Volume 06 - Welding, Brazing and Soldering Part 16 pps

Circuit board soldering and structural applications are described below as general examples. Circuit Board Soldering. First, flux is applied to the leads and circuit board, unless flux-cored soldering wire is used, in which case this step is omitted. The coated area should extend beyond the immediate joint to ensure adequate wetting by the solder. Next, the soldering iron is made to contact the lead (never the circuit board land), and the solder wire is fed so that it contacts the opposite side of the lead. Melting of the solder indicates that the lead has reached temperature. The wire, followed by the iron tip, is then removed from the joint area upon formation of the joint fillet. Structural Applications. The larger thermal mass of typical workpieces requires preheating in order to bring the joint area to the working temperature. Then, the flux is applied to fully cover the hot surface. The joint is further heated and the filler metal is applied to the joint surfaces. Most inorganic and organic fluxes can tolerate the high temperatures required to supply heat to larger members. The rosin-base fluxes can quickly degrade, as indicated by the formation of thick, blackened residues on the surface. Heat and solder are removed when an adequate fillet forms at the joint opening, in order to ensure complete filling of the joint gap. The hot dip coating of parts can be performed by ultrasonic activation, without using a flux. Ultrasonic energy is coupled to the sample through the solder bath (Fig. 22). Optimum coupling is a function of the sample geometry, power level, and workpiece-horn separation. Oxide removal does not occur through simple line-of-sight erosion by the horn (Ref 28). Rather, the oxide is disrupted by the ultrasonic energy that is transferred from the horn into the substrate. Therefore, hidden surfaces can also be coated by the solder. Hot-dipped coatings can be applied to large workpieces or to leads on small electronic devices. In the latter case, care should be taken to ensure that internal connections are not damaged by the ultrasonic energy. This technology has been used to assemble tube-socket joints on heat-exchanger equipment (Ref 29). FIG. 22 ULTRASONIC SOLDERING BATH EQUIPMENT An important component of the solder assembly process is the fixturing that is used to support the workpieces being assembled. Fixturing details are most critical for global heating processes in which the fixture will also be raised to the soldering temperature. The purpose of fixturing is to either anchor the two substrates to prevent movement while the solder is in the molten state or permit the controlled displacement of the substrates to establish specific joint gaps not achieved with the solder preform in the solid state. Lubricants include high-temperature petroleum products or solid lubricants (MoS 2 and graphite). Preloads on the workpieces are provided by springs (coil or Belleville configurations). The joint gap can be established by spacers between the workpieces or particles within the joint itself (Fig. 23). The spacer materials should not cause undesirable reactions with the solder or jeopardize the strength capacity of the joint. FIG. 23 TECHNIQUES TO MAINTAIN JOINT GAPS. (A) BUTT JOINT WITH SPACERS IN THE SOLDER. ( B) BUTT JOINT WITH SPACER IN THE FIXTURE. (C) LAP JOINT CONSTRUCTION Fixture materials should be carefully assessed. Although high vapor pressures generated by fixture metals is not of concern at the relatively low temperatures used in soldering, factors such as thermal expansion and fixture size should be considered. For example, a CTE mismatch between the workpiece and the fixture materials, with improperly designed dimensions, can cause the joint gap to grow or close at soldering temperatures. Fixtures with large thermal mass will lengthen the heating and cooling times for the workpiece, leading to possible flux degradation, base metal or coating erosion, and longer production time. Preferred fixturing materials include oxide ceramics (alumina, beryllia, or mullite), refractory metals (molybdenum, tungsten, or tantalum), and well-oxidized steels (low-carbon and stainless steels). These materials will generally not be wetted by solder (and flux) spillage and can accommodate the elevated temperatures. Fixtures made of machined metals, such as aluminum, copper alloys, or steels, should be annealed in order to relieve any residual stresses that arise from the stock material or machining operations. These internal stresses may cause misalignment of the workpieces or the binding of moving parts, caused by warpage generated at soldering temperatures. Preoxidized surfaces will prevent inadvertent wetting by the solder. Fixtures should be cleaned of organic residues to limit their outgassing in vacuum furnaces, prevent deposition of the contaminants onto the solderable surfaces, and limit spatter by their volatilization at soldering temperatures. Materials that contain or are coated with cadmium or zinc should not be used, because their high vapor pressures (even at