Geometry Profile
The difference in diameter between the entrance hole and the exit hole of a laser-formed microvia is a geometric consequence of the laser ablation process. This characteristic, called laser drill taper, arises because the energy density of the laser beam decreases as it penetrates deeper into the dielectric material. The resulting cone-shaped hole must be controlled to ensure reliable metallization.
Circuit designers must account for this variation to optimize high-density interconnect layers.
Ablation Control
Adjusting the laser spot size, pulse duration, and power profile minimizes the geometric difference between the top and bottom of the via. If the laser drill taper is too pronounced, the bottom pad area becomes too small, reducing the contact zone and increasing the risk of registration errors. Conversely, trying to achieve a perfectly straight wall can cause thermal damage to the target land or cause resin smear.
Optoelectronic control systems on modern drilling machines modulate the energy delivery to strike a balance between throughput and shape consistency. Fabricators also apply multi-shot drilling strategies, where multiple pulses of varying intensity shape the sidewalls without overheating the copper capture pad.
Dimensional Verification
Measurement of the via dimensions is carried out using automated optical inspection and cross-sectional analysis. Scanning electron microscope imaging of sample microvias provides detailed measurements of the top and bottom widths. The taper ratio, calculated as the bottom diameter divided by the top diameter, should typically exceed zero point sixty to allow for reliable chemical cleaning and electroplating.
If the ratio falls below this limit, plating solutions cannot circulate effectively, leading to thin copper deposition or voids at the base of the microvia.
