I’ve got a rule of thumb for picking via sizes, but I don’t remember ever being taught how to choose a size for my vias.
In general, I use vias with 0.5 mm diameter holes and 0.9 mm diameter pads. For traces larger than 0.75 mm, I size the hole to be about 75% of the trace width, with the pad diameter to be a little less than double the hole size. Of course, I’ll do my best to use as few different drill sizes as possible as well.
How about you? Have any guidelines for choosing your via sizes?
For most signal nets I basically use the smallest hole diameter and annular ring size allowed by my manufacturer to keep the design as compact as possible and maximize the space available for routing. Typically this is a 10-mil hole with a 5-mil ring (ie. the pad diameter is 20-mils). But a few mils smaller is supported by many manufacturers.
I like to keep my day to day work at 13 mil drills with 7 mil annular rings, this way they work with OSH Park … obviously when I am dense design I use smaller vias and have them manufactured elsewhere.