Regeneration etchants

Copper etchants do not directly influence the electroless plating process, but are used merely to remove unwanted copper, and should not affect the deposit properties. The costs of waste treatment and disposal have led to disuse of throw-away systems such as chromic—sulfuric acid, ferric chloride, and ammonium persulfate. Newer types of regenerable etchants include cupric chloride, stabilized peroxide, and proprietary ammoniacal etchant baths. 

Cupric chloride (CuClj) ia a strong etch employed for producing circuit patterns on the board. The etchant loses its effectiveness as copper etched from the board reduces the CuClj to cuprous chloride (CuCl). The spent etchant can be regenerated, however, by oxidizing the CuCl, using direct chlorination (DHS 1987). 

Yang, Q. and Kocherginsky, N.M. (2006) Copper recovery and spent ammoniacal etchant regeneration based on hollow fiber supported liquid membrane technology From bench-scale to pilot-scale tests. Journal of Membrane Science, 286, 301. 

CEER process — (Capenhurst electrolytic etchant regeneration process) Electrochemical process for continuous copper removal from printed circuit board etching solutions employing either cupric chloride or ammoniacal etchant. In a cell divided by a cation exchange membrane the etching process is essentially reversed. In case of the cupric chloride etchant the etchant solution is pumped to the anode, the processes are at the... 

The etchant solution is pumped to the cathode of the regeneration cell, electrode reactions are at the... 

Copper Etchant Baths. One of the most promising applications of coupled transport is the renovation of circuit board etchant solutions that contain copper.61 The printed circuit board industry produces more than ten million gal-lons/year of spent etchant solutions in which copper, other salts and etchant chemicals are concentrated. Coupled transport permits continuous on-site removal of copper from the etchant solutions and simultaneous regeneration of the etchant solution. This represents a considerable savings in the costs of manufacturing circuit boards. 

The properties of WC-catalyzed hydrogen electrodes offer the opportunity for another recycling process - the regeneration of cupric chloride etchants from printed circuit manufacturing [26-28]. 

The advantage of the currentless oxydation consists in the complete regeneration of the etchant. The re-oxydation of the CU2CI2 can be performed by inserting oxygen directly into the solution or by oxydation at electrocatalyzed 02-fed electrodes in a short circuited cell. The latter methode prevents the formation of aerosols from gas bubbling. 

In the continuous electrolytic regeneration of cupric chloride etchant the cuprous chloride is oxidised anodically in a cell while the cathode of the cell recovers the copper as a solid flake deposit. The cell, developed by the Electricity Research Council in the UK is divided by a membrane which limits the transport of copper ions, which are in fact complexed, probably mainly as CuCl3 . An economic analysis of the process realised a two year payback on the capital investment. In the case of alternative etchants, such as ferric chloride, continuous regeneration is also feasible. 

The etchant is regenerated by reoxidizing the cuprous chloride to cupric chloride by several chemical choices as illustrated by the following equations ... 

Chlorination. Direct chlorination has been the preferred technique for regeneration of cupric etchant because of its historically low cost, high rate, efficiency in recovery of copper, and pollution control. The cupric chloride-sodium chloride system (Table 34.1, no. 3) is suitable. Figure 34.2 shows a generalized process. Chlorine, hydrochloric acid, and sodium chloride solutions are automatically fed into the system as required. Sensing devices include oxidation-reduction instruments (Cu oxidation state), density (Cu concentration), level sensors. 

The etchant is a solution of cupric chloride and hydrochloric acid (Table 34.1, no. 1). Etchant flows continuously between spray-etching machines and a plating tank. In the plating machine, two processes take place simultaneously copper is plated at the cathode, and regeneration of the spent etchant occurs at the anodes. pf>er recovery may not return copper value and may be expensive, inconvenient, and cause difficulty with recycling. 

These etchants for solder- and tin-plated boards were preferred for many years. More recently, their use has been completely eliminated due to Cr(VI) listing as a critical environmental hazard. Other problems with chromic-sulfuric etchants are difficulty in regeneration, inconsistent etch rate, the low limit of dissolved copper (4 to 6 oz/gal), and dangerous degradation of PVC and polypropylene equipment. Chromic add etchant is suitable for use with solder, tin/nickel, gold, screened vinyl lacquer, and dry or liquid film photoresists. Chromic-sulfuric mixtures etch copper slowly, and additives are needed to increase the etch rate. For example, sodium sulfate and iodine have been used for rate increase. Alkaline etchants have become so well controlled that there is no justification for the risks and costs of chromic acid formulations. 

Processes have been developed which use an electrochemical cell to oxidize to Cr(vi), regenerating the etchant in a convenient manner by recycling the... 

An example is the control of effluent pH - a glass electrode may be used to determine pH and a divided ceil used to introduce OH" or by water electrolysis. An application might be in the chlor-alkah industry for readjusting the pH of the brine leaving the main cells from pH 4 to pH 7. The use of an inert (e.g. platinum) electrode to monitor the redox potential of etchant solutions (and, hence, the state of regeneration) is mentioned in Chapter 9. 

Cupric chloride in hydrochloride acid etchants are faster than the ferric chloride types and regeneration of the etchant is possible using chemical (H2O2 or CI2) or electrochemical treatments (see later). The overall etching process may be represented by ... 

Fi 9.12 The CApcnhurst Electrolytic Etchant Regeneration (CEER) process, (a) Cupric chloride etchant, (b) Atnmoniacal etchant. 

Table 7.4 Typical operating conditions for the regeneration of a Cr(vi) etchant for plastics. (Courtesy General Motors Research Laboratories)...

A major advantage of this etchant is the simple nature of the products, which permit etchant regeneration via CUSO4.5H2O precipitation on cooling or electrochemical methods. The latter approach has the advantage of retaining the sulphuric acid content. 

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