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1、The decomposition is catalyzed by many metals,including Cu,Ni,and Fe.If traces of metal or metal salts are allowed to contaminate piping runs, the piping may also rupture explosively. Facilities should be constructed with pressure relief capabilities. Electrolytic Regeneration.The electrochemical re
2、versal of the etching of copper shown in Eq.34.9 and 34.10 is claimed to be effective and economical.Descriptions of this system are given in the literature.15On a large scale,electrolytic regeneration requires a high investment in equipment and materials, as well as high power consumption.The chemi
3、cal formulation is relatively high in acid content and low in copper,a condition not optimized for etching effec- tiveness or plating efficiency. 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 plati
4、ng 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.Copper recovery may not return copper value and may be expensive,inconvenient,and cause difficulty with recycling. 34.4.2.4Problems wit
5、h Cupric Chloride Systems 1. Slow etch rate.Depending on chemical formulation,cupric chloride is inherently slower than ammonia and must be properly evaluated before production expectations are established. Check to ensure no resist residues or chromate coatings remain on the panel surface before et
6、ching.A slower than usual rate is frequently due to low temperatures,insufficient sump mix- ing,or poor solution chemical control.If temperature and agitation are under control,slow etch- ing with a dark green solution may result from a low cupric ion content, indicating insufficient oxidizer.Acid m
7、ust also be added to clarify cloudy solutions. (Before acid addition, make sure excess oxidizer is not present.) In regenerative systems, the source of oxidation may be de- pleted.It is advisable to analyze specific gravity,free acid,and total chloride on a regular sched- ule and maintain control ch
8、arts to ensure that bath controllers are consistent and operating properly. 2. Sludging.This occurs if acid is low or if water dilution occurs. 3. Etcher goop.This is a common floating accumulation of photoresist compounds leached into and precipitated from etch baths.Accumulations appear to increas
9、e with higher acid formulations. Formation can be limited by proper exposure of photoresists and continu- ous recirculation of the etch bath through carbon filtration.Periodic,complete drain-down of the etching machinery and thorough mechanical and chemical equipment cleaning with a sul- famic acidt
10、ype commercial cleaner can minimize problems. 4. Yellow or white residues on copper surface.Yellow residue is usually cuprous hydrox- ide. It is water insoluble and is left when boards are etched and alkali-cleaned.A white pre- cipitate will probably be cuprous chloride,which can remain after etchin
11、g in solutions that are low in chloride ion and acid.To eliminate both conditions, the solution in which the board is rinsed just before final water-spray rinsing should be 5 percent by volume hydrochloric acid. 5. Waste disposal.Spent or by-product etchant is usually sent off-site for copper recla-
12、 mation.There is usually a fee for this service depending on copper content and distance to the reclamation facility.The solutions must be free of unreacted oxidizer (see previous chlorate system discussion). Etchant can contain traces of zinc, chromium, and arsenic from the foil treatments. 34.4.3S
13、ulfuric AcidHydrogen Sulfuric-peroxide systems are used extensively for copper surface preparation by microetch- ing the surface to provide texture and active surfaces.These formulations have been used in 34.14PRINTED CIRCUITS HANDBOOK Downloaded from Digital Engineering Library McGraw-Hill () Copyr
14、ight 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. ETCHING PROCESS AND TECHNOLOGIES the past for primary circuitization etching but are no longer favored because of slow etch rate and needs for accompanying control of exotherm an
15、d copper crystallization. However, the increasing need for fine-line etching has renewed use possibilities for this and other etchants (see Sec. 34.7.4). In essence, these call for etching foils thinner than 12oz and the control pre- cision offered by slower etchants.They are compatible with metal r
16、esists and many organics. 34.4.3.1Chemistry.Typical constituents of both immersion and spray etchants and their functions are as follows: 1. Hydrogen peroxide is an oxidizing agent that reacts with and dissolves metallic copper. 2. Sulfuric acid makes copper soluble and holds copper sulfate in solut
17、ion. 3. Copper sulfate helps to stabilize etch and recovery rates. 4. Molybdenum ion is an oxidizing agent and rate exaltant.22 5. Aryl sulfonic acids are peroxide stabilizers.23 6. Thiosulfates are rate exaltants and chloride ion controllers that permit lower peroxide content.24 7. Phosphoric acid
18、retains clean solder traces and plated through-holes.25,26 The etching reaction is as follows: Cu + H2O2+ H2SO4 CuSO4+ 2H2O(34.11) 34.4.3.2Properties and Control.The earlier technical problems of slow etch rates, perox- ide decomposition, and foaming in spray systems have been solved, but critical c
19、oncerns still remain.Among these are process overheating, etchant composition balance with by-product recovery,etchant contamination,and the dangers of handling concentrated peroxide solutions. The specific problem of decomposition of peroxide during idle hours has caused disastrous equipment meltdo
