PCB Layout

[tclifford]

I'm currently trying to layout a PCB using PADSPCB. The default VIA has a drill hole of 0.94mm with padsize of 1.397mm.
This seems a little large for the PCB i'm doing and wondered if there were any standard smaller sizes for manufacturable VIAs or any 'rules of thumb' for the size of VIAs and pads with respect to trackwidths.

 

[Lewish]

I work in inches, so here are "my" guidelines in inches. You want a minimum of 0.005 inches of copper around the hole after it is drilled. So, I use a 0.040 pad with a 0.028 drill. The 0.028 is a standard in all PCB fab shops. The 0.005 insures that you get good plating between the hole and the pad and don't have a gap in the annular ring.

The hole diameter is a function of how much current you need to pass thru it. On small low power boards with tiny parts I will use a 0.020 pad with a 0.012 hole and have the fab shop plate the hole shut. On power traces with lots of current I might use a 0.100 pad with a 0.060 hole in it to get lots of current carrying capacity. Bigger holes need bigger annular rings.

That's about it. Find out what the standard drill sizes your fab shop uses are, and size you pads accordingly.

 

[cbarn24050]

Hi, check what your PCB maker can do. As lewish says 0.7mm is easy, really good places can go down to 0.3mm.

 

[47gandalf]

Excuse me for answering in inches instead of millimeters.

I don't think that .037" drill and .055" OD on the via is large. Depending on the components you're using. I've used that drill size for the legs of through hole components for years.

And as a rule the via OD wants to provide a ring of at least .010" radius to reduce dependency on the accurate placement of the drill bit; so the OD should be about .020" larger diameter than the drill size. With a placement error (in aligning the drill bit) up to a few thousandths of an inch, a 10 mil pad helps assure that you'll have continuous metal all around the barrel of the plated through hole (PTH) to ensure reliable via conductivity.

I'm a big fan of PADS although it's a long way from cheap. I like most how it won't let you do some dumb things, like fail to specify what's connected to a given component pin.

If you need the board space you can certainly reduce the via's drill size. I'm using .018" mostly now because it will handle enough current but is only a quarter the size of the bigger hole. Some of my vendors can reliably make PTH with drills down to .012" if you need to go that low.