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September 2015 • The PCB Magazine 83 Monomers Monomers contain at least one double bond to react with a free radical. Suitable monomers belong to the family of polyfunc- tional acrylates and methacrylates, often mod- ified with alkoxy side chains. Their function is to polymerize upon exposure to form a highly crosslinked web, entangling the binder, so that the exposed area becomes less soluble in the developer solution and exhibits sufficient mechanical and chemical resistance to serve as an etch or plating resist. Monomer content in dry film formulations is typically less than 60%. If higher, the dry film's viscosity would be too low. Binders Binder content of dry films tends to be at least 25% to provide the dry consistency and dimensional stability needed. Chemically, the binder, or mixture of binders, is a matrix of polymers and copolymers derived from acry- lates, methacrylates, styrenes and, occasionally, vinylacetates. Important binder characteris- tics that have an effect on end-use application properties (in parentheses) include: molecular weight and weight distribution (strip speed, tenting strength); glass transition temperature (film formation; cold flow resistance; conforma- tion during hot roll lamination); flexibility and tensile strength (mechanical toughness; tenting strength); solubility (strip speed); chemical re- sistance (survival in plating baths; alkaline etch resistance); toxicity (industrial hygiene; biologi- cal waste treatment). The binder has to have sufficient solubility in the coating solution to yield a homogeneous film without phase separations. This does not mean the absence of microstructures in the final dry film photoresist. Monomers with structural features chemically similar to the binder help solubilize the binder during dry film coating. Occasionally, we find solvent swellable "microgels" added to the formulation to control cold flow. Stabilizers (Inhibitors) Stabilizers to prevent premature thermally induced polymerization may be added to the formulation by the resist manufacturer or may already come as an additive with the mono- mers. The level of stabilizer is carefully chosen to achieve the desired stability without lower- ing the photospeed too much. Typical chemical structures include hydroquinones and nitroso dimers. Plasticizers Plasticizers may be added to modify the flexibility and hardness of the resist. This per- tains mainly to the properties of the exposed resist: In the unexposed areas the high abun- dance of monomer plasticizes the resist, but the depletion of monomer during exposure, the polymerization and cross-linking em- brittle the resist, causing it to chip and flake off. The addition of plasticizers counteracts figure 3a: low collimation exposure. figure 3b: high collimation exposure. kArl'S tECh tAlk DRy FILM PHOTORESIST STRuCTuRE AND COMPOSITION continues