US3457151A

US3457151A - Electrolytic cleaning method

Info

Publication number: US3457151A
Application number: US589893A
Authority: US
Inventors: Arnold Kortejarvi
Original assignee: Solutec Corp
Current assignee: Solutec Corp
Priority date: 1966-10-27
Filing date: 1966-10-27
Publication date: 1969-07-22
Anticipated expiration: 1986-07-22

Description

July 22, 1969 A. KORTEJARVI ELECTROLYTIC CLEANING METHOD Filed Oct. 27. 1966 AND F/aZ

+ TRANSFORMER RECT\F\E\Z AND TRANSFORMER //V I/E/V 7'02, flaw/010 Karma/new MQM M' JTTOIZ/VEVS' United States Patent U.S. Cl. 204-130 4 Claims This invention relates to a method and apparatus for cleaning articles and more particularly to a method and apparatus for cleaning conductive and nonconductive articles by electrolysis.

Many processes are known for removing foreign matter that adheres to the surfaces of metallic articles which involve the electrolytic treatment of these articles. Typically, the article to be cleaned is connected to the negative terminal of a direct current power source and is immersed in a solution of a suitable electrolyte with an electrode connected to the positive terminal of the power source. As the current flows between the cathode, which may be the article to be cleaned, and the anode or the electrode connected to the positive terminal, hydrogen bubbles are produced at the cathode. The hydrogen bubbles evolved at the cathode provide a mechanical scrubbing action which dislodges any foreign matter from the surfaces of the article to be cleaned.

In order to clean a number of metallic articles, it has been proposed to utilize a metallic container to support the articles in the bath. The container is connected to the negative terminal of the power source so that during electrolysis, the articles in contact with the container become cathodic whereby hydrogen bubbles are formed on all of the surfaces of the articles, which bubbles as they are released from the surfaces carry away the foreign matter for the desired cleaning function.

While the above-described processes have been found to be satisfactory in some instances for cleaning metallic or electrically conductive articles, they have not always provided a satisfactory and nondeleterious cleaning action for certain metallic and nonmetallic articles, nor have they been adapted to clean completely nonmetallic or nonconductive articles. One reason is that any hydrogen bubbles evolved at the cathodic elements are not generated in intimate proximity to the nonconductive surfaces to produce the required scrubbing action. Also, the electrolyte solutions employed in these processes sometimes comprise chemical substances which make them strongly alkaline, such as caustic soda. Sometimes the electrolytes comprise acids or chlorinated hydrocarbons and the like. These chemicals are, under certain circumstances, harmful to certain nonconductive materials to be cleaned, such as plastics and fabrics.

Accordingly, it is a primary object of the present invention to provide a method and apparatus for cleaning articles electrolytically that overcomes the limitations and disadvantages of the processes heretofore mentioned.

More specifically, it is an object of the present invention to provide a method and apparatus for cleaning articles electrolytically without being harmful thereto.

Another object 9f th,e. n!n i9n isto provide a method andv apparatus for electrolytically c'l eanini'ffiefallic'afid nonmetallic articles which utilizes an improved electrolyte composition.

Still another object of the present invention is the provision of an improved deoxidant dip to remove films of oxidation and deposits of foreign matter or smut that may remain adhered to the surfaces of the articles after electrolytic cleaning thereof.

A further object of the invention is the provision of an improved holder adapted for use in an electrolytic cleaning apparatus for certain articles which, by virtue of their shape and construction, are difiicult to clean.

3,457,151 Patented July 22, 1969 In accordance with one aspect of the presgnt invention, the foregomg and t r.ebieetaateattainssl hvim s an article',"w ichi'sir'i ade gf gnductive means such as priuted cirguit bgard in an electrolytic batlT and causing a current to flow in said bath between a cathodic element closely adjacent the board and an anodic element. The scrubbing action of the hydrogen bubbles generated at the cathodic element and at the conductive portions of the board effectively cleans all of the surfaces thereof.

The cathodic element may take the form of a book-like holder having a pair of pivotally connected frame members constructed of fine wire mesh or wire cloth and provided with fastening means such that when the frame members are pivoted together and fastened, an article such as a printed circuit board may be supported in the holder between the frame members.

