Handbook of Solvents - George Wypych - ChemTech - Ventech!

14.8 Electronic industry 905
has become a standard. Stainless steel stencils with apertures that are either etched or cut
with a laser are used for this purpose. They can also be used for fine-pitch apertures. Studies
have revealed that more than 60% of the defects arising in the SMT process are caused by
inadequate paste printing. This, in return, can be traced to insufficient cleaning that is often
done manually. Consequently, a well working stencil cleaning process is an imperative.
With regard to the
cleaning process it is very
important that no paste or
cleaner residues remain in
a stencil aperture, as this
will inevitably cause mis-
Figure 14.8.6. Left: Residues of SMT adhesive cured in the aperture of a stainless
steel stencil. Right: Stainless steel stencil encrusted with solder paste.
prints (see Figure 14.8.6,
right). The printing of adhesives,
on the other hand,
requires some points to be
taken into consideration since this method has to produce different dot heights. The ratio between
the stencil thickness and a hole opening is the decisive factor with this technology.
Stencils for adhesive application are made of either stainless steel or plastic and are usually
thicker than those used for paste printing. Cleaning is complicated by the thickness of the
stencils (often exceeding 1 mm). Consequently, small holes require a cleaner with intense
dissolving properties to ensure that the adhesives are completely removed (see Figure
14.8.6, left).
Although cleaning of
stencils improves the printing
results drastically, the production
of misprints during
the printing process cannot be
avoided completely. However,
the cleaning of misprinted
assemblies is an
application that is still fre-
Figure 14.8.7. Left: Misprinted PCB, Right: Via filled with solder paste.
quently ignored. This involves
the removal of
misprinted or smeared solder
paste or SMT adhesive. The removal of solder paste as the most widespread application
gives rise to the following demands:
• No solder paste must remain on the surface, or the vias (see Figure 14.8.7).
• The surface must be perfectly clean and dry to ensure immaculate renewed printing.
• The solder flux on double-sided PCBs must also be removed from the side where
soldering has been finished.
14.8.3.2 Compatibility of stencil and cleaning agent
In some cases, repeated washing can damage stencils cleaned in automated washing stations.
This damage usually occurs through degradation of the frame-to-screen glue. This
degradation occurs at the junction of the stencil and the mesh and also at the junction of the
mesh and the frame. The causes for this degradation lay in two factors. First, the high temperatures
that are needed to clean stencils with aqueous cleaners cause thermal stresses