Microstrip Phase Velocity Fundamentals across Multi Layer Glass Stackups
Microstrip phase velocity depends on outer layer inhomogeneous dielectric fields, glass reinforcement architecture, copper roughness, and lamination compression.
Multilayer Glass Stackup
Composite Architecture
Arrangement of different glass weave styles and resin contents within a single circuit board creates a balanced structure optimized for thickness and signal performance. A multilayer glass stackup must be symmetrical around the center axis to prevent the board from warping or twisting during thermal processing. Fabricators select specific combinations of thin and thick glass fabrics to meet the overall height requirement while ensuring enough resin is available to fill the inner layer copper patterns.
This configuration determines the final mechanical and electrical properties of the finished board.
Mechanical Balance
Coefficient of thermal expansion varies between different glass styles, requiring careful placement to maintain flatness. In a multilayer glass stackup, using identical materials for corresponding layers on either side of the core prevents the bimetallic strip effect that occurs when materials expand at different rates. Tight weaves provide better dimensional stability during the etch and lamination phases, while more open weaves allow for better resin flow around heavy copper features.
Designers often mix 1080 glass for its thinness and 7628 glass for its stiffness and cost-effectiveness. The orientation of the glass bundles is also tracked to ensure that the warp and fill directions are consistent throughout the entire height of the board.
Layer Precision
Predicted dielectric thickness depends on the amount of resin squeezed out during the lamination cycle. A multilayer glass stackup uses prepreg layers with known glass-to-resin ratios to calculate the final distance between signal and plane layers. Variations in the stackup height directly impact the characteristic impedance of high-speed traces.
Advanced modeling software accounts for the copper density of the adjacent layers to estimate how much resin will be displaced into the voids between traces.