Viscosity surface-mount adhesives

Furthermore, surface-mount adhesives (SMAs) used in pin transfer must have relatively low viscosities to facilitate pickup by the pins and transfer to the PCB. Because viscosity is so critical, the temperature of the adhesive reservoir must be controlled and preferably maintained between 25 °C and 30 Some SMAs that may be applied by pin transfer are listed in Table 4.17. 

Table 4.23 Viscosity changes vs pot life for a surface-mount adhesive ...

Table 4.23. Viscosity Changes vs Pot Life for a Surface-Mount Adhesive ...

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. ... 

These syringe applicators with very fine needles may apply very small dots or drops down to one hundredth of a milligram. They are widely used in electronics for surface mounting, chips bonding and in jewellery and watches. The application of the adhesive may be controlled with a special camera or an optical fibre detector. The viscosities of the adhesives may range from 100 mPas (cyanoacrylates) to 20,000 mPas (epoxies, silicones). 

For adequate soldering to occur, it is required that the metal surfaces to be joined are free from dirt, oxides, and other contaminates. Intermetallic compounds can only form if the molten solder is able to alloy or react with the exposed metal pad and lead surfaces. To remove metal oxides present on PCB pads, component terminations, and solder balls, requires exposure through the action of a fluxing agent. Wave fluxes are required to perform the same function as fluxes used in surface mount applications with the exception of component retention. In wave applications, SMT components are held onto the PCB by mechanical means, or by the use of an adhesive. Therefore, a lower viscosity, less tacky flux can be used in wave applications compared to surface mount applications. 

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