Hugh:

We're producing a part with +/- 0.001" toleranced holes.  The board is for a customer who believes that the
tolerance is necessary, was willing to work with a fabricator to make the design work, and was willing to
pay for a high precision part.  It can be done, but experimentation and some specification deviations are
necessary to achieve the desired results.  The parts under discussion are relatively small - about 1" X
1.5" with about 50 holes, and the panel is also small - 10 to 20 up on 0.125" FR-4.  Second, the plating
procedures are modified to achieve the desired hole diameter without regard to the minimum copper in the
hole - adjust the drill size and plating conditions to achieve the desired diameter/copper thickness.  Oh
yes, the circuitry is distributed uniformly and not very dense on the board.  Third, plating is controlled
with pin gages.  As you gain experience, the relationship between time and current will be determined, but
the work is pulled from the plating tank when the desired diameter is reached.  Finally the finish is
electroless nickel/immersion gold.  IMO there is no possible way to use HASL.

You can possibly achieve the tolerance on more complex designs if only specific areas have the tight
tolerance and you are given the ability to work with your customer's board designers.  IMO applying such a
tight tolerance to an entire board for normal applications is impossible or at best will be prohibitavely
expensive.  The basic question to be answered is WHY?  What possible parts are going into the plated holes
that require the tolerance?  Even press fit or swaged parts have a tolerance of +/- 0.002 or +0.003/-0.002
on the finished hole diameter.  (If you're talking non-plated holes the tolerance is usually not a
problem.)

Remember that good designs should not specify tolerances any tighter than necessary - wider tolerance
windows translate to lower cost parts.  The old rule of thumb for thru hole parts was to size the finished
hole 0.010" to 0.015" over the lead diameter to provide room for the solder to wick through the hole.

Regards,
Don Vischulis

Hugh Scott Miller wrote:

> Happy Holidays Technetters,
>
> A recent project has entered our shop from a customer that is demanding
> that all finished hole tolerances be held to +/- .001".  I pose the
> following questions to the esteemed Technet panel:
>
> a.  Are hole tolerances of +/- .001" realistic?
> b.  How could a board shop with conventional drilling technology
> in-house adjust its process to accommodate this request?
> c.  What type of advanced hole formation  technology (laser, plasma
> etching, etc.) could be applied to meet this requirement?
> d.  Can a component manufacturer produce components with leads that are
> within this tolerance?
>
> Any and all suggestions would be greatly appreciated.
>
> Regards

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