[PCB_FORUM] Re: NC drill qurery

It depends on how many of these drills you have and their distribution
on the PCB.

If the drills are extremely localized, such as under specific components
and they are not mixed with any other drills of the same size, you can
specify the tolerance on your fab drawing and point out the area.

If they are mixed in with drills of other tolerances, your fab house
will not be able to identify which is which.

My suggestion is that if you have a +/-.002 which is pretty standard for
press fit connectors, I would change my drill size to 19.9 or 21.1 mils
so that your fab house will be able to differentiate those drills and
therefore have control over them. And put a note on your fab drawing
stating the recommended drill size, the finished hole size and tolerance
(all of this should be one the component datasheet) and point to the
areas where those exist. When the tolerance is lower than standard for a
fab house they will have to inspect the size after the plating step.

Leave the +/- .003 holes as twenty. Generally most fab house will be
able to handle +/-.003 as a normal tolerance and if this is specified on
your fab drawing they will process as normal.

Where you have vias and pins with the same finished hole size and
tolerance, I would again suggest that you change your via size, maybe to
14mils which would put you dead center inside your tolerance or give up
on the +.002/-.010 and just accept your fab house standard tolerance.

The short answer to controlling tight drill tolerances is that your fab
house will change drills more often to prevent wear out and the drills
must be inspected after plating to make sure that your requirements have
been met.

 

________________________________

From: icu-pcb-forum-bounce@xxxxxxxxxxxxx
[mailto:icu-pcb-forum-bounce@xxxxxxxxxxxxx] On Behalf Of nagaraj
Sent: Thursday, February 05, 2009 11:13 PM
To: icu-pcb-forum@xxxxxxxxxxxxx
Subject: [PCB_FORUM] NC drill qurery

 

Hi,

I have a query on drilling. If the board has same drill size with
different tolerance say,

20mil drill having tolerance as +/-2mil at some places

20mil drill having tolerance as +/-3mil at some places

20mil drill having tolerance as +2/-10mil at some places(vias)

 

How they get drilled to achieve different tolerance using same 20mil
drill bit? 

 

Looking for a detailed explanation on drilling process in PCB.

 

Thanks,

raj

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