Screen printing adhesive material

In the past, electron beam radiation was applied to produce PSA exclusively however, recent improvements in UV curing technology (precise UV dose control, suitable photoinitiators) permit UV to be used to produce pressure-sensitive adhesives. PSA formulations can vary in consistency from low-viscosity liquids up to solids melting at 80°C (176°F). Therefore, applications may vary from screen printing to roll coating to melt extrusion. Coat weights for most PSA materials vary from 1 to 10 g/m. ... 

When the shear stress of a liquid is directly proportional to the strain rate, as in Fig. 2.4a, the liquid is said to exhibit ideal viscous flow or Newtonian behavior. Most unfilled and capillary underfill adhesives are Newtonian fluids. Materials whose viscosity decreases with increasing shear rate are said to display non-Newtonian behavior or shear thinning (Fig. 2.4b). Non-Newtonian fluids are also referred to as pseudoplastic or thixotropic. For these materials, the shear rate increases faster than the shear stress. Most fllled adhesives that can be screen printed or automatically dispensed for surface-mounting components are thixotropic and non-Newtonian. A second deviation from Newtonian behavior is shear thickening in which viscosity increases with increasing shear rate. This type of non-Newtonian behavior, however, rarely occurs with polymers. ... 

Polyvinyl alcohol solutions are also used as adhesion binders for non-woven fabrics, screen-printing, ferrites, ceramics and some building materials. In powder form, it is also an effective binder for finely divided powders, such as pigments. Mixtures of low dp polyvinyl alcohol with ethylene glycol, however, have low adhesion to substrates and are used as peelable coatings for metals and plastics for protection during storage and transport. 

Screen printing is not suitable for use with low-viscosity products. These fluid materials can, sometimes, be applied by transfer printing methods. One such system utilizes a printing pad of the proper shape immersed in a reservoir of adhesive. Application takes place when the pad is raised up through the liquid and then contacts the substrate positioned above the reservoir. 

Adhesive pastes are non-Newtonian fluids whose viscosity depends upon temperature, time, and shear rate. They are applied to the substrates by means of either stamping (pin transfer), screen printing, or dispensing. The performance of the last technique depends on a key factor the paste rheology. There exists at least six semi-empirical models to describe the rheological response of non-Newtonian fluids [4]. For shear-thinning fluids (thixotropic materials), it has been reported... 

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