CS259694B1 - Copper etching solution - Google Patents

Abstract

The solution relates to a copper etching solution intended especially for the production of printed circuit boards. The aqueous solution contains 10 to 250 g/l of an ammonium salt of a strong acid, 5 to 350 g/l of ammonium carbonate, 0.1 to 50 g/l of ammonium hydroxide and 0.1 to 5 g/l of hydrogen peroxide.

Description

The invention relates to a solution for etching copper, particularly for use in the manufacture of printed circuit boards.

For the production of printed circuit boards by subtractive technique using metal resists, ammoniacal etching solutions are often used, which, in addition to an oxidizing agent such as copper chloride, sodium persulfate or sodium chlorite, contain a significant amount of free ammonia. These solutions are intended for large-scale production of printed circuit boards! in equipment that ensures their optimal use and prevents the penetration of ammonia into the air. They are usually used at elevated temperatures and the solution is sprayed by a system of nozzles perpendicular to the surface of the etched material. The result is very rapid etching of the desired parts of the copper with very little exposure under the resist - underetching.

Etching in printed circuit board manufacturing is often defined using the so-called etching factor, which is the ratio of the depth of the etched metal to the etching length.

At a very small value of the etching factor, that is, high underetching, the thickness of the conductive connections on the board is significantly reduced.

In the case of boards with very thin conductive connections, there is a risk of complete disconnection of the connections.

Another danger in the case of the frequently used tin resist is the breakage of the protruding parts of the resist in question. The released tin particles can short-circuit individual parts of the circuits during further processing of the board and thus be the cause of very serious malfunctions or even the destruction of the entire board equipped with components.

The primary requirement is therefore the production of printed circuit boards with the highest possible etching factor. Etching is influenced by a number of external and internal conditions, such as the composition and temperature of the etching solution, hydrodynamic conditions, etc. Common practice is to use solutions with a high etching rate at elevated temperatures by spraying perpendicular to the surface of the etched surface. Such solutions are commonly known, but they provide good results only in combination with complex and expensive equipment that is able to ensure optimal conditions for their use. In the case of laboratory and small-scale production, the use of such expensive equipment is unprofitable, and without spraying and complex regeneration equipment, the use of such solutions in laboratory conditions is problematic. Therefore, traditional etching solutions such as ferric chloride or chromic sulfuric acid continue to be used for these applications.

The above-mentioned shortcomings are eliminated by the solution for etching copper, especially for use in the production of printed circuit boards, according to the invention. Its essence lies in the fact that it is formed by an aqueous solution of 10 to 250 g/l of an ammonium salt of a strong acid, expediently ammonium sulfate, ammonium nitrate or ammonium chloride, 5 to 350 g/l of ammonium carbonate, 0.1 to 50 g/l of ammonium hydroxide and 0.1 to 5 g/l of hydrogen peroxide.

The basic advantage of the invention lies in the particular suitability of the solution for etching copper at laboratory temperature and by immersion.

The copper etching solution, its advantages are further described in more detail based on comparative tests and the attached tables 1 and II.

A series of etching solutions were prepared for comparative tests. The composition of all the baths used is contained in Table I. Solutions 1, 2, 3 correspond to the bath according to the invention, the others correspond to known types of etching baths and are given only for comparison of properties. A test pattern was used for the measurement, consisting of three parallel conductive connections with a thickness of 0.3 mm, separated by gaps of 0.3 mm and a length of 100 mm. This test pattern was created using a photoresist with a tin coating with a thickness of approximately micrometers. All baths were used under the same conditions. Etching was carried out by immersion in a vertical position with occasional movement of the plates at a temperature of 23 ° C. After etching the entire pattern, that is, after a clear separation of all conductive connections along the entire length, the plate was removed from the bath and quickly rinsed with a stream of water. The test was performed on conventional single-sided laminated copper boards with a thickness of 30 micrometers.

Since direct measurement of the etching factor is difficult for practical reasons, the following procedure was chosen to evaluate the effects of etching baths. After etching the test pattern, the tin resist was removed from the surface of the conductive pattern and the location with the narrowest conductive connection on each of the three parallel conductive connections was determined. This value was measured using a measuring microscope and the average of these three values was taken as the basis for the calculation. The values contained in Table II were obtained by calculation:

N - 100 M/T, where T = line thickness on metal resist before etching

M - line thickness at the narrowest point of the joint after etching the pattern and removing the resist

The value N therefore indicates the thickness of the conductive joint at the narrowest point as a percentage of the thickness of the joint defined by the tin resist. The value 100 /%/ would therefore represent an ideal case without reducing the thickness of the joint by etching the resist. The value 0 means complete etching of the joint. The experimental design shows that not only the etching of individual baths was compared, but also the uniformity of the etching along the entire length of the test pattern. This criterion is of greater importance for the practical use of etching baths for immersion etching than the evaluation of the etching factor itself.

Table I: Composition of the baths used for the tests /data are in g per liter of solution/

1 2 ..................3 . _NH4C1 250 nh ₄ no ₃ 120 /nh ₄ / ₂ sat ₄ 20 /nh ₄ / ₂ co ₅ 5 /NH4/ ₂ C0 ₅ 250 /NH ₄ / ₂ 0θ5 250 nh ₄ oh 50 nh ₄ oh ' 3 nh ₄ oh 10 h ₂ o ₂ 1 h ₂ o ₂ 1 h ₂ o ₂ 3 4 5 6 /NH4/2SO4 250 /NH ₄ / ₂ SQ ₄ 120 On ₂ S ₂ 0g 200 / _{NH4 / 2CO5} 120 /níi ₄ / ₂ co ₅ 120 /nh ₄ / ₂ co ₅ 120 nh ₄ oh 70 nh ₄ oh 3 nh ₄ oh 30 ^h 2°2 1 ^H 2°2 10 7 8 9 • · ^At 2^2°8 120 CuCl ₂ 150 FeGl ₅ 25Ó _NH4OH 50 NI-I ₄ C1 50 HC1 20 · no ₄ oh 30

nh ₄ ci ñh ₄ oh h ₂ o ₂

120 *

Table II: Properties of the baths used

Bathroom No. 1 2 3 4 5 6 7 6 9 ' 10 N /%/ 87 ‘93 90 81 65 76 70 40 45 60

The values in Table II show that under the given conditions, etching solutions with low hydrogen peroxide and free ammonia content, i.e. baths 1, 2, 3 according to the invention, provided the best results. For practical reasons, these baths were continuously supplemented with hydrogen peroxide and ammonia during the etching process so that the content of these components did not exceed the concentrations given in Table I.

The baths according to the invention provide uniform etching along the entire length of the conductive connections, have low underetching and a high etching factor. Due to the low content of free ammonia, they can be used in laboratory conditions. The lower etching rate allows for good visual monitoring of the etching process.

Claims (1)

OBJECT OF THE INVENTION Copper etching solution, in particular for use in the manufacture of printed circuit boards, characterized in that it consists of an aqueous solution of 10 to 250 g / l of an ammonium salt of a strong acid, preferably ammonium sulphate, ammonium nitrate or ammonium chloride, 5 to 350 g / l ammonium carbonate, 0.1 to 50 g / l ammonium hydroxide and 0.1 to 5 g / l hydrogen peroxide.