 The color change is believed to be due to the formation of 24' a precipitate,stannous sulfide. The precipitate will ~e-deposit on any parts being plated during the transitional period, causing grey _ 26 blaclc deposits and occasionally a rust-colored dusty deposit.
27 These deposits can easily be removed by a light brushing of the 28 part with water. However, a dust-free, deposit-free operation _~ . ( 11 ) .. ... . .

1037~07, r,,,, ~ may be accomplished by cvmpletely removing the precipitate by 2 filtering the h~t bath solution through a 10 mlcron glass 3 1 filter. Again, it is noted that if the solution cools below -~

~s 4 ¦ 50C, it will salt out.

5 ¦ The bath should remain covered when not in use to

6 1l avoid iron or alkaline contamination.

7 When the copper surface being plated becomes grey and 9 spotty, the bath is depleted and should be either discarded or re-activated.
10 I The effective life of the tin plating bath of course, 11 ; depends upon many factors. It has been found, however, that ; 12 when the bath is operated at its preferred conditions, e.g.
13 60C~ 5C and it is at its preferred formulation, the bath 14 will plate 30-35 square feet of copper area per gallon of 15 1 bath with tin 70-80 millionths inch thick.
16 ~
17 11 :

18 l~ The preferred operating con-~;1 19 ~ ditions and bath formulations (in Ex~nple 2 below) were t 20 ¦ used in the compilation of data and it was 21¦ found that lt would take about 40 minutes of ~
221 immersion plating time to achieve a tin coating of 80 -23~ millionths of an inch ~hickness.
241 After the panels have plated to the desired thickness, the rack is transferred to a water rinse. The use of warm water is recommended to ensure complete ~eNewe~ of plating salts and to avoid staining upon drying. Poor rinsing is the primary cause of ( 12 ) --- ..... __ ; 1037Z(1'7 I ~stained anl dull tin plated circuits. A typical eLfective 2 rinsing operation comprises a warm water rinse of 100-120F for 3 five minutes.
After rinsing, the panels may be routinely air dried, or more preferably be either forced air dried using clean air or a 6 warm oven bake operated at temperatures of approximately 150-300~ .
7 The no~mal thickness of tin plating (about 80 millionths inch) will withstand optional mild brushing such as wire brushing 9 or optional light pumice brushing. Such optional wire brushing will provide a pleasing shiny appearance and minimize fingerprint 11 as well as other stains. In addition, optional wire brushing pro-12 vides the most solderable surface. Optional Scotch-brite brushing 13 will also yield fine results when set at as light a pressure as 14 possible. In all such optional brushing operations, the machines should be thoroughly cleaned and free of contaminants such as 16 sulfuric acid, copperbrite, etc. Such contaminants can eventually 17 oxidize the tin surface.
; 18 EX~LE

Precleanin~ Cycles 21 The typical process will begin with a pre-cleaning cycle to 22 insure that the copper or other metallic surfaces to be plated are 23 grease and oxide free. The cleaning cycle used usually depends 24 upon the degree of contamination of the surface. A typical mild pre-cleaning cycle would comprise the following steps for the 26 designated time periods:

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1037Z~7 :~
(a) Altrex Soak* 5 minutes (150F-180F) (b) Water rinse 1 minute (c) 10% H2S04 dip 30 seconds 4 (d) Water rinse 1 minute ~i .
' S ., , While the above pre-cleaning cycle is usually quite 7 adequate a stronger cleaning cycle is also often uqed consisting of the following steps:
(a) Ammonium persulfate dip 30 seconds (25% APS at 1~0F) 11 (b) Water rinse 1 minute 12 (e) 10% H2SC4 dip 15 seconds 13 (d) Water rinse 1 minute (allow to drain) 14 `
EXAMPLE _II

17 An immersion tin plating bath was prepared in accord-18 anee with the present invention as indieated below:
19 Stannous ehloride21 g/l 20 Thiourea** 90 g/l 21 Concentrated HCl 22 (37% aqueous)36 ml/l ' FC-98 0.5 g/l 23 .
24 *** Potassium polysulfides 0.1 g/l Deionized water Balanee ... .__ .. _ . . .... _, ._ _ !'' ~ 26 ~ Altrex is thè ~n~-o~ mild alkaline detergent manufactured by Wyandotte Chemicals Corp., Wyandotte, Michigan.
`27 ** Chemically pure grade *** Sulfurated potash as manufactured by the Fisher Scientific Company.
( 14 ) : . -, ....... ... ~ .. --:

