Buildup HDI Stackup Design and Layer Thickness Control
Stackup thickness control requires calculating pressed prepreg heights over local copper patterns to hold impedance and microvia drilling tolerances.
Laser Stop Target Pad
Abutment Feature
Internal copper landing areas on printed circuit board layers serve as mechanical and thermal barriers during laser drilling of blind microvias. A laser stop target pad absorbs laser pulse energy to halt downward hole ablation precisely at the designated dielectric interface layer. This structure ensures accurate microvia depth without damaging underlying circuit copper.
Energy Absorption
Laser drilling systems vaporize unreinforced resin and glass fibers to create microvias between adjacent copper layers. Carbon dioxide lasers reflect off solid copper surfaces due to wavelength absorption characteristics, effectively stopping ablation when the copper pad face is reached. Ultraviolet lasers remove metal slowly, requiring controlled pulse energy density to clean organic residues without burning through the target pad copper thickness.
Target pads must possess sufficient surface area to accommodate laser beam spot size plus mechanical beam positioning drift. Excessive energy density causes cratering or micro-cracks in the target pad structure. Insufficient laser energy leaves resin residue on the target pad, causing interfacial separation during assembly thermal cycling.
Alignment Tolerance
Cross-sectional inspection measures target pad indentation depth and remaining copper thickness. Acceptable target pads demonstrate complete resin removal across the landing area without micro-cracking or copper foil penetration under IPC-6012 standards.