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Copper etching with cupric chloride and regeneration of waste etchant
Journal of Materials Processing Technology 175 (2006) 63–68
Copper etching with cupric chloride and regeneration of waste etchant O. Cakir Department of Mechanical Engineering, Dicle University, 21280 Diyarbakir, Turkey
Abstract Copper etching is considerably important process in electronics industry, particularly in the fabrication of printed circuit board. Various etchants can be used for this purpose, but nowadays cupric chloride is more accepted etchant, because of its high etch rate and easy regeneration properties. In the present study, copper etching process with cupric chloride etchant was investigated. In the etching of copper, the most important etching parameter is etch rate, therefore the investigation was based on the various effects on etch rate. The influence of etchant concentration, additives and etching temperature were examined. It is also important to regeneration/recycle of waste etchant from environmental point of view. Thus, various cupric chloride etchant regeneration processes were investigated. © 2005 Elsevier B.V. All rights reserved. Keywords: Copper etching; Cupric chloride; Etch rate; Waste regeneration
1. Introduction Corrosion damages metals under normal circumstances. It can be an efficient machining process if it is controlled. This type of machining process is generally named etching, which is one of the important nontraditional machining processes to manufacture geometrically complex machine parts from thin and flat streets. The process uses a liquid chemical solution called an etchant. The etchant attacks to remove selected areas of metal part. The process is extensively used to manufacture components such as printed circuit boards, shadow masks for cathode-ray tubes, integrated circuit lead layouts, fluorescent indicators, jewelleries and nameplates, etc. for electronics, aerospace, precision engineering and decorative industries. The etching process was first practiced as an art form in the Ancient Egypt in around 2500 b.c. It was applied to produce jewelleries made of copper etched with citric acid. Nowadays, the process is an essential part of manufacturing micron-size machine parts. The etching process is the major machining step in the chemical and photochemical machining process [1–4]. Copper is an important metal for various engineering applications. Its wide usage is due to its excellent electrical and thermal conductivity, easy fabrication, and good strength and fatigue properties. E-mail address: [email protected]. 0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.04.024
Copper etching is the main process in the production of printed circuit boards. Historically, the first used etchant in the printed circuit boards (PCBs) fabrication was ferric chloride (FeCl3 ) and it is still used as an etchant for small-scale production lines. However, in last two decades, various new etchants have been introduced for copper etching. Some of them found more applications then the others; one of them was cupric chloride (CuCl2 ). Nowadays, CuCl2 have been a major etchant in copper etching process, due to its advantages [5]. In this study, copper etching with using CuCl2 was investigated. The effects of etchant concentration, etching temperature, chemical additions to main etchant and oxidation redox potential on the etch rate, undercut and etch factor of copper were examined. It is also important that the regeneration/recycle and etched metal recovery of waste etchant should be available because of environmental restrictions; therefore, the regeneration processes for waste CuCl2 have been determined.
2. Etchants for copper etching The ideal etchant solution for copper should have some properties as follows [6–9]: (a) high etch rate;
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O. Cakir / Journal of Materials Processing Technology 175 (2006) 63–68
Table 1 Etchants and their properties in the etching of copper Etchant
Etching temperature (◦ C)
Etch rate (m/min)
Undercut
Dissolved copper capacity
Regeneration and metal recovery
Cupric chloride (CuCl2 ) Ferric chloride (FeCl3 ) Alkaline etchants Hydrogen peroxide/sulphuric acid (H2 O2 + H2 SO4 ) Ammonium persulhate [(NH4 )2 S2 O8 ] Chromic-sulphuric acid [CrO3 + H2 SO4 ]
50–54 43–49 43–55 43–55 38–55 26–33
25–50 25–50 30–60 Variable 7 Variable
Low Low Lower Low High Lower
120–140 40–60 140–170 50–90 40–55 50–60
Copper recovery and regeneration Regeneration Copper recovery and regeneration Copper recovery Copper recovery Not available
(b) (c) (d) (e) (f) (g) (h)
minimum undercut; high dissolved copper capacity; stable and easy control of copper etching process; not generate toxic fumes; environmentally acceptable; economic regeneration of waste etchant; economic etched copper recovery from waste etchant.
There are various etchant solutions for copper available commercially. The selection of ideal etchant solution for copper depends on various properties of etchant mentioned above, but environmental and health restrictions have become an important parameter since 1970s. High hazardous properties of any etchant cannot be acceptable anymore because the strict environmental restricting of local and country levels have became increasingly demanding, for forcing copper etching industries into expensive waste minimisation processes. Thus, the manufacturing process has increased. The usable etchants for copper etching are given in Table 1. The properties of each etchant are also mentioned. However, the last three etchants are no longer applied for copper, because they are very corrosive causing environmental pollution and health problems. As a result, basically three etchants have been widely accepted for copper etching. These are CuCl2 , FeCl3 and alkaline etchants [9,10]. CuCl2 is a suitable etchant for any copper etching process. It is also widely used for “print and etch” type PCB at a larger scale. These PCBs are single-sided boards and extensively requires economical etchant. CuCl2 provides high etch rate when compared to FeCl3 . Copper etching with FeCl3 produces CuCl2 during process and it was mentioned that the etching process was completed with the by-product of CuCl2 [11]. It also produces lower undercut than FeCl3 . The copper dissolving capacity of CuCl2 is three times higher than FeCl3 . The full regeneration type of waste CuCl2 makes the etchant more attractive to copper etching industry. It is also used as an etchant for various copper alloys like brass,
bronze, beryllium–copper etching. The other etchant is alkaline etchants which are based on ammoniacal solutions such as ammonium copper chloride or ammonium copper sulphate. The main different factor between CuCl2 and alkaline etchants is that the user of copper etching process depends on the alkaline etchant supplier. The application of CuCl2 would provide more flexible advantages compared to alkaline etchants. Moreover, alkaline etchants are used for copper only. They cannot be used for copper alloys because of alloying elements, which make regeneration and recycling of waste alkaline etchants complex. They are also expensive therefore they are widely used for bulk copper etching process. Table 2 indicates some properties of CuCl2 , FeCl3 and alkaline etchants [9,10].
