Cleaning Strategies for Electronics Production

By Controlled Environments

Created 2011-02-01 00:00

Doris Schulz

Obtaining requirements-oriented cleanliness in a cost-optimized way.
Particles, residual flux material, processing media, and fingerprints—tiny elements—can severely damage electronic products. Thus, requirements-oriented cleanliness is a requisite. This can be achieved by means of an adapted cleaning concept in an effective, reproducible, and environmentally friendly manner.
More and more sophisticated components, as well as increasing requirements regarding reliability and lifetime of electronic components, require solutions providing for particulate and film-like contaminations being removed in a gentle, efficient, and reproducible manner. That is because even fractional contaminations may result in costly scrap, malfunctions, or electronic systems failure. Regardless of whether wafers, printed circuit boards, contacts, or MIDs have to be cleaned, the industry offers different solutions, such as wet chemical processes, cleaning with carbon dioxide, as well as plasma procedures, all of which can be used to obtain the required cleanliness in a cost-efficient manner.
ULTRASOUND—HIGHLY VERSATILE
Wet chemical ultrasonic cleaning with solvents, modified alcohols, or aqueous media provides for a broad field of application in the field of electronics production. Thus, particles, flux material residua, and other film-like contaminations can be removed from metal electronic components, printed circuit boards, wafers, and more. Along with the cleaning medium, the frequency of the electrical signals generated by the ultrasound generator is decisive for the cleaning effect, at which the oscillating system transmits these signals as sound waves into the liquid bath. In this, the following is generally applicable: the lower the frequency of the electrical signals, the higher the energy released by the sound waves.
An example of this application is the cleaning of printed circuit boards after soldering in order to achieve good adhesion values for subsequent coating with protective varnish. In doing so, it is first and foremost important to remove flux material residua and any existing fingerprints. A typical process comprises two ultrasonic immersion cleaning steps, during which the work piece carrier is moved additionally. This is followed by two immersion rinsing procedures with deionised water and drying.
Depending on the result intended, cleaning baths with different frequencies may also be required. This can be the case when cleaning glass wafers for example. Here, the cleaning procedures required in accordance with the production step are implemented with ultrasonic frequencies between 40 kHz and one megahertz. The latter uses multiple-stage, aqueous cleaning of the polished substrates before evaporating the conductive layers. In this, the wafers placed in specifically designed cleaning racks initially pass through three ultrasonic immersion cleaning baths containing a highly alkaline to neutral cleaning agent and intermediary rinsing phases in each case, at which the same are also implemented with ultrasound. During the subsequent three-stage immersion rinsing, as well as final infrared drying, any possibility of particulate accumulation on the wafers has to be avoided. In order to ensure this, highly purified water is used in the rinsing steps; additionally the drying and unloading of the wafers is carried out in a Class 100 cleanroom.
The ideal “combination” of cleaning agent and ultrasonic frequency can be determined on the basis of cleaning tests depending on the manufacturers’ system and media.


COMPRESSED CO₂—A DRY ALTERNATIVE
This cleaning technology using compressed carbon dioxide is an addition to the wet chemical procedures. The innovative method simultaneously complies with the requirements for environmentally friendly, dry, and residue-free techniques. The term compressed carbon dioxide means the phase of CO₂ liquefied by means of pressure, in other words the supercritical phase of CO₂, in which the medium is characterized by very good solvent characteristics when compared to numerous covalent contaminates, such as greases and oils. Supercritical CO₂ is distinguished by low viscosity and low interfacial tension, which results in an improved gap penetration capability. This allows for cleaning components with extremely complex geometries, such as extremely small drill holes and narrow gaps. In the field of electronics production, this technology has promising potential in regard to cleaning complete printed circuit boards and assemblies, removing flux material residua, as well as removing oils and greases from metal components such as contacts. Depending on the phase the environmentally neutral carbon dioxide is used in, the process temperature is between 15 and 31 degrees Celsius. Thus, the procedure is also suitable for the treatment of temperature-sensitive materials. As CO₂ will sublime immediately at ambient pressure, the components are completely dry upon completion of the cleaning procedure. Thanks to the direct transition to the gaseous phase there will be no solvent residua remaining on the components and no secondary waste materials will be generated.


BATTLING DIRT WITH ICE-COLD POWER
Liquid carbon dioxide is also used as the medium in CO₂ snow jet cleaning; in the form of the finest snow crystals. Due to the interaction of chemical, thermal, and mechanical properties the non-toxic and nonflammable CO₂ snow will remove film-like and particulate contaminates without leaving any residua, even selectively on functional areas such as contact points. As the cleaning procedure is of a dry nature, energy-intensive drying processes are not applicable here either. The technology allows for reliable manual or fully automated cleaning in line with the requirements for different applications in the field of electronics production, e.g. before bonding procedures, equipping printed circuit boards, and foil printed circuit boards, as well as producing MID structures.
The procedure provides for a positive additional effect in the field of MID production using the LDS technology (laser direct structuring), in which a specific additive is added to the thermoplastic synthetic material adapted to the application. In order to activate the same, the laser beam induces a physical-chemical reaction. In this, the additive is split open within the polymer matrix and acts as a catalyst during the subsequent reductive copper-plating process. Upon laser structuring, active ablation residua remain on the surface that are also metalized and that may also cause problems. These residua can be removed by means of the CO₂ snow jet technology, through which the fine snow crystals additionally level the roughened LDS structures. This results in a simplified design and connection technology for LDS-MIDs, such as wire bonding, equipment with non-embedded chips, or in the field of flip-chip technology. Another advantage is that the cleaning module can be integrated directly into the laser structuring system.
PLASMA—CLEANING AND ACTIVATION
Plasma, a gaseous mixture of atoms, molecules, ions, and free electrons, allows for efficient surface treatment of electronic components and parts made of different materials. In this, organic contaminations such as oils and greases are cleaned and simultaneously the surface is activated. This double function is based on a physical and chemical reaction of the procedure. Depending on the application case, lowpressure plasmas or inline-compatible atmospheric pressure plasmas are used. Using the former, it is possible to implement both oxidizing and reducing processes. Within the oxidizing plasma, organic contaminations such as greases, oils, and adhesive residua can be removed before soldering or bonding. Reducing plasma processes are mainly used to optimize bond connections by means of reducing galvanically applied metal layers. In the electronics industry, processes of surface cleaning and activation by means of atmospheric pressure plasmas are used before printing, before gluing, or before pouring electronic printed circuit boards and semiconductors, in the field of optoelectronic component production, as well as before wire bonding. A joint project also deals with barrier coatings by means of inline-compatible atmospheric pressure plasmas for selective ageing and corrosion protection of electronic components.