Printed Wiring Boards

Printed wiring boards (PWBs) are the interconnective electrical framework and physical structure that hold together the various electronic components of a printed circuit board. The major categories of PWBs are single-sided, double-sided, multilayer and flexible. The complexity and spacing requirements of ever increasingly dense and smaller boards have required that both sides of the board be covered with underlying circuits. Single-sided boards met early calculator and simple consumer electronic devices requirements, but portable notebook computers, personal digital assistants and personal music systems have required double-sided and multilayer PWBs. The processing of the patterning of PWBs is essentially a photolithographic process that involves selectively depositing and removing layers of materials on a dielectric substrate that acts as the electrical “wiring” that is etched or deposited on the printed wiring board.

Multilayer boards contain two or more pieces of dielectric material with circuitry that are stacked up and bonded together. Electrical connections are established from one side to the other, and to the inner layer circuitry, by drilled holes which are subsequently plated through with copper. The dielectric substrate most commonly used is fibreglass sheets (epoxy/fibreglass laminate). Other materials are glass (with polyimide, Teflon or triazine resins) and paper covered with phenolic resin. In the United States, laminated boards are categorized based on their fire-extinguishing properties; drilling, punching and machining properties; properties of moisture absorption; chemical and heat resistance; and mechanical strength (Sober 1995). The FR-4 (epoxy resin and glass cloth substrate) is widely used for high-technology applications.

The actual PWB process involves numerous steps and a wide variety of chemical agents. Table 1 illustrates a typical multilayer process and the EHS issues associated with this process. The primary differences between a single-sided and double-sided board is that the single-sided starts with raw material clad only on one side with copper, and omits the electroless copper plating step. The standard double-sided board has a solder mask over bare copper and is plated through the holes; the board has gold-coated contacts and a component legend. The majority of PWBs are multilayer boards, which are double-sided with internal layers that have been fabricated and sandwiched inside the laminate package and then processed almost identically to a double-layer board.

Table 1. PWB process: Environmental, health and safety issues

Cl₂ = chlorine gas; CO = carboon monoxide; CuSO₄ = copper sulphate; H₂O₂ = hydrogen peroxide;H₂SO₄ = sulphuric acid; H₃PO₄ = phosphoric acid; HBR = hydrobromic acid; HCl = hydrochloric acid; HNO₃ = nitric acid; K₂CO₃ = potassium carbonate; KMNO₄ = potassium permanganate; NA₃PO₄ = sodium phosphate; NH₄Cl = ammonium chloride; NH₄OH = ammonium hydroxide; NiSO₄ = nickel sulphate; Pb = lead; Sn = tin; SnCl₂ = stannous chloride; UV = ultraviolet; VOCs = volatile organic compounds.

Printed Circuit Board Assembly

Printed circuit board (PCB) assembly involves the hard attachment of electronic components to the PWB through the use of lead/tin solder (in a wave solder machine or applied as a paste and then reflowed in a low-temperature furnace) or epoxy resins (cured in a low-temperature furnace). The underlying PWB (single-sided, double-sided, multilayer or flexible) will determine the densities of components that can be attached. Numerous process and reliability issues form the basis for the selection of the PCB assembly processes that will be utilized. The major technological processes are: total surface mounting technology (SMT), mixed technology (includes both SMT and plated through hole (PTH)) and underside attachment.

Typically in modern electronics/computer assembly facilities, the mixed technology is utilized, with some components being surface mounted and other connectors/components being soldered on using through-hole technology or solder reflowing. A “typical” mixed technology process is discussed below, wherein a surface mount process involving adhesive attach, wave soldering and reflow soldering is utilized. With mixed technology, it is sometimes possible to reflow surface mount components (SMCs) on the top side of a double-sided board and wave solder the SMCs on the underside. Such a process is particularly useful when the surface mount and through-hole technologies must be mixed on a single board, which is the norm in current electronics manufacturing. The first step is to mount the SMCs to the top side of the board, using the solder reflow process. Next, the through-hole components are inserted. The board is then inverted, and the underside SMCs are mounted adhesively to the board. Wave soldering of both through-hole components and underside SMCs is the final step.

