Rheological Parameter
Shear behavior dictates how polymer encapsulates fine pitch component leads during automated dispensing and printing cycles. Viscous fluid flow determines the rate at which conformal coating materials settle across printed circuit board assemblies before UV curing locks the film in place. Manufacturers measure this internal fluid friction using rotational viscometers to verify that underfill liquids penetrate microscopic gaps beneath flip chip arrays without leaving voids.
Thermal profiles alter this property because elevated temperatures decrease molecular resistance inside solder paste matrices during stencil printing. Stencil aperture geometry restricts lateral movement when high solids content pastes exhibit excessive shear thinning under squeezy blade pressure.
Dispensing Mechanics
Fluid dynamics govern the volumetric displacement required during automated micro needle jetting operations. Shear stress applied by positive displacement pumps forces uncured epoxy through narrow fluid paths onto designated surface mount pads. Pressure differentials across the dispensing nozzle dictate the kinetic energy transferred to the droplet during flight toward the target substrate.
Surface tension forces counteract the forward momentum immediately upon contact with the copper pad to prevent bridging between adjacent terminals. Nozzle clogging occurs when particulate fillers agglomerate inside the delivery channel because local velocity drops below the critical shear threshold.
Adhesion Defect
Void formation arises from incomplete displacement of trapped air during capillary underfill processes beneath ball grid array packages. Process engineers adjust dispensing speed to match the spreading rate of the liquid encapsulant across masked solder masks. Low shear viscosity promotes rapid wetting along component edges but increases the risk of excessive bleed out onto neighboring test points.
Post application reflow ovens apply controlled heating rates to lower internal fluid resistance and allow trapped volatiles to escape safely through open fillets. Proper material selection prevents delamination failure during thermal cycling by ensuring cured interfaces withstand mechanical stresses generated by mismatched thermal expansion coefficients.
