Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C18/00—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating
  • C23C18/16—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating by reduction or substitution, e.g. electroless plating
  • C23C18/18—Pretreatment of the material to be coated
  • C23C18/1803—Pretreatment of the material to be coated of metallic material surfaces or of a non-specific material surfaces
  • C23C18/1824—Pretreatment of the material to be coated of metallic material surfaces or of a non-specific material surfaces by chemical pretreatment
  • C23C18/1837—Multistep pretreatment
  • C23C18/1844—Multistep pretreatment with use of organic or inorganic compounds other than metals, first
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C18/00—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating
  • C23C18/16—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating by reduction or substitution, e.g. electroless plating
  • C23C18/52—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating by reduction or substitution, e.g. electroless plating using reducing agents for coating with metallic material not provided for in a single one of groups C23C18/32 - C23C18/50
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C18/00—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating
  • C23C18/54—Contact plating, i.e. electroless electrochemical plating

Definitions

• Electrolytic plating is the production of adherent deposits of metals on conductive surfaces carried out by passage 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 is further characterized by the catalytic nature of the surface which enables the metal to be plated to any thickness.
• such solutions comprise a solvent, a supply of ions of the metal to be deposited, an agent capable of reducing the ions of the metal to be deposited, a complexing agent for the ions of the metal to be deposited, and a pH regulator.
• Immersion plating or contact plating depends, however, upon a galvanic displacement reaction.
• the current instead of 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 to to 50 millionths of an inch.
• deposition stops as soon as the base metal is entirely covered forming a very thin deposit.
• This invention is concerned with immersion plating and its attendant advantages which include, among others: immersion deposits which are decidedly adherent; deposits with considerable resistance to corrosion; the production of dense impervious deposits; and the ability to deposit metal on closely defined areas of metallized surface.
• the high quality of solderability provided by this invention endures for a period in excess of six months of storage under normal stock room conditions.
• the chemical resistance of the tin plate of the present invention is surprisingly excellent.
• the tin plate remains solderable after exposure to normal printed circuit processing chemicals i.e., chromic acid,
• the present invention has the following objects:
• immersion tin plating bath compositions for depositing a smooth, even tin coating on metallized surfaces, comprising a soluble stannous salt, a sulfur component, a mineral acid, and a wetting agent.
• the metal of the substrate 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 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, and a wetting agent, 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.
• a still further preferred embodiment of the invention is in a process for the manufacture of printed circuit 3 boards having a smooth, even tin coating over areas of clean copper circuitry having grease-free and oxidefree copper surfaces, comprising the steps of:
• the Bath Immersion tin baths are not new and have been used for many years, particularly in decorative plating.
• the combination of a stannous salt and HCl has been known, but such a bath proves inadequate in the plating of tin over metal circuitry.
• the tin plated surface was found to be porous and crystalline on the copper substrate. It has been now discovered that by adding a wetting agent to this composition, a beautiful, smooth plate can be achieved which yields exceptionally improved tin thickness.
• a sulfur component aids in removal of impurities and secondary reaction products and generally enhances the stability of the bath.
• the tin bath 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 efficient bath.
• stannous salts found operable in the present invention include soluble organic and inorganic acid salts of tin. While applicant does not limit himself to any specific stannous salt, illustrative of those contemplated within this invention are stannous salts of halides, nitrates, acetate, boron-fluoride complexes, and sulfates.
• wetting agents include fluorinated carboxylic acids such as FC-98, manufactured by the Minnesota Mining and Manufacturing Company and the Triton-X series of wetting agents manufactured by the Rohm and Haas Company.
• the acids effective in the present invention are strong inorganic and organic acids.
• the preferred inorganic acids are the mineral acids such as hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid.
• Useful organic acids include, for example, acetic acid and formic acid.
• the sulfur component useful in the present invention includes organic and inorganic sulfur-containing compounds.
• the sulfur component comprises at least two sulfur-containing compounds.
• organic compounds include aliphatic sulfur-nitrogen compounds, such as thiocarbamates, e.g. thiourea; S-membered heterocyclics containing S-N in the 5-membered ring, such as, thiazoles and isothiazoles; dithiols, e.g., 1,2-ethanedithiol; 6-membered heterocyclics containing S-N in the ring such as thiazines, e.g., 1,2-benzisothiazine, benzothiazine; thioamine acids such as methionine, cystine, cysteine; and, thio derivatives of alkyl glycols.
• inorganic compounds include alkali metal sulfides, alkali metal thiocyanates and alkali metal dithionates.
• alkali metal polysulfides should be present within a certain limited range of concentra- It has been found that when a fresh bath has been I prepared and a commercial imported thiourea has been used as the sulfur component, a greenish-brown precipitate is formed. Articles in the plating bath plate unacceptably for about the first hour. It has been found that when this precipitate is eliminated as by filtration, plating can be accomplished, but an unacceptable crystalline tin deposit forms.
• the tin bath composition is capable of depositing a smooth, even tin coating.
• the immersion tin plating bath may be formulated in either of two procedures. First, de-ionized or distilled water in an amount equivalent to about percent 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 the chemicals are added, the remainder of water is added to bring the bath to full volume.
• the Plating Process in order for successful tin plating to be accomplished, the copper or other metallized surface on a panel or board must be free from grease and oxide films.
• the industry generally uses many types of cleaning cycles.
• the treatment afforded the surface to be plated depends upon the cleanliness of the material to be treated and associate factors. Scrubbing with conventional alkaline cleaners is used to remove heavy soils. Oxides may be removed from the metal surfaces by application thereto of a dilute acid solution such as dilute sulfuric hydrochloric acid, or a light etching solution such as a 25% solution of ammonium persulfate in water. Often both of these solutions may be employed, separated with a water rinse step.
• the treatment period and temperature of this cleaning cycle are significant, in that elevated temperatures and extended periods of time may result in removal not only of the oxide materials but of the metal itself.
• the panel or board containing the metal surface is rinsed thoroughly after this cleaning step with water to remove all residue of etching compounds. Care should be taken to avoid the formation of further oxide film during rinsing as a result of air oxidation.
• a sanding operation with a fine abrasive can also be used to remove oxides.