soldering temperatures) may cause them to contaminate the joint area, as well as to poison vacuum and inert-atmosphere furnace systems. The workpiece temperature should be monitored, at least in the prototype stages of process development, in order to document temperature conditions. In furnace operations, it is preferred that the controlling thermocouple contact the substrates to confirm the desired temperature profile at the joint area. The relatively low temperatures of soldering allow the use of inexpensive thermocouples, such as types J, T, and K, for many furnace cycles. The thermocouples should not interfere with the fixtures or the filling of the joint. An undesirable spread of the molten solder on the substrate surface can be prevented by a number of "solder-stop" products. High-temperature tapes, which are popular in the electronics industry to prevent solder wetting of areas on printed circuit boards, are suited for many structural applications. Another product is a slurry made from powders that are mixed with water or alcohol and painted on the surface areas that are to be free of solder. The vehicle evaporates, leaving a coating that prevents spreading by the solder. Detailed descriptions of the individual soldering processes, equipment, and applications are provided in the Section "Solid-State Welding, Brazing, and Soldering Processes" in this Volume. References cited in this section 28. P. VIANCO AND F. HOSKING, ANALYSIS OF ULTRASONIC TINNING, NEPCON WEST, FEB 19 92, P 1718 29. J. SCHUSTER AND R. CHILKO, ULTRASONIC SOLDERING OF ALUMINUM HEAT EXCHANGERS, WELD. J., OCT 1975, P 711 General Soldering Paul T. Vianco, Sandia National Laboratories Postassembly Cleaning Procedures After the soldering operation, the workpiece is cleaned, primarily to remove flux residues that can cause corrosion of the part while in storage or during service. Other cleaning procedures include the removal of solder-stop materials, as well as stray solder particles, which can interfere with mechanical or electrical performance of the assembly. Flux residues should be removed as soon as possible after the soldering process, because their ability to be removed decreases with time, whereas their tenacity and potential for corrosive damage increase with time. Cleaning fluids and solvents to be used are determined by the particular flux residues. Guidelines are presented in the section "Fluxes" in this article. The selection of organic solvents is rapidly changing as chlorofluorocarbon materials are phased out by environmental regulations and new materials (and processes) become qualified as replacements. Some general practices must be considered when establishing cleaning procedures. First, assess the compatibility of substrate materials and filler metals with the cleaning solutions. Organic solvents are benign toward metal surfaces. Alkaline solutions used to neutralize strongly acidic fluxes can affect some base-metal finishes (for example, copper alloys, iron alloys, and some steels). When in doubt, samples of the substrate should be exposed to the cleaning agent prior to use on final assemblies. Second, determine whether the cleaning solutions leave undesirable residues. Mineral deposits from tap water can corrode or stain substrate surfaces (especially in the continued presence of water vapor). Tap-water rinses should be followed with rinses in either deionized or distilled water. All traces of water can be removed by a final alcohol rinse. Isopropyl alcohol is generally used. Denatured alcohol and acetone should be avoided, because they also can leave residues. Third, the drying of solvent or other cleaning-solution residues should utilize dry, clean gas, such as bottled or cryogenic nitrogen gas. Compressed "house" air may contain water particles or compressor oils that can quickly recontaminate the workpiece. Fourth, limit the contact of the assembly with oily rags and fingerprints. Many instances of cosmetic staining or pitting of the workpiece surface have been traced to fingerprints. Fifth, test for the effectiveness of postcleaning operations. Unlike the well-specified procedures used by the electronics industry, such procedures are not well standardized for structural applications. Temperature-humidity chambers can be used to assess the propensity for corrosion on the workpiece (a destructive test). Cleaning effectiveness can be enhanced by thermal and mechanical assistance. Cleaning solvents and solutions have higher solubility for residues at elevated temperatures. Caution should be observed when heating solutions because of the generation of vapors that can result in health or fire hazards. The use of solvent vapors at their boiling point in vapor degreasers can remove residues from remote locations on the workpiece. However, the solvents that have been popular for vapor degreasing are being restricted from use by environmental statutes. Mechanical agitation of cleaning solutions is obtained by ultrasonic activation, high-energy sprays, and manual scouring procedures. Ultrasonics are very effective for loosening residues, particularly in hidden