3. Copper etching with cupric chloride CuCl2 is rather messy, yellow-brown colour, hygroscopic solid. It is sold as the green crystalline dihydrate salt (CuCl2 + 2H2 O). It is commonly used acidic compound and comprises the copper salt, water and hydrochloric acid (HCl) [2]. The etching of copper with CuCl2 can be simply expressed by the following chemical reaction: CuCl2 + Cu → 2CuCl
(1)
The uncoated copper surface gets attacked by CuCl2 . The one copper atom reacts with one cupric ion and forms two atoms of the cuprous ions as follows: Cu2+ + Cu0 → 2Cu+ cupric
metal
(2)
cuprous
This simple chemical reaction explains main copper etching process with CuCl2 , but the additions of various chemical solutions to main etchant would make etchant complex and thus etching mechanism.
Table 2 Some etchants characteristics in PCB fabrication [9] Etchant
Corrosiveness
Neutralisation and disposal problems
Toxicity
Operational cost∗
Cupric chloride (CuCl2 ) Ferric chloride (FeCl3 ) Alkaline etchants
High High High
Low Medium Medium
Medium Low Medium
Low Medium High
*
Include disposal cost.
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The copper etching with CuCl2 etchant solution is strongly controlled during process. The most commonly used control technology applies a colourimetric type etch chemistry controller. It is cheap and reasonably reliable. However, it is assumed this control type that there should be no free acid [12,13]. 3.1. Effect of etchant concentration The concentration or molarity of CuCl2 etchant solution is an important parameter. It is commonly represented by Baum´e (B´e) degree, which is depends on specific gravity of etchant. The calculation of Baum´e degree can be done as follows [2]: B´e = 145
specific gravity − 1 specific gravity
(3)
The higher Baum´e means high molarity of etchant. It is reported that the optimum Baum´e degree or molarity of CuCl2 should be around 32–33 ◦ B´e or 2.33–2.5 mol. It was noticed that this value of Baum´e produced a stable and high etch rate for copper etching, lower of this value would give low etch rate with poor line resolution and higher etchant would produce slow etch rate with a smooth line resolution. It is also mentioned that the undercut would decreases with increasing Baum´e degrees [14]. 3.2. Effect of etching temperature In general, the etching temperature for any etchant provides advantage in any metal etching process. In case of copper etching with CuCl2 it observed, using higher etching temperature increases the etch rate. Therefore, it is aimed to use the possible highest etching temperature during process. However, the selection of etching temperature depends on etching machine. The etchant chemicals are extremely corrosive to all common metals; therefore, the etching machine chamber is usually made of plastic. The only metal used in the machine is titanium [13]. The possible maximum etching temperature in commercial etching machines allowed up to 50–55 ◦ C. It was obtained that the etch rate of copper at 32 ◦ B´e (2.33 mol) CuCl2 and 20 and 50 ◦ C etching temperatures were 6.6 and 10 m/min, respectively [15]. The high etch rate and low undercut would produce a desired high etch factor. The calculation of etch factor is given as follows (Fig. 1) [2]: etch factor =
D D = (A − B)/2 U
(4)
where D is depth of etch (factor of etch rate), U undercut, A slotwidth after etching and B is slotwidth before etching.
Fig. 1. Etch factor.
3.3. Effect of oxidation redox potential In ideal copper etching process, the oxidation redox potential (ORP) of CuCl2 is continuously monitored. The ORP indicates the relationship of cupric ions to cuprous ions. It is measured as the relative conductivity of etchant and expressed in millivolt. As copper is etched, the etchant changes from a cupric to cuprous state. Decrease of the ORP causes low etch rate of copper. The addition of HCl and an oxidizer to the etchant introduces free chlorine. This makes cuprous ions to cupric form. The optimum ORP for CuCl2 is recommended between 510 and 530 mV [14]. 3.4. Effects of chemical additives The additions of various chemical additives to CuCl2 etchant produce better copper etching properties. Basically, when CuCl2 solution is prepared, the small amount of HCl is added to the main etchant [15]. As the addition of HCl to CuCl2 is completed, the chemical reaction of copper etching becomes as follows: CuCl2 + 2HCl + Cu → 2CuCl + 2HCl
(5)
Copper etching with CuCl2 would produce CuCl and this stops further etching. The addition of HCl reacts with CuCl and makes it CuCl2 , so that the etching continuous. Although the addition of HCl to CuCl2 makes the etchant complex, the combination of CuCl2 + HCl produces high etch rate in copper etching process. This positive effect is shown in Table 3. It is also noticed that the copper dissolve capacity of CuCl2 increases by HCl addition [16]. It is also known that HCl addition increases undercut which is not desired. Therefore, the etch factor will be lower. Probably, the most important chemical addition to CuCl2 is HCl, but the level of this addition is depending on the Table 3 Effect of HCl addition to CuCl2 [on etch rate of copper 13] CuCl2 (◦ B´e)
Etching temperature (◦ C)
HCl addition (N)
Etch rate (m/min)
33 (2.5)a 33 (2.5)a 33 (2.5)a
49 49 49
1 (1)a 2 (2)a 3 (3)a
25 31 38
a
Values are in mol.
