Re: gEDA-user: home-made 4 layer board idea

[DJ Delorie <dj@xxxxxxxxxxx>]

> my favorite layout guy once explained the notion of datum lines (I
> think that's what they are called).  Anyway, if you use an arbitrary
> line (datum) in the center of your board, and reference all
> measurements to it, the tolerances are only from that imaginary 1/2
> way point to the edges.  It's better than if the reference is on an
> edge, then the tolerance adds up along the entire length.

When fabbing your own boards, there's a few places where accuracy or
precision are lost.

First, printers aren't perfect.  Each printer has a characteristic
imprecision.  Mine makes plots slightly thinner (like, 0.1%) than
true.  As long as all your plots are oriented the same way, these
should cancel.

Second, when you etch a double sided board, misaligning the two masks
causes inaccuracy in the traces on those two planes.

Third, when you laminate the core with the outer layers, misaligning
those causes more inaccuracy.

Lastly, if you don't drill exactly in the right spot, each hole can
add inaccuracy.  Especially when the drills are used to align the
various layers.

In my case, when laminating the masks, the datum edge is always the
first one through the laminator.  I don't have much choice about that.


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