DE1929420A1 - Metal-free etching soln. for copper coatings - Google Patents

Abstract

Ammonium chloride and/or nitrate are used in a solution containing chloride and NH4OH. Pref. concentrations are chloride : 0.1 mol/l to saturation (specf. 0.375 mol/l); NH4 salt: 0.2 mol/l to saturation (spec. 1.5 mol/l); NH4OH: to pH 9.0 or above (spec. 3.0 mol/l). Alternatively the total chloride and/or nitrate concentration may be 1.0 mol/l and NH4HCO3 added to give 0.75 mol/l. The solution is pref. used (for etching printed circuits) at room temp. and as the copper in the solution builds up the temp. is gradually increased to 55 degrees C, with periodic NH4OH addition to maintain pH. When the amount of copper exceeds 45 g/l the solution may be aerated.

Description

Copper etch solution and method for dissolving swabs The invention relates to etching solutions for dissolving copper and methods for causing such Solutions, in particular solutions and methods for dissolving copper coatings or Plating of various, both metallic and non-metallic substrates.

The etching solution given here belongs to the so-called "non-metallic" Type, i.e. it is one that does not itself contain metal ions (apart from the dissolved Copper) to effect the etching process. Many etching solutions contain metal ions, for example iron or chromium ions, which cause difficulties in their removal of the solution used. "Non-metallic" type etching solutions are already available known and in use. For example, U.S. Patent 3,231,503 describes an ammoniacal chlorite solution, which is in the alkaline range from pH 8 to 13 to the Dissolving swab is used. According to this patent has such a Etching system parts, index the when using the usually Chromate or ferric chloride systems used for copper cause difficulties the removal of lye can be avoided. This patent also recommends especially the addition of ammonium carbonate as a buffer to the chlorite system for reduction of attack on copper-coated or clad steel substrates and mentions that instead of the bi-carbonate, other ammonium salts can also be used.

It has now been found that ammoniacal chlorite solutions in the alcoholic Area that replaces all or at least part of the ammonium carbonate Ammonium chloride or ammonium nitrate contain an essential and quite unexpected one Ver.

show improvement in their copper dissolving abilities, which is only possible with ammonium chloride or nitrate addition occurs. The solution is particularly suitable for the production of printed circuit boards. Such modified solutions have an up to 507 higher etching speed, up to 28% greater practical plucking resolution capacities per liter of solution and a significantly improved stability and service life, especially with pH control, as possible with the previously known system. these chloride / nitrate solutions also have higher solubility limits for the ingredients, so the solutions with higher X concentration can be used, resulting in a higher resolution capacity and a more economical operation.

Although the replacement of all the known ammonium bicarbonate Etching solution system by ammonium chloride or nitrate (or mixtures thereof) to the. highest etching speeds and the highest Copper dissolving capacity such a system has two disadvantages for certain applications. Of the first of these is a tendency to blacken solder. Since solder is often used as a masking agent or for other purposes on composite printed circuit boards to be etched is present and the blackening or discoloration of the solder the subsequent solderability such a phenomenon should be avoided if possible it depends on the solderability. The replacement of all ammonium bicarbonate by Chloride / nitrate also has the effect of essentially reducing the system's buffer capacity is turned off. If the solution is used up during a day, as is customary in technical spraying applications for printed circuit boards is common, there is no problem of pH stability; however, if the solution is over If something is to be stored for a longer period of time, there may be a lack of buffering of the solution be a disadvantage. For these two reasons, it is sometimes advisable to use a Use solution in which only part of the anionic bicarbonate is replaced by ammonium chloride or nitrate or a mixture of these is replaced. According to the invention therefore, such a partial replacement of the bicarbonate continues to be proposed.

Each of the bicarbonate-chloride-nitrate systems specified here in accordance with the invention enables such operation that its life after exhaustion the primary oxidizer, i.e.

is increased, namely the chlorite, / first by increasing the temperature to use copper dissolved in a bivalent state as a secondary oxidizing agent for the further dissolution of copper, whereby the bivalent copper becomes the monovalent Copper is converted. Secondly can be the useful I, lifetime of the system by ventilation at an elevated temperature immediately before or during the last section of the life of the etching bath can be extended. Under these Operating conditions, of course, the oxygen in the air serves as an oxidizing agent. One Ventilation before exhaustion of the primary oxidizing agent (chlorite) does not help Advantage, and preferably, the ventilation is postponed until the secondary oxidant (divalent copper) is essentially depleted. This fact shows through a reduction in the etching rate obtained and is used as a guide to determine when to ventilate.