20、wns, and thus requires thermal management while equipment is otherwise not in operation. 34.4.3.3Closed-Loop Systems.Production facilities require continuous recirculation of the etchant through the etching tank or machine and the copper sulfate recovery operation. Etchant replenishment is controlle
21、d by chemical analysis and by additions of concentrates. Copper sulfate recovery is based on lowering the solubility of CuSO4? 5H2O by decreas- ing the etchant temperature to 50 to 70F.Crystallizer equipment and conveyorized discharge has been available for this process. 34.4.3.4Problems Encountered
22、 with Peroxide Systems 1. Reduced etch rates.This problem can be caused by operating conditions, solution imbal- ance,or chloride contamination. 2. Under- and overetching.A review of etching conditions, solution control, and the resist stripping process may show deviations from normal. In the case o
23、f immersion etching, the solution and panel agitation may need to be increased.When spray-etching,a check of noz- zles and line clogging is indicated. 3. Temperature changes.Recirculating water rates and thermostats need to be examined regularly.Overheating may be due to high copper content,contamin
24、ation,or rapid perox- ide decomposition. 4. Copper sulfate recovery stoppage.Examine solution balance, heat exchanger, and other recovery equipment.Exit conveyor jamming difficulties from thermal excursions and vary- ing crystal size and liquid content cause major shop floor problems. ETCHING PROCES
25、S AND TECHNOLOGIES34.15 Downloaded from Digital Engineering Library McGraw-Hill () Copyright 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. ETCHING PROCESS AND TECHNOLOGIES 34.4.4Persulfates Ammonium, sodium, and potassium persulf
26、ates modified by certain catalysts have been adopted for the etching of copper in PC manufacturing.Continuous regenerative systems and a batch system using ammonium persulfate are no longer used.Wide use is made of persulfates as a microetch for innerlayer oxide coating and copper electroless and pl
27、ating processes. Per- sulfate solutions allow all common types of resists on boards including solder, tin, tin/nickel, screened inks, and photosensitive films. Formulations of ammonium persulfate catalyzed with mercuric chloride are no longer used, primarily because of environmental issues, costs, a
28、nd improved alternatives.In general,persulfate etchants are unstable and will exhibit decomposi- tion,lower etch rates vs.copper content,and lower useful copper capacity. 34.4.4.1Chemistry.Ammonium,potassium,and persulfates are stable salts of persulfuric acid (H2S2O8).When these salts are dissolved
29、 in water,the persulfate ion (S2O82) is formed.It is the most powerful oxidant of the commonly used peroxy compounds. During copper etch- ing,persulfate oxidizes metallic copper to cupric ion as shown: Cu + Na2S2O8 CuSO4+ Na2SO4(34.12) Persulfate solutions hydrolyze to form peroxy monosulfate ion (H
30、SO41) and, subsequently, hydrogen peroxide and oxygen.This hydrolysis is acid catalyzed and accounts for the insta- bility of acidic persulfate etching solutions. Ammonium persulfate solution,normally made up at 20 percent,is acidic.Hydrolysis reac- tions and etchant use cause a reduction of the pH
31、from 4 to 2.The persulfate concentration is lowered,and hydrated cupric ammonium sulfate CuSO4? (NH4)2SO4? 6H2O is formed.This precipitate may interfere with etching. Solid persulfate compounds are stable and do not deteriorate if stored dry in closed con- tainers.Solution makeup composition include
32、s various catalysts,including organic matter and transition metals (Fe,Cr,Cu,Pb,Ag,etc.).Materials for storage must be chosen carefully.Per- sulfates should not be mixed with reducing agents or oxidizable organics. The useful capacity of the etchant is about 7 oz/gal copper at 100 to 130F.Above 5 oz
33、/gal of copper,it is necessary to keep the solutions at 130F to prevent salt crystallization.The etch rate of a solution containing 7 oz/gal of dissolved copper is 0.00027 in/min at 118F. 34.4.4.2Batch Operation.Sodium persulfate is preferred because it has minimal disposal problems and somewhat hig
34、her copper capacity and etch rates. A composition of 3 lb/gal sodium persulfate with 15 ppm of HgCl2,1 gal of proprietary additive,and 57 ml/gal of H3PO4 has been successfully used for batch-type spray etching.27Etch rates vary throughout bath life and range from 0.0018 to 0.0006 in/min for copper c
35、ontent of 0 to 7 oz/gal.Prepared solutions must be aged for 16 to 72 h before etching when proprietary additives are used. 34.4.4.3Problems with Persulfates 1. Low etch rates.Since the solution may decompose, it will be necessary to replace the bath.If solution is new,add more catalyst and check for
36、 iron contamination. 2. Salt crystallization.Salts crystallize on the board and cause streaks,damage the solder plate, and plug the spray nozzles or filters.When copper content is high,blue salts may precipitate. 3. White films on solder surface.This may occur normally or when the lead content in th
37、e solder plate is too high. 4. Black film on solder.This condition can result when the solder alloy is high in tin. If sol- der reflow or component soldering is to follow, activate by tin immersion or with solder brighteners.Adjust phosphoric acid content in etchant and solder-plating conditions. 5.
