Handbook of Solvents - George Wypych - ChemTech - Ventech!

14.8 Electronic industry 907
• Environmental and personal safety rules.
In some cases even pure water can be used as the cleaning medium (presupposing that
the contaminants are fully water-soluble). In most cases, however, water requires the addition
of chemical or mechanical energy to obtain good cleaning performance. This can come
in the form of:
• A cosolvent,
• A chemical activator,
• Elevated temperature,
• Mechanical action, or
• High-pressure spray or ultrasonics.
Contaminants such as oils, greases and SMT adhesives that feature high organic matter
content are not readily soluble in water. Adding chemical activators such as saponifiers
to the formulation can increase their solubility. Unfortunately, saponifiers require in most
cases elevated temperatures (38-60°C) in order to react with the contamination. However,
temperatures in this range will most likely damage the stencil. Saponifiers improve cleaning
by reducing the surface tension of the cleaning solution, permitting the agent to penetrate
void spaces and stand-off gaps that pure water cannot reach. The saponifiers in
water-based cleaning agents are either organic or inorganic. Organic saponifiers are, in
most cases, fully miscible with water and can easily be rinsed from the stencil. Any residues
remaining after rinsing evaporate during drying. Inorganic saponifiers are usually water-soluble
materials applied as a solution in water. They are nonvolatile, and residues remaining
after rinsing might leave contamination on the substrate. The alkaline saponifier is
consumed during the cleaning process and requires constant replenishment. In addition, the
high pH value and the elevated application temperatures can also cause an oxidative attack
of the aluminum frame of the stencil.
Elevated temperatures are also commonly used to increase cleaning performance. An
increase in temperature will lead to a corresponding increase in the solubilizing properties
of the cleaning medium. There is of course a point at which the thermal stress induced by
high temperature cleaning can damage the stencil adhesion.
Lastly, high pressures or ultrasonics are used as a way to remove contaminants by
transferring mechanical energy to particulate contamination. Pressure helps to force the
cleaning agent into void spaces and stand-off gaps. Pressure can only marginally improve
cleaning performance when the contaminant is not in particulate form or when solubility is
the limiting factor.
Water-based cleaners based on MPC technology
Satisfactory results in stencil cleaning with aqueous systems can only be achieved for the removal
of solder paste. Most aqueous systems used to remove adhesives failed in the past because
the water caused the adhesives to set.
Micro Phase Cleaning (MPC) 2,3,12 refers to the use of a cleaning agent formulated to
undergo a phase change at elevated temperature. When heated above a threshold temperature
(typically 100-120 o F, 40-50°C) a microphase cleaner changes from a clear colorless solution
to a turbid milky mixture.
The phase transition produces a cleaning mixture that exhibits the properties of both
solvent-based and water-based cleaners. “Water-like” properties allow the cleaning agent
to effectively dissolve ionic contaminants and remove them from the substrate’s surface.