US 3824136 A
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United States Patent Ofice 3,824,135 Patented July 16, 1974 3,824,136 ETCHING F ALUMINUM FLUIDIC DEVICES Kenneth E. Gilbert, Port Jervis, N.Y., assignor to the United States of America as represented by the Secretary of the Army No Drawing. Filed Oct. 6, 1972, Ser. No. 295,784 Int. Cl. C23f 1/02 US. Cl. 156-8 Claims ABSTRACT OF THE DISCLOSURE A method for chemical milling of thin aluminum sheets, such as wafers for fluidic control systems, which produces smoother cuts and minimizes rough edges and undercutting, utilizes an aqueous etching solution of about 815 Baum containing about 75-150 grams FeCl and about 6.5-11.5 gram HCl per liter.
The invention described herein may be manufactured, used and licensed by or the Government for governmental purposes without the payment to me of any royalty thereon.
BACKGROUND OF THE INVENTION Aluminum fluidic wafers have found wide application for fluidic control systems, e.g. aluminum fluidic fuzing systems, which are characterized by small size, light weight and resistance to radiation. Trends toward miniaturization require that fluidic wafers possess passages of less than 5 mils width. Miniature aluminum wafers must be interchangeable and hence must possess accurately formed passages, which can be formed either by mechanical or chemical milling. Mechanical milling requires delicate machining techniques, which entail high capital equipment, tooling and production costs. Chemical milling of aluminum provides a high production at low cost, and is commonly accomplished by contacting the thin metal wafer, coated with a suitable resist of the desired design, with an etching solution of 42% B. FeCl containing small amounts of hydrochloric acid. However, such etching solutions are not entirely satisfactory for producing fluidic aluminum wafers with passages of 5 mils width or less, since their action is difiicult to control so as to provide reproducible results, and can produce cuts having more or less jagged edges, rough and irregular inner wall surfaces and severe tindercutting.
Reproducibility of results is desirable not only to minimize rejects but primarily to produce fluidic aluminum wafers of a particular passage design, which are interchangeable in an assembly without deviation of performance.
SUMMARY OF THE INVENTION In accordance with this invention, there is provided an improved process for chemical milling of fluidic aluminum wafers and the like, which produces cuts possessing smooth wall surfaces with minimum of jagged edges and undercutting and is economical, readily controlled and highly reproducible.
More specifically, this invention provides a process, which comprises contacting, e.g. spraying thin aluminum sheets, e.g. aluminum fluidic wafers, coated as usual with a sutiable resist of desired design, with an etching solution consisting essentially of FeCl HCl and water of between about 8 and 15 Baum density, said solution consisting essentially of from about 75 to 150 grams FeCl and from about 6.5 to 11.5 grams HCI per liter of solution.
DESCRIPTION OF PREFERRED EMBODIMENTS The concentration of the aqueous ferric chloride etching solution employed in the process of this invention is rather critical, and ranges from about to 150 grams FeCl in combination with from about 6.5 to 11.5 grams HCl per liter of water solution, corresponding to a density between about 8 and 15 Baum. The small content of hydrochloric acid inhibits the deposition of metallic iron on the aluminum and thus promotes the action of the ferric chloride solution. However, when the concentration of ferric chloride or hydrochloric acid is increased above the upper limit of said ranges, the reaction becomes increasingly vigorous and difficult to control and produces cuts having progressively jagged edges and rough inner Wall surfaces; and when the concentration of ferric chloride or hydrochloric acid is reduced below said limits, the etching reaction is too slow and progressively produces undercutting of the metal and/or breaks down the resist from the surface of the aluminum.
The temperature of the etching solution employed in the process of this invention is preferably maintained at between about F. and F., whereby optimum results are obtained.
The novel etching solutions are particularly effective for producing smooth cuts in aluminum wafers of from about 1 to about 5 mils thickness. With substantially thicker aluminum wafers, such as 10 mils or more, undercutting becomes noticeable.