To guard against injury to metallic and nonmetallic articles and to obtain efficient cleaning thereof by the electrolytic cleaning process of this invention, the bath comprises an aqueous solution of a mildly alkaline electrolyte.

The bath utilized in the present invention may also comprise a wetting agent.

Furthermore, in the practice of the present invention the bath may be heated by suitable means during electrolysis.

In addition, to remove any surface deposits of foreign matter or oxide films commonly referred to as smut that are left on the articles after electrolysis, the articles may be dipped in a solution comprising a deoxidant.

Other objects and advantages of the invention will hereinafter become more fully apparent from the following detailed description when considered in connection with the annexed drawing in which:

FIG. 1 is a perspective view of one embodiment of an electrolytic cleaning apparatus constructed in accordance with the teachings of the present invention;

FIG. 2 shows a sectional view, somewhat diagrammatically, of the embodiment of FIG. 1 wherein the tank containing the bath is fabricated of a conductive material and forms the anode of the electrolytic circuit;

FIG. 3 illustrates a sectional view, somewhat diagrammatically, of another embodiment of the apparatus of FIG. 1 wherein the tank is fabricated of a nonconductive material and wherein an anodic element is immersed in an electrolyte solution or bath 11. Immersed in the bath is a basket-like holder for articles to be cleaned (not shown) which is connected to the negative terminal of a source of electric current (not shown) by means of a lead 14. When the bath is electrolyzed by causing current to flow from the basket 13 acting as a cathode to an anode which may be the tank (not shown) metallic and/ or metallic articles supported by the basket 13 are cleaned by the combined action of hydrogen bubbles evolved at the basket and chemical agents comprising the bath 11.

A suitable cabinet structure 15 conveniently houses the various elements of the apparatus and is provided with an on-off switch 16, an indicator light 17, and a control knob 18 to vary the current passing through the bath, and a thermostatically controlled bath heater (not shown). Electrical power, which may be standard house current volts 60 cycle A.C.), is supplied to the apparatus by means of a common plug and cord unit 19.

In FIG. 2, tank 12 and holder 13 are shown in section and the electrical circuit is shown schematically. Tank 12 is preferably fabricated of an electrically conductive material that is highly resistant to the corrosive action of the electrolyte solution, such as stainless steel or Monel. The basket-like holder 13 is made of the same or of an equivalent material as the tank and is constructed in the form of a fine or closely spaced wire mesh. Basket 13 is provided with compartments formed by a plurality of vertically oriented integral partitions 21 made of the same wire mesh material as the sides and bottom of the basket, and is particularly yvell adapted to support a plurality of conductive and nonconductive articles, such as the printed circuit board assemblies 22, 23, and 24.

The tank 12 is made anodic and the wire mesh basket 13 is made cathodic by connecting them respectively to the positive and the negative terminals of a variable source of direct electric current. The DC. source may be in the form of a conventional rectifier and autotransformer unit 25 (not shown in detail) for converting the AC. supply. Any other suitable arrangement may also be used to supply the electrical power required to practice the process of the present invention.

In order to raise the temperature of the electrolytic bath 11 to the proper level, a heating element 26 controlled by a thermostat 27 is provided which may be conveniently connected to the A.C. power source. of course, other means may be utilized to heat the bath if desired.

By providing the cathodic basket 13 with the vertical mesh partitions 21, an abundant supply of hydrogen bubbles will be produced during electrolysis in close proximity to the nonconductive surfaces of the article to provide an improved scrubbing action. Also since the printed circuit board assemblies are supported on end or upright, restrictions on the upward movement of the hydrogen bubbles is kept to a minimum thereby further enhancing the scrubbing action. In comparison, electrolytic cleaning processes of the prior art depend upon the formation of hydrogen bubbles directly on the surfaces of the article to provide the cleaning action.

In the embodiment shown in FIG. 3, the tank 12 is made of a nonconductive material which is unaffected by the chemical action of the electrolyte solution, such as glass, certain plastics, or the like, and which may be transparent so that the cleaning process may be observed from outside. Since the tank is a nonconductor, an electrode 28 is immersed in an electrolytic bath 11 along with an article holder 29. The electrode 28 and the holder 29 are connected to the positive and negative terminals of the power source 25, as in the embodiment of FIG. 1, thereby making them anodic and cathodic, respectively. Cathodic holder 29 is especially useful to electrolytically clean a relatively flat metallic and/or nonmetallic article, such as a printed circuit board 37.

