Handbook of Solvents - George Wypych - ChemTech - Ventech!

14.8 Electronic industry 911
• External heaters often may not be needed when using aqueous medium, because of
the bath heating that the ultrasonic energy accomplishes.
However, because of this heating, these units are not recommended for use with solvent
cleaners. Also, precise control of the ultrasonic energy is required to prevent possible
damage to the stencil and apertures. Comparatively, also, ultrasonic machines are more expensive
than either type of spray equipment. The size, and thus the cost, is directly proportional
to the size of the stencil to be cleaned.
14.8.3.6 Stencil cleaning in screen printing machines
Today, many screen printers incorporate an automatic cleaning system for the undersides of
stencils. During the process a wiper is moistened with a cleaner that wipes the stencil. On
high-end printers this unit is programmable to permit subsequent dry wiping and vacuum
drying. Cleaning the underside of stencils may not replace machine stencil cleaning, but it
does extend the time, in some cases significantly, until machine cleaning becomes necessary.
A disadvantage of these automatic-cleaning units is the fact that only solder paste removal
is possible. The main problem associated with cleaning the stencil underside of adhesive
stencils is that it does not remove the adhesive from the stencil holes. This means that
only the surface is cleaned, while the holes are untouched. Cleaning agents suitable for this
application already exist.
14.8.3.7 Summary
The cleaning process, which a user finally selects, depends to a major extent on the operating
conditions. Each process has its own specific advantages and disadvantages, so users are
supported by advice and testing. These can be accomplished under tightly simulated process
conditions by a technical center.
14.8.4 CLEANING AGENTS AND PROCESS TECHNOLOGY AVAILABLE FOR
CLEANING PCBs
14.8.4.1 Flux remove and aqueous process
With the banning of CFCs - enforced in Germany since January 1993 - users have been confronted
with the problem of ensuring cost effective elimination of ozone depleting chemicals
from the production process while having to satisfy constantly rising quality
requirements. In the electronic industry it is customary to set certain quality standards in the
manufacturing process of electronic assemblies. Nevertheless, it is often not enough to
check the quality of a part immediately after it has been manufactured as long-term quality
must also be taken into account. However, technically proficient and efficient processes are
available that can exclude long-term risks and even reduce overall costs.
A large number of companies have decided in favor of no-clean processes. Though,
with the expansion of packing density and increased quality demands, more and more companies
are now returning to cleaning processes.
14.8.4.1.1 The limits of a no-clean process
The no-clean concept (i.e., no cleaning after soldering) proceeds from the following assumptions:
(1) The used soldering fluxes have a low solid content of approx. 2 to 3% and
contain strong activators. These activators, however, are critical as they can easily form
highly conductive electrolytes under humid climatic conditions.