Multilayer circuit board having microporous layers and method for making same

DETAILED DESCRIPTION Referring to FIG.
1, the first step in constructing a representative single-sided printed wiring board (PWB) using mass seeding is to hot-roller laminate a layer 10 of photoprocessable material approximately 0.
0015 inch thick on a copper plate 12 approximately 0.
062 inch thick.
The preferred photoprocessable material is an aqueous-processable, moderately hydrophilic, dry film photopolymerizable composition as disclosed in Tecle et al.
, European Patent Application No.
87113013.
4 published Mar.
16, 1988 (hereby incorporated by reference).
This composition consists of an ethylenically unsaturated monomer, an initiator activatable by actinic radiation, at least one preformed, water-soluble, polymeric binder, and particles of an inorganic material transparent to actinic radiation which are chemically bonded to the binder.
It has a wetting tension of at least 52 dynes/cm (e.
g.
, 52-56 dynes/cm) as measured according to ASTM D2578-67.
The wetting tension is an indication of the hydrophilicity/hydrophobicity of the material; the higher the wetting tension the more hydrophilic the material is.
As disclosed in Tecle et al.
, preferred fillers are silica, silicate, alumina, or carbonate particles (or derivatives thereof) in which at least 95% of the particles have diameters between 0.
1 and 15 microns.
Preferred binders include Carboset 525 (methylmethacrylate/ethyl acrylate/-acrylic acid), Blendex 491, Lucite 47 KNL, Amphomer amphoteric pentapolymer (tert-octyl-acrylamide/methyl methacrylate/-hydroxypropyl-methacrylate/tertbutylaminoethyl methacrylate/acrylic acid) acrylic binder, and PVP K-90 (polyvinylpyrrolidone).
Examples of preferred monomers include acrylate, diacrylate, and triacrylate monomers, e.
g.
, pentaerythritol triacrylate and trimethylol propane triacrylate, which may be used alone or in combination with each other.
Preferred initiators are free radical generating addition polymerization initiators activatable by actinic light, e.
g.
, benzophenone, Michler's ketone, diethyl hydroxylamine, and 3-mercapto-1,2,4-triazole