Last amended: Novermber 5, 2001
MANUFACTURING OPERATIONS
Printed circuit board manufacturing (PCBM) is a complex industry requiring numerous processing steps to produce single or multi-layer printed circuit boards. Although some of the manufacturing steps emit no pollutants or only very low levels of pollutants into the atmosphere, several processes used in this industry have the potential to emit significant quantities of Precursor Organic Compounds (POC). A few process steps may also emit toxic compounds or Hazardous Air Pollutants (HAP) as defined by EPA. The Bay Area Air Quality Management District regulates these air pollution sources by imposing emission standards or organic content limits and by requiring Permits to Operate for selected printed circuit board manufacturing processes.
This permit handbook chapter describes the permitting procedures for printed circuit board manufacturing operations. The first two sections of this chapter describe the types of processes that may be encountered at a printed circuit board manufacturing facility and the air emissions associated with these processes. The next two sections discuss Bay Area Air Quality Management District permit requirements for this industry and items necessary for a permit application. The fifth section is an engineering evaluation template and includes typical equipment descriptions, sample emission calculations, applicable regulatory requirements, and sample permit conditions.
The manufacturing of printed circuit boards requires numerous steps to produce the final product. These processes may be electrical, chemical, mechanical, or optical in nature. Although printed circuit boards may be either single-layer or multilayer, most facilities in the Bay Area are capable of producing multilayer boards. Therefore, this section will describe multilayer printed circuit board operations. Typical processing steps encountered at PCBM facilities are discussed in sections A through D. Section E contains a process flow diagram for a typical multilayer printed circuit board manufacturing operation.
A. Inner Layer Circuit Formation
Multilayer printed circuit boards contain several individual inner layers. These inner layers are thin fiberglass epoxy sheets or "cores," with detailed circuit patterns on each core. This section describes the processes required to form the circuit patterns on an inner layer core.
1. Core Preparation Steps
The raw materials are thin fiberglass epoxy cores covered with copper foil on each side. The cores are often processed through several surface preparation steps to promote adhesion of a resist coating to the core. These surface preparation steps may include subjecting the cores to weak acid solutions, pumice scrubs and water rinses which clean and lightly abrade the copper foil surfaces. The cores may then be dried in ovens before proceeding to the next step.
2. Resist Application Operations
Resists contain photosensitive organic monomers and other volatile organic compounds. Upon exposure to ultraviolet light, the photosensitive organic monomers will polymerize and harden. There are two types of resist in use today: dry film resists and liquid resists.
a. Dry film resist comes in large rolls containing a thin film of resist covered by a protective mylar film. Laminators use heat and pressure to apply the dry film resist to both sides of the core material simultaneously.
b. Liquid resists may be applied to the entire core surface (one side at a time or both sides at the same time) or just to certain areas of the core surface. There are a number of application techniques including: squeegees, rollers, sprays, and silk screens. After application of liquid resist, the coated cores are usually tack dried in ovens.
3. Exposure Units
The photosensitive nature of resists enable printed circuit board manufacturers to define the precise location of the circuit pathways on the core surface by selectively exposing certain areas of the resist to ultraviolet (UV) light. This selective exposure is accomplished with photo design tools at electrically powered image units. Organic monomers in the exposed areas polymerize and harden while the resist that was not exposed to the UV light remains soft. For the positive photoresist process, the desired circuit trace pattern is exposed to the UV light. For inner layer circuit formation, most PCBM facilities use a positive photoresist process.
4. Developers
After the circuit design has been imprinted on the resist covered cores at the Exposure Units, the cores are sent to the Develop, Etch, Strip (DES) lines to reveal the final circuit pattern. The developer is typically a series of baths equipped with a conveyor. The first bath normally contains an aquous solution of potassium carbonate and the remaining baths are water rinses. The soft unpolymerized resist is dissolved by the potassium carbonate solution, revealing the underlying copper foil.
5. Etchers
This copper foil is then removed at the Etcher. The remaining polymerized resist protects the desired copper circuit traces during the etching process. Most inner layer etchers use either cupric chloride solutions or ammoniacal/alkaline solutions to remove the unwanted copper foil.
