Solder resist coatings (maskants) are heat-resistant polymer coatings applied to selected areas of a PWB prior to solder coating and component assembly. Solder maskants protect portions of the board during subsequent soldering and reflowing of solder. They are useful in preventing solder from splattering onto conductor traces or bridging conductors. Other benefits of solder masking include: reducing solder-pot contamination from copper or gold, protecting the circuit from handling and environmental damage, and providing isolation between components and conductor lines or vias when components are mounted directly over the conductor lines. Solder maskants are generally based on epoxy, acrylic, or epoxy-acrylic resins formulated with fillers and additives to render them screen printable in selected areas.
They are heat cured and must withstand solder-exposure temperatures as
high as 230°C (as high as 260°C for the no-lead solders)[39] without
chemical or physical degradation or loss of adhesion. Solder maskants
are generally left on the board augmenting the moisture protection and
electrical properties of the board material itself. Colorants are
incorporated to aid in inspection and to distinguish one supplier’s
product from another.
Photoimageable liquid and dry-film solder maskants are also available.
These may be cured in selected areas by exposing the fully coated board
to uv light through a photomask. Tests for the qualification and
testing of solder maskants for PWBs are specified in IPC-SM-840.
Permanent solder maskants have found new applications with the advent
of ball grid array (BGA), flip-chip, and chip-scale packages (CSP). The
solder maskant is applied to the bottom of