• the boards or panels containing the metal surfaces are usually transported from process to process on racks.
• special precaution must be taken as to the choice of the material of these racks.
• Polypropylene or coated stainless steel racks are recommended. Uncoated stainless steel racks can be used for short runs, but as the bath contains a sulfur compound, caution should be taken to prevent contamination of the rack and fouling of the bath.
• Racks made of iron and other metals easily attacked by corrosive acids such as hydrochloric acid should also be avoided.
• the immersion tin bath must be agitated when in use to prevent localized starved spots. Air agitation should not be used but, rack agitation proves quite effective. Mild agitation for a minute upon entering the bath solution ensures uniform coverage. Also found very effective is the use of a propellor mixer, sufficient to circulate solution through a rack without introducing air.
• the tin plating bath of the present invention is generally operated at a temperature of 5080C. Storing the bath composition at temperatures of 50C or higher tends to accelerate the decomposition of thiourea. However, it should be noted that at temperatures below about 50C, the chemicals begin to salt out of the solution.
• the bath Upon formulation and heating, the bath should be a pale green color. The color will gradually'turn to a coffee color, usually after two to three hours. During this transitional period of coffee color, parts should not be plated.
• the color change is believed to be due to the formation of a precipitate, stannous sulfide.
• the precipitate will do-deposit on any parts being plated during the transitional period, causing grey-black deposits and occasionally a rust-colored dusty deposit. These deposits can easily be removed by a light brushing of the part with water.
• a dust-free, deposit-free operation may be accomplished by completely removing the precipitate by filtering the hot bath solution through a micron glass filter. Again, it is noted that if the solution cools below 50C, it will salt out.
• the bath should remain covered when not in use to avoid iron or alkaline contamination.
• the bath is depleted and should be either discarded or re-activated.
• the effective life of the tin plating bath depends upon many factors. lt has been found, however, that when the bath is operated at its preferred conditions, e.g. 60C i 5C and it is at its preferred formulation, the bath will plate 3035 square feet of copper area per gallon of bath with tin 70-80 millionths inch thick.
• Example 2 The preferred operating conditions and bath form ulations (in Example 2 below) were used in the compilation of data for this analysis. It is therefore seen that it would take about 40 minutes of immersion plating time to achieve a tin coating of millionths of an inch thickness.
• the rack is transferred to a water rinse.
• warm water is recommended to ensure complete removel of plating salts and to avoid staining upon drying. Poor rinsing is the primary cause of stained and dull tin plated circuits.
• a typical effective rinsing operation comprises a warm water rinse of lOO-12OF for five minutes.
• the panels may be routinely air dried, or more preferably be either forced air dried using clean air or a warm oven bake operated at temperatures of approximately l50-300F.
• tin plating (about 80 millionths inch) will withstand optional mild brushing such as wire brushing or optional light pumice brushing. Such optional wire brushing will provide a pleasing shiny appearance and minimize fingerprint as well as other stains. In addition, optional wire brushing provides the most solderable surface. Optional Scotchbrite brushing will also yield fine results when set at as light a pressure as possible. In all such optional brushing operations, the machines should be thoroughly cleaned and free of contaminants such as sulfuric acid, copperbrite, etc. Such contaminants can eventually oxidize the tin surface.
• FC-98 is a fluorinated carboxylic acid wetting agent manufactured by the Minnesota Mining and Manufacturing Company.
• the bath formulated above was 12 liters (3.18 gallons).
• circuit boards were immersed on racks into the tin plating bath formulated above. These circuit boards contained the following copper surface areas:
• the bath of the above formulation operated at the above conditions yielded a tin' plate of 80 millionths inch thick over 32 square feet of copper per gallon of bath.
• the components in the preferred composition of the tin bath may be present in the following ranges of concentration based upon the total bath composition:
• the sulfur component comprises an alkali metal polysulfide and at least one other sulfur compound as described earlier.
• the ratio between the polysulfide and the other sulfur compounds comprising the sulfur component can vary widely. For example,
• solder masks and legends can be applied either before or after tin plating. If the solder mask is applied before tin plating, traditional and customary techniques may be employed but with special precaution to employ a sufficient cure of the mask before the plating operation. The solder mask should be applied over a clean, wire brushed surface. A single pass through the gas-fired oven at 250F is not a sufficient cure to withstand the subsequent contact with the tin bath. Insufficient cure will cause the solder mask to blister in the 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 procedure:
• a. 2 passes through a gas-fired oven at 250F b. 30 minutes at 250F oven bake c. 15 minutes at 320F oven bake
• Application of the solder mask over the plated tin is done in the samemanner as solder masking over solder plate. However, the tin under the solder mask will reflow upon prolonged exposure to molten solder, in excess of 8 seconds. 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 they adhere to copper.
• 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 first screen a clear mask over the nickel-gold fingers. Then the boards are cleaned in accordance with the precleaning cycle hereinabove discussed and plated with tin. After the tin plate and rinsing procedures, the fingers may be stripped by conventional procedures including Blakeslee strip.
• An alternative procedure for tin plating a board containing nickel-gold fingers is to first tape the fingers in the conventional manner. Commonly available platers tape may be used and applied firmly to fingers to avoid solution creepage. The boards are then cleaned in accordance wtih the pre-cleaning cycle hereinabove described, tin plated, rinsed and finally the tape is removed.
• the high quality of solderability provided by this invention extends for long periods of time.
• 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 Soluble stannous salt 15-30 g/l Sulfur component 15-120 g/l Mineral acid 25-50 ml/l Wetting agent 0.1-l g/l Water Balance said sulfur component comprising at least two sulfurcontaining compounds selected from the group consisting of organic sulfur compounds and inorganic sulfur compounds, said organic sulfur compounds selected from the group consisting of aliphatic sulfur-nitrogen compounds, and 6-membered heterocyclic compounds containing S-N in the ring, dithiols, thio derivatives of alkyl glycols, and thioamine acids, and said inorganic sulfur compounds selected from the group consisting of alkali metal sulfides, alkali metal thiocyanates, and alkali metal dithionates.
• soluble stannous salt is selected from the group consisting of stannous salts of halides, sulfate, fluoborate, nitrate, and acetate;
• sulfur component comprises a mixture of alkali metal polysultides and at least one other sulfur-containing compound;
• wetting agent is selected from the group consisting of organic anionic, non-ionic and cationic surfactants;
• mineral acid is selected from the 110 group consisting of hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid.
• stannous salt is stannous chloride
• sulfur component contains thiourea and alkali metal polysulfides
• mineral acid is hydrochloric acid
• said wetting agent is a fluorinated carboxylic acid.