The cathodic holder 29 is more clearly illustrated in FIG. 4, and comprises a pair of generally

rectangular metal frames

31 and 32 of substantially equal size. The frames are pivotally connected or hinged together along one edge of each by means of rings 33. Within each frame and secured thereto is a metal wire cloth or a fine wire mesh material, which is preferably made of stainless steel or other highly corrosion resistant metal. To secure the frames together face-to-face, in closely adjacent relation with the board 37 sandwiched therebetween an appropriate releasable fastener assembly 34 is provided on frame 32 to cooperate with a loop member 35 secured to frame 31. To assist in manipulating the holder 29 in and out of the electrolytic bath, a handle 36 made of a conductive material is attached to frame member 31'and serves as a convenient part of the holder to which the negative lead 14 is connected. During the cleaning process, circuit board 37 will be completely encased by the wire mesh of the holder 29. In this position the conductive as well as the nonconductive surfaces will be in intimate contact with the mesh such that during electrolysis, the

evolution of hydrogen bubbles will take place immediately adjacent the surfaces and thus provide an effective cleaning action.

The baths employed in many of the prior art electrolytic cleaning processes and apparatus include sulfuric acid, caustic soda and other similarly strong chemicals in relatively high concentrations which are often injurious to certain nonmetallic and metallic materials that are ineorporated in printed circuit boards, microminiature circuitry and related articles.

To electrolytically clean these articles as well as others which may be made of such materials, the electrolyte solution used in the present invention comprises an improved chemical composition that is not harmful to these materials. The improved composition also provides a highly efiicient cleansing action, enables relatively high current densities to be reached, and has a long bath life.

In order to accomplish these results and others, the bath 11 should be heated to a temperature in the range between F.-212 F. with the optimum being between F.- F., and the constituents thereof should be present in about the proportions indicated in the following table:

TABLE 1 Trisodium phosphate oz./gal 3-15 Ammonium carbonate oz./gal 36-4 Alkali metal phosphate oz./gal.. 34-6 Alkali metal metasilicate or aluminate oz./gal 16-3 Ammonium hydroxide ml./gal 1-3 Wetting agent grams/gal 6-4 The above mixture is preferably made up with distilled water and mixed with a high speed mixer to make up a concentrated electrolyte solution. When it is desirable to color the electrolyte, an agent such as Aluminum Bordeaux R. L. Dye may be used. This concentrate is normally then diluted approximately 1:1 with ordinary tap water to form the bath.

The components of the improved bath act together as a group in such a way as to provide the treated article with clean surfaces to insure high adhesion for subsequent plating or soldering procedures.

The trisodium phosphate and the alkali metal phosphate such as potassium or sodium phosphate and potassium or sodium hexametaphosphate, for example, soften hard waters which are often the most available for making up the solution and maintain the calcium and magnesium salts in solution, while permitting the best possible solution of the chemical products used. They are also effective conditioners of the bath.

The ammonium carbonate and the ammonium hydroxide impart high electrical conductivity to the solution and resultant low electrical resistivity and low voltage drop across the terminals.

The ammonium hydroxide has been found to remove grease by emulsification or saponification, and when electrolyzed, produces a copious evolution of hydrogen and oxygen which facilitates the removal of the foreign materials from the surfaces of the article. Also the ammonium hydroxide has been found to remove smut formed on articles comprising copper and/or brass. However, the ammonium hydroxide may be deleted from the bath without any significant reduction in its cleansing power especially when the articles being cleaned comprise aluminum and/or stainless steel.

The alkali metal metasilicate or aluminate, such as sodium metasilicate or aluminate has been found to be advantageous because during electrolysis, one of the agents used decomposes, producing an element which remains in semi-colloidal form which acts mechanically as an abrasive when it is dispersed in the solution.