The major technical mixed technology process steps include:

• pre- and post-cleaning

• solder paste and adhesive application (screen print and placement (SMT and PTH))

• component insertion

• adhesive cure and solder reflow

• fluxing (PTH)

• wave soldering (PTH)

• inspection and touch-up

• testing

• reworking and repairing

• support operations—stencil cleaning.

A brief discussion of the important environmental, health and safety implications for each process step is provided below.

Pre- and post-cleaning

Commercial PWBs are typically purchased from a PWB supplier and have been pre-cleaned with de-ionized (DI) water solution to remove all surface contaminants. Prior to the concerns regarding stratospheric ozone layer depletion, an ozone depleting substance, such as a chlorofluorocarbon (CFC), would be used as a final clean, or even pre-clean by the electronic device manufacturer. At the end of the PCB assembly process, the use of a chlorofluorocarbon “vapour degreasing” operation to remove residues from the flux/wave soldering operation was typical. Again due to concerns about ozone depletion and tight regulatory controls on the production of CFCs, process changes were made that allowed the complete PWB assemblies to by-pass cleaning or use only a DI water cleaning.

Solder paste and adhesive application (stencil print and placement) and component insertion

The application of lead/tin solder paste to the PWB surface allows the surface mount component to be attached to the PWB and is key to the SMT process. The solder material acts as a mechanical linkage for electrical and thermal conduction and as a coating for surface protection and enhanced solderability. The solder paste is made up of approximately 70 to 90% non-volatile matter (on a weight per weight or weight per volume basis):

• lead/tin solder

• a blend of modified resins (rosin acids or mildly activated rosin)

• activators (in the case of “no clean” products, mixtures of amine hydrohalides and acids or just carboxylic acids).

Solvents (volatile matter) make-up the remainder of the product (typically an alcohol and glycol ether mixture that is a proprietary blend).

The solder paste is printed through a stencil, which is an exact pattern of the surface design that is to be added to the PWB surface. The solder paste is pushed through the apertures in the stencil onto the pad sites on the PWB by means of a squeegee that slowly traverses the stencil. The stencil is then lifted away, leaving the paste deposits on the appropriate pads on the board. The components are then inserted on the PWB. The primary EHS hazards relate to the housekeeping and personal hygiene of the operators that apply the solder paste to the stencil surface, clean the squeegee and clean the stencils. The concentration of lead in the solder and the tendency of the dried solder paste to adhere to the skin and equipment/facility work surfaces requires the use of protective gloves, good clean-up of work surfaces, safe disposal of contaminated clean-up materials (and environmental handling) and strict personal hygiene by the operators (e.g., handwashing with soap prior to eating, drinking or applying cosmetics). Airborne exposure levels are typically below the detection limit for lead, and if good housekeeping/personal hygiene is used, blood lead readings are at background levels.

The adhesive application involves the automated dispensing of small quantities of an epoxy resin (typically a bisphenol A-epichlorhydrin mixture) onto the PWB surface and then “picking and placing” the component and inserting it through the epoxy resin onto the PWB. The EHS hazards primarily relate to the mechanical safety hazards of the “pick and place” units, due to their automated mechanical assemblies, component shuttles on the rear of the units and potential for serious injury if appropriate guarding, light curtains and hardware interlocks are not present.

Adhesive cure and solder reflow

The components that were attached by stencil printing or adhesive application are then carried on a fixed-height mechanical conveyor to an in-line reflow furnace that “sets off” the solder by reflowing the solder paste at approximately 200 to 400°C. The components that were attached by the epoxy adhesive are also run through a furnace that is downline of the solder reflow and is typically run at 130 to 160^oC. The solvent components of the solder paste and epoxy resin are driven off during the furnace process, but the lead/tin component is not volatilized. A spider-web type residue will build up in the exhaust duct of the reflow furnace, and a metal mesh filter can be used to prevent this. PWBs can occasionally get caught in the conveyor system and will overheat in the furnace, causing objectionable odours.

