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.