The three stages of the copper etching reaction that play a role in the processes described are represented by the following reaction equations: a) 2 Cu + NaClO2 + 2 NH4Cl + 4 NH4OH 2 Cu (NH3) 4Cl2 + NaCl + 6H2O b) Cu + Cu (NH3) 4Cl2 - 2 Cu (NI-13) 2Cl c) 2 Cu (NH3) 2Cl + 2 ° 2 + 2 NH4C1 + 2 NH4OH 2 Cu (NH3) 4C12 + 3H20 The divalent copper present in the complex formed according to equation a) is available for further oxidation of metallic copper according to equation b) in order to form the monovalent (copper-I) complex. which in turn is then oxidized by aeration according to c) Obviously, the same conditions apply if ammonium nitrate is used instead of or in conjunction with the chloride and / or the dicarbonate. The conversion of the amount of happen, if not all of the copper, to the divalent state as well as the increase in the copper concentration in the solution to a point where the reaction (etching) rates are too slow for practical technical purposes, determine the point where the Solution needs to be replaced.

The improvement over the one from US Pat. No. 3,231,503 known system is best explained by comparing the results, obtained with that system and according to the invention, as in the following Table 1 listed. Table 1 mol / l mol / l mol / l mol / l mm / min. Cu capacity% of theo no. ° C NaClO2 NH4OH NH4HCO3 NH4Cl. Etching before the retic Temp.

NH4NO3 speed draining capacity or mixture 1 0.375 3.0 1.5 0 25 0.00330 81.6g / l 85.8 2 0.375 3.0 0.75 1.0 25 0.00457 93.6 g / l 98.4 3 0.375 3.0 0 1.5 25 0.00533 93.6 g / l 98.4 4 0.375 3.0 1.5 0 40 0.00660 81.6 g / l 85.8 5 0.375 3.0 0.75 1.0 40 0.00914 93.6 g / l 98.4 6 0.375 3.0 0 1.5 40 0.00991 93.6 g / l 98.4 7 0.375 3.0 1.5 0 55 0.00813 81.6 g / l 85.8 8 0.375 3.0 0.75 1.0 55 0.01092 93.6 g / l 98.4 9 0.375 3.0 0 1.5 55 0.01168 93.6 g / l 98.4 In the The figures given in Table 1 are without ventilation, their effects in the following is described. obtain.

Examples 1, 4 and 7 of the table above give the experience with the known system of the aforementioned US Pat. No. 3,231,503. The other examples in the table show the significantly improved etching rate and total copper capacity of the system according to the invention.

Using the solutions of the invention, the copper clad printed circuit boards or other documents with the etching solution by immersion in a tank or bath or by spraying on the surface of the pads brought into contact. The solution is initially kept at room temperature, however the rate of etching decreases as the amount of dissolved swab in the solution increases and it is therefore advisable to gradually adjust the temperature of the solution to compensate to increase. As a general rule, it is recommended to raise the temperature of the bath, if that is to remove all copper from a standard 56.7 g copper clad plate required time approaches 30 minutes. Temperature increases can be up to one Maximum value of about 55QC can be made. That is called the practical upper limit considered, since ammonia escapes rapidly from the system at higher temperatures In order to maintain a good etching rate, it is also useful in the Add ammonium hydroxide to the solution from time to time so that its pH is not significant falls below 9.0 and preferably not below 9.8.

When the amount of dissolved copper exceeds about 82.5 g / l, is it is generally advisable to drain the solution and start over, albeit with Aeration this limit can be increased to about 105 g / L, as explained below. The dissolved copper contained in the used solution is readily precipitated, by adjusting the solution to essentially neutral reaction, either by Addition of an excess of water or of dilute sulfuric acid except for one pH of about 6.0. After standing, the copper hydroxide settles out of the solution and is generally at least 99% recovered. The supernatant solution can be easily decanted and drained without risk of wastewater contamination, whereby the copper content remains as sludge in the settling tank and is used for recovery of the metal can be worked up.