Fluxing

To form a reliable solder joint at the PWB surface and the component lead, both must be free of oxidation and must remain so even at the elevated temperatures used in soldering. Also, the molten solder alloy must wet the surfaces of the metals to be joined. This means the solder flux must react with and remove metal oxides from the surfaces to be joined and prevent the re-oxidation of the cleaned surfaces. It also requires that the residues be either non-corrosive or easily removable. Fluxes for soldering electronic equipment fall into three broad categories, commonly known as rosin-based fluxes, organic or water-soluble fluxes and solvent-removable synthetic fluxes. Newer, low-solids “no clean” or non-volatile organic compound (NVOC) fluxes fall into the middle category.

Rosin-based fluxes

The rosin-based fluxes are the most commonly used fluxes in the electronics industry, either as spray flux or foam flux. The fluxer may be contained either internal to the wave soldering equipment or as a stand-alone unit positioned at the infeed to the unit. As a base, rosin-based fluxes have natural rosin, or colophony, the translucent, amber-coloured rosin obtained after turpentine has been distilled from the oleoresin and canal resin of pine trees. The resin is collected, heated and distilled, which removes any solid particles, resulting in a purified form of the natural product. It is a homogeneous material with a single melting point.

Colophony is a mixture of approximately 90% resin acid, which is mostly abietic acid (a non-water soluble, organic acid) with 10% neutral materials such as stilbene derivatives and various hydrocarbons. Figure 1 provides the chemical structures for abietic and pimaric acids.

Figure 1. Abietic & pimaric acids

The active constituent is abietic acid, which at soldering temperature is chemically active and attacks the copper oxide on the PWB surface, forming copper abiet. Rosin-based fluxes have three components: the solvent or vehicle, the rosin and the activator. The solvent simply acts as a vehicle for the flux. To be effective the rosin must be applied to the board in a liquid state. This is accomplished by dissolving the rosin and activator in a solvent system, typically isopropyl alcohol (IPA) or multicomponent mixtures of alcohols (IPA, methanol or ethanol). Then the flux is either foamed onto the bottom surface of the PCB through the addition of air or nitrogen, or sprayed in a “low-solids” mixture which has a higher solvent content. These solvent components have different evaporation rates, and a thinner must be added to the flux mixture to maintain a constituent flux composition. The primary categories of rosin-based fluxes are: rosin mildly active (RMA), which are the typical fluxes in use, to which a mild activator is added; and rosin active (RA), to which a more aggressive activator has been added.

The primary EHS hazard of all the rosin-based fluxes is the alcohol solvent base. Safety hazards relate to flammability in storage and use, classification and handling as a hazardous waste, air emissions and treatment systems required to remove the VOCs and industrial hygiene issues related to inhalation and skin (dermal) exposure. Each of these items requires a different control strategy, employee education and training and permits/regulatory compliance (Association of the Electronics, Telecommunications and Business Equipment Industries 1991).

During the wave soldering process, the flux is heated to 183 to 399°C; airborne products generated include aliphatic aldehydes, such as formaldehyde. Many fluxes also contain an organic amine hydrochloride activator, which helps clean the area being soldered and releases hydrochloric acid when heated. Other gaseous components include benzene, toluene, styrene, phenol, chlorophenol and isopropyl alcohol. In addition to the gaseous components of heated flux, a significant amount of particulates are created, ranging in size from 0.01 micron to 1.0 micron, known as colophony fumes. These particulate materials have been found to be respiratory irritants and also respiratory sensitizers in sensitive individuals (Hausen, Krohn and Budianto 1990). In the United Kingdom, airborne exposure standards require that colophony fume levels be controlled to the lowest levels attainable (Health and Safety Commission 1992). Additionally, the American Conference of Governmental Industrial Hygienists (ACGIH) has established a separate threshold limit value for the pyrolysis products of rosin core solder of 0.1 mg/m³, measured as formaldehyde (ACGIH 1994). The Lead Industries Association, Inc. identifies acetone, methyl alcohol, aliphatic aldehydes (measured as formaldehyde), carbon dioxide, carbon monoxide, methane, ethane, abietic acid and related diterpene acids as typical decomposition products of rosin core soldering (Lead Industries Association 1990).

