Handbook of Solvents - George Wypych - ChemTech - Ventech!

912 Martin Hanek, Norbert Löw, Andreas Mühlbauer
Figure 14.8.11. Soldering flux residues on a component.
Figure 14.8.12. Dentride growth between connecting
contacts.
(2) Reduction of the resin content and quantity of activators simultaneously diminishes
the risk of leakage current, electro-migration and corrosion.
After soldering, cleaning removes all residues from the pc-boards, whereas the
no-clean soldering process leaves residues on the pc-boards surface, thereby exposing them
to the influence of humidity. Consequently, the presence of soldering flux residues on assemblies
(Figure 14.8.11) can cause significant functional disruptions. To guarantee quality
assurance with no-clean fluxes, the standard tests previously employed in the days of CFC
cleaning where transferred accordingly to no-clean pc-boards, including:
• Testing for impurity,
• Testing for surface resistance (SIR test),
• Testing for electro-migration,
• Testing for bondability, and
• Testing under climatic stresses.
Testing for ionic contamination and establishing the surface resistance usually produces
good results with pc-boards soldered in a no-clean process. Nevertheless, the values
are still significantly poorer than with cleaned assemblies. The migration and precipitation
of metal ions can cause the dangerous growth of dendrites (Figure 14.8.12). These metal
dendrites are outstanding electric conductors that diminish the surface resistance and, in the
long-term, result in short-circuits. Coating can significantly reduce the risk of electro-migration.
However, this demands a clean surface, otherwise homogeneous adhesion of the
coating is not assured.
14.8.4.1.2 Different cleaning media and cleaning processes
When selecting a cleaning process, the cleaning chemistry is the most sensitive parameter,
because the cleaning agent has to be adapted to the chemical ingredients of the contamination.
Only after determining the most suitable chemistry, should the method of application
be evaluated. In principle cleaning processes can be split into three separate categories:
(1) Semi-aqueous cleaning,
(2) Aqueous cleaning, and
(3) Solvent cleaning.