Dear board designers,

OK, here's some mistakes I've made.

(1) Put the most expensive part on the board almost touching the mounting
screws. When (not if) the screwdriver slipped off the screw and smashed into the
board, guess which part acted like a magnet ? It is annoying to replace
components even when their cost is insignificant, so now I try to keep all
components at least 3r away from the center of the hole, where r is the radius
of the screw head.

(2) Just like Jeff, I also connected a relay coil with the wrong polarity.

(3) On a prototype board that I *knew* we were going to attach glomper clips to
all the big chips for testing, I neglected to leave enough room between chips
for those glomper clips.

(4) Took the DXF drawing from mechanical engineering and layed out the entire
board within a strangely-shaped board outline, and sent it off to be prototyped.
Then I found out this was a "bottom view", not a "top view". The raw, empty
board fit just fine, but all the tall components projected in the wrong
direction and intersected the metal case; and none of the connectors fit quite
right.

(5) We planned to install sockets on the first few boards so software
development could pop in new PROMs easily as they worked, then when they
released the code, just solder PROMs directly to the board on the production
line. I made sure the footprint would accept either one. But the plastic socket
stuck out so far beyond the pads that it wouldn't even fit between its
neighbors. (The amazing Justin N. was able to pull those neighbors off, then
somehow cram them all on the prototype and hand-solder them. Um, this was
surface mount, and the components were practically touching, so it was
impossible to squeeze a soldering iron between them).

(6) I wanted a new board similar to another board I had done. I deleted a bunch
of components and made other changes and hit "save" before I realized that I
forgot to archive that earlier design. Oops.

(7) Get pins 1, 2, 3 shuffled around on a SOT-23. To avoid that mistake in the
future, my schematics have a crude drawing of what the package looks like for
each kind of transistor, with a schematic transistor symbol in the middle and
source/drain/gate appropriately connected to the correct pin or tab.

Brad Velander <[EMAIL PROTECTED]> on 2001-02-26 11:15:56 AM mentioned:
> Would all SOT23 devices fit one standard/global SOT23
> footprint? Not a chance, in the professionally designed footprint standard
> that our company uses there are over one dozen variations of a SOT23
> footprint to cover all manufacturers and devices that they have encountered
> over the life of their footprint design standard. Need a SOT23 footprint?
> You had better pull out the manufacturers datasheets and ensure that you
> have got exactly the right footprint for that manufacture and that
> manufacturers particular part type (this does not even include the fact that
> some manufacturers alter the pin numbering scheme around on SOT23s). In some
> cases, a standard SOT23 footprint is not even a reality across one
> manufacturers part offerings, depends on the packaging facility that
> packaged the dies.

I have only 2 SOT-23 footprints in my footprint library (IR reflow and wave
solder), both with the pins labeled
    3
  =====
  1   2
(top view).

If a manufacturer has some wierd pinout on a SOT-23 device; I just re-alter it
back to (my) standard. Then I make a new schematic symbol, and fix up pin 1,2,3
on the schematic symbol, *not* the footprint, so I can link that schematic
symbol to my standard SOT-23 footprint.

Am I missing something crucial here ? Have I just been lucky that my one IR
reflow SOT-23 footprint happens to fit every SOT-23 device I've used so far ?

"Jeff Stout" <[EMAIL PROTECTED]> on 2001-01-31 02:14:07 PM started an interesting
thread I'd like to continue.
...
> Before sending a new board out, I always check my work
> for mistakes I've made on PCB I've designed in the past.
> Although I've had a bad netlist once (Protel's fault), most
> of the mistakes on PCB I've designed have been of the
> human generated variety.
...
> 7. Made a footprint for a relay, but did not know that the
> coil for the relay was polarized.  I didn't even know that it
> was possible for relays have polarized coils.
...
> If you have made big mistakes (and are not too embarrassed
> to talk about it) that have cost a lot of money, I think
> many people on this list would like to hear about it.
>
> Only by knowing where the danger spots are can you
> have any hope avoiding them.
>
> Jeff Stout

--
David Cary



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