Organic fluxes

Organic fluxes, sometimes called intermediate fluxes or water-soluble fluxes, are composites that are more active than the rosin-based fluxes and less corrosive than acid fluxes used in the metal-working industries. The general active compounds of this class of fluxes fall into three groups:

• acids (e.g., stearic, glutamic, lactic, citric)

• halogens (e.g., hydrochlorides, bromides, hydrazine)

• amides and amines (e.g., urea, triethanolamine).

These materials and other parts of the formulation, such as surfactants to assist in reducing the solder surface tension, are dissolved in polyethylene glycol, organic solvents, water or usually a mixture of several of these. Organic fluxes must be considered corrosive, but can be cleaned off easily, with no more than hot water.

Synthetic activated (AS) fluxes

Whereas rosin-based fluxes are solid materials dissolved in a solvent, AS fluxes are usually totally liquid formulas (solvent + flux). The solvent carrier is driven off during the preheating phase of wave soldering, leaving a wet and oily residue on the PWB surface, which must be cleaned off immediately following soldering. The primary attribute of AS fluxes is their ability to be removed by the use of a suitable solvent, typically fluorocarbon based. With restrictions on the use of ozone-depleting substances such as fluorocarbons (Freon TF, Freon TMS and so on), the required use of these cleaning materials has severely restricted the use of this class of fluxes.

Low-solids “no clean” or non-VOC fluxes

The need for the elimination of the post-soldering cleaning of corrosive or tacky flux residues with fluorocarbon solvents has lead to the widespread usage of a new class of fluxes. These fluxes are similar in activity to the RMA fluxes and have a solids content of approximately 15%. The solids content is a measure of viscosity and equals the ratio of flux to solvent. The lower the solids contents, the higher the percentage of solvent. The higher the solids content, the more active the flux, and the more potential for needing a post-soldering cleaning step. Low-solids flux (LSF) is commonly used in the electronics industry and typically does not require the post-cleaning step. From an environmental air-emission perspective, the LSF eliminated the need for fluorocarbon vapour degreasing of wave soldered boards, but with their higher solvent content, they increased the quantity of alcohol-based solvents evaporated, resulting in higher VOC levels. VOC air-emission levels are tightly controlled in the United States, and in many locations worldwide. This situation was addressed by the introduction of “no clean” fluxes, which are water based (rather than solvent based) but contain similar activators and fluxing rosins. The primary active ingredients are dicarboxylic acid based (2 to 3%), typically glutaric, succinic and adipic acids. Surfactants and corrosion inhibitors (approximately 1%) are also included, resulting in a pH (acidity) of 3.0 to 3.5. These fluxes virtually eliminate VOC air emissions and other EHS hazards associated with using solvent-based fluxes. The decomposition products noted in rosin-based fluxes are still applicable, and the mild pH does require that the flux-handling equipment be acid resistant. Some anecdotal evidence points to potential dermal or respiratory problems from the dried, mildly acidic dicarboxylic acids and corrosion inhibitors that may become a residue on board carriers, carts and internal surfaces of wave soldering equipment utilizing these compounds. Also, the water component of these fluxes may not get adequately evaporated prior to hitting the molten solder pot, which can lead to splattering of the hot solder.

Wave soldering

The addition of flux to the bottom surface of the PWB can be accomplished either by a fluxer located internal to the wave soldering unit or a stand-alone unit at the entry to the wave soldering unit. Figure 2 provides a schematic representation of a standard wave soldering unit with the fluxer located internally. Either configuration is used to foam or spray the flux onto the PWB.