6. Resist Strippers
The Stripping step removes the hard polymerized resist from the remaining copper foil circuit pattern. Resist strippers use aqueous solutions of water miscible organic compounds to soften and dissolve the polymerized resist. These strippers can be operated continuously using enclosed conveyorized equipment or in batch mode using dip tanks that are covered when not in use. The stripping solution may be classified as either aqueous for low percentages of organic compounds or semi-aqueous for higher percentages of organic compounds.
7. Oxide Line
After resist stripping, the cores are processed at an Oxide Line to prepare the copper surfaces for panel assembly and lamination. The Oxide Line consists of a series of baths. The first baths use inorganic solutions to clean and micro-etch the remaining copper on the cores. The next baths add an oxide layer to the surface of the copper. Typically this oxide layer is cupric oxide, which is black; but other oxides such as white tin oxide may be added. The oxidation process is considered to be a surface preparation step.
The inner layer cores must be assembled together to form a multilayer printed circuit board panel. This section discusses the equipment and procedures used to assemble a panel and prepare it for outer layer circuit formation.
1. Lamination Press
A "book" is created by placing: a layer of copper foil, a layer of "pre-preg" (a sheet of epoxy material), alternating layers of the inner layer cores and pre-preg, and a final layer of copper foil; between press plates. The book layers are joined together at a Lamination Press by subjecting the book to high heat and pressure. The pre-preg melts, joining the layers together and forming a rigid panel.
2. Panel Curing Ovens
After lamination, the rigid panel may be heat cured in an oven. These ovens are usually electrically powered.
3. Panel Trimming and Drilling
The circuit board panel edges may be cleaned of excess pre-preg, shaped, and smoothed by routers, saws, and buffers. In addition, small holes are drilled through the panels to join the individual layers together.
4. Drill Hole Cleaning (Desmear)
The hole surfaces must be cleaned, made conductive, and then plated to form a complete circuit through the entire panel. The drilling process leaves an epoxy residue, or "drill smear," in the holes which must be removed before copper can be plated in the drill holes. Removing this drill smear, or desmearing, is accomplished by either wet chemical methods or by a gaseous plasma process.
a. The wet chemical methods typically include an organic/water conditioner bath to soften the pre-preg residue, a permanganate bath to dissolve the softened epoxy, a dilute hydrogen peroxide solution to neutralize the permanganate, and several water rinse steps.
b. The plasma process uses an ionized gas under vacuum to chemically react with the drill smear and etch the hole surfaces. The gas components, processing time, temperature, pressure, and RF power level are used to control the reaction. The gas components and reaction products can vary considerably from one operation to another.
5. Making Holes Conductive
After cleaning, the drill holes must be made conductive so that copper can be plated on the hole surfaces. There are several methods currently available to make the drill holes conductive.
a. Metalization of the drill holes is accomplished by electroless copper plating. The plating baths consist of water, metal ions, catalyst, reducer, complexing agents, and stabilizers. Formaldehyde is sometimes used as a reducing agent in electroless copper plating solutions.
b. Alternative Metalization processes such as "shadow," "crimson," or "black hole" add other types of conductive materials, such as graphite, to the hole surfaces.
6. Panel Preparation
In order to prepare the panels to receive resist on the outer layers of the panel, the panels are subjected to anti-oxidant baths, cleaners, and pumice scrubs similar to the Core Preparation step discussed in Section I.A.1. above. Some facilities also add an anti-tarnish coating at this step.
C. Outer Layer Circuit Formation
This section describes the processes used to create the printed circuit design on the two outer layers of the panel.
1. Resist Application Operations
The types of resist and application methods used by the PCBM industry for Outer Layer Circuit Formation are the same as the Resist Application Operations discussed above in Section I.A.2. for Inner Layer Circuit Formation.
2. Exposure Units
The resist that has been applied to the outer layers of a circuit board panel is selectively exposed to UV light using photo design tools and electric Exposure Units similar to those discussed in Section I.A.3. for inner layer circuit formation. However, the outer layer circuits are normally produced using a negative photoresist process. In a negative photoresist process, the resist covering the desired circuit traces remains soft because it was not exposed to the UV light, while the resist covering the undesired copper areas is exposed to the UV light and polymerized.
3. Developer
The unexposed resist is removed by a Developer to reveal the copper foil circuit pattern. As described in Section I.A.4., this developer normally uses potassium carbonate to dissolve the soft unpolymerized resist.