locations. Although generally safe for the cleaning of structural members, care must be exercised when using ultrasonics on electronic assemblies because of the possible damage to internal connections. The use of sprays or jets to force the cleaning solution into crevices and hidden areas of the workpiece can increase cleaning efficiency. Batch and in-line equipment based on spray and jet technology is currently available. Because the cleaning material passes through a jet, aerosols and mists are generated, which may create an explosion hazard. Manual scouring can remove residues on exposed surfaces only. Cleaning with sandpaper or vapor blasting metal surfaces with abrasive particles should be avoided in the postprocess cleaning steps for three reasons: First, the base-metal oxide layer protects the surface from corrosion or excessive oxidation later in service. For example, stainless steels are particularly susceptible to corrosion attack after abrasive treatments, particularly those that use steel wool or a steel brush. Second, solders are generally much softer than the base metals. Therefore, inadvertent damage can be easily done to the joint fillets, possibly jeopardizing monotonic strength and fatigue resistance. Third, abrasive grit particles can become embedded in the substrate and, particularly, in the softer solder. Dislodged particles can damage mechanical actuators that are part of the soldered assembly. The abrasive particles can also deteriorate surface solderability of the workpiece during subsequent assembly, repair, or rework procedures. Damage occurs readily to circuit boards (solder masks, coatings, and the laminate itself) by abrasive particles. Finally, storage of the parts must be considered. The extent of storage control depends on factors such as the type of assembly and its service requirements, the cost of rework, repair or scrapping of damaged parts, and the environment of the factory. Acute contamination or corrosion of finished parts can be prevented by their enclosure in bags. Popular containers are polyethylene plastic bags, which can provide short-term storage (<1 year) when properly sealed. Solderability may deteriorate for longer storage times because of the outgassing of polymer components contained in the plastic. Only new bags should be considered for the storage of solderable parts because contamination of the bag interior from previous usage may deteriorate solderability. Plastic bag material should be a minimum 0.1 mm (0.004 in.) thick. The bags can be filled with an inert gas (nitrogen) to further limit oxidation of solderable surfaces. Paper bags should be considered only for the transport of devices because their inability to be sealed from the environment, as well as contamination of solderable surfaces by chemicals in the paper, limit their appropriateness as storage containers. Metal foils can be used as storage media, although greater effort is required to seal the material. The use of desiccants that comply with MIL-D-3464E, "Desiccants, Activated, Bagged, Packaging Use and Static Dehumidification," is acceptable. More elaborate and expensive storage measures include the construction of special facilities that control temperature, humidity, and air-particulate counts. The prevention of mechanical damage that results from the movement of inventory must also be addressed. Storage conditions are of particular concern for electronic devices and circuit boards in which solderability loss can severely hinder subsequent automated assembly processes. General Soldering Paul T. Vianco, Sandia National Laboratories Postassembly Cleaning Procedures After the soldering operation, the workpiece is cleaned, primarily to remove flux residues that can cause corrosion of the part while in storage or during service. Other cleaning procedures include the removal of solder-stop materials, as well as stray solder particles, which can interfere with mechanical or electrical performance of the assembly. Flux residues should be removed as soon as possible after the soldering process, because their ability to be removed decreases with time, whereas their tenacity and potential for corrosive damage increase with time. Cleaning fluids and solvents to be used are determined by the particular flux residues. Guidelines are presented in the section "Fluxes" in this article. The selection of organic solvents is rapidly changing as chlorofluorocarbon materials are phased out by environmental regulations and new materials (and processes) become qualified as replacements. Some general practices must be considered when establishing cleaning procedures. First, assess the compatibility of substrate materials and filler metals with the cleaning solutions. Organic solvents are benign toward metal surfaces. Alkaline solutions used to neutralize strongly acidic fluxes can affect some base-metal finishes (for example, copper alloys, iron alloys, and some steels). When in doubt, samples of the substrate should be exposed to the cleaning agent prior to use on final assemblies. Second, determine whether the cleaning solutions leave undesirable