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etching machine again. The high HCl addition would react with etching machine parts, particularly attacks titanium, and cause some undesired corrosion. The addition of ammonium chloride (NH4 Cl), to CuCl2 provides high etch rate and copper dissolve capacity as the same of HCl addition. It is reported that CuCl2 with 15% NH4 Cl would produce four times higher etch rate than CuCl2 only [6]. It is mentioned that the other chemical additives such as monomethanol amine, ethanol, acetonitrile, aceton, dimethyl formamide to CuCl2 etchant increase etch rate of copper [17]. The effect of chemical additives to CuCl2 seems advantageous, the selection of ideal additive would be based on chemical reaction of copper etching which should make simpler and controllable. This is also important factor from the regeneration of waste CuCl2 etchant point of view. The complex etchant solution would make the regeneration process difficult and expensive. Therefore, it is a general approach that the etchant should be kept as simple as possible. As a result, the addition of HCl to CuCl2 would make more sense.
4. Regeneration of waste cupric chloride etchant The waste etchant is very corrosive and considered “hazardous waste”, because of its acidic (pH of waste CuCl2 etchant is less than 2.5) preposition in accordance with one of the standard descriptions. The waste CuCl2 etchant contains copper ions and their limitations on the environment are very strict, because they are heavy metals [18–20]. The environmental issues have become very important phenomena in the industrialised countries since the beginning of 1970s. Strict environmental legislation has been introduced and etching industry has been extensively affected. The users of etching process relied on end-of-pipe treatment and/or disposed for hazardous waste etchant generated before [20,21]. This option has been not an optimal strategy for managing waste CuCl2 etchant anymore. Because: (a) The potential liabilities involved with the handling and disposal of waste CuCl2 etchant have arisen and will continue to increase. (b) Waste etchant disposal cost has gone up significantly due to restrictions placed on land disposal. As a result, the etching industry has faced with a challenge of finding alternative methods for managing by-product of hazardous waste CuCl2 etchant. Thus, the waste etchant minimisation or even elimination was considered seriously. The regeneration processes for waste CuCl2 etchant have been developed to solve waste problems of etching industry. The use of regeneration process would also brought other benefits like saving from equipment and operation costs and significantly reducing liabilities associated with the disposal of hazardous waste etchant [20]. The regeneration of waste etchant is a waste treatment process used by etching industry. The waste etchant is ox-
Table 4 Regeneration processes for waste CuCl2 etchant used for copper etching Chlorine regeneration 2CuCl + Cl2 → 2CuCl2 Sodium chlorate regeneration 2CuCl + 23 NaClO3 + 2HCl → 2CuCl2 + 23 NaCl + 2H2 O Hydrogen peroxide regeneration 2CuCl + H2 O2 + 2HCl → 2CuCl2 + 2H2 O Oxygen regeneration 2CuCl + 21 O2 + 2HCl → 2CuCl2 + 2H2 O Electrolysis 2CuCl + 2HCl → 2CuCl2 Anode: Cu+ − e− → Cu2+ oxidation Cathode: H3 O+ ± e− → 21 H2 ↑ +H2 O Electrodialysis Anode: Cu+ + e− → Cu2+ oxidation Cathode: Cu+ − e− → Cu0 reduction
idised back to reusable or starting form by regeneration process. The benefits of any regeneration process can be expressed below [14]: (a) (b) (c) (d) (e)
lower etchant cost; continuous and stable etch rate; higher productivity; less equipment downtime; lower labour costs.
The full regeneration property of waste CuCl2 etchant is one of the most important advantages. There are various regeneration processes are commercially available for waste CuCl2 etchant. Chemical regeneration processes such as chlorine, sodium chlorate, hydrogen peroxide and oxygen mainly use various chemical additives into waste etchant. They are widely used by etching companies due to the pure price of CuCl2 . The chemical regeneration of waste CuCl2 produces twice the volume of CuCl2 than the starting volume of the etchant. This surplus CuCl2 can be sold to the other copper etching companies for further utilising. It is also sold to plastic producers for blue colouring of plastics. The chemical regeneration processes for waste CuCl2 etchant are shown in Table 4 [22–24]. It is noticed that the chlorine gas regeneration seems an ideal regeneration process for waste CuCl2 etchant. It is also a cheaper regeneration process than hydrogen peroxide and sodium chlorate regeneration methods [14]. Chemical regeneration processes mentioned above produce as amount of etchant in excess amount of starting quantity in copper etching. This causes further problems such as disposal of excess regenerated etchant. These regeneration processes do not also deal with the removal of etched copper from waste etchant. To solve waste regeneration and etched copper recovery together can be accomplished by using electrolysis and electrodialysis processes.