4. Electrolytic Copper Plating Lines
Additional layers of copper are added to the copper foil circuit pattern by electrolytic copper plating. The electrolytic copper plating line includes several cleaning or etching baths, rinses, and the copper sulfate plating bath.
5. Tin and Tin/Lead Plating Lines
The new copper circuit design is protected from subsequent processing steps by adding a second layer of metal. Most PCBM facilities use tin or a tin/lead alloy for this step. The tin or tin/lead alloy is added at a plating line which may include cleaning baths, etching baths, rinses, and electrolytic plating baths.
6. Resist Strippers
Resist Strippers similar to those discussed in Section I.A.6 are used to dissolve and remove the polymerized resist to reveal the unwanted copper foil.
7. Etchers
As described in Section I.A.5, unwanted copper foil is removed by Ammoniacal Etchers.
8. Tin and Tin/Lead Strippers
The protective layer of tin (or tin/lead) is removed from the copper circuits using either a chemical stripping process or an electrolytic stripping process. Both processes normally use only inorganic solutions. Occasionally this step is skipped per customer requirements, and the tin/lead plate is left on the panel for reflow.
This section discusses the miscellaneous processing steps that are required to finish the printed circuit board for shipment, alternative technologies, and ancillary operations. These operations may not be found at all PCBM facilities.
1. Solder Mask
A coating called a "solder mask" is applied to the panels to protect all areas of the board except holes and pads that need solder or tips that require gold plating.
Some facilities use wet solder masks, which are applied by silk screen, tack dried in ovens, and UV cured. These types of coatings are discussed in Permit Handbook Section 5 Chapter 7. The ovens for these coatings are discussed in Permit Handbook Section 7 Chapter 5.
Most PCBM facilities now use a liquid photoimageable (LPI) type solder mask. The LPI masks are uncured monomers with low percentages of volatile organic compounds. LPI masks are applied to the entire panel using many application methods including: squeegees, rollers, or sprays applied to one side of the panel at a time or to both sides at the same time. After application, the LPI mask is tack dried in ovens. The LPI solder masks are exposed to UV light and developed using equipment described in Sections I.A.3 and I.A.4.
2. Solder Application
Solder may be applied to the holes and pads where components will be attached to the boards. The older technology involves a three step process: plate, flux, and reflow. The tin/lead plating process was discussed in Section I.C.5. Next, the panels are sent to a fluxer which lightly oxidizes the solder plate surface to prepare it for reflow. Flux can be applied using manual methods or conveyorized rollers or dip tanks. The tin/lead plate is melted in place on the panel during the reflow process, where panels are immersed in reflow oils and heated to about 425° F. The heat burns off residual flux and melts the solder. The flux and reflow operations may be contained in a single piece of equipment.
3. Hot Air Leveling (HAL)
Hot Air Leveling (HAL) is an alternative means of applying solder to a printed circuit board panel. Panels are normally pre-cleaned using conveyorized equipment and aqueous inorganic solutions. Fluxes are applied by manual or conveyorized methods to aid the solder coating process. Solder is coated on the panel by immersing the panel in a hot solder pot (~ 480° F). Excess solder is removed by hot air knives as the panel exits the solder pot. Panels are often cleaned again, usually with hot or deionized water.
4. Legend Stenciling
Legends and other identifiers are printed on the circuit boards using coatings and stencils. These type of operations are discussed in Permit Handbook Section 5, Chapter 7.
5. Gold Plating
Some PCBM facilities apply gold to panel tips using an electrolytic gold plating bath.
6. Miscellaneous Coating Operations
Various types of protective coatings may be used in place of solder masking or may be added to the printed circuit boards before shipping. Coating operations on multiple substrates are discussed in Permit Handbook Section 5 Chapter 7.
7. Solder Mask Stripping
Occasionally, printed circuit board manufacturers will need to remove the solder mask after it has been applied to correct errors. The solder mask is normally stripped from the boards at a dip tank.
