Characterizing Dielectric Loss in Adhesiveless Polyimide Substrates
Adhesiveless polyimide substrates eliminate high-loss acrylic adhesives, dropping dielectric loss tangents to 0.002 at 10 GHz when paired with smooth rolled copper.
Very Low Profile Copper
Foil Surface
Electrodeposited copper foils utilized in high frequency printed circuit boards are engineered with controlled surface textures to reduce signal loss at elevated frequencies. The very low profile copper describes a grade of foil with a surface roughness between two and three micrometers, providing a balance between low skin-effect loss and high adhesive peel strength. This foil minimizes the increase in trace resistance that occurs when high speed currents are confined to the conductor outer boundaries.
Material specifiers select this copper type when laying out high density boards that must operate at high data rates.
High Frequency Conductivity
Skin depth decreases as the signal frequency rises, forcing the current to flow closer to the outer surface of the copper. When very low profile copper is used on the laminate, the electrical signal encounters a smoother surface than it would on standard copper foil. This smoother surface reduces the effective resistive path length, thereby decreasing the overall conductor attenuation.
Consequently, this foil allows for longer signal trace runs and improved signal integrity in high speed digital and rf layouts, especially in boards that run gigabit ethernet and PCIe signals.
Adhesion Mechanism
Mechanical bonding of the copper foil to the dielectric substrate is maintained through micro-roughness and surface treatment. Fabricators must ensure that the peel strength of this foil is sufficient to prevent the traces from lifting during reflow soldering or hand reworking. While smoother copper generally has lower adhesion, very low profile copper balances this by using specialized chemical bonding agents.
Mid Loss Laminates That Outrun Their Price Class
Mid-loss laminates paired with low-roughness copper foil match high-cost ultra-low-loss performance at a fraction of the raw panel price.