Drilling Geometry
Laser-ablated vertical channels connect high-density circuit layers to enable signal routing across restricted internal distances. A microvia occupies a diameter of 150 micrometers or less and maintains a target aspect ratio to facilitate reliable copper electroplating. This structure eliminates the footprint penalty of through-hole connections within multilayer laminates.
Fabrication limits depend upon the laser wavelength and the dielectric material composition.
Processing Sequence
Sequential lamination allows the formation of these connections before the final outer layer is bonded. Manufacturers drill the dielectric while leaving the underlying copper foil intact to act as a target for the beam. Chemical desmear processes then remove resin debris from the cavity wall to prepare for metallization.
Electroless copper covers the interior of the hole to provide a conductive surface for subsequent electrolytic plating. Voids within the barrel represent a high risk for thermal stress failure during reflow cycles.
Reliability Factor
Thermal expansion differences between the copper barrel and the surrounding laminate create mechanical stress on the interface during high-temperature assembly. Fatigue cracking occurs when the coefficient of expansion mismatch exceeds the ductility of the plated copper. Inspection standards define limits for plating thickness at the shoulder and the base of the channel.
Consistent monitoring of the grain structure during the plating process minimizes the probability of barrel fracture. Proper aspect ratio control ensures that chemistry reaches the bottom of the structure to prevent open circuits. Electrical continuity fails when the plating fails to achieve complete coverage of the sidewalls.
