Thick film pastes types

Dispersions can behave either as pure viscous substances or viscoelastic materials. In the case of viscous substances, the deformation does not recover after the stress has been removed. Part of the deformation can be recovered after the removal of shear stress for viscoelastic materials. Thick-film pastes may exhibit both time-dependent and viscoelastic flow behavior. The classification of flow behavior is shown in Table 8.21. Various types of flow behavior are shown in Fig. 8.79. This figure shows the plots of shear stress versus shear rate and corresponding plots of viscosity versus shear rate. 

Shear Thickening. Dispersions showing an increase in viscosity with increasing shear rate are called shear thickening or dilatant materials. This type of flow behavior is usually exhibited by dispersions of rigid particles at high concentrations. Thick-film pastes rarely show this type of flow behavior. 

Flow Curves. In this type of testing, shear stress or shear rate is changed as a function of time and resulting shear rate or shear stress is measured. This type of test is generally used to survey flow behavior over one to two orders of magnitude of shear rate range. Thixotropic behavior of pastes can be determined from the hysteresis in shear stress-shear rate plots. There are several phenomenological models available to curve-fit the experimental data. Flow curves for various thick-film pastes are shown in Fig. 8.81. 

Creep Test. In a creep test, a constant shear stress is apphed to a sample for a certain period of time and the resulting deformation is monitored. This type of test is used to characterize viscoelastic properties of the paste. Creep tests can also be used to measure paste viscosity at very low shear rates (approximately 10 to 10" s ). In a strain test, a step shear rate is applied to the sample and the resulting stress relaxation is measured. Examples of creep tests for thick film pastes are shown in Fig. 8.82. 

As supplied, adhesives can be found in the form of low viscosity liquids, viscous pastes, thin or thick films, semisolids, or solids. Before application to a substrate, an adhesive need not be sticky or otherwise particularly adherent. A distinct exception is the pressure-sensitive adhesive (PSA), which is inherently tacky when first made. Such an adhesive is applied as a thin film with or without a backing, the combination of the adhesive and the backing defining an adhesive tape. The PSA remains throughout its useful lifetime essentially the same material it was when first made. All other forms and types of adhesives undergo a transformation which is central to their function as an adhesive. This transformation is usually carried out through imposition of time, heat, or radiation, either actively or passively. 

Metallization methods for alumina ceramics using conductive paste can be broadly classified into two types thick film processing and cofiring (HTCC). Although both methods share many points of similarity in the printing process, big differences can be seen in the other manufacturing processes. 

The glass frit type of thick film gold paste that has jnst been described is commonly used for electronic devices and sensors. The particle size of the gold powder is 3 xm or less, and particles will tend to be spherical in shape in order to achieve a dense film after firing. The finished electrode thickness after printing will usnaUy range from 1 to 20 pm, depending on particle size, metal content of the paste, etc. 

The process for the thennal sensor network is as follows. Organic diodes, to be used as sheet-type thermal sensors, are manufactured on an ITO-coated PEN film. A 30-mn thick p-type semiconductor of copper phthalocyanine (CuPc) and a 50-nm thick n-type semiconductor of 3,4,9,10-perylene-tetracarboxylic-diimide (PTCDI) are deposited by vacuum sublimation. A 150-mn thick gold film is then deposited to form cathode electrodes having an area of 0.19 mm. The film with the organic diodes is coated with a 2-pm thick parylene layer and the electronic interconnections are made by the method similar to that mentioned before. The diode film is also mechanically processed to form net-shaped structures. Finally, to complete the thermal sensor network, we laminated the transistor and diode net films together with silver paste patterned by a microdispenser. This is shown in Figure 6.3.11. 

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