PTH Drill sizes

[Freighternut]

Is there a rule of thumb for calculating drill sizes for PTH given a component lead size?

Also I am a bit confused as to wether the drill NC file should show finished hole size or pre-plate size, does this vary between PCB manufacturers?

[ejeffrey]

Finished hole sizes is the standard at every manufacturer I have used, I assume it is nearly universal.  The should specify what they want in their artwork guidelines, along with their standard drill sizes.

Check your data sheets, they usually have a preferred footprint with hole sizes.  Just round that up to the nearest standard drill size of your board house.

[mikeselectricstuff]

General rule 0.8mm for leaded parts and ICs, 1.0mm for 0.1" pin-headers etc.
You want at least 0.3mm above the lead size for easy insertion, but it isn't  too critical and going bigger is rarely a problem unless tight alignment is important.

As well as lead dia itself, size of part, number of pins and likelihood of bent pins are other considerations - inserting a multi-pin part into close-fitting holes can be near impossible if there are misaligned pins - oversized holes make things a lot easier.

Minimizing the number of different sizes is also good as this can sometimes influence cost.

You spec the finished size - PCB maker will adjust for their plating thickness.

[Freighternut]

Thanks Mike and ejeffrey, most helpful as usual.

[KashmirGR]

There are recommended hole sizes for PTH components based on IPC rules.  IPC has rulesvfor three classes of hole size (least, nominal and most).  There are also formulas to calculate hole diameter.  Here is a lead to one page that discusses calculations:

http://en.wikibooks.org/wiki/Practical_Electronics/PCB_Layout

Your pcb vendor may have preferred sizes as well.  If you look at pcbexpress.com for example, they have set sizes for through holes (you can find the table on their site). 

Mentor Graphics offers a footprint viewer for free that has many footprints, which I believe has both SMT and PTH components. Search for IPC land viewer.