Source Reduction of Chlorinated Solvents in Electronics Industry

Transcription

1 Source Reduction of Chlorinated Solvents in Electronics Industry Azita Yazdani, P.E. President Pollution Prevention International 8230 Beverly Blvd., Suite 14 A Los Angeles CA I. Introduction Chlorinated solvents are a class of interrelated chemicals that have gone through major regulatory scrutiny in recent years. A number of chlorinated solvents are used in the electronics industry in printed circuit board (PCB) fabrication and assembly; semiconductor wafer fabrication and assembly; and critical cleaning applications. The growth in the electronics industry in the last few decades has been significant. This growth has resulted in a similar growth in the use of chlorinated solvents for cleaning purposes. The three major chlorinated solvents used in electronics industry are 1,1,2-trichloro- 1,2,2,-trifluoroethane (CFC-113), l,l,l-trichloroethane (TCA), and methylene chloride (METH). The chlorinated solvents are used in degreasing of PCB and semiconductors, stripping and developing photoresist, critical cleaning, and in-situ preparation of etchants. It should be noted that some small quantities (<l metric ton) of perchlorethylene (PERC) and trichloroethylene (TCE) are also used in the electronics application. In this paper, the use of chlorinated solvents in PCB fabrication and assembly is discussed. CFC-113 is a fully halogenated CFC and had been regulated by EPA to satisfy provisions of the Montreal Protocol to protect the ozone layer (Fed. Reg., Dec & Aug. 1988; Montreal Protocol, Sept. 1987). TCA has recently been added to the list of substances covered by the Montreal Protocol. The production of TCA is to be discontinued by the year 2005 and CFC-113 by the year In the U.S., the CFC Title passed by congress in August 3, 1990, has even more stringent phase down dates for the U.S.. Table 1.1 summarizes the Montreal Protocol and CFC Title phase-out schedules. METH the other chlorinated solvent used in the electronics industry has been under intense regulatory pressure. METH has shown positive animal carcinogenicity and Occupational Safety and Health Administration (OSHA) is planning to lower its Permissible Exposure Level (PEL) in the near future. In California, METH is regulated as a toxic air contaminant and users have to pay high emission fees for its use. It is expected that the Clean Air Act will regulate METH as a toxic air. Table 1.2 shows the chlorinated solvent usage in the PCB fabrication and assembly in

2 Table 1.1 Phase Out Schedule for Ozone Depleting Substances % of Base Year CFC-113 TCA Base Year Mont. Prot.a CFC Title Mont. Prot. a CFC Title I % 80% 80% 75% 80% 65% 50% 50% 50% 40% 15% 15% 15% 15% 15% 15% 0% 0% 100% 100% 100% 90% 100% 85% 70% 70% 70% 50% 70% 50% 70% 50% 70% 50% 30% 20% 30% 20% 30% 0% 0% 0% a is the Montreal Protocol. Note: 1986 is the Base Year for CFC-113 TCA. and 1989 is the Base Year for Source: Du Pont, 1990; HSIA,

3 metric ton Table 1.2 Chlorinated Solvent Use in Electronics Industry- PCB Fabrication & Assembly SOURCE: INDUSTRY ESTIMATES