Figure 2. Wave solder unit schematic

Preheating

The flux carriers must be evaporated prior to soldering. This is accomplished by using high-temperature preheaters to drive off the liquid components. Two basic types of preheaters are in use: radiant (hot rod) and volumetric (hot air). The radiant heaters are common in the United States and present the potential for ignition of excess flux or solvent or the decomposition of a PWB should it become immobilized under the preheater. Local exhaust ventilation is provided on the fluxer/preheater side of the wave soldering unit to capture and exhaust the solvent/flux materials evaporated during these operations.

Soldering

The solder alloy (typically 63% tin to 37% lead) is contained in a large reservoir called the solder pot, and is heated electrically to maintain the solder in a molten state. The heaters include a powerful bulk heater to do the initial melt and a smaller regulated heat supply to control the temperature thermostatically.

Successful board-level soldering requires that the design of the solder pot and recirculation pump systems continually provide a consistent “wave” of fresh solder. With soldering, the pure solder becomes contaminated with oxidized lead/tin compounds, metallic impurities and flux decomposition products. This dross forms on the surface of the molten solder, and the more dross formed, the more of a tendency for additional formation. Dross is harmful to the soldering process and the solder wave. If enough forms in the pot, it can get pulled into the recirculation pump and cause impeller abrasion. Wave solder operators are required to de-dross the wave on a routine basis. This process involves the operator straining the solidified dross from the molten solder and collecting the residues for reclaim/recycling. The process of de-drossing involves the operator physically opening up the rear access door (typically a gulf-wing configuration) adjacent to the solder pot and manually scooping out the hot dross. During this process, visible emissions are liberated from the pot which are highly irritating to the eyes, nose and throat of the operator. The operator is required to wear thermal gloves, an apron, safety glasses and a face shield and respiratory protection (for lead/tin particulate, corrosive gases (HCl) and aliphatic aldehyde (formaldehyde)). Local exhaust ventilation is provided from the interior of the wave soldering unit, but the solder pot is mechanically withdrawn from the main cabinet to allow the operator direct access to both sides of the hot pot. Once withdrawn, the local exhaust duct that is mounted in the cabinet becomes ineffective for removing the liberated materials. The primary health and safety hazards are: thermal burns from hot solder, respiratory exposure to materials noted above, back injuries from handling heavy solder ingots and dross drums and exposure to lead/tin solder residues/fine particulate during maintenance activities.

During the actual soldering process, the access doors are closed and the interior of the wave soldering unit is under a negative pressure due to the local exhaust ventilation provided on the flux and solder pot sides of the wave. This ventilation and the operating temperatures of the solder pot (typically 302 to 316°C, which is just above the melting point of solder), result in the minimal formation of lead fumes. The primary exposure to lead/tin particulate comes during the de-drossing and equipment maintenance activities, from the agitation of the dross in the pot, transfer to the reclaim vessel and clean-up of solder residues. Fine lead/tin particulate is formed during the de-drossing operation and can be released into the workroom and breathing zone of the wave solder operator. Various engineering control strategies have been devised to minimize these potential lead particulate exposures, including the incorporation of local exhaust ventilation to the reclaim vessel (see figure 3), use of HEPA vacuums for residue clean-up and flexible exhaust ducts with articulating arms to position ventilation at the hot pot during de-drossing. The use of brooms or brushes for sweeping up solder residues must be prohibited. Stringent housekeeping and personal hygiene practices must also be required. During wave solder equipment maintenance operations (which are done on a weekly, monthly, quarterly and annual basis), various components of the hot pot are either cleaned within the equipment or removed and cleaned in a locally exhausted hood. These cleaning operations may involve physically scraping or mechanically cleaning (using an electric drill and wire brush attachment) the solder pump and baffles. High levels of lead particulate are generated during the mechanical cleaning process, and the process should be performed in a locally exhausted enclosure.