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Table 5 Comparison of regeneration processes for waste CuCl2 etchant
5. Conclusion
Regeneration process
Comments
Chlorine regeneration
The cheapest regeneration process Simple regeneration process Water addition necessary Consideration on safety
Sodium chlorate regeneration
More expensive than chlorine regeneration Eliminating chlorine addition Improvement on the etch rate Solid etchant addition necessary
Hydrogen peroxide regeneration
The most expensive regeneration process Safer than chlorine regeneration Solid etchant necessary
Oxygen regeneration
Very attractive regeneration process Much safer than chlorine regeneration Water addition necessary
Electrolysis
Expensive equipment and materials Flammable H2 release Water addition necessary
Electrodialysis
Expensive equipment and materials Etched copper recovery simultaneous Investment amortization by selling recovered copper
Copper etching is an important nontraditional machining process to produce complex machine parts from thin sheet for various industries. This process is commonly used in the production of PCBs. Copper is etched by different etchants, but CuCl2 seems to have more advantages than the other etchants. Copper etching with CuCl2 produces a stable and high etch rate and low undercut. This indicates the desired high etch factor. The selection of etchant parameters in copper etching with CuCl2 should be completed to obtain optimum etching result. The etching temperature should be selected as the highest possible temperature the etching machine allowed. This would be around 50–55 ◦ C. The molarity or Baum´e degree of CuCl2 etchant should be around 2.33–2.5 mol or 32–33 ◦ B´e. The ORP of CuCl2 should be strictly controlled and kept around 510–530 mV. The maintaining of the ORP can be completed by adding HCl or chlorine. These etching parameters would provide a desired copper etching result. The regeneration of waste CuCl2 etchant is also important factor to be considered for copper etching. Environmental regulations are forcing etching companies to use one of suitable regeneration processes. The advantage of full regeneration of waste CuCl2 etchant minimizes or eliminates environmental pollution occurred during and after etching process. This would also reduce operational costs. Copper etching with CuCl2 etchant and a suitable regeneration process of waste etchant should be carried out simultaneously. This would make the overall manufacturing cost low and environmentally friendly etching process.
The electrolysis process is an electrochemical separation technique where the waste etchant is regenerated, but no etched metal recovery occurs. The electrolytic cell is divided into anode and cathode compartments by means of an anion–permeable ion exchange membrane. The membrane allows only negatively charged ions to pass between catholyte and anolyte, and therefore, stops newly produced cupric ions at the anode from being reduced back to cuprous at the cathode. Chlorine ions come from the addition of HCl to the regeneration system and join with cuprous ions. As a result, CuCl2 is obtained. The electrolysis process uses a high level current. It produces hydrogen gas, which is released from the system. This is environmentally undesirable. It also increases the etchant concentration, which is controlled by water addition. This causes a disposal problem of excess etchant [25]. The electrodialysis process is a new promising regeneration method and is variation of electrolysis method. The cation selective membrane divides the electrodialysis cell into two halves, the anoylte and the catholyte. The waste CuCl2 etchant is reoxidised on the anode and etched copper is recovered on the cathode by passing a dc electric through the waste etchant. The waste etchant is regenerated and copper recovery is obtained simultaneously [25]. All possible regeneration processes for waste CuCl2 etchant have advantages and disadvantages. However, a few of them can be considered as environmentally acceptable. The comparison of regeneration processes is given in Table 5.
References [1] R. Ueda, Corros. Eng. 38 (1989) 271–282. [2] D.M. Allen, The Principle and Practices of Photochemical Machining and Photoetching, Adam Hilger Publication, 1986. [3] W.T. Harris, Chemical Milling, Oxford University Press, 1976. [4] J.W. Dini, American Machinist (Special Report), 128 (1984) 113–128. [5] C.F. Coombs Jr. (Ed.), Printed Circuit Boards, McGraw-Hill, 1995. [6] J.O.E. Clark, Marconi Rev. 26 (1961) 135–152. [7] S. Gowri, K.S. Indira, B.A. Shenoi, Met. Finish. 64 (1966) 54– 59. [8] E.C. Jubb, Plating 139 (1964) 2202–2211. [9] M.J. Thomas, Etching, MacDermid (GB) Ltd., 28 pp. (Technical Note). [10] W.C. Bosshart, Printed Circuit Boards—Design and Technology, Tata McGraw-Hill Publications, 1985. [11] E.B. Saubestre, Indust. Eng. Chem. 51 (1959) 288–290. [12] R. Sedlak, http://www.pcbfab.com/ineretch.html (1999). [13] A. Latham, Electronic Production, 11 (1988) 101,103,107. [14] R.E. Markle, Indust. Finish. (May) (1982) 47–49. [15] O. Cakir, Proceedings of the First Mechanical Engineering Congress, Istanbul, Turkey, June 4–6, 1997, pp. 464–468 (in Turkish). [16] Kenneth Mirski, Proc. Tech. Prog. Nat. Electron. Pack./Prod. Conf., USA (1981) 196–207.
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[17] E.B. Khobotova, S.D. Gorobels, V.I. Larin, Zashchita Metallov 23 (1987) 980–982. [18] M.D. La Grega, P.L. Buckingham, J.C. Evans, The Environmental Resources Management Group, Hazardous Waste Management, McGraw-Hill Inc., Singapore, 1994. [19] T. Higgins, Pollution Prevention Handbook, CRC Press Inc., USA, 1995. [20] R.D. Ross, Industrial Pollution Control Handbook, McGraw-Hill Inc., USA, 1971 (Chapter 7).
[21] J.M. Avery, P.G. Moleux, Printed Circuits Handbook, McGraw-Hill Inc., USA, 1996 (Chapter 34). [22] D.G. Barrett, PCMI J. (46) (1991) 15–17. [23] D.M. Allen, O. Cakir, PCMI J. (52) (1993) 4–7. [24] O. C¸akir, Proceedings of the International Symposium on Municipal and Industrial Wastes and their Treatment in 2000 (Treatment 2000), Istanbul, Turkey, May 17–20, 2000, pp. 396– 401. [25] O. Cakir, Ph.D. Thesis, Cranfield University, UK, 1996.
