Studies Of Alkali Developable Uv-Cure Solder Mask For Printed Circuit Board

INTRODUCTION

Aqueous alkali developable UV-cure liquid photo-irnagable ink (LPI).
is a new type of, solder mask which is applied over copper or tin/lead.
circuitry by screen printing. It can be well dissolved in dilute aqueous
alkali before photocuring, ,but once exposed to UV, it has excellent alkali
and solder heat resistances. This negative-acting photocurable screen ink
has some advantages compared with traditional solder mask. It can be
designed to achieve high resolution and definition which are important
for the printed circuit board (PCB) with concentrated weld points. The
definition, however, is almost independent of printing conditions, such as
screen mesh and printing technology. If the film is found to be imperfect,
it can be easily removed without damaging the PCB at any stage before the
final postcuring operation. Omission of the screen pattern-made process
which is inevitable for other kinds of solder mask reduces greatly the
product cost, especially for small batch production. Therefore, the
commercial potential in PCB fabrication industry for a new solder mask
model based on aqueous alkali developable UV ink has been apparent In
recent ten years, the solder mask market increases by about 20% every
year, but 40% of it is for producing 'concentrated printed weld board
(PWB). Some patents and papers about the study of LPI have been
reported, and a few successful application has been achieved since 1984.
The resin as a main component of LPI is usually obtained by the reaction
of the hydroxyl group contained in UV-sensitive oligomer or monomer
with acid anhydride.

In this article, the characteristics of LPI with oligomer, which is
'obtained by the condensed reactions of phenolic aldehyde epoxy resin with
anhydride 'and acrylic acid,, and ,the effects of predrying conditions,
concentration of developer, developing time, and heat postcuring process
on the properties of its film were investigated in detail.