Figure 3. Dross cart with vacuum cover

Inspection, touch-up and testing

Visual inspection and touch-up functions are conducted after wave soldering and involve the use of magnifying lenses/task lights for fine inspection and touch-up of imperfections. The touch-up function may involve the use of a stick-solder hand-held soldering iron and rosin core solder or brushing on a small amount of liquid flux and lead/tin wire solder. The visual fumes from the stick soldering involve breakdown products from the flux. Small quantities of lead/tin solder bead that did not adhere to the solder joint may present a housekeeping and personal hygiene issue. Either a fan adjacent to the workstation for general dilution ventilation away from the operator’s breathing zone or a more sophisticated fume exhaust system that captures the breakdown products at the tip of the soldering iron or adjacent to the operation should be provided. The fumes are then routed to an air scrubber exhaust system that incorporates HEPA filtration for particulates and activated carbon gas adsorption for the aliphatic aldehydes and hydrochloric acid gases. The effectiveness of these soldering exhaust systems is highly dependent on capture velocities, proximity to the point of fume generation and lack of cross drafts at the work surface. The electrical testing of the completed PCB requires specialized test equipment and software.

Reworking and repairing

Based on the results of the board testing, defective boards are evaluated for specific component failures and replaced. This reworking of the boards may involve stick soldering. If primary components on the PCB such as the microprocessor need replacement, a rework solder pot is used for immersing that portion of the board housing the defective component or joint in a small solder pot, removing the component and then inserting a new functional component back onto the board. If the component is smaller or more easily removed, an air vac system that uses hot air for heating the solder joint and vacuum for removing the solder is employed. The rework solder pot is housed within a locally exhausted enclosure that provides sufficient exhaust velocity to capture the flux decomposition products formed when the liquid solder is brushed on the board and solder contact made. This pot also forms dross and requires de-drossing equipment and procedures (on a much smaller scale). The air vac system does not require being housed within an enclosure, but the lead/tin solder removed must be handled as a hazardous waste and reclaimed/recycled.

Support operations—stencil cleaning

The first step in the PCB assembly process involved the use of a stencil for providing the pattern of bonding locations for the lead/tin solder paste to be squeegeed through. Typically, the stencil’s openings start to become clogged and the lead/tin solder paste residues must be removed on a per shift basis. A pre-cleaning is usually performed at the screen printer to capture gross contamination on the board, by wiping the board surface with a dilute alcohol mixture and disposable wipes. To completely remove the remaining residues a wet-cleaning process is required. In a system similar to a large dishwasher, hot water (57°C) and a chemical solution of dilute aliphatic amines (monoethanol amine) is used to chemically remove the solder paste from the stencil. Significant quantities of lead/tin solder are washed off the board and either deposited in the wash chamber or in solution in the water effluent. This effluent requires filtration or chemical removal of lead and pH adjustment for the corrosive aliphatic amines (using hydrochloric acid). Newer closed system stencil cleaners utilize the same wash solution until it is spent. The solution is transferred to a distillation unit, and the volatiles are distilled off until a semi-liquid residue is formed. This residue is then handled as a lead/tin-contaminated hazardous waste.

Computer Assembly Process

Once the final PCB is assembled, it is transferred to the systems assembly operation for incorporation into the final computer product. This operation is typically very labour intensive, with the component parts to be assembled supplied to the individual workstations on staging carts along the mechanized assembly line. The major health and safety hazards relate to materials movement and staging (fork-lifts, manual lifting), ergonomic implications of the assembly process (range of motion, insertion force required to “set” components, installation of screws and connectors) and final packaging, shrink wrapping and shipping. A typical computer assembly process involves:

• chassis/case preparation

• PCB (mother and daughter board) insertion

• primary component (floppy drive, hard drive, power supply, CD-ROM drive) insertion

• display assembly (portables only)

• mouse and keyboard insertion (portables only)

• cabling, connectors and speakers

• top cover assembly

• software downloading

• test

• rework

• battery charging (portables only) and packaging

• shrink wrapping and shipping.

The only chemicals that may be used in the assembly process involve the final cleaning of the computer case or monitor. Typically, a dilute solution of isopropyl alcohol and water or a commercial mixture of cleaners (e.g., Simple Green—a dilute butyl cellosolve and water solution) is used.

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