residues. Mineral deposits from tap water can corrode or stain substrate surfaces (especially in the continued presence of water vapor). Tap-water rinses should be followed with rinses in either deionized or distilled water. All traces of water can be removed by a final alcohol rinse. Isopropyl alcohol is generally used. Denatured alcohol and acetone should be avoided, because they also can leave residues. Third, the drying of solvent or other cleaning-solution residues should utilize dry, clean gas, such as bottled or cryogenic nitrogen gas. Compressed "house" air may contain water particles or compressor oils that can quickly recontaminate the workpiece. Fourth, limit the contact of the assembly with oily rags and fingerprints. Many instances of cosmetic staining or pitting of the workpiece surface have been traced to fingerprints. Fifth, test for the effectiveness of postcleaning operations. Unlike the well-specified procedures used by the electronics industry, such procedures are not well standardized for structural applications. Temperature-humidity chambers can be used to assess the propensity for corrosion on the workpiece (a destructive test). Cleaning effectiveness can be enhanced by thermal and mechanical assistance. Cleaning solvents and solutions have higher solubility for residues at elevated temperatures. Caution should be observed when heating solutions because of the generation of vapors that can result in health or fire hazards. The use of solvent vapors at their boiling point in vapor degreasers can remove residues from remote locations on the workpiece. However, the solvents that have been popular for vapor degreasing are being restricted from use by environmental statutes. Mechanical agitation of cleaning solutions is obtained by ultrasonic activation, high-energy sprays, and manual scouring procedures. Ultrasonics are very effective for loosening residues, particularly in hidden locations. Although generally safe for the cleaning of structural members, care must be exercised when using ultrasonics on electronic assemblies because of the possible damage to internal connections. The use of sprays or jets to force the cleaning solution into crevices and hidden areas of the workpiece can increase cleaning efficiency. Batch and in-line equipment based on spray and jet technology is currently available. Because the cleaning material passes through a jet, aerosols and mists are generated, which may create an explosion hazard. Manual scouring can remove residues on exposed surfaces only. Cleaning with sandpaper or vapor blasting metal surfaces with abrasive particles should be avoided in the postprocess cleaning steps for three reasons: First, the base-metal oxide layer protects the surface from corrosion or excessive oxidation later in service. For example, stainless steels are particularly susceptible to corrosion attack after abrasive treatments, particularly those that use steel wool or a steel brush. Second, solders are generally much softer than the base metals. Therefore, inadvertent damage can be easily done to the joint fillets, possibly jeopardizing monotonic strength and fatigue resistance. Third, abrasive grit particles can become embedded in the substrate and, particularly, in the softer solder. Dislodged particles can damage mechanical actuators that are part of the soldered assembly. The abrasive particles can also deteriorate surface solderability of the workpiece during subsequent assembly, repair, or rework procedures. Damage occurs readily to circuit boards (solder masks, coatings, and the laminate itself) by abrasive particles. Finally, storage of the parts must be considered. The extent of storage control depends on factors such as the type of assembly and its service requirements, the cost of rework, repair or scrapping of damaged parts, and the environment of the factory. Acute contamination or corrosion of finished parts can be prevented by their enclosure in bags. Popular containers are polyethylene plastic bags, which can provide short-term storage (<1 year) when properly sealed. Solderability may deteriorate for longer storage times because of the outgassing of polymer components contained in the plastic. Only new bags should be considered for the storage of solderable parts because contamination of the bag interior from previous usage may deteriorate solderability. Plastic bag material should be a minimum 0.1 mm (0.004 in.) thick. The bags can be filled with an inert gas (nitrogen) to further limit oxidation of solderable surfaces. Paper bags should be considered only for the transport of devices because their inability to be sealed from the environment, as well as contamination of solderable surfaces by chemicals in the paper, limit their appropriateness as storage containers. Metal foils can be used as storage media, although greater effort is required to seal the material. The use of desiccants that comply with MIL-D-3464E, "Desiccants, Activated, Bagged, Packaging Use and Static Dehumidification," is acceptable. More elaborate and expensive storage measures include the construction of special