Analytical Models for Viscous Squeeze Flow Trace Swim in Multilayer Buildups
Analytical squeeze flow models predict trace swim by calculating hydrodynamic drag and side-wall pressure differentials across fine copper features during lamination.
Non Newtonian Polymer Flow
Fluid Rheology
Polymer resins and solder fluxes do not follow a simple linear relationship between force and flow rate. In the context of PCB materials, non Newtonian polymer flow describes how the viscosity of a liquid changes based on the speed at which it is moved. Most resins used in laminates exhibit shear-thinning behavior where the material becomes less viscous under high pressure.
This allows the resin to fill tiny gaps between copper traces during fabrication.
Dynamic Viscosity
Speed and temperature are the primary factors that control the movement of the resin within the press. When the viscosity drops during the lamination cycle, non Newtonian polymer flow ensures the epoxy reaches every corner of the internal stackup. Once the mechanical pressure stops or the speed of movement slows, the viscosity increases again.
This change prevents the resin from flowing out of the board entirely before it can cure. Laboratory rheometers map these changes and predict how a new material will behave in the factory.
Fabrication Control
Precise control of the press cycle is necessary to exploit these fluid properties without causing defects. Improper management of non Newtonian polymer flow can result in resin-starved areas or thick spots in the board. Reliable manufacturing depends on matching the press profile to the specific rheology of the resin.