The wetting agent, which is preferably a fatty alcohol sulfate, sulfonated oil or another equivalent anionic surfactant, improves the penetrating power of the solution so that the cleansing composition penetrates even into the most flrgrm persulfate lbs./gal

minute crevices or interstices in the article, thereby facilitating the removal of the foreign elements or other unwanted surface impurities from the article. It has been discovered that it also has a tendency to decrease the danger of a back E.M.F. in the solution during electrolysis since it decreases the surface tension of the liquid, thereby enabling the gases produced to pass readily from the solution. One example of such a wetting agent that has been used with excellent results is the sodium lauryl sulfate product Duponol ME Dry.

In order to remove various smut films and deposits of foreign matter which may remain on the surfaces of the articles after they have been eletrolytically cleaned, a separate cold deoxidant dip is provided, usually contained in a separate tank (not shown), having the following ingredients present in about the proportions indicated in the table below:

* TABLE 2 1-5 Copper sulfate grams/gal 520 Wetting agent grams/gal..- 2-

This mixture is preferably made up with distilled water and mixed with a high speed mixer. The resulting concentrated deoxidant formula may then be diluted withtap water in any desirable ratio, but normally in the ratio of 1:1.

The ammonium persulfate is a strong oxidizing agent and is effectively employed to remove the smut films. The copper sulfate has been found to act as a catalyst to assist in the removal of the foreign matter. The wetting agent such as the material used in the bath above, increases the smut removing action of the deoxidant due to its emulsifying and surface tension reduction qualities.

The articles being cleaned are immersed in the tank 12 which has been filled with a hot solution comprising various proportions and concentrations of the above components of the bath depending on the nature and conditionof the articles. The bath is electrolyzed for a period from a few seconds to five minutes at a current density between about to 35 amperes per square foot of article surface. By heating the bath to the temperature range set forth above, an active metallic surface for subsequent plating or soldering operations is insured without requiring any intermediate steps except possibly a water rinse.

After the electrolytic cleaning operation, if smut films remain on the surfaces of the articles, they may be dipped in the cold deoxidant solution for a short period of time whereupon all surfaces will be completely clean.

A specific example of the process of this invention is as follows: A printed circuit board was secured within cathodic holder 29 and was immersed in metal tank 12 containing a quantity of anelectrolytic bath that had been heated to about 180 F. and which comprised the following constituents:

Trisodium phosphate oz 9 Ammonium carbonate ..oz 1 Sodium phosphate oz 1 Sodium hexametaphosphate oz 2 Sodium metasilicate oz 1 Ammonium hydroxide ml 2 Sodium lauryl sulfate (Duponol ME Dry) grams 4 quantity of a cold deoxidant solution which comprised the following constituents:

' Ammonium persulfate lbs 3 Copper sulfate grams.. 10 Sodium lauryl sulfate (Duponol ME Dry) do 4 The deoxidant was made up according to the following I formula:

The ammonium persulfate was dissolved in gal. hot F. F.) distilled water, 100 grams of copper sulfate (reagent quality) was dissolved in hot water to make 500 ml., 60 ml. of which was then added to the ammonium persulfate solution, 4 grams of the fatty alcohol sulfate was mixed with the solution, and then distilled water was added to make one gallon of concentrate. The concentrate was then diluted 1:1 with ordinary tap water.

After the deoxidant dip, the board was drained and dried. Successful cleaning of the printed circuit board was accomplished in less than five minutes. All foreign contaminates including flux, grease, ink, metallic debris, etc., were removed.

The process of this invention is not harmful to microminiature circuitry or semiconductor devices and therefore it would not be harmful to similar electronic devices produced by various techniques and during various stages of manufacture due to the unique chemical characteristics of the bath.

To insure safety during the cleaning process, D.C. potential across the electrodes is normally varied between 0-24 volts.

The use of the above chemicals is preferred, but it is clear that other equivalent chemicals, proportions and concentrated solutions thereof may also be used if desired, depending upon the type of article being cleaned and the extent of the contamination of the surfaces.

The partitioned wire mesh basket 13 utilized in the present invention provides a highly effective holder for subjecting a plurality of certain hard to clean articles to an electrolytic cleaning process, particularly in the case of printed circuit boards. The basket is equally well adapted for use as an anodic element in a DC. electrolytic circiut and may also be used in an AC. circuit.

