Jim,
If no one finds a spec that covers that, the rule of thumb is .010" over the lead diameter. You can go down to .008", but it also has to do with the level of manufacturing and your hole tolerance.
.005" is the bare minimum. The old "common" tolerance used for the hole size was +/-.005", so if you specify .010" over and the hole is at the minimum of the tolerance, you would be left with .005" over.
In addition, there is the mechanical tolerance. If you have, for example, a transformer with fixed leads, it has about a +/-.010" tolerance in lead to lead spacing.
You either deal with that during assembly or add that into the hole size. You can divide the between the hole sizes. +.005" for one hole and +.005" for the other.
That would give you .015" over the lead dia. Those leads usually have a large tolerance too and are square. You wouldn't really want to add in all the tolerances, but a healthy mix is necessary.
Probably none of this is news to you, but is one reason that IPC can't really specify more than the minimum clearance over the lead for solder flow. The rest is up to the application.
I have a handy dandy calculator for determining the pad sizes according to your requirements and can calculate for square post too. www.pcbdr.com. the resource calculator can also be used online of you have the Excel Plug in. (if you don't it won't work. Simple)
Chris Robertson
Author "PCB Designer's Reference"
[log in to unmask]
Hi everyone,
Does anyone know of an IPC standard that will tell what a plated thru-hole size should be depending on the diameter of the component that will be inserted. Example would be a pin header, or a certain wire size. Basically, how much bigger do you make the hole to accommodate the size needed so that you will get a good solder joint, and designed well for manufacturing.
Thanks,
Jim
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