. - , 1 In the above formulation FC-98 is a fluorinated 2 carboxylic acid wetting agent manufactured by the Minnesota 3 Mining and Manufacturing Company. The bath formulated above 4 was 12 liters (3.18 gallons).
After being pre-cleaned in accordance with the mild S pre-cleaning cycle of Example I above, circuit boards were 7 immersed on racks into the tin plating bath formulated above~
These circuit boards con~ained the following copper surface 9 areas:
,, 10 .
11 NoO of Boards Dimensions= Copper Surface Area 12 24 pcs~ 2 1/2" x 8" 6.67 sq. ft.
13135 pcs. 6 " x 8" 89.33 sq. fto 14 31 pcs. sample circuits 5000 sq, ft.
lOlo~OO sq~.-ft.-16 .
17 The bath was operated a~ a temperature of 65C with 18 initial agitation by vibration of the racks. The boards were 19 removed from the bath after 40 minutes and several microsections were made of the plated boards. The results indicated the 21 average plated thickness of the tin was 80 millionths inchu 22 The yield of the bath was then calculated, based upon the volume of the bath, 12 liters or 3.18 gallons, and the 24 average surface plate thickness of 80 millionths inch:
101 s~. ft = 32 sq. ft./gal.
26 3,18 gal.

..... _ .. _ '' 37ZO~ ~
¦ Therefore, the bath of the above formulation operated at the 2 ¦ above conditions yielded a tin plate of 80 millionths inch thick -3 over 32 square feet of copper per gallon of bathO
4 To determine the optimum and maximum lie of the plating bath additional experiments were made increasing the surface area S of copper in the tin bath. It was found that when more than 101 sqO fto o copper surface was immersed into the 3.18 gallon bath, the copper surace became grey and spotty indicating the bath was depleted before it could adequately plate the copper surfaces.
Therefore, it is seen that 32 sq. ft. of copper surface per ll gallon of bath represents an optimum yield.
12 While the above examples are illustra~ive of the tin plating 13 bath composition and process of tin plating, variations of the 14 process and compositions have proved e~ually as effective. ~ -For example, the components in the preferxed composition of the 16 tin bath may be present in the following ranges of concentra~ion 17 based upon the total bath composition:
18 Stannous salt 15-30 g/l 19 Sulfur component 15-120 g/l Mineral Acid 25-50 ml/l 21 We~ting agent 1 10 gfl 22 Wa~er Balance 23 It is noted that ~he sulfur component comprises an alkali 24 metal polysulfide and at least one other sulfur compound as described earlier. The ratio between the polysulfide and the 26 other sulfur compounds comprising the sulfur component can 27 vary widelyO For example, where polysulfides are present with ',:`' .

( 16 ) .. ___ _ - . ., 1 ¦only one other sulfur-containing compound, the xatio of the~
2 ¦former to the latter may be between about .004% to 1.3%.
3 ¦Solder masks and legends can be applied either before or after 4 ¦ tin plating. If the solder mask is appli~d before tin plating, ¦ traditional and customary techniques may be employed but with 6 special precaution to employ a sufficient cure of the mask 7 before the plating operationO The solder mask should be applied over a clean, wire brushed surface. A single pass 9 through the gas-fired oven at 250F is not a sufficient cure to ~0 withstand the subsequent contact with the tin bath. In-11 sufficient cure will cause the solder mask to blister in the 12 tin bath. The following bakes enumerated below are merely illustrative of the minimum bakes which have proved quite adequate in protecting the solder mask during tin bath lS procedure:

17 (a) 2 passes through a gas-fired oven at 250F
18 (b) 30 minutes at 250F oven bake (c) 15 minutes at 320F oven bake 21 Application of the solder mask over the plated tin is done 24, 27 ( 17 ) .
., . ' , .
_ . _ . .

1037Z0~7 :
, in the same manner as solder maslcing over solder plate.
owever, the tin under the solder mask will re-flow upon pro- -~: longed exnosure to mo]ten solder, in exccss of 8 ~eco~
The tin re-flow causes the solder mask to wrinkle. This is ,-- the same phenomenon as observed with the mask over solder. -, Legends are best applied after tin plating. The problem -~
with legends applied prior to tin plating is limited to where ~;- legends are applied directly to a copper surface. The tin ; bath tends to lift off legends where thev adhere to copper.
If the legend is applied over the epoxy mask or the base material only, no lifting will occur in the tin bath. Legends ,~ .
generally will remain their normal color during 40 minutes , ., ~ .
exposure to the tin bath. IJhite legend may tint a very nale green, but the color change is almost imperceptible.
The tin plating process of the present invention may be accomplished on circuit boards containing areas of nickel gold plating. The preferred procedure is to f;rst screen a cl~ar mask over the nickel-gold fingers. Then the boards are cleaned ' in accordance with the pre-cleaning cycle hereinabove discussed and plate with tin. After the tin plate and rinsing procedures, the fingers may be stripped by conventional procedures including Blakeslee strip~ which is a process for cleaning a surface using a machine such as a Blalceslee Printed Circllit Board Stripning Machine manufactured by the Baron Blakeslee Com~any. Ihe milclline comprises a conveyor on which the board to be cleaned is sprayed with methylene chloride and simultaneously brushed.
; An alternative procedure for tin plating a board containing nickel-gold fingers is to first tape the fingers in the conven-tional manner. Commonly available platers tape may be used and applied firmly to fingers to avoid solution creepage. The boards ; are then cleaned in accordance with the pre-cleaning cvcle here-inabove described, tin plated, rinsed and finally the tape is .~';
~ dr ~ - 18 , ¦removed. 10;~7Z07 ¦ It has been found that high quality solderability has 3 1 been achieved after exposure of 50-80 millionths inch of tin 4 1 plate after the following conditions:
(1) Humidity conditioning at 35C and 907O relative 6 1 humidity for 96 hrs;
7 ¦ (2) Baking at 320F for 1 hour, or 250F for 2 hours or :
3 passes through Gas Fired Oven at 250 F or 120F
9 or 3 hours;
(3) Exposure to 35C temperature and 90% relative 11 humidity for 10 days O