facilities that control temperature, humidity, and air-particulate counts. The prevention of mechanical damage that results from the movement of inventory must also be addressed. Storage conditions are of particular concern for electronic devices and circuit boards in which solderability loss can severely hinder subsequent automated assembly processes. General Soldering Paul T. Vianco, Sandia National Laboratories Inspection Because inspection necessarily implies a determination of defects in the workpiece, the type and number of defects that jeopardize the service requirements of the product should be documented. The search for, and rework of, noncritical defects (such as unnecessary cosmetic attributes) simply increases product cost, the amount of scrapped product, as well as the likelihood of heat damage to the substrates. Defects and their acceptance limits should be defined in conjunction with the design engineer, who understands service requirements and codes, and the manufacturing engineer, who understands the process limitations. Several defect types and testing procedures are summarized below. The defects include: • INCOMPLETE FILLING OF THE JOINT, CAUSED BY POOR SOLDERABILITY (NONWETTIN G OR DEWETTING), AN INADEQUATE QUANTITY OF FILLER M ETAL, LOW REFLOW TEMPERATURE, FLUX ENTRAPMENT (HOLES AND VOIDS), OR POOR JOIN T DESIGN (REENTRANT CORNERS OR BLIND HOLES THAT PREVENT THE ESCAPE OF GASES) • POOR FILLET GEOMETRY, CAUSED BY POOR SOLDERABILITY OF THE BAS E METALS, AN INADEQUATE SUPPLY OF FILLER METAL FOR THE GIVEN JOINT GEOMETRY, OR INSUFFICIENT COVERAGE OF THE SURFACES BY THE FLUX COATING • CRACKS IN THE SOLDER FILLET, WHICH SIGNIFY PART MOVEMENT DURING THE SOLIDIFICATION PROCESS, SOLIDIFICATION SHRINKAGE OF THE SOLD ER METAL, OR, IN THE WORST CASE, A MAJOR FLAW IN THE JOINT DESIGN IN WHICH RESIDUAL STRESSES (FOR EXAMPLE, CTE MISMATCH) OVERLOAD THE SOLDER JOI NT. THE LATTER CONDITION CAN CAUSE CRACKS IN THE SUBSTRATES, PARTICU LARLY IN BRITTLE MATERIALS, SUCH AS CERAMICS OR REFRACTORY METALS • GRAINY OR VERY DULL FILLET SURFACES, WHICH INDICATE EXCE SSIVE SOLDER CONTAMINATION, SUCH AS THAT ARISING FROM THE DISSOLUTION OF THE BASE- METAL MATERIALS AND SURFACE COATINGS, INADEQUATE HEATING OF THE SUBSTRATES, OR SUBSTRATE MOVEMENT DURING SOLIDIFICATION. (HO WEVER, SOME SOLDERS NORMALLY EXHIBIT A DULL FINISH UPON SOLIDIFICATION. SLOW COOLING, ESPECIALLY IN VAPOR-PHASE SOLDERING, CREATES DULL JOINTS) • FLUX RESIDUES, WHICH SUGGEST INADEQUATE CLEANING PROCE DURES OR THE OVERHEATING OF THE FLUX TO PRODUCE VERY TENACIOUS RESIDUES. LARG ER PARTICLES MAY BE EMBEDDED IN THE SOLDER FILLET. • DISCOLORATION IN THE BASE METAL, WHICH SIGNIFIES EXCESSIVE HEAT EXPOSURE Nondestructive inspection techniques range from visual assessment to elaborate tests involving complex equipment and procedures. The principal nondestructive testing methods are discussed below. Visual inspection is the most widely used technique. Pictorial guides help the inspector qualify the particular joint configuration. Because the inspector can only infer joint quality from external observations, some destructive assessments should accompany the visual evaluations on early prototypes to correlate with subsequent inspector observations on the actual product. Low-power microscopes (<70×) are used. Radiographic (x-ray) and ultrasonic inspection are used to visualize the interior sections of the joint. X-ray radiographs are used to detect mass differences in the joint, which indicate areas that are absent of solder, such as gas voids or entrapped flux. Ultrasonic techniques detect discontinuities in the path between the receiver and the transmitter (transmission mode) or in the reflected path (reflection mode). Besides unfilled sections of the gaps, this process also detects discontinuities such as poor bonding or cracks. Infrared (thermal transfer) imaging is based on the transfer of heat across the joint gap. Voids and, to a lesser extent, cracks, have a lower thermal conductivity than the continuous base metal-solder-base metal path. The decreased thermal conduction of these defects is imaged by an infrared-sensitive camera that views the opposite side of a heated substrate. This technique has relatively poor resolution because of lateral heat conduction within the materials. Proof testing involves subjecting the solder joint to a mechanical load that exceeds the service design load. The joint is then inspected for damage using nondestructive techniques. In the prototype phase of process development, this procedure may be considered "destructive." Pressure and vacuum testing represent other nondestructive techniques. In pressure testing, which is detailed in the ASME Boiler and Pressure Vessel Code, a positive pressure is applied to conduit joints or soldered vessels. The joint is either coated with a soapy solution or submerged in a water bath before being pressurized. Leaks are identified by the formation of bubbles on the workpiece surface. Air can be replaced by helium or a halogen gas, and special detection equipment can be used to locate leaks in the joints. Vacuum checks