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INTRODUCTION
Epoxy resins are used in many assembly processes of electronics for the mounting or
encapsulating of components and modules. The resins should process easily and cure
rapidly at moderate temperatures in order to attain short cycle times, tolerant manufacturing
processes and low thermal stressing of substrates. These demands generate the
high interest in one component epoxy resins having a long shelf life and being curable
by UV initiation and/or thermally at low temperatures.
For the UV initiated cure the well-known triarylsulfonium salts 1J are available, which
photolytically generate protons to initiate the cationic epoxide polymerization. For thermal
curing a number of initiators such as benzylthiolanium 2J, dialkyl-2,5-dihydroxyphenylsulfonium
3], dialkyl-4-hydroxyphenylsulfonium 4] or benzyl-4-hydroxyphenylmethyl-
sulfonium 5] salts have recently been described, some of which show a rather
favourable temperature dependency of the starter activity when combined with cationically
polymerizable monomers. The shelf lives and the curing profiles of selected
epoxy formulations containing a number of benzylthiolanium salts Ill have been determined.
Based on these investigations UV and thermal ly curing (dual cure) systems have been
developed, for which a broad range of applications exist in electronics, amongst them
conformal coatings and globe tops for bare chips 6].
The dual curing has been realized by two chemical paths: By the combination of triarylsulfonium
salts with thermoreactive benzylthiolanium salts and by the sensitization of
the benzylthiolanium salts.
