pcb design - What pad hole (drill) size is appropriate for a given through-hole lead diameter?

I am making some new through-hole parts in Eagle for components that are not already in the various libraries. I've realized that the drill size needs to be a little larger than the lead diameter, but I'm not sure by how much.

With some research, I found the following information:

• "It depends" based on whether the component is being hand or machine soldered


• add 6 mil to the lead diameter


• 7 to 15 mil (diametric gap) for 63/37 solder


• 5 to 10 mil (diametric gap) for lead-free/RoHS solder

Is there a rule of thumb or guide to support this information? Someone referred to the Industry Standard for Printed Board Design (IPC-2221), but the IPC apparently only provides the table of contents of the document unless you pay $100US.

I'm planning on soldering the components by hand using 63/37 solder.

Answer

You need the pin or wire to be able to fit thru the hole, but otherwise tighter is better.

First, you look at the specs from your board house. They will give you the tolerance of final finished hole diameters from what you specify. In some cases, they will round to the nearest drill size, with then a resulting diameter range for each such drill. In other words, it is best to stick to a set of discrete hole sizes. Check with your board house, but .020, .025, .029, .035, .040, .046, .052, .061, .067, .079, .093, .110, .125 inches is otherwise a good list to stick to. If your board house guarantees finished hole diamter is ±3 mil, for example, from one of these standard drill sizes, then the first would be .017-.023, the second .022-.028, etc. Note that these ranges overlap a little for common tolerance values.

Now look at the datasheet for your part and see what the maximum lead diameter can be. If it's a round lead, it will tell you this directly. If it is a rectangular lead, you have to do the math to find the maximum possible diagonal. Either way, you end up with the minimum diameter hole the lead will fit into.

Now look thru your list of hole sizes and compare the minimum guaranteed size for each of them to the maximum diameter of the lead. Specify the smallest drill size where the minimum diameter hole is larger than the maximum diamter lead. If both come out to the same value, use the next higher drill size.