of conduit also require special equipment to detect the leakage of trace gases, such as helium, into the conduit or vessel. Mass spectrometers can be used to detect and identify the passage of other gas species into the vacuum. The detector monitors the inside of the conduit (vacuum environment) while the gas is passed along the exterior walls. Fluorescent dye penetration involves the introduction of a fluorescent dye to one side of the joint. The opposite side is then inspected for dye leakage, which would indicate a continuous path of cracks or voids. Destructive testing techniques include all types of mechanical testing techniques as well as metallographic sectioning for microstructural examination. Mechanical testing involves numerous techniques that have been standardized to examine tensile, shear, peel, impact, and torsion strengths, as well as the fatigue life of adhesive joints (Table 10). The various strength measurements can be extended to quantify defects in the joints, such as voids or cracks that cannot support a load, or a microstructural modification to the solder metal, which changes its strength. Inspection of the fracture surface provides critical information on the failure mechanism, as well as data on void content and nonwetting that are caused by poor solderability. Additional information is available in the article "Evaluation and Quality Control of Soldered Joints" in this Volume. General Soldering Paul T. Vianco, Sandia National Laboratories Environmental, Safety, and Health Issues Both the materials and processes used in soldering present some general, as well as unique, environmental, safety, and health considerations. There is an increased awareness of the environmental damage caused by manufacturing processes. Applicable rules and regulations can be set by local, state, and federal legislation, as well as regulations developed through the Environmental Protection Agency (EPA). The environmental concerns that relate to soldering operations include: • RELEASE OF CLEANING SOLVENTS AND SOLUTIONS INTO THE LIQUID WASTE STREAM • VENTING OF SOLVENT FUMES INTO THE ATMOSPHERE • DISPOSAL OF HEAVY-METAL SOLID WASTES, SUCH AS SOLDER DROSS AND SCRAP MATERIAL Rules and regulations that govern worker safety are established by local, state, and federal employment statutes. The federal agency responsible for worker safety is the Occupational Safety and Health Administration (OSHA). Industrial organizations, such as the National Fire Protection Agency, the Underwriters Laboratory, and many insurance companies, have developed excellent safety guidelines for industrial practices. Some safety concerns in soldering operations include: • BURNS FROM MOLTEN METAL AND ELEVATED-TEMPERATURE PROCESSING EQUIPMENT • BURNS FROM THE IGNITION OF FLAMMABLE SOLVENTS USED IN CLEANING PROCESSES OR AS FLUX VEHICLES • CHEMICAL BURNS FROM CORROSIVE ACIDS IN CLEANING SOLUTIONS AND FLUXES, THE LATTER OF WHICH INCLUDES SPATTER DURING SOME SOLDER PROCESSES • CHEMICAL BURNS FROM ALKALINE MATERIALS USED IN POSTASSEMBLY CLEANING An extensive effort has been made to identify and control the worker health risks of industrial soldering operations. Unlike safety issues, which tend to address acute dangers, health concerns can manifest themselves in accumulated toxicity to the worker which may not be identified for months or years. Besides the legislative bodies, regulations are also set by agencies such as OSHA, EPA, and the U.S. Mine Safety and Health Administration (MSHA). In addition, various guidelines are provided by organizations such as the Food and Drug Administration (FDA) and the National Institutes of Health National Cancer Institute. Some of the health issues associated with soldering operations are: • EXPOSURE TO ORGANIC SOLVENT FUMES (ACETONE, ALCOHOLS) IN FLUXES AND CLEANING COMPOUNDS OR THEIR HEAT-GENERATED BY-PRODUCTS (FOR EXAMPLE, ALDEHYDES FROM ROSIN-BASE FLUXES) • ILLNESS FROM THE INHALATION OF ACID OR ALKALINE FUMES USED IN CLEANING SOLUTIONS • HEAVY-METAL TOXICITY FROM MOLTEN SOLDER FUMES, BASE-METAL COATINGS, AND FROM THE BASE METALS THEMSELVES (AIRBORNE PARTICLES OF BERYLLIUM, CHROMIUM STEELS, AND SUCH) Exposure to toxins occurs through skin absorption, ingestion, or inhalation. The use of protective clothing and face protection, as well as the practice of good hygiene habits, will prevent the absorption and ingestion, respectively, of toxic materials. Because the most likely source of accidental exposure is inhalation, some guidelines and exposure limits are described below for materials and processes used in soldering operations. Air-sampling procedures have been established and should be used to certify compliance with these specifications. The exposure limits provide some insight into the relative hazard presented by each of the listed materials. It is important to note that regulations and exposure limits can change frequently as data are accumulated by the appropriate agencies. Therefore, it is recommended that the reader consult the necessary documents or agency representative to ensure compliance with the most recent guidelines. The