4 II. ELECTRONICS PRODUCTION PROCESSES In what follows, the PCB fabrication and assembly processes and the processes that use chlorinated solvents are discussed in detail. PCB Fabrication There are two types of PCBs. Rigid boards are made of glass, epoxy, plastics, ceramics, and paper-based phenolic resin. Flexible boards account for about 90 percent of the market and can be single sided, double sided or multi-layered. Most boards are clad with copper and the desired circuit pattern is formed by a thin layer of conductive metal. There are three methods for producing PCBs. The first is the additive method which involves electroless plating on the bare boards to buildup circuit layers. Only small fraction --5 to 10 percent-- of the boards are manufactured by this method. The second method, the subtractive techniques, accounts for a significant majority, perhaps 80 percent of the board manufacturing. It involves etching the board, or removing metal from a metal clad board in the desired circuit patterns. The semiadditive method, not widely used today, is a combination technique. The bare board is metalized as in the additive method, but is then etched to produce the desired pattern as in the subtractive method. Figure 2.1 shows the steps in the process of the printed circuit board production. The left and right side illustrate the additive and subtractive processes, respectively. The boards are cut from sheet material and holes are drilled in them to give points of attachment for the components that will eventually be mounted to the boards. The boards are then sanded and cleaned. Mechanical cleaning is followed by chemical cleaning with acids and then with chlorinated solvents. The surface of the board is prepared and plated with electroless copper to obtain a continuous metallic surface in the subtractive process. In the additive and semi-additive techniques, soldered and tin electroplate are used as masks. Screening offset printing or photlitrography is used to transfer the circuit pattern from a master to the board. Solder plating is then used to further define the image. The boards are exposed to light and in the developing step, the unexposed resist is dissolved and removed by the developer when a negative photoresist is used. Dry film photoresist is applied to the copper substrate of the PCB and the desired circuitry is imprinted by shinning high intensity light through a photomask. If a negative photoresist is used, the areas of the film exposed to the light will polymerize whereas the unexposed areas will not. If positive photoresist is used the reverse is true. The developer dissolves and carries away the unpolymerized material. Three types of photoresist-- solvent, aqueous and semi-aqueous-- are used today. The solvent photoresist is developed with TCA and stripped with METH. Aqueous photoresist is developed with a mixture of alkaline 154 4

5 Figure 1 -PRINTED CIRCUIT BOARD PRODUCTION ADDITIVE PROCESS SUBTRACTIVE PROCESS 155

6 slats and other components and stripped without the use of chlorinated stripped with alkylamines, glycol ethers, and other water soluble components. When a positive resist is employed, the exposed resist is removed. Finally, the exposed copper is etched away leaving the transferred patterns of the circuitry. The electroplating step, the resist application step and etch steps are repeated until the boards have been completely fabricated. Following the etching step, the remaining photoresist is stripped. PCB Assembly In the assembly process, electrical components are placed on the boards, the boards are fluxed, and the components are attached and soldered to the boards. In wave, dip, and drag soldering, a molten solder acts as a heat source and supply of solder. In wave soldering, the board is fluxed and passed through a wave solder which adheres only to the metal leads. In hand soldering, a cold solder is deposited and later heated. The flux is applied to reduce the surface tension so the solder will flow evenly. Three major types of flux are employed. Rosin (or resin) flux which accounts for about 55 percent of the flux used is composed of abietic acid and can only be removed with chlorinated solvents. Organic acid flux -- representing 40 percent of the market is cleaned with aqueous cleaners. They are stronger than rosin fluxes and can be corrosive. The remaining 5 percent of the flux is synthetically activated flux which must be cleaned with chlorinated solvents. Activators are halide compounds and provide the same strong fluxing action of organic acid fluxes, but are not as corrosive as organic acid fluxes. Table 2.1 provides a summary of different fluxes available on the market and their characteristic. As can be noted only rosin(r) and mildly activated flux (RMA) are approved for military use. III. INDUSTRY TRENDS - PRESENT AND FUTURE In recent years, there has been a clear trend to move away from solventbased photoresist in PCB manufacturing. It is estimated that only 25 percent of the market still used solvent photoresist in 1985 (SRI, 1985). Another source estimates that 40 percent of the market is held by semi-aqueous photoresist (Foecke, 1988). Another major technological change in the industry has been the movement toward surface mount technology (SMT). In the traditional PCB, devices are connected to the board by drilling holes in the board, inserting and crimping the leads and then soldering the components to the board. Surface mount devices are small and have no connector leads. Because holes in the boards are not needed with these devices, components may he more densely packed on the board. The movement toward SMT means that 156 6