Copper etching with cupric chloride and regeneration of waste etchant O. Cakir Department of Mechanical Engineering, Dicle University, 21280 Diyarbakir, Turkey
Abstract Copper etching is considerably important process in electronics industry, particularly in the fabrication of printed circuit board. Various etchants can be used for this purpose, but nowadays cupric chloride is more accepted etchant, because of its high etch rate and easy regeneration properties. In the present study, copper etching process with cupric chloride etchant was investigated. In the etching of copper, the most important etching parameter is etch rate, therefore the investigation was based on the various effects on etch rate. The influence of etchant concentration, additives and etching temperature were examined. It is also important to regeneration/recycle of waste etchant from environmental point of view. Thus, various cupric chloride etchant regeneration processes were investigated. © 2005 Elsevier B.V. All rights reserved. Keywords: Copper etching; Cupric chloride; Etch rate; Waste regeneration
1. Introduction Corrosion damages metals under normal circumstances. It can be an efficient machining process if it is controlled. This type of machining process is generally named etching, which is one of the important nontraditional machining processes to manufacture geometrically complex machine parts from thin and flat streets. The process uses a liquid chemical solution called an etchant. The etchant attacks to remove selected areas of metal part. The process is extensively used to manufacture components such as printed circuit boards, shadow masks for cathode-ray tubes, integrated circuit lead layouts, fluorescent indicators, jewelleries and nameplates, etc. for electronics, aerospace, precision engineering and decorative industries. The etching process was first practiced as an art form in the Ancient Egypt in around 2500 b.c. It was applied to produce jewelleries made of copper etched with citric acid. Nowadays, the process is an essential part of manufacturing micron-size machine parts. The etching process is the major machining step in the chemical and photochemical machining process [1–4]. Copper is an important metal for various engineering applications. Its wide usage is due to its excellent electrical and thermal conductivity, easy fabrication, and good strength and fatigue properties. E-mail address: [email protected]. 0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.04.024
Copper etching is the main process in the production of printed circuit boards. Historically, the first used etchant in the printed circuit boards (PCBs) fabrication was ferric chloride (FeCl3 ) and it is still used as an etchant for small-scale production lines. However, in last two decades, various new etchants have been introduced for copper etching. Some of them found more applications then the others; one of them was cupric chloride (CuCl2 ). Nowadays, CuCl2 have been a major etchant in copper etching process, due to its advantages [5]. In this study, copper etching with using CuCl2 was investigated. The effects of etchant concentration, etching temperature, chemical additions to main etchant and oxidation redox potential on the etch rate, undercut and etch factor of copper were examined. It is also important that the regeneration/recycle and etched metal recovery of waste etchant should be available because of environmental restrictions; therefore, the regeneration processes for waste CuCl2 have been determined.
2. Etchants for copper etching The ideal etchant solution for copper should have some properties as follows [6–9]: (a) high etch rate;
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O. Cakir / Journal of Materials Processing Technology 175 (2006) 63–68
Table 1 Etchants and their properties in the etching of copper Etchant
Etching temperature (◦ C)
Etch rate (m/min)
Undercut
Dissolved copper capacity
Regeneration and metal recovery
Cupric chloride (CuCl2 ) Ferric chloride (FeCl3 ) Alkaline etchants Hydrogen peroxide/sulphuric acid (H2 O2 + H2 SO4 ) Ammonium persulhate [(NH4 )2 S2 O8 ] Chromic-sulphuric acid [CrO3 + H2 SO4 ]
50–54 43–49 43–55 43–55 38–55 26–33
25–50 25–50 30–60 Variable 7 Variable
Low Low Lower Low High Lower
120–140 40–60 140–170 50–90 40–55 50–60
Copper recovery and regeneration Regeneration Copper recovery and regeneration Copper recovery Copper recovery Not available
(b) (c) (d) (e) (f) (g) (h)
minimum undercut; high dissolved copper capacity; stable and easy control of copper etching process; not generate toxic fumes; environmentally acceptable; economic regeneration of waste etchant; economic etched copper recovery from waste etchant.
There are various etchant solutions for copper available commercially. The selection of ideal etchant solution for copper depends on various properties of etchant mentioned above, but environmental and health restrictions have become an important parameter since 1970s. High hazardous properties of any etchant cannot be acceptable anymore because the strict environmental restricting of local and country levels have became increasingly demanding, for forcing copper etching industries into expensive waste minimisation processes. Thus, the manufacturing process has increased. The usable etchants for copper etching are given in Table 1. The properties of each etchant are also mentioned. However, the last three etchants are no longer applied for copper, because they are very corrosive causing environmental pollution and health problems. As a result, basically three etchants have been widely accepted for copper etching. These are CuCl2 , FeCl3 and alkaline etchants [9,10]. CuCl2 is a suitable etchant for any copper etching process. It is also widely used for “print and etch” type PCB at a larger scale. These PCBs are single-sided boards and extensively requires economical etchant. CuCl2 provides high etch rate when compared to FeCl3 . Copper etching with FeCl3 produces CuCl2 during process and it was mentioned that the etching process was completed with the by-product of CuCl2 [11]. It also produces lower undercut than FeCl3 . The copper dissolving capacity of CuCl2 is three times higher than FeCl3 . The full regeneration type of waste CuCl2 makes the etchant more attractive to copper etching industry. It is also used as an etchant for various copper alloys like brass,
bronze, beryllium–copper etching. The other etchant is alkaline etchants which are based on ammoniacal solutions such as ammonium copper chloride or ammonium copper sulphate. The main different factor between CuCl2 and alkaline etchants is that the user of copper etching process depends on the alkaline etchant supplier. The application of CuCl2 would provide more flexible advantages compared to alkaline etchants. Moreover, alkaline etchants are used for copper only. They cannot be used for copper alloys because of alloying elements, which make regeneration and recycling of waste alkaline etchants complex. They are also expensive therefore they are widely used for bulk copper etching process. Table 2 indicates some properties of CuCl2 , FeCl3 and alkaline etchants [9,10].
