Z Axis Permittivity Variations in Standard High Speed Multilayer Stackups
Out-of-plane permittivity in glass-reinforced multilayer stackups runs lower than datasheet figures, requiring Z-axis test methods to hold impedance.
Copper Volume Resin Fill
Lamination Planning
Design parameters specifying the volume of resin required to fill the voids between copper features on internal circuit layers guide the calculation of prepreg requirements. Achieving an adequate copper volume resin fill ensures that the liquefied resin during lamination completely occupies the empty spaces between trace patterns and plane clearances. This calculation determines the thickness and style of the prepreg sheets needed to prevent dry spots and laminate voids.
Calculation Procedure
Prepreg sheets of known resin content flow under temperature and pressure to encapsulate the adjacent copper layers. Calculating the copper volume resin fill involves subtracting the volume of the etched copper patterns from the total volume of the layer to find the empty space that must be filled. When the resin volume of the selected prepreg is lower than this calculated deficit, the laminate suffers from micro-voids and potential delamination.
Engineers use mathematical models to balance the fiber-to-resin ratio while ensuring that the physical thickness of the finished board matches the impedance specifications. This step is critical in high-density interconnect designs with heavy copper weights where the volume of air to be displaced by the resin is high.
Defect Prevention
Inadequate resin flow leads to structural weaknesses and trapped moisture that can cause board failure during solder reflow. Ensuring proper copper volume resin fill also prevents the formation of conductive anodic filaments along the glass fibers. By verifying these calculations before lay-up, fabricators ensure a robust internal bond that withstands thermal stress testing.