Fillets(teardrops)

Ron for small vias say 25 mil round or pads for that matter filllets serve a few purposes that come to mind. The first is they aid in
the transition of high-speed clocks running down a trace to a load. A load can be a pin on a device. In particular high speed clocks
running at 100''s of Mhz or more.

The other thing is that on small diameter pads they add more copper and a transition from the pad to the trace. This can be handy
to have during rework on fine pitch pads. The extra copper and transition can mean the difference to a pad or trace becomming de-laminated from the substrate during re-work.

The mechanical fit from a trace to a pad is stronger if you use a fillet.

Also in Flex circuits a fillit can aid in removing mechanical stress between the trace and a pad.  

I have seen fillets used alot in flex circuits and on single and double sided boards over the years. 

Scott 

Trade ya'.  My MEs are campaigning to senior management on subdividing a high-speed design to 5 individual cards with a passive backplane...and cheap berg headers. Round enclosures with "screw-on caps" take less than a year to certify as explosion proof.  But...

Fillets are an old concept. Helped improve yield when the via pad had to be smaller than twice the size of the drill.  That board was scrap if the edge of the hole "broke" past the edge of the pad.

I believe registration problems have been solved to profitably tackle jobs more common having 1 and 0.8mm BGA fanouts. The domestic fabricators I use today don't add fillets as they still get 100% yield.

Should be safe to allow fabricators to balance out copper distribution with thieving, provided it is kept 100 mils away from traces on adjacent signal layers.  And, they should be allowed to add fillets in CAM (Valor Genesis definitely does this.)