Rheological Testing Conditions for Thin Prepreg Viscosity
Dynamic parallel plate rheometry under controlled 2 °C/min thermal ramps defines the minimum viscosity window for thin prepreg lamination success
Dynamic Complex Viscosity
Viscoelastic Metric
Mathematical measurement of the total resistance to flow exhibited by a fluid under oscillatory shear conditions provides data for evaluating polymer behaviour during lamination. This physical quantity, known as dynamic complex viscosity, combines both the in-phase elastic response and the out-of-phase viscous response of a resin system. Testing occurs during the ramp-up phase of lamination to ensure that the composite material flows adequately around copper features.
The measurement stops being useful once gelation begins and the material transforms into a cross-linked solid.
Shearing Deformation
Shearing deformation determines how the resin responds to high temperatures and mechanical stress during the wet-out stage. When sinusoidal oscillation is applied to a B-stage prepreg sample between parallel plates, the resulting deformation is measured at controlled heating rates. High-frequency inputs help simulate the rapid thermal transitions of high-volume assembly lines.
A low measurement means the resin flows too easily, which can cause excessive squeeze-out and dry spots on the finished circuit board. Conversely, high values prevent the resin from filling the internal gaps. Instrument calibration and sample thickness must be held constant to avoid erroneous readings that could lead to miscalculated laminating press programs.
Flow Limits
Flow limits are determined by monitoring the minimum point on the viscosity curve. As temperature rises during the lamination cycle, the dynamic complex viscosity of the unreacted epoxy drops before the curing agent initiates cross-linking. Accurate control over this window ensures void-free encapsulation of internal copper traces.
Process engineers adjust press recipes to align the pressure application with this specific fluidity window.