Figure 1. Acid cupric chloride etching, mass balance (qualitative).

In-situ regeneration of etchant and on-site recovery of copper or copper salts is environmentally appealing (see Ref. 1) and may also make economic sense, given a certain scale of operation (Ref. 5). The regeneration of Cu²⁺ (cupric ions) from Cu¹⁺ (cuprous ions) in the etch chamber, or in a regeneration chamber that is close-looped to the etch chamber, is common practice.

Figure 2. Cupric chloride etch chemistry.

Examples are the re-oxidation of cuprous ions with oxygen in ammoniacal etchers (Ref. 8-12), the oxidation of complexed cuprous chloride to cupric chloride with chlorine (Ref. 13-16), hydrogen peroxide, or chlorate in acid etchers, and the oxidation of ferrous chloride to ferric chloride with chlorine (Ref. 17, 18).

Figure 3. Ammoniacal etching, mass balance (qualitative).

However, to maintain a desirable etch rate, a dynamic equilibrium concentration of etchant and reaction products needs to be maintained by bleeding off a steady stream of etch chemistry (see material balances and etch chemistry for acid and ammoniacal etching, Fig. 1-4).

Figure 4. Ammoniacal etching chemistry.

The use of hydrogen peroxide as full-etch is rare but it is quite common as a micro-etch. The full-etch example I have in mind was practiced in an environmentally sensitive area in Southern Germany, but was abandoned due to high cost and the high level of analytical controls required to maintain stabilizers at the proper level. Nevertheless, copper sulfate-based hydrogen peroxide etchants are ideal for on-site recovery: high purity copper sulfate is precipitated in a chiller, isolated, and sold or can be used for acid copper plating bath make-up. Hydrogen peroxide based etchants are becoming quite popular in reducing the copper thickness on laminate for the production of high density interconnections. These etchants contain proprietary additives (e.g., Vantage 1700S Copper Etchant) to enhance the etch uniformity.

On-site Recovery Options for Acid and Ammoniacal Etchants

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(see also commercial examples under Ref. 1, and Overviews of Ref. 4 & 6)

On-site recovery of spent cupric chloride typically involves electro-winning, i.e., the plating out of metallic copper from spent etchant on the cathode of an electro-winning cell (e.g., FSL, Ref. 1-3). A few issues to be aware of: from an electro-winning perspective, it is desirable to recover from spent etchant with high free acid normality to obtain high plating rates. However, the normality of the acid in the etcher should be limited to <3N because higher concentrations may attack the photoresist. Some electro-winning systems rely on the brittleness of the plating copper to facilitate the removal of the copper metal from the cathode. If organic impurities that act like brighteners are dragged into the electro-winning cell, the recovered copper may become too ductil to be easily scraped off the cathode.

The ARS system, ARS Resource Recovery (Ref. 1), can recover copper from acid as well as alkaline spent etchant streams.

One system (Mecer®, Ref. 1, 2, 10, 11), uses an organic, non-miscible phase, containing strong complexing agents, to extract the copper from the spent ammoniacal etchant. The copper is then extracted into sulfuric acid electrolyte from which copper metal is recovered in an electro-winning cell.

The CuRE process is also based on solvent extraction. It can combine acidic and ammoniacal spent copper etchant. After solvent extraction and re-extraction into an aqueous sulfuric acid phase, copper sulfate is precipitated.

Figure 5. Ammoniacal etchant replenishment and recovery loops (source: www.elo-chem.de).

Another system for ammoniacal etchant re-generation and on-site recovery is offered by Elo-chem (Ref. 1, 7, 12). It consists of two separate loops (see schematic of Fig. 5):

1. One loop for the regeneration of the etching solution in the etching machine (Regeneration I).
2. A second loop for the electrolytic recovery of copper from this etching solution (Regeneration II).

The first loop introduces oxygen from the air in a controlled way to maintain the oxidizer (oxygen) at a known, high level. In most other systems, the oxygen concentration is not a well-controlled, independent variable, but is the result of the vent air flow and machine design features such as air intake and exhaust locations. As etchant consumption per time unit changes due to board sizes and copper area per board surface, etch rates will begin to fluctuate, and, unless the conveyor speed is adjusted, over-etching or under-etching will result.

