Re: [CAD_CAM_EDM_DRO] Re: Cleaning Circuit Boards and "Static" rant

> At 02:07 AM 11/27/00 -0000, you wrote:
> >
> >There are some components that cannot have protection circuits
> >because it would compromise important specifications. Some of these
> >are low-leakage FETs or FET input op-amps. These are special cases
> >and it is unlikely they would ever appear in consumer gear.
>
> These were op-amps, CMOS analog mixed signal chips etc. Standard
> in mid-to-high range consumer and industrial gear, really. Some bipolar
> chips
> and even LEDs are now static sensitive because the dimensions have
> continued to shrink.
>
> >Secondarily, even if sensetive circuits are used, surrounding
> >circuitry, if designed properly, will protect them against I/O
> >induced faults.
>
> Definitely. They are mostly vulnerable during assembly.
> Even if the circuitry isn't designed all that properly, when they are
> assembled
> they are much more immune to damage.
>
> I still wouldn't advise grabbing ahold of your $400 graphics card, walking
> across a nylon carpet wearing rubber-soled shoes in winter, and arcing
> the gold fingers to grounded metal, though.
>
> ;-)
>
> Just using a wood bench with a concrete floor is enough to avoid many
> problems, especially if there is some reasonable humidity, it doesn't
> have to be South Florida.

I just had an interesting experience re: static. I was working on
the DAC board of the parallel port motion control board set. I
have a big metal bench where I do the assembling, soldering, rework,
etc. I have a smaller, particle-board topped bench behind the big
one which has a computer and a logic analyzer on it, it is the
'test bench'. I had just changed a couple parts on the board,
and swung my chair around to plug the board in to the little
motherboard that was made for it. The power supply was off,
but the motherboard was still connected to the computer's parallel
port, and the computer was running. When I placed the board
into the card edge connector, there were visible sparks on several
of the contacts! I sort of jumped with surprise, checked that the
power supply was off, and then thought "Well, now how many
chips am I going to have to replace?" Amazingly, the entire
board (and everything else connected, too) worked. (In fact,
that component change had fixed the trouble I was having, too!)

So, it appears that components have gotten a LOT tougher
in recent years! Most of the external signal lines go to a Xilinx CPLD
chip, so their static protection circuits are definitely working!
This is the first time I've ever seen circuits survive visible sparks
with no damage. I must have moved my shoes on the carpet
(or maybe my rear on the chair) to build up that big a charge.

Jon