# 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
• 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.

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.

For most through-hole parts, I specify .035" finished hole size. This is suitable for DIP ICs, 1/4 watt resistors, small transistor and capacitors.

.040" is needed for post headers, and larger holes are required for TO-220 transistors, larger diodes and electrolytic capacitors.

The 7 to 15 mil oversize suggested sounds reasonable to me.

If you are using a prototype board shop, you will usually be limited to their standard drill sizes, unless you are willing to pay a premium for custom sizes.

In the IPC 7251 it is documented that:

Hole Diameter = Max Lead diameter plus the level value (Min 0.15 - Max 0.25)

On this website they added some extra info:

Minimum Hole Size is calculated according to equations below:

Minimum Hole Size = Maximum Lead Diameter + 0.25mm (for Level A of IPC-2222) Minimum Hole Size = Maximum Lead Diameter + 0.20mm (for Level B of IPC-2222) Minimum Hole Size = Maximum Lead Diameter + 0.15mm (for Level C of IPC-2222)

Hope this helps.

I am not sure about the machine placement needs (it will probably need wider holes), but there are several considerations:

1. metalized holes must to be drilled wider in order to accept the metal layer and still to be wider than the lead. Talk with your PCB maker about this.

2. For not metalized holes, simply add 0.1mm to the lead diameter. Note that too big clearances will reduce the reliability of the PCB, because the solder has pretty bad mechanical properties.

3. For square or rectangle leads - add 0.1mm to the diagonal of the rectangle.

4. For very big flat/square/rectangle leads, better use slot/square holes, not round. (because of the reliability reasons).

• For metalized holes you usually indicate the finished diameter, so as a client you don't have to care about the larger diameter. – amadeus Oct 14 '13 at 9:16
• @amadeus - it depends. If you have to provide the CAM file for the drilling machine, you have to set there the proper tool codes (diameters). – johnfound Oct 14 '13 at 9:21

You need to allow for both the lead size and tolerance and the hole size tolerance. Normally I allow a tolerance of +/-0.1mm for holes 0.3mm diameter to 2mm diameter.

Hence for leads which are round or not far off square add 0.1 minimum to the lead diameter / diagonal + the diameter/diagonal tolerance.