7 Table 2.1 CHARACTERISTICS OF FLUX a S= chlorinated solvent soluble; W-water soluble; WS=water soluble with saponifier b Cleaning may not be required. Source: Markenstein, 1983; Morrison and Wolf,

8 solvent used in the defluxing the assembled PCB must be able to penetrate the smaller crevices. Some believe that there has been a movement away from aqueous defluxing systems toward chlorinated solvents since the smaller contact angle prevent removal under the surface mount devices. IV. SOURCE REDUCTION OPTIONS A variety of source reduction options can be adopted to eliminate or reduce the use of chlorinated solvents in the electronics industry. These include chemical substitution, process and product modification, recovery and reuse of vapors, recovery and reuse of waste solvent, equipment improvements, operating practices and housekeeping measures. In this section, a short discussion of chemical substitution and process modification options has been provided for PCB fabrication and assembly. PCB FABRICATION As mentioned earlier, PCB fabrication involves the use of TCA for developing photoresist and the use of METH for stripping photoresist. Conversion to Aqueous and Semi-aqueous There seems to be strong economic reasons that favor the use of semiaqueous and aqueous phased photoresists. Solvent developing photoresist and semi-aqueous resists apparently give comparable resolution. It has been noted that the effluent from the stripping operations, would precipitate metal components that require pretreatment prior to sewer discharge. In the case of aqueous systems, neutralization would be required prior to sewer discharge. In the past military specifications have been one of the major barriers to wider adoption of these non-solvent systems. Military specification have since been changed and when equipment is replaced, it is replaced by semi-aqueous systems. Developing and stripping with semi-aqueous systems are believed to be less costly that solvent systems. PCB ASSEMBLY The two major source reduction options in this category are chemical substitution and process modification. Chemical Substitution CFC-113 and TCA are the primary solvents used for removing the flux from the PCB after components have been soldered on them. Flux residue is not the only contaminant that needs to be removed from PCB. In general, contaminants can be classified into three groups; nonpolar such as oils, greases, rosin and waxes; polar such as rosin flux activators, sodium chloride, soldering, plating, and etching salts; and particulates such as dust, machining, drilling and punching fragments. As mentioned 158 8

9 earlier, there are a variety of different fluxes available on the market. These were shown in table 2.1. Rosin fluxes are mixtures of isomeric acids including abietic acid. Generally, they are the poorest conductors and the least corrosive of the flux types. Activators, which have traditionally been halide compounds, are used to increase the wetting ability of the flux. Only rosin (R) and mildly activated flux (RMA) are approved for military use. Nonactivated rosin fluxes (R), however, are not suitable for PC boards, but only for component manufacture and solderability testing. RMA flux is used by the communications industry. Super activated rosin fluxes (RSA) are used for challenging solder problems. Organic acid (OA) fluxes are water soluble; they are stronger than rosin flux and can be corrosive. They allow faster rates and result in fewer reject boards. Synthetically activated (SA) fluxes provide the same strong fluxing action of organic acid fluxes, but are removed with solvents instead of water. They are not as corrosive as organic acid fluxes. Although resin flux is a derivative of rosin flux, it has a significantly different structure and is water soluble. Low solids fluxes which require little or no cleaning, have less solids--2 to 5 percent rather than 15 to 35 percent. Another significant factor that technically influence the choice of solvent is compatibility with the substrate. Although CFC-113 is not a particularly good solvent for removing abietic acid, it is compatible with virtually all materials of construction for PCB. When it is combined with alcohol in the azeotrope, it is one of the best compromise solvents. Only certain solvents are presently allowed for use as defluxers under the military specification that governs cleaning (DOD 2000). These include various alcohols, TCA, CFC-113 and combinations of alcohols and the solvents. Aqueous cleaners can be used only with the permission of the contract officer. Water soluble (organic acid) flux is not permitted under the specification. In the following section, the three classes of potential chemical substitutes in PCB assembly are explained. These are flammable solvents, combustible solvents, and hydrochlorofluorocarbons. Flammable solvents - Low molecular weight solvents such as aliphatic, aromatic and oxygenated solvents are possible chemical substitutes for chlorinated solvents. Most of these solvents show good solvency characteristics, however, they have several limitations such as low vapor pressure, flammability and are considered precursors to photochemical smog. They may be used to clean nonpolar contaminants such as grease, oils, waxes, resins and polymers. These chemicals are presently used as simple substitute for halogenated solvents in cold cleaning applications. No major equipment modification is required, the solvents show high solvency for common contaminants. Organic solvents have several limitations, They cannot be used in enclosed systems because of solvent vapor build up. These solvent 9 159