The book-like wire mesh holder 29 is extremely well suited for electrolytic. cleaning processes when the major portions of the surface of the article to be cleaned are nonconductive and when the entire article is non conductive. This is by virtue of the fact that the fine metal wire mesh surrounds the article and is in intimate contact with the surfaces thereof so that during electrolysis, an abundant supply of bubbles will be evolved as close to these surfaces as is feasible. As with basket 13, holder 29 is well adapted for use in other types of eleectrolytic circuits.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of the invention and that numerous modifications or alterations may be made therein without departing from the spirit and the scope of the invention as set forth in the appended claims.

Having thus described the invention, what is claimed is: 1. A method of cleaning articles which comprises the steps of:

immersing an article to be cleaned in a bath comprising an aqueous solution of an alakline electrolyte,

electrolyzing said bath by passing an electric current between cathodic means adjacent said article and anodic means, and immersing said article in a dip comprising an aqueous solution of a deoxidant, said bath comprising the following ingredients:

Alkali metal phosphate oz./gal 3%4-21 (of which 3 to 15 oz./gal. is trisodium phosphate) 1 Amonium carbonate oz./gal Vz-4 Alkali metal metalsilicate or said dip comprises an aqueous solution of the following ingredients:

Ammonium persulfate lbs./gal 1-5 Copper sulfate grams/gal 5-20 Wetting agent grams/gal 2-6 2. The method of cleaning according to claim 1 further comprising the step of:

heating said bath to a tempertaure of between 120 F.-

said bath being electrolyzed at said temperature for a period from thirty seconds to five minutes at a current density between -35 amperes per square foot.

3. The method of cleaning according ot claim 1,

wherein:

said alkali metal phosphate comprises a compound selected from the group consisting of potassium phosphate, sodium phosphate, potassium hexametaphosphate and sodium hexametaphosphate,

said alkali metal metasilicate or aluimnate is selected from the group consisting of sodium metasilicate and sodium aluminate, and

said wetting agent is selected from the group consisting of fatty alcohol sulfates and sulfonated oils.

4. A method of cleaning conductive and nonconductive articles comprises the steps of:

placing the article to be cleaned in a conductive support means,

immersing said support means in a quantity of an electrolytic bath contained in a tank, heating said bath to a temperature of about 170l90 F.,

passing an electric current between said support means as a cathode and said tank as an anode for a period of about one minute and at a current density of about 20 amperes per square foot,

said bath being formed of a diluted aqueous solution comprising a composition in about the following proportions per gallon of water:

Trisodium phosphate oz 9 Ammonium carbonate oz 1 Sodium phosphate oz.. 1 Sodium hexametaphosphate oz 2 Sodium metasilicate --oz 1 Ammonium hydroxide ml 2 Fatty alcohol sulfate ..grams 2 and immresing said article in a quantity of a deoxidant dip contained in a vessel,

said clip being formed of a diluted aqueous solution comprising a composition in about the following proportions per gallon of water:

Ammonium persulfate lbs 3 Copper sulfate grams 10 Fatty alcohol sulfate do.. 4

References Cited UNITED STATES PATENTS 2,801,978 8/1957 Perlman 252102 2,775,535 12/ 1956 Poole 204-141 2,482,486 8/1945 Irish 204285 2,314,285 3/1943 Morgan 252-156 JOHN H. MACK, Primary Examiner SIDNEY S. KANTER, Assistant Examiner U.S. Cl. X.R.

Claims

1. A METHOD OF CLEANING ARTICLES WHICH COMPRISES THE STEPS OF: IMMERSING AN ARTICLE TO BE CLEANED IN A BATH COMPRISING AN AQUEOUS SOLUTION OF AN ALAKLINE ELECTROLYTE, ELECTROLYZING SAID BATH BY PASSING AN ELECTRIC CURRENT BETWEEN CATHODIC MEANS ADJACENT SAID ARTICLE AND ANODIC MEANS, AND IMMERSING SAID ARTICLE IN A DIP COMPRISING AN AQUEOUS SOLUTION OF A DEOXIDANT, SAID BATH COMPRISING THE FOLLOWING INGREDIENTS: