Handbook of Solvents - George Wypych - ChemTech - Ventech!

898 Martin Hanek, Norbert Löw, Andreas Mühlbauer
sure, is in most cases necessary to assure complete and residue-free removal of the contamination.
However, since pure deionized water reveals a relatively high surface tension, the
wetting ability is extremely low. 8 Consequently, being confronted with the situation of constant
increase of the packaging densities on PCBs, flux residues especially, between or under
components, are very difficult to be removed. 8 As a result this presently predominant
cleaning process will be in decline in the future.
14.8.2.1.2 Cleaning with alkaline water-based media
The principle of this kind of cleaning process is based on the presence of saponifiers in the
cleaning fluid. These saponifiers are able to react with the non-soluble organic acids, that
are the main ingredients of the flux residues. In other words, the non-soluble flux residues
are transformed into water-soluble soaps due to a chemical reaction between the saponifier
and the flux. This reaction is called saponification. For such a cleaning process an effective
rinse with deionized water is imperative to minimize the level of ionic contamination. 1,5
However, the presence of alkaline components in the aqueous cleaning fluid may
cause the phenomenon of corrosion on different metals (Al, Cu, Sn) resulting in the visible
dulling or discoloration of solder joints and pads. 9,10
The chemical nature of the saponifiers is either organic or inorganic. Organic saponifiers
are for example monoethanolamine or morpholine. 5 These soft bases transform the insoluble
organic carboxylic acids to soluble soaps acting, in addition, as a kind of buffer to
stabilize the pH of the cleaning solution. However, the above mentioned organic amines
have an associated odor, and more important, are classified as volatile organic compounds
(VOCs). Inorganic saponifiers based on the system sodium carbonate/sodium hydrogen
carbonate are also very attractive alternatives exhibiting sufficient cleaning ability at
pH-values below 11. By using these chemistries the VOC and odor issue can be avoided. 5
Besides organic or inorganic saponifiers a large scale of different anionic, cationic or
nonionic surfactants are present in water-based alkaline cleaning solutions. The main reason
for adding surfactants is the requirement for lower surface tension in order to increase
the wetting of the substrates. However, using surfactants in cleaning median in high-pressure
spray in air cleaning applications, the issue of foaming has to be taken into consideration.
14.8.2.1.3 Aqueous-based cleaning agents containing water soluble organic
components
There is a large number of products on the market that are mixtures of water and water-soluble
organic solvents. These media are recommended for the removal of solder paste, SMT
adhesives and flux residues from stencils, misprints and populated reflowed PCBs. 11
The water-soluble organic solvents are predominantly natural long chain alcohols,
glycol ether derivatives or furfuryl alcohol. The basic principle of the cleaning process using
such fluids is based on the removal of lipophilic contamination such as oil, grease, flux
or adhesive through the organic components of the mixtures. After the removal from the
surface of the substrates, the lipophilic contamination precipitates out of the water-based
cleaning agent and can be removed by using a filtration devices. In order to guarantee a sufficient
cleaning performance, these mixtures contain organic components up to 50%. Consequently,
these formulations tend to be classified as cleaning agents with high content of
volatile organic compounds (VOCs).