The system is extremely stable, especially if you have a good pH setting is maintained at or above pH 9.0 so that no precipitation in the etch tanks or settling occurs. That is true even if the solution is between hours of operation stands for some time, especially when their temperature, if they are above around initially 350 C is immediately cooled below this point if the period of non-use exceeds about 3 hours The solution is not critical with regard to the oxidant concentration, since only 0.1 mol / l sodium chlorite already provide a certain caustic effect, although obviously the etching speed is slow. At the other border, the Concentration up to the saturation limit of the chlorite in the solution. Preferably becomes at a concentration range of about 0.375 mol / l worked. The ammonium ion concentration should always be that of the copper in the solution It will exceed 1 and in general there will be an excess over the copper in one Molar ratio of at least 2: 1 is preferred. This will of course go through the ammonium hydroxide and the ammonium chlorite, nitrate and bicarbonate used delivered. Since the required amount of ammonium hydroxide is mainly due to the requirement to maintain a pH of the solution of at least 9.0 is determined im the use of about 3 moles of base per liter is generally recommended.

The ammonium chloride, ammonium nitrate and ammonium bicarbonate concentrations in such a system should be at least 0.2 mol / l and preferably 2.0 mol / l, to achieve the highest possible etching speed and copper capacity. A practical one upper limit for the ammonium chloride or ammonium nitrate and optionally ammonium bicarbonate is generally about 4.0 mol / l. Ammonium chloride is opposed to nitrate preferred, simply for the sake of storing dry ammonium nitrate associated dangers. However, the effectiveness of the nitrate in the etching solution is the same as well as that of chloride, and the storage problem of nitrate can be taken for granted be circumvented by using a concentrated solution instead of dry salt used When the solution is at least partially exhausted, i.e. to the point where the primary oxidizer is consumed and the concentration of solute Copper is above about 45 g / l, further operation is significantly improved, by stirring with air while maintaining the temperature at 550 ° C. That will explained by the following table 2, in which a solution with the composition of Example No. 8 of Table 1 was used.

Table 2 Cu in solution (g / l) Etching rate Etching rate before aeration (mm / min.) after aeration (mm / min.) 44.9 g / l 0.00381 0.00432 59.9 g / l 0.00279 0.00534 74.9 g / L 0.00203 0.00381 Although the etching rate is 44.9 g / i in the table above shows some improvement with ventilation, it is assumed that the difference is primarily due to measurement errors, since the Speeds should theoretically be the same because at this concentration all of the copper would likely be in divalent form. Both This condition no longer applies to both higher levels of exhaustion, and one notices that the etching speed is almost doubled.

The copper dissolution capacity of the solutions with aeration is as above specified, no longer limited by the original chlorite concentration, since another oxidizing agent, namely air, is introduced. Under such conditions the ammonia content is decisive, and is in the examples given above the theoretical capacity of the solutions is about 142.4 g / l. Limit solubility problems however, the practical capacity in normal operation to about 104.9 g / l. Although the increase in capacity thus obtained is not too great, is the increase the etching speed is of considerable practical importance, especially in the case of automated ones Procedures.

Claims (5)

Patent claims 1.) Copper etching solution, which is essentially water, sodium chlorite, ammonium hydroxide and an ammonium salt, namely ammonium chloride, ammonium nitrate, or ammonium bicarbonate or contains a mixture of these salts, ammonium nitrate and / or ammonium chloride are always present and the chlorite concentration from about 0.1 mol / l to saturation and the ammonium salt concentration is from about 0.2 molar to saturation and Ammonium hydroxide is present in such an amount that the solution has a pH of at least 9.0 owns. 2.) copper etching solution according to claim 1, characterized in that the Sodium chlorite concentration 0.375 molar, the ammonium salt concentration 1.5 molar ammonium chloride or ammonium nitrate or mixtures of the two and the ammonium hydroxide concentration 3.0 molar is. 3.) copper etching solution according to claim 1, characterized in that the Ammonium chlorite concentration 0.375 M lar, the ammonium salt concentration 0.75 Molar in terms of ammonium bicarbonate and 1.0 molar in terms of ammonium chloride or Ammonium nitrate or mixtures of these two salts. Document 4.) Verb preserve to dissolve copper from a copper-coated / characterized in that a) an etching solution of the composition specified in claim 1 is made and initially held at about room temperature; b) the one to be etched copper-bearing base with etching solution sufficient long in touch brought to dissolve the desired amount of copper therefrom; c) the temperature of the etching solution with increasing content of the etching solution in dissolved copper and decreasing The etching rate of the solution is gradually increased to not above 550 C and d) from time to time ammonium hydroxide is added to the solution to bring the pH to to hold at least about 9.0. 5.) A method for dissolving copper according to claim 4, characterized in that that the solution is also aerated when the dissolved copper content is about 45 exceeds g / l.