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Solder printing can broadly be divided into 3 categories.
1.1 Dispensing
1.2 Screen printing
1.3 Stencil printing
Solder paste is dispensed by
squeezing through the needle of a syringe. Here paste is dispensed sequentially
on one land/ pad at a time. generally, dispensing is a manual or semi-automatic
operation. Dispensing is appropriate when screening can not be used as in
repair/ rework, prototype development, and prototype development or low volume
production.
Dispensing is not considered for its speed of application, but for its
versatility. For example, dispensing method can be used to print different
materials, like adhesive, flux, and permitting the same tool to be used. Also,
there is no vendor turn around time as in case of stencils or screens.
Dispensing may also have
special application for fine pitch packages that require lower paste thickness
than other components (i.e. lower than 8 mil) such as fine pitch SOICs, PLCCs).
Though screen / stencil printing are dominant methods, dispensing is widely used
for specialized applications for which screen printing is not viable.
A screen made of a woven wire mesh is stretched over a frame with a
glued-on photosensitive emulsion. The wire mesh supports the emulsion, which is
etched where solder paste must flow
to reach the substrate surface. The screens/ stencils have etched openings to
match the land patterns on the substrate(PCB). The screen is attached to a
aluminum frame, which imparts rigidity and tension, hence a flat but flexible
screen surface. The wire mesh is usually made of polyester or stainless steel.
Polyester materials are more resilient than stainless steel and last longer.
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