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In the electronics industry, assembly refers to incorporating electronic components that perform electrical functions into equipment or devices. More specifically, it generally refers to soldering electronic components onto printed circuit boards (PCBs). Terms such as assembly and packaging are also used in related contexts.
Cleaning plays an important role in electronics assembly processes. Electronic components used in electronic equipment can broadly be classified into through-hole components (or leaded components) and surface-mount devices (SMDs). Through-hole components are mounted by inserting their leads into holes in a PCB. SMDs are mounted directly onto the surface of a PCB without inserting leads through holes, a method known as SMT (Surface Mount Technology). Most components used in modern digital equipment are now surface-mount devices, although leaded components continue to be used in applications such as power circuits and locations carrying higher currents.
The process shown below uses an example of mixed assembly in which through-hole components are mounted after surface-mount assembly, highlighting where cleaning is required throughout the process.
For products requiring particularly high quality, purchased PCBs may be water-cleaned to remove burrs and contamination. In PCB manufacturing processes where resist ink or identification markings are screen-printed, inks adhering to the openings of screen or mesh stencils must also be cleaned.
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Stencil printing is widely used for solder paste printing. Solder paste adhering to openings in metal stencils, screen stencils, and mesh screens must be removed by cleaning. If cleaning is neglected, crushed or dried solder paste can adhere firmly to the stencil, and accumulated paste can reduce the amount transferred to the PCB, resulting in soldering defects.
As assembly density has increased, printing patterns have become finer. Cleaning that could previously be handled by manual wiping is increasingly performed with equipment such as ultrasonic or spray cleaning systems. When cleaning metal stencils and mesh screens, it is necessary to evaluate not only solder paste removability but also compatibility with stencil-frame adhesives, screen emulsions, and other stencil materials.
The squeegee is another component of the printing machine that contacts and spreads solder paste, and paste adhering to the squeegee must also be cleaned.
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Surface-mount components are placed at high speed onto the solder paste printed in the previous process using equipment known as a pick-and-place machine. Dust or debris adhering to the component pickup nozzles can interfere with component pickup, making regular nozzle cleaning essential.
Adhesive may also be screen-printed in advance to secure components, in which case adhesive adhering to the screen stencil or metal stencil must be cleaned. Cleaning is also required during manufacture of the surface-mount components themselves, including screen stencils, mesh screens, and cylinder rolls used to print thick-film materials such as silver-palladium and nickel pastes.
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Surface-mount components are placed onto solder paste and heated in a reflow oven, where the solder paste melts and bonds the components to the PCB pads. During reflow, gases generated from the solder paste can condense on cooler internal surfaces and heat-exchange components inside the oven, forming hardened flux deposits. These deposits can reduce heat-transfer performance or, when heavily accumulated, drip onto workpieces and cause defects. Regular maintenance and cleaning of reflow oven components are therefore important.
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Flux is applied using a spray fluxer. Because flux accumulates inside the spray fluxer during operation, periodic cleaning is required.
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In mixed assembly, through-hole components are wave-soldered after the surface-mount processes described in Steps 3 and 4. A heat-resistant carrier pallet, also known as a flow pallet or wave-solder pallet, is fitted to the PCB so that previously mounted surface-mount components and other areas that should not contact molten solder are masked, while only the through-hole component leads and required soldering areas remain exposed. The assembly then passes over the raised waves of molten solder in the wave soldering bath. Flux can accumulate around the edges of the pallet openings and promote solder creep into masked areas, making periodic cleaning of carrier pallets important.
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Flux residue remaining after soldering can affect subsequent circuit performance. As assembly density increases, smaller chips, narrower pitches, and fine-pitch assembly are increasingly common. Flux cleaning may also be required in applications demanding high reliability, such as aerospace and automotive electronics. In subsequent processes such as wire bonding, underfill application, conformal coating, and other resin applications, flux residue can interfere with adhesion. Cleaning may also be required to remove solder balls or to prevent contact problems with pin checkers during inspection.
As lead-free solder has become widely adopted, solder pastes based mainly on tin, silver, and copper generally require higher soldering temperatures than conventional tin-lead eutectic solder. Higher processing temperatures can bake or carbonize flux residue. Thixotropic and other viscosity-control additives can also have lower solubility in cleaning agents, while stronger or increased activator content can produce reaction residues and metal salts that inhibit cleaning. These factors make continued improvement and development of flux cleaners important as residues become more difficult to remove.
Electronics cleaning has also undergone major environmental transitions. CFCs and 1,1,1-trichloroethane, which offered good drying properties and were widely used for cleaning, were phased out because of their impact on the ozone layer. HCFC alternatives were subsequently restricted and phased out, while semi-aqueous cleaning systems have also faced challenges such as wastewater treatment and frequent bath replacement. Cleaning technologies have therefore continued to evolve.
Flux cleaning is performed after electronic components have already been mounted, so compatibility with components must also be considered. The key challenge is balancing removal of increasingly difficult flux residues with compatibility for components that may be sensitive to ultrasonics or solvents, such as crystal devices, aluminum electrolytic capacitors, and resin connectors.
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Mounted components are inspected using image-recognition inspection equipment or visual inspection. If an assembly defect is found, rework may be performed, followed by additional cleaning as required.
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This is an overview of a typical electronics assembly process. Cleaning is involved at many stages and can play an important role in maintaining assembly quality.
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