US3373113A - Process for etching copper printed circuits - Google Patents

Description

3,373,113 PROCESS FOR ETCHING COPPER PRINTED CIRCUITS Karl Achenbach, Frankfurt am Main, Germany, assignor to FMC Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed Aug. 18, 1965, Ser. No. 480,750 Claims priority, application (grmany, Aug. 22, 1964,

5 Claims. (Cl: 252-112 ABSTRACT OF THE DISCLOSURE Process for etching printed copper circuits in which an aqueous solution of a mineral acid such as H 80 or HNO, containing H as. activator and H PO, as stabilizer for the B 0, is employed as the etching liquid.

The present invention concerns a process for etching the portions of the copper printed circuits which are not protected by printing ink.

.In the production of printed circuits, an insulating backing is first provided with a copper coating 35 or 70 m. thick which is then printed with an etch resistant printing ink on the portions which should be retained as conductor lines after the etching. After the resist has been printed on, the etching is effected by spraying the printed plates with an etching liquid or by dipping such printed plates in an etching bath in order to dissolve the copper which is not protected by the printed resist.

Usual etching liquids are strong mineral acids such as H 80, and HNO;,, as well as chromic acid, iron chloride or ammonium persulfate solutions. The use of such solutions, however, entail certain disadvantages.

In order to be able to carry out the etching procedure in an acceptable short period, it is necessary to use the mineral acids in high concentration which requires use of corrosion proof apparatus. This is also true of iron chloride and chromic acid. As it mostly is necessary to operate at higher temperatures, the resulting vapors are burdensome to the operating personnel. With iron chloride, which is used at a concentration of 42" B. insoluble reaction products are formed which settle out as slime like deposits which when sprays are employed can clog the pumps and/or the spray nozzles. As the sediments in addition have an abrasive action on the resist coatings, such baths usually are only utilized to 60% of their theoretical Cu dissolving capacity.

Recovery of copper from the spent etching solutions or their preparation for satisfactory disposal is very costly, especially with iron chloride or chromic acid etching solutions.

While several disadvantages could be avoided by the use of ammonium persulfate, its use still has several drawbacks. As ammonium persulfate only attacks the copper slowly, it is necessary to add activators or the solution is activated with heat. In spite of this the etching periods necessary, even though not very great, are still longer than those required for iron chloride. Ammonium persulfate solutions can only effectively be used in a concentration range of 150 to 250 g./liter of water.

Therefore the capacity of such solutions for take up of Cu is correspondingly smaller than when iron chloride solutions are employed. The recovery of copper from the spent ammonium persulfate solutions is somewhat simpler than from'spent iron chloride or chromic acid solutions. It, for example, can be etfected by cementation with aluminum or by electrolysis, nevertheless the removal of the remaining ammonium ions requires additional measures.

3,3 73,1 13 Patented Mar. 12, 1968 As dilute mineral acids, such as, dilute H 80 or HNO; only attack copper slowly they do not come into consider'ation for etching printed circuits where complete dissolution in the shortest time po'ssible is required. It is only through addition of an oxidizing agent, such as, H O, or other peroxygen compounds, such as, persulfates or perborates that dissolution of copper can be accelerated when dilute mineral acids are used. Baths of this type have been suggested in the literature for the surface treatment of metals especially for polishing such surfaces. See Surface Treatment of Metals with Peroxygen Compounds, Plating 42, 1955, pages 561- 66. In such surface treatments metal ions go into solu' tion which decompose the active oxygen c'atalytically and in a short time reduces the activity of the baths to a considerable degree. This is especially true when higher temperatures of, for example, 40-60 C. are employed. Application of baths of this composition for etching printed circuits therefore appeared completely out of the question, as in such etching process considerably higher copper concentrations must be reckoned with, especially with a view to the proper industrial efliciency of such process. In order to provide for optimum use of the active substance of printed circuit etching baths copper concentrations of 40 to 60 g./Cu/l. are no rarity. An effective protection of H 0; containing ba'ths against the decomposing action of Cu-ions at such concentration was previously unknown. As a result it previously appeared impossible to employ H O, containing mineral acid b'aths for etching of printed circuits.

According to the invention it was found that despite these considerations, H O, activated mineral acid baths could be successfully used for etching printed circuits when certain definite requirements are met. The essence of the process according to the invention resides in the use of aqueous H O, containing mineral acid solutions which in addition contain phosphoric acid, as stabilizer, for the etching of printed circuits.

It was furthermore found that if the etching velocity is to be accelerated still further this can effectively be accomplished by the addition of other activators, namely, salts of heavy metals which are more noble than cop per, such as, for example, HgCI The activating effects of such heavy metal salts are known in connection with etching processes with ammonium persulfate. The etching baths according to the invention, even in the presence of such additional activ'ators, possess sufficient stability for use in the etching process although it was to be expected that an accelerated decomposition of the bath would take place.

The process according to the invention is illustrated by the embodiments thereof given in the following ex ample Example An etching bath of the following composition per liter was employed.

0.14 kg. H conc. (D: 1.84)

0.15 kg. H 0, (35% by wt.)

0.05 kg. H PO, (84.5% by wt., D=1.70)

1.0 ml. HgCl, solution (6.8 g. HgCl /l. corr. to 5 p.p.m.

3 sulfate containing etching baths as can be seen from the following table:

Etching time in minutes and seconds G. Cu/l Bath according Ammoniumto example persulfate bath 6,1401 77m" grzsl! 13 18' 2130" 33' Analogous results were obtained when the HgCl solution employed in the bath composition was replaced by the following:

1 ml AgNO; solution (9.93 g. AgNO /l.=6.3 p.p.m. Ag 1 m. Rh (80 solution (3.45 g.

Rh (SO .12H 0/l.=5 ppm. Rm 1 In]. HQPtC-le solution (2.12 g.

l-l-,,PtCl,.6H O/l.=8 p.p.m. Pt

1 claim:

1. In a process for etching printed copper circuits which comprises contacting the printed copper circuit with an aqueous etching solution of a mineral acid selected from the group consisting of sulfuric acid and nitric acid containing H O, as an activator for the mineral acid, the step of incorporating an effective stabilizing amount of H PO, as a stabilizer for the H 0 in said aqueous etching solution.

2. The process of claim 1 in which said aqueous mineral acid is dilute sulfuric acid containing 0.01 to 0.7 kg. of sulfuric acid per liter.

3. The process of claim 2 in which said dilute sulfuric acid solution by weight contains 15-10% of H 0 and 1-10% of the H PO 4. The process of claim 2 in which said dilute sulfuric acid solution by weight contains 3.5-7% of H 0; (100%) and about 5% of the H PO 5. The process of claim 4 in which said dilute aqueous sulfuric acid solution contains about 0.1 to 0.2 kg. of sulfuric acid per liter References Cited UNITED STATES PATENTS 2,923,608 2/1960 Margulies 156-18 2,978,301 4/1961 Margulies 25279.1 2,982,625 5/ 1961 Saubestre.

FOREIGN PATENTS 955,000 4/1964 Great Britain.

LEON D. ROSDOL. Primary Examiner.

I. T FEDIGAN, M. WEINBLATT,

Assistant Examineris.