8. Screen and Stencil Manufacture and Cleaning
The screens used for liquid resist applications or wet solder mask applications and stencils used for legend coating are often manufactured on site. Most screens and legend stencils are made from a type of dry photoimageable film. The film is cut to size and exposed to UV Light to define the stencil or screen image. The rigid mesh screens that will hold the stencil image design are sprayed with a solution to prepare them to receive the stencil. The stencils are then laid on the screens and hand pressed to dry them. A hot water and peroxide solution is used to develop the stencil by stripping away all unexposed film.
After use, the mesh screens are normally cleaned with bleach to dissolve the stencils. Some facilities also use a citrus type cleaner.
9. Solvent Cleaning Operations
Organic solvents may be used for wipe cleaning on the printed circuit boards, work areas, or to clean processing equipment such as stripper tanks or organic coating application equipment. Permit Handbook Section, 6 Chapter 3 discusses wipe cleaning operations in more detail.
Solvent dip tanks or vapor cleaners may also be used at PCBM facilities. Cold Solvent Cleaning and Vapor Solvent Cleaning are discussed in Permit Handbook Section 6, Chapters 1 and 2, respectively.
10. Test Equipment
Most PCBM facilities use ionic test equipment, which may contain isopropyl alcohol, for quality control and assurance.
11. Liquid Storage Equipment
PCBM facilities have various sizes of storage vessels for the chemicals and solutions they use to produce printed circuit boards and for the waste products they generate during processing. Permit Handbook Section 4 discusses the permit requirements for storage equipment in more detail.
12. Waste Treatment Operations
Most of the waste treatment operations involve metals removal by precipitation and filtering and neutralization of waste water.
The diagram below illustrates one example of a process flow diagram for a multilayer printed circuit board manufacturing operation. Variations in the type and order of these processes are possible.
INSERT FLOW DIAGRAM (PCBFLOW.VSD)
Many printed circuit board manufacturing processes use coatings or solutions containing organic compounds which could be emitted into the atmosphere. Other processes may emit toxic inorganic compounds such as ammonia or lead. Since many of the Bay Area Air Quality Management District’s permit requirements for this industry are based on the emission rates of precursor organic compounds or toxic compounds, these emission rates must be determined before the permit requirements can be assessed. Sections A through D below discuss the air emissions potential for each process described in Section I. Emission factors and emission calculation methods are presented for any processes that may have significant organic or toxic emissions. Section E below discusses the types of air pollution abatement equipment that may be used by the printed circuit board manufacturing industry.
A. Inner Layer Circuit Formation
1. Core Preparation Steps
The materials used at the core preparation steps do not contain any organic compounds. These processes use only weak concentrations of inorganic acids, which have low vapor pressures and are not expected to emit any significant quantities of pollutants into the atmosphere.
2. Resist Application Operations
a. Dry film.
Dry film resist contains low quantities of volatile organic compounds which may include methylene chloride or other EPA Hazardous Air Pollutants (HAP). The heat of the laminators will drive off these volatile organic compounds from the film during the resist application process. Since this process has the potential to emit a toxic compound, even low quantities of emissions may be significant.
Organic emissions from the Laminator can be calculated from the gross usage rate of dry film (typically expressed as million square feet of film per year) and an emission factor (pounds of organic vapors per million square feet of film). This emission factor can vary depending on the type of film and the film thickness. Therefore, emissions should be calculated using the total organic vapor emission factor and toxic chemical concentrations listed on the vendor's MSDS for the specific type and thickness of film in use at the site, if they are available. If this data is not available, the following factors may be used:
Total Organic Compounds = 50 lb./MM ft2 of film
Methylene Chloride (3% of total organic vapors) = 1.5 lb./MM ft2 of film
b. Liquid resists.
Liquid resists typically contain significant quantities of organic compounds. The total organic emissions for the application and drying of liquid resists can be calculated from projected coating usage rates and the VOC Content of the coating, which is normally listed on the coating’s MSDS. For example, a facility plans to use 500 gallons per year of liquid resist. The MSDS for this coating indicates that the VOC Content is 3.5 pounds of organic compounds per gallon of coating, as applied. The total organic emissions for this operation would be:
(500 gal/yr) ´ ( 3.5 lb organics/gal) = 1750 lb/yr of organic compounds
3. Exposure Units
Nearly all of the volatile organic compounds in resists are expected to be emitted during the lamination process or during liquid resist application and tack drying. No significant levels of organic emissions are expected from these imaging units.
