Handbook of Solvents - George Wypych - ChemTech - Ventech!

904 Martin Hanek, Norbert Löw, Andreas Mühlbauer
• Good material compatibility with respect to materials commonly found on
assemblies.
14.8.3 CLEANING OF TOOLS AND AUXILIARIES
The prohibition of CFCs in 1993 in Europe found the electronic industry scrambling to undertake
appropriate actions. Various alternative technologies have asserted themselves during
the conversion to environmentally friendly processes. Basically, two courses have
evolved as alternative to the use of CFCs for changing applications:
• New soldering techniques that avoid cleaning as a process have been developed and
introduced in the electronic industry.
• New environmentally friendly cleaning processes have been introduced.
A large number of European companies decided in favor of no-clean processes, in
spite of the associated disadvantages. In the USA cleaning has not be abandoned; it is still an
important step in the SMT production line. However, with the increase of packing density
and increased quality demands, more and more European companies are now returning to
cleaning processes. In general, the purpose of changing is for the removal of contamination
which might otherwise harm the operation of the electronic circuit assembly.
Ensuring high reliability for a circuit assembly by effective cleaning is extremely
important in such domains as the military, aviation, telecommunications, and in the automotive
industry. The need for change to high reliability can be expected to grow appreciably
because of different factors. These factors range from more complicated assemblies
with higher density to the greatly accelerating use of conformal coatings in the automotive,
niche computer, consumer electronics, and telecommunications markets. Dramatic growth
is expected to occur in the USA, Europe and Asia. The applications where cleaning is necessary
can be divided into three main applications:
• the removal of flux residues from printed circuit assemblies (PCBs) after reflow or
wave soldering;
• the removal of solder paste and/or SMT-adhesive from stencils screens or misprints,
and
• the tool cleaning (dispensing nozzles, solder frames, cooling coils from reflow or
wave soldering ovens, and squeegees).
14.8.3.1 Cleaning substrates and contamination
In recent years stencil and misprint cleaning has become a significant issue for many users.
Normally, in a typical PCB assembly line, the solder paste is printed through screens or
stencils. As an alternative to glue dispensing, specially formulated SMT adhesives can now
be printed through a stencil. Consequently, the stencils must be cleaned periodically to
maintain the quality and yield of the process. However, the removal of SMT adhesive gives
rise to completely new demands on the cleaning process and the condition of the stencils.
The following specifies the points that must be observed in connection with stencil cleaning.
Great importance has therefore been given to comparison of different cleaning chemistries,
cleaning equipment, and comparison of manual and automated cleaning.
Ever-increasing component density and the result expected from the printed image
give rise to stringent requirements regarding stencil cleanliness. A flawless printed image is
very important because misprinted boards can no longer be used without cleaning. This can
be very expensive, particularly if components have already been mounted onto one side of a
board. As a consequence, stencil printing for the application of solder paste on SMT boards