Aspect ratio is board thickness divided by drilled hole diameter. Plating solution struggles to circulate in narrow, deep holes, so copper thins in the middle of the barrel as the ratio climbs. Many fabricators treat 8 to 1 or 10 to 1 as standard for through holes, with higher ratios as a special process, and microvias are held to about 1 to 1. Thick backplanes with small vias are where designers run into the limit.
3.2 mm board with 0.25 mm drills is almost 13 to 1, the fab will want bigger vias or they'll no-bid on aspect ratio.
Also heard as
- AR
- drill aspect ratio
Related terms
PTH
Plated through-hole: a drilled hole whose walls are copper-plated to connect every layer it passes through.
Microvia
Very small laser-drilled via, typically 0.15 mm or less across, spanning one thin dielectric layer.
Plating
Electrochemical deposition of copper or other metals on a board, building hole walls and surface conductors.