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Questions relating to designing PCBs
By Thric
#102896
Hi all,
I am trying to understand why the holes on my board look sooo much bigger than the holes I see in my Gerber files. how can I make sure that the diameter is large enough or small enough to fit my componets? Is there some rule or trick to this?

Thanks
User avatar
By leon_heller
#102899
A useful rule of thumb is to make the holes 10 mil wider than the lead.
By Thric
#102902
Thanks for the reply,
I'm using parts from the Sparkfun Library so I assume your suggestion has already done. its just kindaof unnerving seeing the hole size. I guess i'll check the data sheets and compare the two.

Thanks
By skriim
#103770
You should remember that according to the Gerber standard (RS-274X, if I'm not mistaken) a hole's diameter is really a drill diameter your board manufacturer will use. If this hole should be plated (it usually should be), the plating will decrease its diameter by 0.1...0.2mm (depends on manufacturing process), also you'd left a gap of ~0.1mm to be able to insert your component without a hammer =). So if a component's lead diameter is 0.8mm, you should use 1...1.1mm hole for it in your CAD. But some manufacturers think that only fools follow standards; they claim that the diameter you set to be the one for a plated hole. This is a possible reason of getting bigger holes than supposed.
By MichaelN
#103783
skriim wrote:...some manufacturers think that only fools follow standards; they claim that the diameter you set to be the one for a plated hole. This is a possible reason of getting bigger holes than supposed.
The only manufacturer I'm aware of that follows what you've said is Olimex. All other places I've used specify the FINISHED hole diameter.
By waltr
#103796
All other places I've used specify the FINISHED hole diameter.
Same here.
In addition I always have fab notes specifying which IPC standard to use and that all holes dimension are finished sizes.
By skriim
#103812
If your manufacturer prefers getting finished holes' diameters, then the rule of thumb "+10mil to the lead diameter" seems to be unreasonably large.
By MichaelN
#103873
leon_heller wrote:... Samtec has this table for their connectors:
http://www.samtec.com/standard_products ... sizes.aspx

10 mil is roughly in the middle of the table.
Thanks Leon, that's useful. However, the actual clearances in that table are 6 mil for circular, and about the same for the square & rectangular section connectors. This is about what I try to use, and agree that 10 mil is a bit too big.