US2791516A

US2791516A - Electroless plating

Info

Publication number: US2791516A
Application number: US380858A
Authority: US
Inventors: Clarence G Chambers; Robert A Spaulding
Original assignee: General Motors Corp
Current assignee: Motors Liquidation Co
Priority date: 1953-09-17
Filing date: 1953-09-17
Publication date: 1957-05-07
Anticipated expiration: 1974-05-07

Description

May 7, 1957 c. G. CHAMBERS ETAL ELECTROLESS F'LATING Filed Sept. 1'7. 1953 HEFT EXK'HNER mir/Nq United ELECTROLESS PLATING Application September 17, 1953, Serial No. 380,858

9 Claims. (Cl. 117-102) This invention relates to improvements in the deposition of metal from chemical reduction type plating solutions.

It is known that nickel and other metals can be deposited without the use of electrical current by immersing an article to be coated into an aqueous solution of a nickel or other metal salt and a reducing agent such as sodium hypophosphite or potassium hypophosphite. There are two types of such plating solutions, namely, alkaline and acid, of which the acid solution is generally better adapted for commercial plating operations. The plating by chemical reduction is usually conducted at an elevated temperature, typically about 90 C.

One' of the diicul'ties heretofore encountered in the commercial adaptation of chemical reduction plating has been the inability to preclude deposition of metal on the plating equipment with the result that in many instances an undesirable metallic deposit is produced throughout the plating equipment. Not only does such a deposit deplete the metal content of the plating solution but it also impairs heat transfer and acts as a catalyst increasing the undesired deposition of metal.

The exact nature of the chemical plating reaction is not clearly understood at present. However, it has been observed that certain metals, such as lead, copper, brass or platinum which would not ordinarily be coated by the oxidation-reduction plating reaction, do become susceptible to being coated when, for example, momentarily brought in contact with aluminum or other reaction initiating material. Inasmuch as the exact nature of such an initiating reaction is not clearly understood the failure to control lthe deposition of nickel and other metals deposited by chemical reduction methods in certain cases is a serious problem even though the plating equipment is formed from a material which theoretically should not become coated during the plating reaction. The problem, of course, is even more serious in large scale reduction plating operations which employ plating tanks, piping and Vother equipment formed of metals, such as iron, steel, nickel, aluminum and alloys of these metals, which are readily coated with the metal being deposited.

A principal object of the present invention is to provide a chemical reduction plating process in which the aforementioned difticulties are minimized. A further object is the provision of an improved chemical reduction plating process. A still further object is the provision of an improved plating operation in which maintenance of proper solution concentration is facilitated while effectively controlling the metallic deposition. These and other objects and advantages of the invention will more fully appear from the following description taken in connection with the accompanying drawing which illustrates in schematic form apparatus for the operation of a chemical reduction plating process in accordance with the present invention.

Generally, we have discovered that undesirable metallic deposition throughout the plating equipment can effectively be minimized and precluded entirely in many arent M 2,791,516 ce Patented May 7, 1957 applications by constantly recirculating the plating solution and by quench cooling a portion of this solution from the high bath temperature required for optimum plating to a temperature at which little or no plating occurs and thereafter recirculating the cooled solution into the plating tank. By proper control, using this technique, sufficient plating solution is quenched to preclude metallic deposition throughout the equipment; yet an excessive amount of heat energy is not wasted lin so doing.

In the drawing the single figure is a schematic diagram of apparatus used in performing the plating process.

For the continuous operation of a plating solution, quench cooled in accordance with the invention, the maintenance of proper solution concentration is contemplated by continuously or intermittently introducing into the circulating plating solution the required amounts. of bath ingredients. This addition preferably is made by charging a lfilter, used in chemical reduction plating operations, with proper amounts of bath additives to maintain the desired concentration.

Referring now more particularly to the accompanying drawing, which illustrates preferred apparatus comprising at least one plating tank, a heat exchanger, a filter and a pump connected in series, and control valves for carrying out the practice of the invention, articles which are to be coated with nickel or other metal are immersed in a reduction plating solution in plating tanks indicated at 1t) and 12. Although two tanks are illustrated, it will be understood, of course, that one or any number of tanks may be employed, a plurality of tanks merely being connected in parallel, each in series with the heat exchanger, filter, and pump.

