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 Ablation
Photon Erosion
High energy light beams remove material from a circuit board to create precise holes or patterns. The laser ablation process is the standard method for drilling microvias in high density interconnect boards where mechanical drills are too large. This technique can target specific depths with high accuracy, stopping exactly at the copper layer below the dielectric.
Microvia Formation
Modern fabrication facilities employ two different types of lasers to handle the various materials found in a PCB. A laser ablation step typically uses ultraviolet light for high precision or carbon dioxide lasers for faster material removal in thicker resins. This choice depends on the glass weave and the thickness of the dielectric being removed.
Using light instead of a physical drill bit prevents mechanical stress on the laminate and allows for much smaller hole diameters.
Thermal Impact
Intense heat from the beam vaporizes the material, but it can also leave behind carbon debris or melt the surrounding resin. The laser ablation parameters must be tuned to minimize the heat affected zone and ensure the walls of the hole are clean for subsequent plating. Proper cleaning after the laser step is required to remove any residue that could prevent the copper from bonding to the target pad.
Failure to clean the hole results in high resistance connections or total circuit failure.