3. Copper etching with cupric chloride CuCl2 is rather messy, yellow-brown colour, hygroscopic solid. It is sold as the green crystalline dihydrate salt (CuCl2 + 2H2 O). It is commonly used acidic compound and comprises the copper salt, water and hydrochloric acid (HCl) [2]. The etching of copper with CuCl2 can be simply expressed by the following chemical reaction: CuCl2 + Cu → 2CuCl
(1)
The uncoated copper surface gets attacked by CuCl2 . The one copper atom reacts with one cupric ion and forms two atoms of the cuprous ions as follows: Cu2+ + Cu0 → 2Cu+ cupric
metal
(2)
cuprous
This simple chemical reaction explains main copper etching process with CuCl2 , but the additions of various chemical solutions to main etchant would make etchant complex and thus etching mechanism.
Table 2 Some etchants characteristics in PCB fabrication [9] Etchant
Corrosiveness
Neutralisation and disposal problems
Toxicity
Operational cost∗
Cupric chloride (CuCl2 ) Ferric chloride (FeCl3 ) Alkaline etchants
High High High
Low Medium Medium
Medium Low Medium
Low Medium High
*
Include disposal cost.
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The copper etching with CuCl2 etchant solution is strongly controlled during process. The most commonly used control technology applies a colourimetric type etch chemistry controller. It is cheap and reasonably reliable. However, it is assumed this control type that there should be no free acid [12,13]. 3.1. Effect of etchant concentration The concentration or molarity of CuCl2 etchant solution is an important parameter. It is commonly represented by Baum´e (B´e) degree, which is depends on specific gravity of etchant. The calculation of Baum´e degree can be done as follows [2]: B´e = 145
specific gravity − 1 specific gravity
(3)
The higher Baum´e means high molarity of etchant. It is reported that the optimum Baum´e degree or molarity of CuCl2 should be around 32–33 ◦ B´e or 2.33–2.5 mol. It was noticed that this value of Baum´e produced a stable and high etch rate for copper etching, lower of this value would give low etch rate with poor line resolution and higher etchant would produce slow etch rate with a smooth line resolution. It is also mentioned that the undercut would decreases with increasing Baum´e degrees [14]. 3.2. Effect of etching temperature In general, the etching temperature for any etchant provides advantage in any metal etching process. In case of copper etching with CuCl2 it observed, using higher etching temperature increases the etch rate. Therefore, it is aimed to use the possible highest etching temperature during process. However, the selection of etching temperature depends on etching machine. The etchant chemicals are extremely corrosive to all common metals; therefore, the etching machine chamber is usually made of plastic. The only metal used in the machine is titanium [13]. The possible maximum etching temperature in commercial etching machines allowed up to 50–55 ◦ C. It was obtained that the etch rate of copper at 32 ◦ B´e (2.33 mol) CuCl2 and 20 and 50 ◦ C etching temperatures were 6.6 and 10 m/min, respectively [15]. The high etch rate and low undercut would produce a desired high etch factor. The calculation of etch factor is given as follows (Fig. 1) [2]: etch factor =
D D = (A − B)/2 U
(4)
where D is depth of etch (factor of etch rate), U undercut, A slotwidth after etching and B is slotwidth before etching.
Fig. 1. Etch factor.
3.3. Effect of oxidation redox potential In ideal copper etching process, the oxidation redox potential (ORP) of CuCl2 is continuously monitored. The ORP indicates the relationship of cupric ions to cuprous ions. It is measured as the relative conductivity of etchant and expressed in millivolt. As copper is etched, the etchant changes from a cupric to cuprous state. Decrease of the ORP causes low etch rate of copper. The addition of HCl and an oxidizer to the etchant introduces free chlorine. This makes cuprous ions to cupric form. The optimum ORP for CuCl2 is recommended between 510 and 530 mV [14]. 3.4. Effects of chemical additives The additions of various chemical additives to CuCl2 etchant produce better copper etching properties. Basically, when CuCl2 solution is prepared, the small amount of HCl is added to the main etchant [15]. As the addition of HCl to CuCl2 is completed, the chemical reaction of copper etching becomes as follows: CuCl2 + 2HCl + Cu → 2CuCl + 2HCl
(5)
Copper etching with CuCl2 would produce CuCl and this stops further etching. The addition of HCl reacts with CuCl and makes it CuCl2 , so that the etching continuous. Although the addition of HCl to CuCl2 makes the etchant complex, the combination of CuCl2 + HCl produces high etch rate in copper etching process. This positive effect is shown in Table 3. It is also noticed that the copper dissolve capacity of CuCl2 increases by HCl addition [16]. It is also known that HCl addition increases undercut which is not desired. Therefore, the etch factor will be lower. Probably, the most important chemical addition to CuCl2 is HCl, but the level of this addition is depending on the Table 3 Effect of HCl addition to CuCl2 [on etch rate of copper 13] CuCl2 (◦ B´e)
Etching temperature (◦ C)
HCl addition (N)
Etch rate (m/min)
33 (2.5)a 33 (2.5)a 33 (2.5)a
49 49 49
1 (1)a 2 (2)a 3 (3)a
25 31 38
a
Values are in mol.