exposure limits are expressed as the time-weighted average (TWA), which is equivalent to the threshold limit value (TLV) established by the American Conference of Governmental Industrial Hygienists (ACGIH). The TWA limits were also accepted by OSHA in 1972 as the legal permissible exposure limits (PEL). The TWA value represents the time- averaged airborne concentration of the material over an 8-h period, which is assumed to be the exposure period per day. Fluxes and Cleaning Agents. The primary concern when using rosin-base fluxes are the thermal breakdown products generated upon their exposure to the molten solder temperature. These reaction products are primarily the aliphatic aldehydes, typically measured by the equivalent formaldehyde concentration (Ref 30). The adopted TWAs for these substances, together with values for organic solvents used as flux vehicles or cleaning agents, are listed in Table 38. No limits are available for the typical constituents of organic acid fluxes (lactic acid, benzoic acid, or glutamic acid) or any suspected thermal breakdown products. The exposure levels of fumes from the inorganic acids, whether used as the flux or precleaning material, and from sodium hydroxide (neutralizing agent) are listed in Table 39. New regulations and exposure limits change as new data become available. The reader should remain aware of updates to these guidelines for current practices. TABLE 38 TWA LIMITS FOR ORGANIC SUBSTANCES USED IN SOLDER PROCESSING MATERIAL TWA STANDARD, MG/M 3 COMMENTS FORMALDEHYDE (AS A ROSIN PYROLYSIS PRODUCT) 0.1 REPRESENTATIVE OF ROSIN FLUX BY-PRODUCTS FROM HEAT EXPOSURE LSOPROPYL ALCOHOL 980 SKIN EXPOSURE (A) ETHYL ALCOHOL 1900 METHYL ALCOHOL 260 SKIN EXPOSURE METHYL CHLORIDE 105 LEVEL OF INTENDED CHANGE AS OF 1981 METHYLENE CHLORIDE (DICHLOROMETHANE) 360 LEVEL OF INTENDED CHANGE AS OF 1981 CARBON TETRACHLORIDE 30 BENZENE 30 SKIN EXPOSURE; SUSPECTED CARCINOGEN. LEVEL OF INTENDED CHANGE AS OF 1981 ACETONE 1780 LEVEL OF INTENDED CHANGE AS OF 1981 ETHYLENE GLYCOL 10 (B) PARTICULATE Source: Ref 30 (A) INDICATES THAT A CUTANEOUS ROUTE (MUCOUS MEMBRANES, EYES, ET C.) CAN ALSO CONTRIBUTE TO OVERALL EXPOSURE. (B) VAPOR, 125 MG/M 3 . TABLE 39 TWA LIMITS FOR INORGANIC ACIDS AND ALKALINES MATERIAL TWA STANDARD, MG/MM 3 HYDROCHLORIC ACID (HCL) 7 SULFURIC ACID (H 2 SO 4 ) 1 NITRIC ACID (HNO 3 ) 5 HYDROFLUORIC ACID (HF) 2.5 PHOSPHORIC ACID (H 3 PO 4 ) 1 SODIUM HYDROXIDE (NAOH) 2 ZINC CHLORIDE (ZNCL 2 ) 1 AMMONIUM CHLORIDE (NH 4 CL) 10 Source: Ref 30 Solder and Base-Metal Constituents. The acute and chronic side effects of human exposure to metals are becoming better understood by the medical community. Toxicity hazards associated with the so-called "heavy" metals (lead, cadmium, antimony, tin, and others) used in the soldering processes are of particular concern to the health and regulatory agencies. Two mechanisms are responsible for airborne particulates: evaporation of metals by heat energy, which is applied to the molten solder or substrate, and mechanical agitation, which releases dust or abrasive particles. Molten solder is generally used at temperatures well below the boiling point of either the solder or the substrate material. Exceptions are workpieces constructed with high vapor pressure metals or coatings, such as cadmium or zinc, or solders that contain these elements. Molten solders, like other liquids, release greater amounts of metal vapors as their temperature increases toward their vaporization point. Manual soldering processes, which exhibit the least degree of temperature control, are most susceptible to overheating of the solder and are therefore most likely to increase levels of toxic fumes in the atmosphere. Unfortunately, such processes also require the operator to be in close proximity to the workpiece. Therefore, proper ventilation must be maintained in the work area to minimize worker exposure. Mechanical agitation of the molten solder for prolonged processing times, during which the solder is liquid, can accelerate the release of metal fumes. Airborne metal fumes and particulates are also caused by the agitation of solder dross and through abrasion of the solid metals. The dross that forms on the surface of solder baths can be a source of airborne particulates of tin, lead, or antimony, for example, when it is removed for disposal. Base-metal particles are generated by the use of abrasive precleaning techniques, such as grit or sand-blasting techniques. Base-metal particles and solder particles can also be generated when similar techniques are used to remove residues from the finished joints. Table 40 gives the TWA values of metals that are used in solders or that compose substrate materials. Particulates of lead, cadmium, indium, platinum, silver, and chromium VI compounds are particularly hazardous, whereas those of zinc, copper, tin, and aluminum are relatively benign. Data are not available for bismuth and gold, a commonly used finish layer. TABLE 40 TWA LIMITS FOR METAL PARTICULATES MATERIAL TWA STANDARD, MG/M 3 COMMENTS LEAD 0.05 FUMES AND DUST; ACTION LEVEL, 0.03 TIN 10 TIN OXIDE AS TIN, NUISANCE; FOR PARTICULATE, AUTOMATIC LEVEL SET AT 10 MG/M 3(A) ANTIMONY 0.5 INCLUDES ALL COMPOUNDS SILVER 0.1 METAL DUST AND FUMES (B) INDIUM 0.1 INDIUM AND COMPOUNDS BISMUTH NONE AVAILABLE . . . ZINC 5 ZINC OXIDE FUMES CADMIUM 0.05 SUSPECTED CARCINOGEN GOLD NONE AVAILABLE . . . PLATINUM 0.002 . . . COPPER 1 DUST AND MIST ALUMINUM 10 OXIDE, NUISANCE PARTICULATE NICKEL 1 (C) METAL IRON 5 IRON OXIDE FUMES [...]