10 cannot be used in vapor degreasing applications because of their flammability. Another disadvantage of these solvents is that they have short atmospheric lifetimes and they form precursors that contribute to photochemical smog. It is worth noting that most of these chemicals despite their widespread use for many years in commerce have not been adequately tested. A good example is isopropyl alcohol. EPA has only recently required manufacturers to test the chemical, despite the many years of use. Combustible solvents- High molecular weight organic solvents such as terpenes, N-methyl pyrrolidene (NMP), dibasic esters (DBE), and alkyl acetates are very new to the solvent cleaning market and are not widely used. These chemicals are considered biodegradable substances by their manufacturers and are rated combustible due to their flash point by the National Fire protection Association. The flash point of terpenes is 47 o C (117 O F). These substances, because of their combustible nature should be used at room temperature. Furthermore, a terpene supplier contends that the parts need to be washed with water afterwards to remove the terpene residue, which are to heavy to volatilize. This in itself may pose some problems since the stream may have to be handled properly (i.e. pretreated) prior to sewer discharge. To overcome the flammability problem, nitrogen inerting systems are proposed which will add to the expense of the unit. It should be noted that at this time not much is known about the toxicity of these chemicals. A major ingredient of terpene -d-limonene- has shown positive animal carcinogenicity in male rats (NTP, 1989). EPA has recently issued a test rule on NMP. DBE and alkyl acetates have not been tested for chronic toxicity. Additionally it is not clear if heavy organic hydrocarbons can be recycled for reuse. At this time the spent baths are incinerated. Hvdrochlorofluorocarbons (HCFC)- The new HCFCs that are under investigation as a replacement for CFC-113 and other chlorinated solvents are HCFC-113, HCFC 141b and HCFC-225 which has shown good solvent power, stability, low surface tension, and low ozone depletion potential, because it contains hydrogen. CFC-123 has a low boiling point (27.7 o C) which is advantageous for some applications such as cold cleaning but disadvantageous for others. A consortium of CFC producers has organized a toxicity testing program to test these chemicals for chronic toxicity effects. The results of HCFC-123 and HCFC-14lb should be available in the 1992/1993 time frame. The testing of HCFC-225 has just been initiated and the results will not be available until later. New HCFCs would not be commercially available for the next five years. Process Modification There are three process modifications that could be effective in PCB assembly. Aqueous cleaning, and no clean flux or inert gas soldering are explained below. Aqueous Cleaning- Water can be used to deflux PCB in two distinct ways First-, water can be used employed with organic acid fluxes which are