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• Chemical & Material Sciences (AREA)
• General Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Electrochemistry (AREA)
• Chemically Coating (AREA)
• Manufacturing Of Printed Wiring (AREA)

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Citations (4)

• 1973
  • 1973-07-24 US US382056A patent/US3917486A/en not_active Expired - Lifetime
• 1974
  • 1974-04-26 CA CA199,110A patent/CA1037207A/en not_active Expired
  • 1974-05-17 JP JP5607274A patent/JPS5624713B2/ja not_active Expired
  • 1974-07-11 CH CH958174A patent/CH606499A5/xx not_active IP Right Cessation
  • 1974-07-15 GB GB3120874A patent/GB1436645A/en not_active Expired
  • 1974-07-19 FR FR7425180A patent/FR2238772B1/fr not_active Expired
  • 1974-07-22 SE SE7409520A patent/SE411228B/xx unknown
  • 1974-07-23 DK DK397374A patent/DK397374A/da not_active Application Discontinuation
  • 1974-07-23 IT IT52221/74A patent/IT1016948B/it active
  • 1974-07-24 AT AT609174A patent/AT332188B/de not_active IP Right Cessation
  • 1974-07-24 NL NL7410019A patent/NL7410019A/xx not_active Application Discontinuation

Patent Citations (4)

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