Modeling Slip Velocity and Pressure Gradients across Smooth Copper Foils
Modelling slip velocity and pressure gradients across smooth copper foils prevents resin starvation and locks dielectric thickness tolerances during lamination.
Shear Rate Dependence
Fluid Characteristic
Non-Newtonian fluids exhibit a change in viscosity based on the speed at which the material is stirred or pushed through an opening. The concept of shear rate dependence explains why solder paste becomes thinner and easier to spread as the squeegee moves faster across the stencil. This property, known as shear thinning, is necessary for achieving a clean release from the apertures.
Once the force is removed, the viscosity must increase rapidly to prevent the deposit from losing its shape.
Printing Accuracy
Squeegee speed directly controls the force applied to the paste. Because of shear rate dependence, a slow stroke might result in insufficient filling of small apertures. Conversely, a stroke that is too fast can cause the paste to skip over the holes entirely.
High-Speed Dispensing
Dispensing needles and jetting valves also require materials with a high degree of sensitivity to flow speed. For these applications, the shear rate dependence allows a thick epoxy to be ejected as a precise droplet through a microscopic nozzle. The high shear at the nozzle tip lowers the resistance, while the low shear after exit keeps the dot from spreading.
Engineers must calibrate the pressure and pulse duration to match the specific thinning profile of the adhesive. Monitoring the consistency of the dot size provides an indirect measure of the material stability.