4. Developers
Most developers use an aqueous potassium carbonate solution. This process does not result in any emissions to the atmosphere.
Some processes require water miscible organic compounds to develop the resist. Although the organic content of these solutions is typically low, developers using water miscible organic compounds will emit organic compounds into the atmosphere. The amount of organic emissions can be calculated using the method described in Section II.A.6 below for Resist Strippers.
5. Etchers
Copper foil etchers using cupric chloride solutions do not emit any significant quantities of pollutants into the atmosphere.
Etchers using ammoniacal/alkaline based solutions could emit significant quantities of ammonia (NH3), a District toxic air contaminant. Ammonia is the only pollutant of concern from these etchers. Most facilities use two types of ammoniacal etchants: starter solutions and make up or replenisher solutions. These solutions typically contain copper ions, ammonium hydroxide (AH), ammonium chloride (AC), and low percentages of other ammonium salts. The ammonium ions dissociate in water forming ammonia which then reacts with copper ions to form complex cupric ammonia salts. These copper/ammonia salts build up in the etchant solution until the etchant solution reaches a saturation point or becomes "loaded with copper."
The ammonia emission rate from the Etcher can be calculated using the following equation:
AE = [ ( AH/35.05 + AC/53.59 ) ´ 0.1703 ´ r ] - ( 0.067 ´ l )
AE = ammonia emission factor, pounds of NH3 per gallon of etchant solution used
AH = weight percent of ammonium hydroxide (NH4OH) in the etchant, from MSDS
AC = weight percent of ammonium chloride (NH4Cl) in the etchant, from MSDS
r = density of the etchant solution, pounds per gallon, from MSDS
l = copper loading capacity, ounces of copper per gallon of etchant, from MSDS or site specific operating specifications
This ammonia emission factor equation was developed based on the following assumptions:
a. All of the ammonium hydroxide and ammonium chloride in the etchant solutions completely dissociate to form ammonia, water, hydrogen ions and chloride ions.
b. The concentration of the other ammonium salts are negligible.
c. All of the ammonia either reacts with the copper or completely evaporates from the etchant solutions.
The resulting emission factor will provide a conservative estimate of emissions because spent etchant solutions typically contain some residual ammonia and ammonium ions. (There was not enough data available to establish a minimum residual ammonia content, therefore it was assumed to be zero. If the PCBM facilities or etchant solution recyclers can provide sufficient data to establish a minimum residual ammonia/ammonium content, this equation may be adjusted to subtract off the residual ammonia content.) Appendix A contains a more detailed discussion of the development of the ammonia emission factor equation.
6. Resist Strippers
As described in Section I, resist strippers are used to remove polymerized resist from both inner layer details and the outer layers of printed circuit boards. The resist stripping solution is a mixture of water and water miscible organic compounds.
Some of the organic compounds in the stripping solution will evaporate during the stripping process. However, several of the organic components of stripping solutions (e.g., butyl cellosolve and monoethanol amine) have low vapor pressures, even at the elevated operating temperature (~130° F) of the strippers. Therefore, it is not accurate to assume that all of the organic compounds in the stripping solution evaporate completely into the atmosphere. This statement is confirmed by the fact that organic compounds with low vapor pressures have been found in wastewater samples from several printed circuit board manufacturing facilities.
Although source testing is the most accurate method for determining emissions from resist strippers, this method can only be used for existing operations. Therefore, the BAAQMD has developed an emission calculation procedure for estimating emissions from Resist Strippers. The emission calculation procedure is based on three major assumptions:
a. The organic constituents of the solution obey Raoult’s Law: the partial vapor pressure of a component above a liquid mixture is equal to the mole fraction of the component in the liquid mixture times the vapor pressure of the pure component at the temperature of the liquid mixture.
b. The vapor exhaust stream from the system is saturated with organic compounds.
c. The emissions are at steady state.
These three assumptions were used to develop a theoretical emissions calculations procedure based on the concentrations of each organic compound in the stripper solution (as it is used in the stripper), the operating temperature of the stripper, the exhaust flow rate from the stripper, and the operating hours for the stripper.
The total amount of each organic compound available for evaporation was determined based on concentration data and the gross usage rate of the solution. The emissions from the stripper are assumed to be the lower of these two emission rates (theoretical emissions versus total organic compounds available).
