Obtaining requirements-oriented cleanliness in a cost-optimised way

21 JUNE 2011

Particles, residua of flux material, and processing media, fingerprints – tiny elements which 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.

by Doris Schulz

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, amongst others, from metal electronic components, printed circuit boards, as well as wafers. 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 application is for example 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 possibly existing fingerprints. A typical process comprises two ultrasonic immersion cleaning steps, during which the workpiece carrier are moved additionally. This is followed by two immersion rinsing procedures with deionised water and drying.
Depending on the result intended, cleaning baths with different frequency 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 40kHz and one megahertz. The latter is used 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 drying and unloading of the wafers are carried out in a class 100 clean room.

Polished glass wafers are cleaned in an ultrasonic cleaning system with several immersion and rinsing baths before the conductive layers are evaporated. Highly alkaline to neutral cleaning agents are used as medium. Image source: UCM

The ideal “combination” of cleaning agent and ultrasonic frequency can be determined on the basis of cleaning tests with system respectively medium manufacturers.

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 respectively the supercritical phase of CO2, in which the medium is characterised by very good solvent characteristics when compared to numerous covalent contaminations, such as greases and oils. Supercritical CO2 is distinguished by low viscosity and low interfacial tension, which results in an improved gap penetration capability.

High-frequency ultrasonic systems with 250 and 500kHz, as well as mega sound are available for gently cleaning sensitive components and fine structures in the electronic and semi-conductor industries. Image source: Weber Ultrasconics

This allows for cleaning components with extremely complex geometries, for example extremely small drill-holes and most narrow gaps. In the field of electronics production, this technology has promising potential as regards 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 medium in CO2 snow jet cleaning – but in form of finest snow crystals. Due to the interaction of chemical, thermal, and mechanical

The CO2 snow jet cleaning technology allows for removing particles after direct laser structuring in a dry and gentle manner. On the basis of the easy automation capabilities the cleaning process can be integrated into the laser system or positioned directly downstream. Image source: acp

properties the non-toxic and non-inflammable CO2 snow will remove film-like and particulate contaminations without leaving any residua, even selectively on functional areas such as contact points for example. 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 the most 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 catalyst during the subsequent reductive copper-plating process. Upon laser structuring, active ablation residua remain on the surface that are also metallised and that may also cause problems. These residua can be removed by means of the CO2 snow jet technology, at 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, low-pressure plasmas or inline-compatible atmospheric pressure plasmas are used. Using the former, it is possible to implement both oxidising and reducing processes. Within the oxidising plasma organic contaminations such as greases, oils, and adhesive residua can be removed before soldering or bonding. Reducing plasma processes are mainly used to optimise 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 pouring electronic printed circuit boards and semi-conductors, in the field of optoelectronic component production, as well as before wire bonding for example.

Along with the ideal cleaning effect, the CO2 snow jet procedure has the advantage of levelling the roughened LDS structures. This simplifies the design and connection technology. Image source: acp

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.

Info box

parts2clean 2011
Which cleaning technology can be used to obtain the required degree of cleanliness for the corresponding electronic products in a reproducible and efficient way? What is the potential of special procedures in regard to cleaning and activation? parts2clean will provide answers to these and many other questions regarding component and surface cleaning. The 8th leading international trade fair for cleaning within production and maintenance processes will take place at the exhibition centre directly adjacent to Stuttgart International Airport (Germany) from the 25th through the 27th of October, 2011. The exhibition portfolio encompasses systems, processes and process media, parts baskets, workpiece carriers, handling technology, process automation, cleanroom technology, quality assurance, test methods, analysis procedures, media treatment and disposal, job-shop cleaning, research and technical literature.

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