Prepreg Melt Viscosity Dynamic Range during Thin Core Lamination Profiles
Matching prepreg melt viscosity minima to press pressure ramps prevents thin-core distortion and resin starvation in high-density multilayer lamination.
Prepreg Melt Viscosity
Resin Flowability
Minimum resistance to flow reached by the resin in a partially cured glass reinforced sheet during the lamination process determines how well the material fills the circuit features. A low prepreg melt viscosity is necessary to achieve a void free bond between layers of a multilayer board. This property is highly sensitive to the temperature ramp and the age of the material.
Lamination Window
Heat from the press causes the epoxy to liquefy and flow around the copper traces. If the prepreg melt viscosity remains too high, the resin will not penetrate the tight spaces of a high density design. This lack of flow results in air pockets and weak adhesion at the interface of the layers.
Conversely, resin that is too fluid will be squeezed out of the stack, leaving the board too thin for the designed impedance.
Quality Control
Testing the flow of incoming material ensures that the press cycles do not need to be adjusted for every batch. Monitoring the prepreg melt viscosity helps to detect material that has begun to age or that was not properly stored in a refrigerated environment. Consistent flow characteristics are the foundation of a stable and repeatable lamination process.