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etching machine again. The high HCl addition would react with etching machine parts, particularly attacks titanium, and cause some undesired corrosion. The addition of ammonium chloride (NH4 Cl), to CuCl2 provides high etch rate and copper dissolve capacity as the same of HCl addition. It is reported that CuCl2 with 15% NH4 Cl would produce four times higher etch rate than CuCl2 only [6]. It is mentioned that the other chemical additives such as monomethanol amine, ethanol, acetonitrile, aceton, dimethyl formamide to CuCl2 etchant increase etch rate of copper [17]. The effect of chemical additives to CuCl2 seems advantageous, the selection of ideal additive would be based on chemical reaction of copper etching which should make simpler and controllable. This is also important factor from the regeneration of waste CuCl2 etchant point of view. The complex etchant solution would make the regeneration process difficult and expensive. Therefore, it is a general approach that the etchant should be kept as simple as possible. As a result, the addition of HCl to CuCl2 would make more sense.
4. Regeneration of waste cupric chloride etchant The waste etchant is very corrosive and considered “hazardous waste”, because of its acidic (pH of waste CuCl2 etchant is less than 2.5) preposition in accordance with one of the standard descriptions. The waste CuCl2 etchant contains copper ions and their limitations on the environment are very strict, because they are heavy metals [18–20]. The environmental issues have become very important phenomena in the industrialised countries since the beginning of 1970s. Strict environmental legislation has been introduced and etching industry has been extensively affected. The users of etching process relied on end-of-pipe treatment and/or disposed for hazardous waste etchant generated before [20,21]. This option has been not an optimal strategy for managing waste CuCl2 etchant anymore. Because: (a) The potential liabilities involved with the handling and disposal of waste CuCl2 etchant have arisen and will continue to increase. (b) Waste etchant disposal cost has gone up significantly due to restrictions placed on land disposal. As a result, the etching industry has faced with a challenge of finding alternative methods for managing by-product of hazardous waste CuCl2 etchant. Thus, the waste etchant minimisation or even elimination was considered seriously. The regeneration processes for waste CuCl2 etchant have been developed to solve waste problems of etching industry. The use of regeneration process would also brought other benefits like saving from equipment and operation costs and significantly reducing liabilities associated with the disposal of hazardous waste etchant [20]. The regeneration of waste etchant is a waste treatment process used by etching industry. The waste etchant is ox-
Table 4 Regeneration processes for waste CuCl2 etchant used for copper etching Chlorine regeneration 2CuCl + Cl2 → 2CuCl2 Sodium chlorate regeneration 2CuCl + 23 NaClO3 + 2HCl → 2CuCl2 + 23 NaCl + 2H2 O Hydrogen peroxide regeneration 2CuCl + H2 O2 + 2HCl → 2CuCl2 + 2H2 O Oxygen regeneration 2CuCl + 21 O2 + 2HCl → 2CuCl2 + 2H2 O Electrolysis 2CuCl + 2HCl → 2CuCl2 Anode: Cu+ − e− → Cu2+ oxidation Cathode: H3 O+ ± e− → 21 H2 ↑ +H2 O Electrodialysis Anode: Cu+ + e− → Cu2+ oxidation Cathode: Cu+ − e− → Cu0 reduction
idised back to reusable or starting form by regeneration process. The benefits of any regeneration process can be expressed below [14]: (a) (b) (c) (d) (e)
lower etchant cost; continuous and stable etch rate; higher productivity; less equipment downtime; lower labour costs.
The full regeneration property of waste CuCl2 etchant is one of the most important advantages. There are various regeneration processes are commercially available for waste CuCl2 etchant. Chemical regeneration processes such as chlorine, sodium chlorate, hydrogen peroxide and oxygen mainly use various chemical additives into waste etchant. They are widely used by etching companies due to the pure price of CuCl2 . The chemical regeneration of waste CuCl2 produces twice the volume of CuCl2 than the starting volume of the etchant. This surplus CuCl2 can be sold to the other copper etching companies for further utilising. It is also sold to plastic producers for blue colouring of plastics. The chemical regeneration processes for waste CuCl2 etchant are shown in Table 4 [22–24]. It is noticed that the chlorine gas regeneration seems an ideal regeneration process for waste CuCl2 etchant. It is also a cheaper regeneration process than hydrogen peroxide and sodium chlorate regeneration methods [14]. Chemical regeneration processes mentioned above produce as amount of etchant in excess amount of starting quantity in copper etching. This causes further problems such as disposal of excess regenerated etchant. These regeneration processes do not also deal with the removal of etched copper from waste etchant. To solve waste regeneration and etched copper recovery together can be accomplished by using electrolysis and electrodialysis processes.
