Cleaning in electronics manufacturing

Increasing demand for reliability, continuing miniaturisation and the growing number of faults in electronic components manufactured in no-clean processes have put the focus back on cleaning in electronics manufacturing. This article looks at different solutions that help ensure optimal cleaning of components.

The development of no-clean fluxes and soldering pastes had turned the attention away from the need to clean components in electronics manufacturing. For many components that are only used in non-critical atmospheric environments, this mostly poses no problem. However, where they are used in adverse environments (such as humid or fluctuating temperatures), the protective layer applied in the no-clean process can be gradually eroded, releasing ionising substances that promote electromigration and dendritic growth. This occurs chiefly in narrow spaces beneath components and between their connections or other contact surfaces.

Surface cleanliness

Protective coatings (conformal coatings), progressive miniaturisation, wire bonding and the increased use of high-voltage components all call for a high level of surface cleanliness. A further aspect is the use of lead-free solders, containing a higher proportion of fluxes and more aggressive activators that can cause problems. Cleaning of electronic components also involves removing potentially hazardous impurities such as fluxes, residues of soldering agents and adhesives, and such contaminants as dust and residue from previous manufacturing stages.

Choosing the right cleansing agent

A key factor in achieving economy and efficiency in the cleaning process is the selection of a suitable cleansing agent. Selection criteria include the nature and quantity of the impurities to be removed, and the subject material. Cleaning agents currently used in electronics manufacturing include solvents, water-based media containing alkaline surfactants and water-based, tenside-free cleaning agents.

The electronics industry mainly uses solvents containing non-halogenated hydrocarbons, modified alcohols or hydrofluorethers (HFEs). HFEs were developed as an alternative to the previously preferred chlorofluorocarbons (CFCs), after CFC manufacturing was stopped about 20 years ago due to its high potential for breaking down the ozone layer. Non-inflammable HFEs have similar properties to CFCs, but pose no danger to ozone, do not persist in the atmosphere and have low greenhouse potential. At the same time, they offer physical properties that are in demand for cleaning of electronics, such as relatively high density, low viscosity and low surface tension. These solvents are used in mono-solvent, co-solvent and bi-solvent systems.

A mono-solvent system uses a pure HFE or an azeotrope - a mixture of two or more components that vaporises without changing its chemical composition. It is used to remove slight impurities such as light oils, halogen compounds, residue of easy-clean solvent, particles and dust.

The co-solvent system consists of an HFE combined with a low-volatility organic solvent as solubility promoter. The solubility promoter removes impurities from the surface of the work piece, and the HFE rinses away the solvent and the impurities from the components. Cleaning by co-solvent procedure is extremely versatile and also gives good results with the most stubborn impurities such as heavy oils, grease, waxes, NC-flux residues, adhesives and hot-melt glues. Choice of a low-volatility organic solvent allows material compatibility to be tested. Co-solvent and bi-solvent systems differ mainly in that, for the co-solvent system, the solvent and the rinsing agent are mixed together, while in the bi-solvent process they are kept separate.

Optimising a process by adapting the plant technology

To ensure efficient and reproducible cleaning, it is essential to match the cleaning agent to the plant technology. That is why so many different cleaning systems, such as dipping plants with ultrasound or pressure-flooding and spray-cleaning plants, are available. Solvents are used today in totally enclosed cleaning units. Ultrasound cleaning with solvents or water-based media offers a wide range of applications in electronics manufacturing.

Another factor that influences the cleaning action, in addition to the cleansing agent, is the frequency of the electrical signals from the ultrasound generator, which converts the oscillating system to soundwaves in the fluid bath. In general, the lower the electrical signal frequency, the greater the energy released by the soundwaves. There are multifrequency systems that permit acoustic irradiation of the goods to be cleaned at different frequencies; the optimum combination of cleansing agent and ultrasound frequency can be determined from cleaning tests, carried out by the suppliers of the equipment or the cleaning agent.

In selecting the type of cleaning equipment to use, key questions are: What throughput must be handled? What space is available in which to set up the equipment? How can the cleaning process be integrated into the manufacturing chain?

Dry alternatives with carbon dioxide

Cleaning with compressed carbon dioxide provides an extension to the wet-chemical process. The term “compressed carbon dioxide” indicates CO₂ that has been converted to liquid form (that is, into its supercritical phase) under pressure. In this form, it possesses excellent properties as a solvent upon a range of nonpolar impurities such as grease and oils. Supercritical CO₂ has low viscosity and low interfacial tension, resulting in improved ability to penetrate crevices. This enables cleaning of components with highly complex geometries such as tiny drilled holes and the narrowest of cracks. In electronics manufacturing, this technology offers the ability to clean such items as complete PCBs and assemblies, removing flux residues and cleaning away oils and grease from metallic components such as contacts. It meets the requirement for an environmentally friendly, dry and residue-free procedure.

Liquid carbon dioxide is also used as the medium in CO₂ snow-jet cleaning - in this case, in the form of minute snow crystals. With its combination of chemical, thermal and mechanical properties, the non-poisonous and non-inflammable CO₂ snow removes surface films and particulate contamination leaving no residue, and can also be used selectively on functional areas such as contact points. Since the cleaning is itself a dry process, there is no need for an energy-intensive subsequent drying procedure. The procedure is employed in the most diverse of applications in electronics manufacturing, such as in preparation for bonding processes, equipping PCBs and foil-PCBs, and in the manufacture of metal-insulator semiconductors (MISs), to allow custom-tailored and reliable manual or fully automated cleaning.

The 4th state of matter

Plasma, a gaseous mixture of atoms, molecules, ions and free electrons, allows efficient surface treatment of electronic parts and components of different materials, simultaneously cleaning away organic impurities such as oils and grease and activating the surface. This double function is based on a physical and chemical reaction during the procedure. Depending on the application in question, low-pressure plasmas or inline-capable atmospheric pressure plasmas can be used. With low-pressure plasmas, both oxidising and reducing processes can be carried out. An oxidising plasma can clean away organic contaminants such as grease, oil and adhesive residue prior to soldering or bonding. A reducing plasma process can be used, for example, to improve bonded connections by reduction of electroplated metallic layers. Surface cleaning and activation by atmospheric pressure plasmas is used in the electronics industry in such tasks as pre-printing, bonding or casting of electronics boards and semiconductors, manufacture of optoelectronic components and prior to wire-bonding.