... ELECTRONIC ASSEMBLIES SOLDER ALLOY, WT% SOLIDUS/LIQUIDUS TEMPERATURES °C °F 521N-48SN 118/118 244/244 50IN-50SN 118/125 244/257 58BI-42SN 43SN-43PB-14BI 97IN-3AG 70SN-18PB-12IN 63SN-37PB 60SN-40PB 62.5SN-36.1PB-1.4AG 50PB-501N 60PB-40IN 96.5SN-3.5AG 95SN-5PB 97.5PB-1.5AG-1.0SN 90PB-10SN 95PB-5SN 138/138 143 /163 143/143 162 /162 183/183 183/190 170/179 180/209 195/225 221/221 233/240 304/309 268/299 270/312... Philadelphia, PA, 1911 1-5 094.) TABLE 5 GUIDELINES AND SPECIFICATIONS FOR SOLDER JOINT DESIGN IPC-D-300G, PRINTED BOARD DIMENSIONS AND TOLERANCES IPC-SM-782, SURFACE MOUNT LAND PATTERNS (CONFIGURATIONS AND DESIGN RULES) IPC-S-815A, GENERAL REQUIREMENTS FOR SOLDERING ELECTRONIC INTERCONNECTIONS IPC-SM-780, COMPONENT PACKAGING AND INTERCONNECTING WITH EMPHASIS ON SURFACE MOUNTING IPC-D-275, DESIGN STANDARD FOR RIGID... through-hole and surface-mount device packages These alloys include Kovar, 29Ni-17Co-0.2Mn-balance Fe; alloy 52, 0.5Mn-0.25Si-50.5ONi-balance Fe; and alloy 42, 0.5Mn-O.25Si-5.5Cr-42Ni-balance Fe Solder wetting of the iron-base alloys requires the removal of a thick, tenacious surface oxide by such procedures as the use of etchants and chemical brighteners or aggressive electropolishing treatments (Ref 16) ... RIGID PRINTED BOARDS AND RIGID PRINTED BOARD ASSEMBLIES IPC-D-322, GUIDELINES FOR SELECTING PRINTED WIRING BOARD SIZES USING STANDARD PANELS IPC-MC-324, PERFORMANCE SPECIFICATION FOR METAL CORE BOARDS IPC-D-330, DESIGN GUIDE IPC-PD-325, ELECTRONIC PACKAGING HANDBOOK IPC-CM-770, PRINTED BOARD COMPONENT MOUNTING IPC-D-279, DESIGN GUIDELINES FOR RELIABLE SURFACE MOUNT TECHNOLOGY IPC-SM-785, GUIDELINES FOR... PRINTING (-8 0 MESH SCREEN) 4 5-7 5 180 0-3 000 STENCIL PRINTING 4 5-7 5 180 0-3 000 BULK 3 0-7 5 120 0-1 800 DISPENSING TECHNIQUE (A) METAL CONTENT, % 85 8 5-9 0 8 0-8 5 VISCOSITY, PA · S(A) 25 0-5 50 40 0-8 00 10 0-4 00 1 PA · S = 1 KCPS Fluxes The role of fluxes in electronic soldering is to reduce thin tarnish layers on the substrates and solder, lower the surface tension of the solder to improve capillary flow and optimize... tin-lead solder material procurement (per federal specification QQ-S-571-E) and for tin-lead solder baths (per IPC-S-815A) that are for hot-dip coatings or PWB assembly Amendment 6 to the QQ-571-E specification indicates a reduction in the maximum antimony content of powdered material to 0.120% in the upcoming QQ-571-F edition The complete list of impurities is found in Amendment 6 of the QQ-571-E... TESTING OF SURFACE MOUNT SOLDER ATTACHMENTS IPC-S- 816, SMT PROCESS GUIDELINE AND CHECKLIST J-STD-001, REQUIREMENTS FOR SOLDERED ELECTRICAL AND ELECTRONIC ASSEMBLIES J-STD-002, SOLDERABILITY TESTS FOR COMPONENT LEADS, TERMINATIONS, LUGS, TERMINALS AND WIRES J-STD-003, SOLDERABILITY TESTS FOR PRINTED BOARDS MIL-C-55302, CONNECTORS, PRINTED CIRCUIT SUBASSEMBLY, AND ACCESSORIES An assessment of solder joint... and federal specifications that quantify flux activity and PWB cleaning procedures and criteria for electronic applications have been established (Table 4) TABLE 4 FLUX ACTIVITY TESTS AND WIRING BOARD CLEANLINESS GUIDELINES SPECIFICATION IPC-TM-650, TEST METHODS MANUAL IPC-S-815A, GENERAL REQUIREMENTS FOR SOLDERING ELECTRONIC INTERCONNECTIONS MIL-F-14256 E, FLUX, SOLDERING, LIQUID (ROSIN-BASE) MIL-STD-2000... EPOXY-ARAMID FIBER 125 POLYIMIDE-ARAMID 250 FIBER POLYIMIDE QUARTZ 250 FIBERGLASS-TEFLON 75 CERAMICS ALUMINA, 86% BERYLLIA, 99.5% SILICON METALS COPPER PLANAR TENSILE MODULUS 106 PSI GPA PLANAR COEFFICIENT OF THERMAL EXPANSION(A), 1 0-6 /K THERMAL CONDUCTIVITY W/M · K 1 0-4 BTU/FT · S · °F 257 482 257 482 1 3-1 8 1 2-1 6 6-8 3-7 0 .16 0.35 0.12 0.15 0.26 0.56 0.19 0.24 17 19 30 28 2.5 2.8 4.4 4.0 482 167 6-8 ... input required to melt the solder may damage some PWB substrates or heat-sensitive devices lndium-Tin Alloys The eutectic and near-eutectic indium-tin solders (52In-48Sn and 50In-50Sn, respectively) are low- melting-point alloys used in the solder attachment of heat-sensitive devices or in the soldering steps that follow the use of tin-lead alloys Because most electronic fluxes are unable to activate at . 58BI-42SN 138/138 281/281 43SN-43PB-14BI 143 /163 289/325 97IN-3AG 143/143 289/289 70SN-18PB-12IN 162 /162 324/324 63SN-37PB 183/183 361/361 60SN-40PB 183/190 361/374 62.5SN-36.1PB-1.4AG. MASS SOLDERING 2 14 HAND IRON SOLDERING 3 14 HAND IRON SOLDERING 4 14 HAND IRON SOLDERING 5 13 OCCASIONAL HAND SOLDERING 6 11 LAYOUT OPERATOR 7 17 MASS SOLDERING 37 19 MASS SOLDERING. PWB substrates or heat-sensitive devices. lndium-Tin Alloys. The eutectic and near-eutectic indium-tin solders (52In-48Sn and 50In-50Sn, respectively) are low- melting-point alloys used in the