An example of this emission calculation procedure is presented in the following spreadsheet. The sample evaluation includes a more detailed discussion of this emission calculation procedure, the spreadsheet calculations methods, and notes on the limitations of this calculation procedure are available.
7. Oxide Line
The solutions used at the Oxide Line
are not expected to contain any organic compounds. The aqueous inorganic
solutions do not emit any significant quantities of air pollutants.
1. Lamination Press
There may be some off gassing of organic compounds from the pre-preg as it melts, but these emissions are expected to be very low. No significant toxic emissions are expected from this source either.
2. Panel Curing Ovens
No additional organic emissions are expected from these heat treatment ovens.
3. Panel Trimming and Drilling
The panel trimming and micro-hole drilling operations will generate particulate emissions. These dust emissions are usually collected by cyclones or baghouses. The particulate emission rates from the control devices are expected to be insignificant.
4. Drill Hole Cleaning (Desmear)
a. Wet Chemical
The conditioner bath that is used to soften the drill smear contains organic compounds. The organic portion of this solution is assumed to be 100% volatile. The permanganate bath and hydrogen peroxide bath do not emit any significant levels of air pollutants.
b. Plasma
Plasma is an electrically charged gas containing ions, electrons, and free radicals. When substrate surfaces are bombarded with the ionized gases a chemical reaction with the substrate occurs, thereby modifying the surface layer. These chemical reactions can be varied for different applications by using different or blended gases in the process. The gas vented from the reaction chamber may contain hydrogen fluoride, organic compounds and nitrogen oxides. Generally, only the emissions of toxic compounds are significant.
A commonly used plasma gas is Tetrafluoromethane (CF4). From the ideal gas law, the molar volume of any gas at 70° F and 1 atm is: 386.7 ft3/lbmol. If it is assumed that all CF4 will react in the process to form Hydrogen Fluoride (HF) according to the equation below, then the HF emission factor will be:
CF4 + 2 H2 + O2 ® CO2 + 4 HF
(1 lbmol CF4/386.7 ft3 CF4) ´ (4 lbmol HF/1 lbmol CF4) ´ (20.01 lb HF/1 lbmol HF)
= 0.21 lb of HF / ft3 of CF4 (at 70° F and 1 atm)
or
(1 lbmol CF4/88.0 lb CF4) ´ (4 lbmol HF/1 lbmol CF4) ´ (20.01 lb HF/1 lbmol HF)
= 0.91 lb of HF / lb of CF4
5. Making Holes Conductive
a. Electroless Copper Plating
Most of the chemicals used in electroless copper plating baths are inorganic with low partial pressures and low toxicity. One exception is formaldehyde, which is sometimes used as a reducing agent in electroless copper plating solutions. Formaldehyde is a hazardous air pollutant and even very low emission rates are considered significant. The total emission rate of formaldehyde at a printed circuit board facility is estimated to be 3.9 E-4 pounds of formaldehyde per gallon of concentrated (37% by weight) formaldehyde solution consumed. Appendix C describes the origin of this emission factor in detail.
b. Alternative Metalization
The currently available alternative metalization processes are not expected to have any significant emissions of toxic compounds or other pollutants.
6. Panel Preparation
Most of the panel preparation processes use weak acids or other inorganic solutions to clean and prepare the panels. These processes do not emit any significant levels of toxic compounds or organic pollutants.
However, the antitarnish coating that is sometimes applied to boards at this stage does contain organic compounds. All of the organic compounds in this coating are assumed to be emitted during application or drying. The VOC Content of the coating should be listed on the coating manufacturer’s MSDS. Organic emissions should be calculated using projected coating usage rates and the coating specific VOC Content.
C. Outer Layer Circuit Formation
1. Resist Application Operations: Refer to Section II.A.2.
2. Exposure Units: Refer to Section II.A.3.
3. Developers: Refer to Section II.A.4.
4. Electrolytic Copper Plating Lines
The electrolytic copper plating line includes several cleaning or etching baths that use water or aqueous inorganic acid solutions. These baths do not emit any significant amounts of toxic compounds.