For proper deposition of nickel by chemical reduction plating, the temperature of the plating solution should be relatively high. Moreover, as mentioned above, in order to remove harmful ingredients either precipitated or mechanically dragged into the bath, the solution is continuously filtered. One of the most difficult problems heretofore encountered has been Ithat of precluding or minimizing the continued deposition of nickel or other metal as the hot plating solution is passed through the filter and other auxiliary equipment.

The practice of the present invention comtemplates circulating plating solution, heated to the desired temperature in a conventional manner as by steam coils (not shown), through plating tanks 10 and l2, a heat exchanger 14, a fluid pump i6 and a filter 18 connected in series with said tanks, and withdrawing hot used plating solution from the plating tanks 10 and l2 through pipes 2i) and 22, the ow being regulated by

valves

24 and 26, and rapidly quench cooling the solution to a temperature at which plating does not occur. This is accomplished in the apparatus illustrated in the drawing by recycling cooled plating solution from the heat exchanger, having a cooling fluid inlet 28 and outlet 30, by means of a pump 16, back through

pipes

32, 34, 36 and 38, control valves 40, 42 and 58, to the heat exchanger where it mixes with the hot plating solution being withdrawn from the plating tanks through

pipes

20 and 22 and serves to immediately quench the hot plating solution to a lower temperature. ln operation, the plating solution from the heat exchanger 14 may be continuously filtered by passing a portion of it through pipe i4 and filter 18 and recycling the remainder through the heat exchanger 14. lf desired, a portion of the circulating bath can be simultaneously recycled to an addition tank 60 through pipe 46 and valve 48.

Although the quantities of solutions and flow rates thereof through pipes generally will be dictated primarily by the metal being deposited, the heat content of the solutions, the temperatures involved and the particular bath composition, in a particular instance excellent results have been obtained by recycling approximately three times the volume of hot plating solution continuously being removed from the plating tank. The temperature of the hot plating solution removed from the plating tanks, typically about 195 F., is thus quickly reduced to about 160 F. by mixing therewith three times its volume of cooled recirculated solution having a temperature of about 145 F. The 160 F. temperature is thereafter reduced to 145 F. by passing the solution through the heat exchanger, a portion of this etiluent being passed to the filter and thence to the plating tank through valves 5t), 52 and

pipes

54 and 56 while the remainder is recycled by the pump 16 to the heat exchanger through pipe 38 and control valve 53 as described above.

The invention contemplates the use, preferably in combination with the aforementioned quench cooling of the hot used plating solution, the original bath makeup use of, or bath maintenance by, one or more water-insoluble salts of the metal to be deposited together with the appropriate reducing agent and/ or other bath additives. In depositing nickel, for example, the proper plating solution concentrations preferably are maintained by addition of water-insoluble nickel salts. In practice, metal salts, as well as other bath ingredients, are added to the bath through an addition tank 60 where the resulting slurry, or solution in case it is desired to employ water-soluble salts, is pumped to the filter 18 and thence either back to the addition tank 60, having an outlet pipe 76 and control valve 78, or to the

plating tanks

10 and 12.

Depending upon the particular type of lter employed it is desirable in many instances to employ a lter aid, such as linely divided asbestos or diatomaceous earth, in order to form a flterable cake. Such a filter aid can be added to the mixture in the addition tank 60 and thence pumped to the lter. In the drawing, the filter 18 is provided with a gas Outlet 62 and control valve 61 to permit discharge of carbon dioxide or other gases formed during gradual dissolution of bath additives into the plating solution. To facilitate cleaning the lter a compressed air source (not shown) can be discharged through the filter 1S by means of valve 64 and inlet pipe 66, valve 68 permitting draining the apparatus when desired.