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Table 5 Comparison of regeneration processes for waste CuCl2 etchant
5. Conclusion
Regeneration process
Comments
Chlorine regeneration
The cheapest regeneration process Simple regeneration process Water addition necessary Consideration on safety
Sodium chlorate regeneration
More expensive than chlorine regeneration Eliminating chlorine addition Improvement on the etch rate Solid etchant addition necessary
Hydrogen peroxide regeneration
The most expensive regeneration process Safer than chlorine regeneration Solid etchant necessary
Oxygen regeneration
Very attractive regeneration process Much safer than chlorine regeneration Water addition necessary
Electrolysis
Expensive equipment and materials Flammable H2 release Water addition necessary
Electrodialysis
Expensive equipment and materials Etched copper recovery simultaneous Investment amortization by selling recovered copper
Copper etching is an important nontraditional machining process to produce complex machine parts from thin sheet for various industries. This process is commonly used in the production of PCBs. Copper is etched by different etchants, but CuCl2 seems to have more advantages than the other etchants. Copper etching with CuCl2 produces a stable and high etch rate and low undercut. This indicates the desired high etch factor. The selection of etchant parameters in copper etching with CuCl2 should be completed to obtain optimum etching result. The etching temperature should be selected as the highest possible temperature the etching machine allowed. This would be around 50–55 ◦ C. The molarity or Baum´e degree of CuCl2 etchant should be around 2.33–2.5 mol or 32–33 ◦ B´e. The ORP of CuCl2 should be strictly controlled and kept around 510–530 mV. The maintaining of the ORP can be completed by adding HCl or chlorine. These etching parameters would provide a desired copper etching result. The regeneration of waste CuCl2 etchant is also important factor to be considered for copper etching. Environmental regulations are forcing etching companies to use one of suitable regeneration processes. The advantage of full regeneration of waste CuCl2 etchant minimizes or eliminates environmental pollution occurred during and after etching process. This would also reduce operational costs. Copper etching with CuCl2 etchant and a suitable regeneration process of waste etchant should be carried out simultaneously. This would make the overall manufacturing cost low and environmentally friendly etching process.
The electrolysis process is an electrochemical separation technique where the waste etchant is regenerated, but no etched metal recovery occurs. The electrolytic cell is divided into anode and cathode compartments by means of an anion–permeable ion exchange membrane. The membrane allows only negatively charged ions to pass between catholyte and anolyte, and therefore, stops newly produced cupric ions at the anode from being reduced back to cuprous at the cathode. Chlorine ions come from the addition of HCl to the regeneration system and join with cuprous ions. As a result, CuCl2 is obtained. The electrolysis process uses a high level current. It produces hydrogen gas, which is released from the system. This is environmentally undesirable. It also increases the etchant concentration, which is controlled by water addition. This causes a disposal problem of excess etchant [25]. The electrodialysis process is a new promising regeneration method and is variation of electrolysis method. The cation selective membrane divides the electrodialysis cell into two halves, the anoylte and the catholyte. The waste CuCl2 etchant is reoxidised on the anode and etched copper is recovered on the cathode by passing a dc electric through the waste etchant. The waste etchant is regenerated and copper recovery is obtained simultaneously [25]. All possible regeneration processes for waste CuCl2 etchant have advantages and disadvantages. However, a few of them can be considered as environmentally acceptable. The comparison of regeneration processes is given in Table 5.
References [1] R. Ueda, Corros. Eng. 38 (1989) 271–282. [2] D.M. Allen, The Principle and Practices of Photochemical Machining and Photoetching, Adam Hilger Publication, 1986. [3] W.T. Harris, Chemical Milling, Oxford University Press, 1976. [4] J.W. Dini, American Machinist (Special Report), 128 (1984) 113–128. [5] C.F. Coombs Jr. (Ed.), Printed Circuit Boards, McGraw-Hill, 1995. [6] J.O.E. Clark, Marconi Rev. 26 (1961) 135–152. [7] S. Gowri, K.S. Indira, B.A. Shenoi, Met. Finish. 64 (1966) 54– 59. [8] E.C. Jubb, Plating 139 (1964) 2202–2211. [9] M.J. Thomas, Etching, MacDermid (GB) Ltd., 28 pp. (Technical Note). [10] W.C. Bosshart, Printed Circuit Boards—Design and Technology, Tata McGraw-Hill Publications, 1985. [11] E.B. Saubestre, Indust. Eng. Chem. 51 (1959) 288–290. [12] R. Sedlak, http://www.pcbfab.com/ineretch.html (1999). [13] A. Latham, Electronic Production, 11 (1988) 101,103,107. [14] R.E. Markle, Indust. Finish. (May) (1982) 47–49. [15] O. Cakir, Proceedings of the First Mechanical Engineering Congress, Istanbul, Turkey, June 4–6, 1997, pp. 464–468 (in Turkish). [16] Kenneth Mirski, Proc. Tech. Prog. Nat. Electron. Pack./Prod. Conf., USA (1981) 196–207.
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[17] E.B. Khobotova, S.D. Gorobels, V.I. Larin, Zashchita Metallov 23 (1987) 980–982. [18] M.D. La Grega, P.L. Buckingham, J.C. Evans, The Environmental Resources Management Group, Hazardous Waste Management, McGraw-Hill Inc., Singapore, 1994. [19] T. Higgins, Pollution Prevention Handbook, CRC Press Inc., USA, 1995. [20] R.D. Ross, Industrial Pollution Control Handbook, McGraw-Hill Inc., USA, 1971 (Chapter 7).
[21] J.M. Avery, P.G. Moleux, Printed Circuits Handbook, McGraw-Hill Inc., USA, 1996 (Chapter 34). [22] D.G. Barrett, PCMI J. (46) (1991) 15–17. [23] D.M. Allen, O. Cakir, PCMI J. (52) (1993) 4–7. [24] O. C¸akir, Proceedings of the International Symposium on Municipal and Industrial Wastes and their Treatment in 2000 (Treatment 2000), Istanbul, Turkey, May 17–20, 2000, pp. 396– 401. [25] O. Cakir, Ph.D. Thesis, Cranfield University, UK, 1996.