Prepreg Rheology Control in Thin Core Laminate Stacks
Precise control of prepreg minimum viscosity and press temperature ramp rates prevents core squeeze out and micro voiding in thin laminate stackups.
High Tg FR4
Thermal Resistance
Reinforced epoxy laminates with a glass transition temperature exceeding 170 degrees Celsius provide enhanced stability for complex circuitry. Using high tg fr4 ensures that the PCB substrate remains in its rigid state during the peak temperatures of lead-free soldering. This material class resists the rapid expansion that occurs once resin enters the rubbery phase.
Manufacturers specify these grades for multilayer boards with high hole density.
Z-Axis Expansion
Controlling the rate of dimensional change in the vertical direction protects plated through holes from mechanical stress. Since high tg fr4 possesses a higher temperature threshold before its expansion rate increases sharply, it reduces the strain on copper barrels. Plating cracks occur less frequently in materials that stay below their transition point during thermal excursions.
Thick boards benefit most from this restricted movement. Reliability improves in harsh environments where temperature cycles are frequent and severe. The increased crosslink density of the resin also provides better resistance to moisture absorption and conductive anodic filament growth.
Fabrication Constraint
Drilling and desmear processes require adjustment to account for the increased hardness of the cured epoxy. A high tg fr4 substrate often demands slower drill speeds to prevent hole wall damage. Chemical dwell times in the etch-back line may increase to achieve the necessary surface topography.
Quality boards result from balanced process parameters.