When water-insoluble salts of the metal to be deposited are used, a slurry is formed in the addition tank 60, valves 40 and 74 are either closed or adjusted to permit the desired liow and the slurry is pumped through valve 78 and pipe 76 to the lter 18 where it deposits over the filter media, formed typically of heat and chemical resistant cloth or iiber. If desired to build up a thicker coating on the filter media, valve 72 and valve 74 are closed and the solution is recycled back to the addition tank 60 through pipe 46 and valve 48. The plating solution being passed through the lter 18 gradually dissolves the metallic salts as the metal already in solution is depleted in the plating operation. Hence the procedure provides an extremely cicient method of controlling the plating operation and maintaining the proper bath concentration.

The present invention is particularly well adapted for use in the chemical reduction deposition of nickel. Typical of a nickel reduction plating bath which may be regulated in operation by the practice of the present invention is the following bath composition employing water-soluble nickel salts where the quantities expressed are per liter of water:

Nickel chloride grams 30 Sodium hypophosphite do l Glycollic acid (70%) -milliliters 35 Sodium hydroxide -grams 15 It will be understood of course that the above bath composition is intended only as an illustration of a typical reduction plating bath and that the present invention is applicable to a wide range of nickel bath compositions. For example, plating baths containing about 5 to 50 grams per liter of a water-soluble nickel salt such as nickel chloride, nickel acetate, nickel sulfate, etc.; approximately 5 to 100 grams per liter of a reducing agent, such as hypophosphorous acid, sodium hypophosphite or other soluble hypophosphites; and about 15 to 100 grams per liter of a butter such as glycollic acid, sodium hydroxy acetate, sodium acetate, sodium citrate, etc. and/or other bath additives can similarly be replenished in accordance with the present invention.

It is to be understood that the present invention also contemplates the formation of an improved nickel chemical reduction plating bath utilizing a water-insoluble nickel salt as an initial ingredient. Such a bath comprises an aqueous mixture of a reducing agent and a Waterinsoluble nickel salt selected from the group consisting of oxy nickel salts such as nickel monoxide, nickel peroxide, nickel sesquioxide, nickelous nickelic oxide and nickelous superoxide; hydroxy nickel salts such as nickelic hydroxide and nickelous hydroxide; and nickel carbonates such as nickel carbonate and basic nickel carbonate. In most instances it is desirable to additionally 'employ a butler agent. A preferred bath of this type comprises nickel carbonate, sodium hypophosphite, and glycolic acid.

A bath of the above-mentioned type can be produced by dissolving the desired water-insoluble nickel salt or salts in glycollic acid and thereafter adding the reducing agent. lf another type buffer is used, for example, sodium acetate, sodium citrate or sodium hydroxy acetate, the nickel salt may first be dissolved in hypophosphorus or phosphorus acid, enough acid being used to obtain the desired initial bath pH.

The following is an example of one reduction plating bath utilizing a water-insoluble nickel salt as an initial ingredient. The quantities expressed are per liter of water.

Nickel carbonate gramsn 15 Glycollic acid (70%) milliliters.- 35 Sodium hypophosphite grams l0 Preferred pH 5.0

A satisfactory range of ingredients comprises a mixture of about 5 to 100 grams per liter of a water-insoluble nickel salt, approximately 15 to 100 grams per liter of a butter such as glycollic acid, sodium hydroxy acetate, sodium acetate, sodium citrate, etc., and about 5 to 100 grams per liter or reducing agent such as sodium hypophosphite, hypophosphorous acid, or other hypophosphites.

We have discovered that excellent results are obtained when the plating tanks, typically of steel or the like, are coated on the interior with porcelain or a heat resistant imprevious organic coating, a particular example being the commercially available phenolic coating known as Heresite, a product of the Heresite & Chemical Co. Supenior results have been obtained when the equipment coming in contact with the hot plating solution is formed of tantalum which is extremely resistant to being-coated by chemical reduction plating.

It is to be understood that, although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

What is claimed is:

l. A method of depositing a metal from a chemical reduction type plating bath, said methodl comprising the steps of immersing an article to be coated into said plating bath which is maintained at an elevated temperature and is continuously recirculated through a filter, said plating bath being cooled just prior to passage through said lter by mixture therewith of cooled plating solution.