38、 Spontaneous decomposition of etch solution.This breakdown is due to contaminated,over- heated, or idle solutions. Ammonium persulfate etchants are unstable, especially at higher temperatures.At about 150F,the solution decomposes quickly.Use it soon after mixing. 34.16PRINTED CIRCUITS HANDBOOK Downl
39、oaded from Digital Engineering Library McGraw-Hill () Copyright 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. ETCHING PROCESS AND TECHNOLOGIES 6. Disposal. The exhausted etchant consists mainly of ammonium or sodium and copper su
40、l- fate with a pH of about 2.Two methods for disposal are: Electrolytic disposition of the copper on the surface of passivated 300 series stainless steel is one disposal method. The spent etchant is acidified with sulfuric acid prior to electrolysis. Once the copper has been removed, the remaining s
41、olution can be diluted, neutralized, checked, and discarded. The copper can be removed from the cathode. Spent sodium persulfate can be treated with caustic soda. Addition of aluminum or iron machine turnings to a slightly acidified solution is another practical but possibly more difficult means of
42、removing the dissolved copper.The reac- tion, especially in the presence of chloride ions, will be violent, and considerable heat will be given off if the solutions are not diluted. 34.4.5Ferric Chloride Ferric chloride solutions are used as etchants for copper,copper alloys,Ni/Fe alloys,and steel i
43、n PC applications,electronics,photoengraving arts,and metal finishing.Current use of ferric chloride etchant in printed wiring fabrication is extremely limited in the United States be- cause of costly disposal of the copper-containing etchant, and the much better commercial support for ammoniacal an
44、d cupric chloride etchants.There is still considerable use for alloy etching and photochemical machining applications. Ferric chloride can be used with screen ink,photoresist,and gold patterns,but it cannot be used with tin or tin/lead resists.However,ferric chloride is an attractive spray etchant b
45、ecause of its ease of use, holding capacity for copper, and ability to be used on an infrequent batch application basis. The composition of the etchant is mainly ferric chloride in water,with concentrations rang- ing from 28 to 42 percent by weight. Free acid is present because of the hydrolysis rea
46、ction and the need to maintain an acid environment. The natural acidity is usually supplemented by additional amounts of HCl (up to 5 percent) to prevent formation of insoluble precipitates of ferric hydroxide. Commercial formulations for copper alloy etching are usually 36 Be, or approximately 4.0
47、lb/gal FeCl3,and may contain antifoam and wetting agents.Customary acid operating HCl content is 1.5 to 2.0 percent. The effects of ferric chloride concentration, dissolved copper content, temperature, and agitation on the rate and quality of etching have been reported in the literature.28,29 34.4.6
48、Chromic-Sulfuric Acids These etchants for solder- and tin-plated boards were preferred for many years. More re- cently, their use has been completely eliminated due to Cr(VI) listing as a critical environ- mental hazard. Other problems with chromic-sulfuric etchants are difficulty in regeneration, i
49、nconsistent etch rate, the low limit of dissolved copper (4 to 6 oz/gal), and dangerous degra- dation of PVC and polypropylene equipment. Chromic acid 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 s