Building PCB's

[Marc Randolph <mrand@pobox.com>]

I hope this doesn't turn into a duplicate email message.  This is a
short version of what I basicly wrote yesterday.  Our mail server
went down and so the original never made it out.  I think I'm going 
to turn on my "autosave sent mail" option.

I'm sorry David had to go through the ordeal with the board shop.
When we order boards (I'm an electrical engineer), we almost always
specify "electrical testing" - which I assume is probably the same
thing that David's shop calls "UL testing."   I'll explain why we
do this.

Board vendors are like chip makers.  When they build a batch
of boards, they get a yield depending on the technology of
the board (line width and spacing). In general, the higher 
the spacing/line widths, the higher the yield.

Knowing that, when you specify you want something electrically 
tested, they are going to charge more because they know they 
will have to produce more than the ten boards you requested
(because their yield is rarely 100%).  Not only that, but by saying
you want it electrically tested, they have to make or configure
a test fixture specially for your board to verify that everything
is 100% electrically correct with the board.  This takes even
more money, not to mention time.

When electrical testing is not specified, the board shop can
produce much closer to the exact number of boards ordered, and
they don't have to dink with a test fixture.  Hence the lower
cost.  The unstated assumption is that the customer is taking 
the risk that the yield will be decent, and this is acceptable
to many customers because they controls the yield through their
control of the line width and spacing.

I'm certainly not defending the way this works - I'm just saying
this is the way the industry operates in my experience.  I've
been bitten by the same thing, but on a much bigger scale.  
Imagine a 10 layer board with over 5000 traces.  We populated 
about 20 boards (1000 parts on each).  So now we have 20 boards,
each worth a couple thousand dollars - and some of them don't 
work because there are opens or shorts on the traces.  The end
result is that there were a few we couldn't find where the
open(s) or short(s) were (or it had too many), so we had to trash 
the boards.  The others we put wires on to jump around the opens.

Basicly, if anyone ever does a board build in the future, beware
that you either need to use big line widths and spacing, or you
need to pay extra for electrical testing.  I wish I could have
told David all of this before he had to go through it.

   Marc

-- 
  Marc Randolph     -    mrand@pobox.com    -     PGP keyID: 0x4C95994D
     If you have any info on the mid-60's car called the Bill Thomas
       Cheetah, or know anyone that might, please contact me.
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