11 water soluble. Second, water can be used to remove rosin fluxes if saponifiers and detergent additives can only be used with permission of the Government Contracting Officer in military applications. organic acid fluxes are currently not allowed under military specifications. One advantage of water/organic acid flux combination is that a highly acid wash may be required to avoid tin and lead hydroxide deposition on the boards. Any rinse water left on the boards can cause corrosion or degradation of the laminate surface. A second disadvantage can occur where acid rinse is not used. Tin and lead ions may be precipitated as hydroxide and these oxides residues left on the boards can cause insulation failure at high humidities. A third disadvantage is that water soluble fluxes, unlike rosin fluxes, do not encapsulate impurities. this can result in electromigration on the boards that the impurities remain (Morrison, Wolf, 1985). Water cleaning systems with saponifiers used to clean rosin fluxes foaming in the rinse tanks. Antifoaming agents are recommended to be employed for these tanks to prevent foaming. in some cases the flux may not be completely removed by saponifiers that can cause a problem. Effluent from water cleaning processes has to meet federal and local standards for lead, ph, biological oxygen demand (BOD), and chemical oxygen demand (COD). Water systems are believed not to adequately clean surface mount boards due to spacings of about or less that 10 mils. This problem can be partially overcome by the use of high pressure spray or ultrasonic systems. No Clean Flux- Flux is generally composed of 15 to 35 percent solids, Low solid fluxes with only 2 to 5 percent solids are being developed and adopted by various users. An advantage of these fluxes is that the solder does not bridge over the densely packed surface mounted components and the small amounts of residue left on the board may reduce or eliminate cleaning requirements. A disadvantage of these fluxes is that they contain ionizing acids and so were not allowed under military specifications. Today however, no clean fluxes are available that contain no halide activators (Deram, 1998). There is no products that leaves "no residue", but the residue left by these fluxes is minimal. Indeed, the manufacturers claim that reliability of clean boards is equal to that of the rosin flux solvent process. The major advantage of no-clean flux is that it makes the cleaning step obsolete. This can lead to significant cost savings for the users. Another advantage of the no-clean flux is that the movement toward surface mount technology is not a concern. A disadvantage of the lowsolids flux is that it is not allowed under military specifications. Fluxless soldering in inert atmosphere eliminates the need for the post solder cleaning. The process is believed to solder electronics components and assemblies without flux, provided that the parts are contacted with solder wave in an inert environment. In order to solder without flux, the solder wave must be inert such a nitrogen based. A solder wave that is not inert will form oxides on the

12 surface. This will result in ineffective soldering. Additionally, component: leads and circuit pads to have a certain finish such as "previously solder coated", "noble metal plated", and "etched and cleaned base metals" (Nowotarski, undated). The systems are believed to be quite expensive. The off-gas from this process need to further studied. It is believed that by products such as formaldehyde may be formed

13 REFERENCES Deram, B., Technical Considerations for use of No-clean Fluxes, Kester Solder, Sept DOD-STD-2000-IB, Military Standards, Soldering Technology, High Quality/ High Reliability, June 10, Du Pont, Chemicals and Pigments, Montreal Protocol Changes/ Du Pont Investments in Substitutes, July 20, Du Pont Dibasic Esters, Brochure. Solvents and Intermediates for Industry, Product Federal Register(FR), Environmental Protection Agency (EPA), Vol. 52, p47485, Dec. 14, FR, EPA, August 1, Foecke, T.L., Waste Minimization in the Electronics Products Industries, Journal of Air Pollution Control Association, Vol. 38, No. 3, p. 283, March Halogenated Solvents Industry Alliance (HSIA), Newsletter, Vol. 6, No. 4, August/Sept Markenstein, H., Solder Flux Developments Expand Choices, Electronics Packaging and Production, P. 39, April Montreal Protocol on Substances that Deplete the Ozone Layer, Final Act, Montreal, Canada, Sept Morrison, P. and Wolf, K. Substitution Analysis: A Case Study if Solvents, Journal of Hazardous Materials, Vol. 10, p. 189, National Toxicology Program, Technical Report on the Toxicology and Carcinogenesis Studies of d-limonene in F 3441N Rats, B6C3Fl Mice, Draft Report, Stanford Research Institute, B. Waterhouse and A. Miller, Integrated Solvents Analysis: Electronics industry Uses, July

14 12th AESF/EPA Conference on Environmental Control for the Surface Finishing Industry Hyatt Orlando Kissimmee, Florida January 28-30, 199l