The second loop contains an electro-winning cell in which pure copper is recovered. Direct electrolysis of the electrolyte without fear of chlorine evolution is possible because the alkaline etch chemistry is based on copper sulfate, not copper chloride. An additive is used to increase the etch rate which is inherently slower with sulfate than with chloride. Both replenishment and copper recovery are controlled by means of density measurements and are linked to the rate of etchant demand.

If on-site recovery is not a suitable option, you may want to check out the IPC Recycle Directory for off-site recovery and reclamation resources. This document is provided by the Reuse/Recycling Subcommittee of the IPC Suppliers Management Council Steering Committee.

References

1. Design for the Environment, A Cooperative Project between the U.S. Environmental Protection Agency and PWB Manufacturers Nationwide, Publication EPA 744-F-95-005. Referencing:
-Mecer System by Cognis, Inc. (Ammoniacal Etchant Recovery System)
-Elo-Chem (Alkaline Regeneration Module, for Ammonium Sulfate based Etchant)
-FSL System, distributed by Finishing Services Limited (Cupric Chloride Regeneration System)
-ARS Resource Recovery, distributed by ARS (Combined System- Cupric Chloride/ Ammonium Chloride)

2. "Praegitzer Challenges Current Delisting Process; Turns Waste into Profit," Bret Bruhn, CiruiTree, February 1998, pg. 26 (FSL acid copper recovery and Mecer system alkaline copper recovery are covered).

3. Etchant Regeneration and Copper Recovery Systems, FSL Product Literature.

4. "Steady-State Regenerative Etching: A Technology Review," Dr. Marshall I Gurian, Electronic Packaging and Production, July 1978, pg. 32.

5. "Processing and Economic Aspects of Etchant Regeneration," R. E. Markle, Plating and Surface Finishing, Vol. 70, Nr. 1, January 1983, pg. 59.

6. "Electrolytic Regeneration of Etching Solutions," M. R. Hillis, PC Fab, June 1983, pg. 32.

7. www.elo-chem.de, ELO-CHEM Recycling-Anlagenbau GmbH
Lippertsreuterstrabe 7
88682 Salem/Rickenbach
Germany
phone: +49 - 7553 - 7051
fax: +49 - 7553 - 7052
Email: c.bernauer@elo-chem.com

8. Ammoniacal Etchant- Onsite Reblending, Ed Shaw, Advanced Recovery Systems, Inc., Mt. View, CA, Proceedings, NECA (North East Circuits Assoc.), Tech Conf. & Exhibit, Nov. 8-9, 1995.

9. "Etchant Reblending and Copper Recovery," Ed Shaw, CircuiTree, March 1998, pg. 122.

10. Mecer® Ammoniacal Etchant Regeneration Systems, Product Literature, Cognis, Inc.

11. The Mecer® Process: Continuous Recycling of Ammoniacal Etch Liquors used in the Manufacture of Circuit Boards for the Electronics Industry, Dr. Hans Reinhardt, Sigma Metallextraktion AB, Sweden, Sigma-MEAB Print MECER317, April 1993.

12. Elo-Chem Recycling Aetzverfahren: Eine Revolution auf dem Gebiet des Aetzens von Leiterplatten, Elo-Chem Aetztechnik GmbH, 7758 Meersburg, Drosteweg 21, Phone: 49 (0) 7532-9080.

13. Elektrolytisches Regenerations - und Kupferrueckgewinnungs-System fuer Kupferchlorid-Aetzmedium, Galvanotechnik, D-7968 Saulgau, 83 (1992), Nr. 10, pg.3566.

14. "Cupric Chloride: Characteristics and Regeneration Methods," James C. Swartzell, PC Fab, August 1980, pg. 4

15. "Cupric Chloride Regeneration," Donald G. Barrett, The Journal (PCMI), Fall 1991, pg. 15.

16. "Electrolytic Regeneration of Cupric Chloride with Full Copper Recovery," Gareth Weed, Finishing Services Limited, PO Box 153307, Irving, Texas 75015.

17. "The Potential of Oxygen for Regeneration of Spent Ferric Chloride Etchant Solutions," David M. Allen and L. T. Ler, The Journal (PCMI), Winter 1995, pg. 3

18. "Experience with Ferric Chloride Regeneration, Dr. Hans Korsee," The Journal, Summer 1992, pg. 11.