Stuart,

Are these impedance boards?  Are they military?  Are they high
reliability?  Have you had assembly problems in the past?  What
tolerances to you need on your hole sizes?  Usually, fab houses have a
+/- .003, which I believe is IPC standard for plated through component
holes.  Via holes usually have a wider tolerance something like +.003/-
holesize.  Non-plated through holes usually have a +/-.005 tolerance.  So
with spreads of 6 to 10 mils, the actual finished hole sizes may blend
across your specified ranges.

What I mean is that it is not practical to specify hole sizes only .003
apart unless you also specify a tight tolerance (and pay more) to get
that specific finished hole size.

Depending on your tolerances for the finished hole sizes, many
fabricators will drill difference finished holes with the same drill bit
anyway since the finished hole size will be well within your specified
tolerance.

If you absolutely have to have those hole sizes then specify tighter
tolerances.  Check with your fabricator to see if they can hold +/-.002.
Check with the assembly house to see what tolerances they need on the
holes.

The more drills needed to fabricate a board, and the tighter the
tolerance, drives the tooling setup times up, total drill times up,
inspection time up, scrap rate up, and hence - cost up.  $$$$$

Hope this helps.


On Sat, 26 Sep 1998 06:51:54 -0700 Stuart Chessen
<[log in to unmask]> writes:

>Over the past year we have upgraded our CAD library. Each
>component  has it own pad and/or hole size in the library. I have
noticed
>this  has doubled the number of different drills needed compared with
>previous boards layout in years past . Some of the drills sizes
>only  vary by .003".
>
>     What problems is this causing for the board fab house?



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