2. In the deposition of nickel from a hot chemical reduction plating bath of the type comprising an aqueous solution of at least one nickel salt and a reducing agent in which said solution is continuously recirculated through a ilter to thereby remove harmful impurities, the improvement which consists of precluding nickel deposition throughout the ilter and associated apparatus as the plating solution circulates therethrough by passing the hot plating solution through a heat exchanger to cause said solution to cool, a portion of the cool plating solution from said heat exchanger being mixed with the hot plating solution as it is withdrawn from said both to thereby rapidly reduce the solution temperature below that temperature at which plating occurs.

3. Apparatus for chemical reduction plating operations, said apparatus comprising .a plating tank, a heat exchanger and a filter connected in series by conduits to permit fluid flow from said tank through said heat exchanger and then through said filter and back to said tank, and means for recycling ya portion of the fluid effluent of said heat exchanger to the inlet of said heat exchanger.

4. Apparatus for chemical reduction plating operations, said apparatus comprising a container for plating solution, a heat exchanger, a filter and a pump connected in series by conduits for the passage of said plating solution therebetween and valve means in said conduits to recirculate a portion of the heat exchanger etlluent plating solution to the inlet of said heat exchanger.

5. Apparatus for carrying out chemical reduction plating operations, said apparatus comprising at least one plating tank, a heat exchanger, a filter and a fluid pump connected in series by conduits to permit fluid ow therebetween, said apparatus having control valve means for alternate and simultaneous tiltening and recycling through said heat exchanger of a portion of said plating solution.

6. Apparatus for carrying out chemical reduction plating operations, said apparatus comprising a container for plating solution and a heat exchanger connected by conduits to permit uid flow therebetween, means tin said apparatus for maintaining proper solution concentration, and means for recycling a portion of the heat exchanger eluent to the inlet of the heat exchanger.

7. In apparatus for use in chemical reduction plating operations, said apparatus comprising at least one container for plating solution, a heat exchanger, a filter and a pump, connected by conduits for series lluid tiow, the improvement which comprises control valve and piping means adapted to withdraw a portion of cooled plating solution from said heat exchanger and admix it with the plating solution passing from the container to heat exchanger.

8. Apparatus .as set forth in claim 7 in which portions of the apparatus in contact with the plating solution are formed of tantalum.

9. In a process for the deposition of nickel from a chemical reduction plating bath, said bath comprising an aqueous solution of at least one nickel salt and a reducing agent, the improvement which comprises withdrawing the solution from said bath and passing it through a lilter charged with at least one nickel salt to thereby replenish the nickel salt in said solution and simultaneously remove solid material from said solution, and then returning said solution to said bath, said solution being rapidly cooled just prior to passage through said filter to a temperature below that at which plating occurs.

References Cited in the file of this patent UNITED STATES PATENTS 2,009,232 Hood July 23, 1935 2,516,349 Smith July 25, 1950 2,516,884 Kyame Aug. 1, 1950 2,658,839 Talnley et al. Nov. l0, 1953

Claims

9. IN A PROCESS FOR THE DEPOSITION OF NICKEL FROM A CHEMICAL REDUCTION PLATING BATH, SAID BATH COMPRISING AN AQUEOUS SOLUTION OF AT LEAST ONE NICKEL SALT AN A REDUCING AGENT, THE IMPROVEMENT WHICH COMPRISES WITHDRAWING THE SOLUTION FROM SAID BATH AND PASSING IT THROUGH A FILTER CHARGED WITH AT LEAST ONE NICKEL SALT THEREBY REPLENISH THE NICKEL SALT IN SAID SOLUTION AND SIMULTANEOUSLY REMOVE SOLID MATERIAL FROM SAID SOLUTION, AND THEN RETURNING SAID SOLUTION TO SAID BATH, SAID SOLUTION BEING RAPIDLY COOLED JUST PRIOR TO PASSAGE THROUGH SAID FILTER TO A TEMPERATURE BELOW THAT AT WHICH PLATING ACCURS.