13 Additionally, the high quality of solderability provided 14 by this invention extends for long periods of time.
The invention in its broadest aspects is not limited 16 to the specific steps, processes and composition shown and 17 described but departures may be made therefrom within the scope 18 of the accompanying claims without departing from the principles of ~he invention and without sacrificing its chief advantages.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An immersion tin plating bath composition for depositing a smooth, even tin coating on metallized surfaces capable of chemically displacing tin from a tin plating bath, said composition comprising a soluble stannous salt, a sulfur component comprising at least two sulfur-containing compounds, a mineral acid, a wetting agent and water.

2. The immersion tin plating bath composition as defined in claim 1 wherein said soluble stannous salt is selected from the group consisting of stannous salts of halides, sulfate, fluoborate, nitrate, and acetate; the sulfur component comprises a mixture of alkali metal polysulfides, present in an amount sufficient to produce said smooth, even coating, and at least one other sulfur-containing compound; 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.

3. The immersion tin plating bath as defined in claim 2 wherein said ingredients are present in the following con-centration ranges:

4. The tin plating bath composition of claim 2 wherein the sulfur component contains alkali metal polysulfides and thiourea.

5. The tin plating bath composition as defined in claim 2 wherein said stannous salt is stannous chloride, said sulfur component contains thiourea and alkali metal polysulfides, and said mineral acid is hydrochloric acid and said wetting agent is a fluorinated carboxylic acid.

6. The tin plating bath composition of claim 5 wherein said fluorinated carboxylic acid is present at a concentration of from about 0.1 to about 5 g/l.

7. The immersion tin plating bath composition of claim 3 wherein said sulfur component contains thiourea and alkali metal polysulfides wherein said alkali metal polysulfides are present in the range of 0.005 to 0.2 g/l of the bath composition.

8. The tin plating bath composition of claim 7 wherein said ingredients are present in the following concentrations:

9. 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 at least two sulfur containing compounds, a mineral acid, a wetting agent and water, 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.

10. The process as defined in claim 9 wherein the tin plating bath composition comprises a stannous salt selected from the group consisting of stannous salts of halides, sulfate, fluo-borate, nitrate and acetate; the sulfur component is a mixture of alkali metal polysulfides and at least one other sulfur-containing compound; 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.

11. The process as defined in claim 9 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.

12. The process of claim 11 wherein the alkali metal polysulfides are present at a concentration of 0.005 to 0.2 g/l of the bath composition.

13. The process as defined in claim 9 wherein said bath is operated at a temperature of 50°-80°C.

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 at least two sulfur-containing compounds, a mineral acid, a wetting agent and water until tin fonns in a continuous coating on said copper circuitry;

(4) (2) rinsing said boards; and (3) drying said boards.

15. The process as defined in claim 14 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 sulfur component contains alkali metal polysulfides and at least one other sulfur-containing compound; 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.

16. The process as defined in claim 14 wherein said tin plating bath comprises stannous chloride, thiourea, alkali metal poly-sulfides, hydrochloric acid and fluorocarboxylic acid wetting agent.

17. The process as defined in claim 14 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.

18. The process as defined in claim 14 wherein said tin plating bath is operated at a temperature of 50°-80°C.

19. The process as defined in claim 14 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.

20. The process as defined in claim 14 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) dippping into a 10% sulfuric acid solution and (4) rinsing said board in water.

21. The process as defined in claim 14 wherein said rinsing step comprises rinsing said circuit boards in warm water at a temperature of about 100°F-120°F.

22. The process as defined in claim 14 wherein said drying step comprises a warm oven bake operated at a temperature between about 200°F-250°F.

23. The process as defined in claim 14 wherein after the drying step said boards are wire brushed.

24. The process as defined in claim 14 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.

25. A process for the manufacture of printed circuit boards having a smooth, even tin coating over areas of clean copper circuitry wherein the improvement comprises the in-clusion in an immersion tin plating bath containing a soluble stannous salt, a mineral acid and water, of a sulfur component comprising at least two sulfur-containing compounds and a wetting agent.