Material Ratio
Compositional ratios quantify the amount of epoxy or other polymer relative to the reinforcement glass within a prepreg or laminate. Higher resin volume content improves the filling of copper voids but reduces the dimensional stability of the board. This ratio is typically expressed as a percentage of the total weight or volume.
Structural Impact
Resin provides the bonding force and the insulation between the copper layers of a multi-layer board. During the lamination process, the liquid resin must flow into the gaps between traces to prevent air bubbles. A high volume of resin ensures that these spaces are filled completely, which is essential for high-voltage applications or dense routing.
However, resin has a higher coefficient of thermal expansion than glass, so a higher percentage increases the Z-axis expansion of the assembly. This expansion puts stress on the copper plating in the vias and can lead to barrel cracking over time. Maintaining the correct balance ensures the board survives the thermal shocks of lead-free soldering.
Dielectric Influence
The electrical properties of the board are a weighted average of the resin and the glass. Increasing the resin percentage lowers the dielectric constant because most resins have a lower permittivity than E-glass. This relationship allows manufacturers to tune the impedance of a substrate by adjusting the material ratio.
