Adhesive pastes Screen printing

Other small molecule species decompose and volatilise. The glass frit fuses, wetting the surface of the functional phase, providing adhesion and sealing of the composite to the substrate. Because of the screen printing process, resolution is modest. Fired film thicknesses, which range from 10 to 50 p.m (0.4 to 2.0 mils), are large compared to thin-fHm microelectronics. Some photosensitive pastes are also ia use. 

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. ... 

Bare die and other chip devices are attached with electrically conductive or nonconductive adhesives to ceramic substrates having defined circuit patterns produced by thin-film vapor deposition and photoetching of metals or by screen-printing and firing of thick-film pastes. With recent advancements in fine-line printed-circuit boards, adhesives are also finding use in attaching bare die to PWBs, a technology known as chip-on-board (COB). 

In surface mounting of components, both solder paste and adhesive need to be dispensed. The solder paste is usually screen printed or stenciled first, followed by selective dispensing of the adhesive between the solder pads. The prime function of... 

Most commercially available anisotropically conductive adhesives are formulated on the bridging concept, as illustrated in Fig. 1. A concentration of conductive particles far below the percolation threshold is dispersed in an adhesive. The composite is applied to the surface either by screen printing a paste or laminating a film. When a device is attached to a PWB, the placement force displaces the adhesive composite such that a layer the thickness of a single particle remains. Individual particles span the gap between device and PWB and form an electrical interconnection. For successful implementation of anisotropically conductive adhesives, the concentration of metal particles must be carefully controlled such that a sufficient number of particles is present to assure reliable electrical conductivity between the PWB and the device (Z direction) while electrical isolation is maintained between adjacent pads (X,Y directions). 

Surface mounting. Surface mounting of components onto PWBs is commonly referred to in the industry as SMT or surface-mount technology. It consists of the attachment of electronic components by automated dispensing or screen printing of solder paste and adhesive onto a PWB, placement of the components, then curing the adhesive and reflowing the... 

One printing technique that will apply adhesive to such shapes is the screen. Silk screens have long been used to apply paste-type inks to flat surfaces and even to simple curved surfaces such as cans and bottles. Many of the paste-type structural adhesives can be suitably deposited in intricate flat shapes by this means. Rather than the conventional silk screens, however, screens made from fine stainless steel wire are more suitable. The thickness of the deposit is determined by the gauge of the wire. In other respects, the method is quite similar to silk screen printing. 

This is a clever modification of flat silk screen printing. The paste adhesive is fed in the center of a rotary perforated drum, it is pumped through a hollow doctor blade into the rotating screen stencil and then through the holes in the drum and applied onto a web or fabric (Fig. 64). The web or fabric is running over a counter roller coated with rubber. 

Thick-film technology is an additive process that utilizes screen printing methods to apply conductive, resistive, and insulating films, initially in the form of a viscous paste, onto a ceramic substrate in the desired pattern. The films are subsequently dried and fired at an elevated temperature to activate the adhesion mechanism to the substrate. 

A cermet thick-film paste has four major ingredients an active element, an adhesion element, an organic binder, and a solvent or thinner. The combination of the organic binder and thinner are often referred to as the vehicle, since it acts as the transport mechanism of the active and adhesion elements to the substrate. When these constituents are mixed together and milled for a period of time, the result is a thick, viscous mixture suitable for screen printing. 

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