The plating bath is an aqueous copper sulfate solution. Electrolytic plating operations can emit low levels of particulate matter into the air due to entrainment of metallic compounds from the plating solution. However, the copper compound emissions from the electrolytic copper plating baths are expected to be well below the risk screening trigger level. Therefore, these emissions are not expected to be significant.
5. Tin and Tin/Lead Plating Lines
As discussed for electrolytic copper plating, the particulate emissions from electrolytic tin plating operations are expected to be low. In addition, tin compounds are not considered to be toxic. Therefore, the emissions from tin plating operations are not significant.
However, the risk screening trigger level for lead is very low (currently 29 pounds per year). Therefore even low levels of particulate emissions from a tin/lead plating operation are potentially significant. Lead emissions from electrolytic tin/lead plating operations are estimated to be 4.7 E-8 pounds of lead per amp-hour. This factor was estimated using the District’s emission factors for chrome plating operations and the assumption that plating solution entrainment rate is dependent on the voltage and current required for the process and the concentration of metal compounds in the solutions. Appendix D describes how this lead emission factor was calculated in more detail.
6. Resist Strippers: Refer to Section II.A.6.
7. Etchers: Refer to Section II.A.5.
8. Tin and Tin/Lead Strippers
The emissions from electrolytic tin and tin/lead stripping operations are assumed to be the same as the emissions discussed above for electrolytic tin and tin/lead plating operations.
The wet chemical stripping processes involve aqueous inorganic solutions and are not expected to emit any toxic compounds.
1. Solder Mask
The solder mask coatings contain organic compounds which are emitted during the application processes and during subsequent drying steps. The total organic compound emission rate from these operations can be calculated based on projected solder mask usage rates and the VOC Content of the coating as listed on the MSDS. Some coatings may contain non-precursor organic solvents such as acetone. The precursor and non-precursor organic compounds should normally be evaluated separately. The organic solvent in the solder mask may also include toxic compounds. The emission rate for each toxic component should be calculated as well.
2. Solder Application (Plate, Flux, and Reflow)
The air emissions due to tin/lead plating operations are discussed in Section II.C.5.
Fluxes are usually water soluble, slightly acidic, organic based solutions. The organic content of fluxes can vary from negligible amounts of VOC to nearly 100% VOC. These fluxes may also contain inorganic acids such as hydrochloric acid (HCl) or hydrobromic acid (HBr). Some of the VOC and acid gases (HCl or HBr) will evaporate at the flux applicator and the remainder will evaporate during the reflow process. Emissions of VOC and acid gases are calculated by assuming 100% evaporation of all organic compounds and 100% evaporation of all acids. The organic content and acid content of the flux is normally found on the flux manufacturer's MSDS.
The reflow oils used at the Reflow Operation may also contain volatile organic compounds. Any organic compound in the oils with a boiling point less than the operating temperature is assumed to completely evaporate during the reflow process. Lead emissions from the reflow operation are not expected to be significant.
3. Hot Air Leveling (HAL)
Fluxes are also applied to printed circuit boards before the hot air leveling process. The emissions due the use of these fluxes should be calculated using the methods discussed above for the solder application operations.
The Hot Air Leveler (HAL) applies molten tin/lead solder to the holes and pads on the printed circuit board. In addition to the organic compounds and acid gases emitted due to the residual flux on the boards, the HAL will generate some lead emissions from the molten tin/lead solder pot and the hot air knives that are used to remove excess solder from the board.
Source testing of a typical HAL indicated that lead emissions would be less than 1 pound per year. It is therefore assumed that lead emissions from most Hot Air Leveling operations will not represent a significant toxic risk.
4. Legend Stenciling
Legend Ink Stenciling is analogous to surface coating operations covered in Section 5 Chapter 7 of the Permit Handbook "Miscellaneous Solvent And Surface Coating Operations." All volatile organic compounds in the coating will be emitted. The total organic compound emission rate from these operations can be calculated based on projected legend ink usage rates and the VOC Content of the ink as listed on the MSDS. Some coatings may contain non-precursor organic solvents such as acetone. The precursor and non-precursor organic compounds should be evaluated separately. The organic solvent in the ink may also include toxic compounds. The emission rate for each toxic component should be calculated as well.
5. Gold Plating
Although the gold plating bath usually contains cyanide compounds, the cyanide is expected to be complexed in the solution and not emitted to the atmosphere in significant amounts.