EXAMPLE Part A. Preparation of the Aluminum For Etching According to the Prior Art Aluminum sheet, 4 mils thick and about 3 inches long and 2 mils wide, was scrubbed with pumice, rinsed with water and then coated with Alodine, which promotes adherence of the resist coat, by immersing it for about 90 seconds at room temperature in a solution of 4.5 grams Alodine, sold by Amchem Chemical Co.,
9 ml. 70.0% nitric acid 950 ml. distilled water The specimen was then coated with a photosensitive resist by dipping it in a solution of 1 125 ml. KMER (Kodak Metal Etch Resist), sold by Eastman Kodak Corp., Rochester, NY.
ml. resist thinner solvent (Kodak Metal Etch Resist Thinner) sold by Eastman Kodak Corp., Rochester, NY.
and prebaking the coated specimen at 500 F. to remove the solvent. Photographic films (transparencies) containing an identical design were placed one in contact with each side of the aluminum specimen so as to provide accurate registration of the two films. Both sides of the specimen were then exposed to UV light for about 15 minutes. The transparencies were removed and the exposed resists were developed by immersion in KMER developer for 2 minutes at room temperature and rinsed with water to remove all undeveloped resist. The specimen was postbaked at 500 F., then scrubbed lightly with a soft bristle (typewriter) brush along with water spray and pumice to remove the Alodine from the areas to be etched, and finally rinsed with water.
Part B. Etching According to the Process of the Iinvention The etching solution employed consisted of 8000 ml. water 200 ml. 38% hydrochloric acid 1800 ml. 42 B. ferric chloride It possessed a density corresponding to about 10.5 Baum and contained approximately 9 grams HCl and 99 grams FeCl per liter of solution.
The aluminum specimen prepared in Part A was placed on a vertical rotary etching machine sold under the trademark Dynamil VRP 70 by Western Technology, Anaheim, Calif., and etched for about 22% minutes with the aforesaid etching solution heated to about 120 F. In the machine the etch solution was continuously pumped as a conical spray onto both sides of the vertically held speci men through spray nozzles mounted on a vertically rotating wheel positioned on each side of the specimen. At the end of this period the specimen had been completely penetrated by removal of the metal exposed along the design. The speicmen was next immersed for one week in a standard commercial stripper solution (sold under the trade name CBI by The Allied Chemical Corp.) to remove the resist and underlying Alodine, then rinsed with water and dried.
Inspection of the specimen under a microscope with 70 magnifications showed that the cut thus produced with the aforesaid etching solution possessed smooth inner walls with no jagged edges and essentially no undercutting.
I wish it to be understood that I do not desire to be limited to the exact method and detail of construction described for obvious modification will occur to persons skilled in the art.
1. In a process for etching a design into aluminum, wherein the aluminum is coated with a resist which exposes the metal surface according to the desired design and the exposed metal is etched with aqueous ferric chloride solution, the improvement which comprises contacting said metal surface with an etching solution consisting essentially of F'eCl HCl and water of between about 8 and 15 Baum density, said solution consisting essentially of from about to 150 grams FeCl and from about 6.5 to 11.5 grams I-ICl per liter of solution.
2. The process according to claim 1, wherein the solution is maintained at a temperature of from about F. to about F.
3. The process of claim 2, wherein the aluminum is a sheet material not more than about 5 mils coated on both sides with identical aligned resist designs, and the etching solution is contacted simultaneously with both sides of said sheet until the exposed metal is completely removed.
4. The process according to claim 3, wherein the solution consists essentially of about 100 grams FeCl and from about 6.5 to 11.5 grams I-ICl per liter of solution.
5. The process according to claim 4, wherein the solution consists essentially of about 100 grams FeCl and about 9 grams of HCl per liter of solution.
References Cited UNITED STATES PATENTS 3,402,083 9/1968 Patsko et a1. l56-22 X 3,679,500 7/1972 Nobuzo kubo et a1. 15611 3,266,962 8/1966 Post 156-22 WILLIAM A. POWELL, Primary Examiner US. Cl. X.R. 156-22; 25279.2