Flow Property
The lowest point of resistance to flow reached by a thermosetting resin during its heating cycle indicates the maximum fluidity of the material. A low dynamic viscosity minimum allows the epoxy to fill the narrow gaps between high density interconnect traces and microvias. This value is reached just before the onset of cross linking increases the molecular weight of the polymer.
Process Impact
Control of the heating rate in the lamination press directly influences the fluid behavior of the prepreg. If the dynamic viscosity minimum is too high, the resin will not flow sufficiently to remove air bubbles or wet the surface of the copper. Conversely, a value that is too low results in the resin being squeezed out of the stack, leading to a thin board and potential delamination.
Most high performance resins are engineered to maintain a stable window for these flow characteristics.
Measurement Method
Specialized rheometers track the viscosity of a sample as the temperature rises at a fixed rate, often twenty degrees celsius per minute. The recorded dynamic viscosity minimum serves as a critical quality control point for incoming prepreg batches. Laboratories compare this figure against the resin manufacturer’s specification to ensure consistency between different lots of material.
