John,

The more accuracy you use at your stage, the better it is for us manufacturing types.  I like to see 4-decimal place accuracy because this data can be used for assembly machine programming and stencil design.  I have run into a "rounding" error many times that can cause inconsistency in the artwork.  For example, most QFP packages are spec'd in mm.  What is commonly referred to as a 20mil QFP is actually a 0.5mm lead pitch package.  That equates to 19.68mils but with 3-place accuracy, rounding occurs making the PCB layout a variable with 20mil (0.020") mixed with an occasional 19mil (0.019") pad-to-pad spacing.  That 4th decimal place accuracy lets the pads be spaced evenly at 19.7mils (0.0197") which more closely matches the actual device being placed.  Assuming a 12mil pad, you typically have about 7.7mils space between the pads.  With this type of variation you end up with spacing (between the pads) down to 7mils on the compromised (19mil spacing) pads.

One of the principle things manufacturing engineers try to eliminate is variations and this example is a variation we cannot control unless we communicate these ideas to the designers early in the design phase.

As many say, just my $0.02 (USD) worth.

Mark


-----Original Message-----
From: John Foster [mailto:[log in to unmask]]
Sent: Wednesday, December 04, 2002 2:12 PM
To: [log in to unmask]
Subject: Question about resolution


It seems that there are a lot of very experienced fabricators in this forum.
I have learned a great deal from reading the posts.

I started laying out boards in 1998, it is not my chosen profession but is
starting to become my chosen profession. I enjoy it very much.  When
I started a old manufacturing engineer gave me a set of guidelines to follow
and something's I am still figuring out on my own. With that said here is my
question to the forum.

Right now I am sending all my gerbers and drills out in millimeters in a 3,5
274x format.

So that means I am calling out things at 1/100,000 of a millimeter. I know
that this has
to be way beyond etching tolerance and what a drill can hold on tolerance.
This is kind
of what I started with. I have begun to change all of my designs to a .05
millimeter grid.
But I still have some old legacy stuff from when I really didn't know what I
was doing.

So my question is what is realistic. At how many decimal places in
millimeters or mills
does it become nonsensical and a waste of time for the guy or program that
has to
round everything off.

What kind of tolerance can a modern drill hold?
At what point does the photo plotter or direct imaging machine run out of
gas?

Once again I have learned a great deal from this forum and would appreciate
any kind of response

Thank You
John Foster

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