Fluid Behavior
Behavior of liquefied resin as it is forced laterally under pressure between circuit board layers governs the fill of gaps and the displacement of internal features. Squeeze flow dynamics describe this transient viscous movement during the hot press cycle when the temperature exceeds the glass transition point of the prepreg. This flow is driven by the applied pressure of the platen and opposes the internal resistance of the resin.
A high-viscosity resin restricts movement, while low viscosity encourages rapid spreading. Proper flow ensures complete encapsulation.
Lamination Consequence
When the resin begins to flow, it must fill the gaps between copper traces without displacing the conductors themselves. Excessive flow speed can exert lateral force on fine-line traces, causing them to shift or wash out, an effect known as trace sweep. Conversely, if the flow stops too early due to rapid curing, resin starvation occurs, leading to micro-voids and delamination during subsequent lead-free assembly.
Understanding this flow behavior allows engineers to select the correct prepreg style for dense layouts. Thermal ramp rates must be adjusted to keep the viscosity in the optimal range.
Flow Measurement
Process engineers use rheological measurements to characterize these dynamics for different prepreg lots. They adjust the press parameters based on the measured viscosity profile of the material. Control of this flow prevents inner-layer registration defects.
Consistent fluid behavior ensures defect-free laminates.