6. Miscellaneous Coating Operations
As with solder mask coatings and legend ink, all organic compounds in these miscellaneous coatings are assumed to be volatile unless the coating manufacturer specifically states otherwise on the MSDS.
7. Solder Mask Stripping
A solvent dip tank is normally used when the solder mask needs to be stripped from the board. Emissions from this operation are calculated based on net solvent. The solvent is generally assumed to be 100% volatile, unless the MSDS states otherwise.
8. Screen and Stencil Manufacture and Cleaning
The UV light exposure machines that are used to imprint the silk screen or stencil image on photoimageable film do not have any air emissions.
Before the exposed film is attached to a screen, many facilities use a screen preparation solution on the rigid mesh screen. The organic content of these screen preparation solutions are normally assumed to be 100% volatile. Emissions from this operation are based on gross screen preparation solution usage and the solution's organic content from the MSDS. Most facilities use water based solutions with less than 1% VOC; however, some facilities may still use solutions with high organic content and high toxic compound concentrations.
Most facilities use a hot water and peroxide solution to develop (or to remove unexposed film) the silk screen or stencil image. This operation does not result in any air emissions. If a developer solution is used that contains organic compounds, the emissions may be calculated in the same manner as the emission from screen preparation solutions discussed above.
After use, the mesh screens are normally cleaned with bleach to dissolve the stencils. Some facilities use a citrus cleaner or water soluble organic compounds instead of bleach to clean their screens. These materials are all normally discharged into the sanitary sewer system. Any organic compounds in these screen cleaners are assumed to be 100% volatile unless the compounds have high initial boiling points. For organic compounds that are 100% volatile, emissions are based on gross screen cleaner usage rate and the organic content of the screen cleaner from the MSDS. If the organic compounds in these cleaners meet the Regulation 8 Rule 16 definition of a low volatility compound (initial boiling point greater than 248° F and at least 180° F greater than the operating temperature) and the organic compound is considered to be highly biodegradable, then the emissions may be assumed to be 10% of the gross organic compound usage rate. The ten percent factor is applicable because the low partial pressures of these cleaning solutions will limit emissions during the cleaning operation and because the remaining organic compounds in solution will be nearly all destroyed biologically in the sewer system treatment processes.
9. Solvent Cleaning Operations
Any organic solvents used for wipe cleaning on the printed circuit boards, work areas, or processing equipment are assumed to be 100% volatile. Emissions are calculated from gross solvent usage and the density of the solvent.
Solvents used in cold cleaner dip tanks or vapor degreasers are also considered to be 100% volatile. However, emissions from these sources are generally based on net solvent usage rates, if available, because the solvent in the machine is periodically removed as waste and replaced with fresh solvent.
10. Test Equipment
Ionic test equipment will emit some isopropyl alcohol. However, these emissions are negligible compared to the larger sources of organic emissions at printed circuit board manufacturing facilities.
11. Liquid Storage Equipment
The organic compound emissions from liquid storage equipment is potentially significant, depending on the size of the tank and the material being stored. Permit Handbook Section 4 Chapter 1 discusses the air emissions from small (< 20,000 gallon capacity) storage tanks.
12. Waste Treatment Operations
The waste water from PCBM facilities contains low concentrations of organic compounds. However, these compounds normally have very low vapor pressures. Therefore the waste treatment operations are not expected to have any organic emissions.
Most of the organic compounds used by printed circuit board manufacturing facilities are water soluble. Therefore, Wet Scrubbers are generally considered to be an appropriate control method for organic emissions from these facilities. Most Wet Scrubbers employ sprays or packing to enhance water contact with gaseous streams and can achieve 50% to 90% control of organic compounds. Scrubber water may be discharged to the sanitary sewer system under permit from local POTWs.
Organic emissions may also be controlled by combustion in a thermal or catalytic oxidizer. However, a concentrator is often required upstream of the combustion device because of the dilute organic concentration in the exhaust streams. A concentrator/combustion control scheme can achieve greater than 98.5% control.
Control of ammonia emissions from etchers
is normally accomplished by Scrubbers using water or weak sulfuric acid
as the scrubbing solutions. These scrubbers can achieve 50% to 99% control
of ammonia emissions.