15 Dec
2001
15 Dec
'01
2:50 p.m.
On Dec 15, 12:28, Matthew Sell wrote:
Typically, the time spent "in the water"
isn't long enough to damage.
Even
items made of steel and iron won't rust if the
water is removed after the
cycle is complete. If they sit overnight, well, that's a different story.
The only production problem I saw with untreated water was with an
electronic test instrument that had a lot of high-impedance signal
interconnects throughout. Many signal lines ran for long distances next
to
each other. While the design of this piece of test
equipment was
questionable, it was our duty to get it to work.
The two biggest problems were contaminants from the water supply used in
the washing process (city water - switched to using a commercial
filtration
system), and humidity (had to paint a sealant on all
of the boards).
That's a well-known problem. Some of the residues from a domestic water
supply -- especially in hard water areas -- are mildly hygroscopic, and as
a result, the boards would acquire very small amounts of moisture on the
surface, especially when exposed to a humid atmosphere. In combination
with the salts in the residue, this makes for leakage across the board,
which could easily upset very high impedance circuits.
I heard of someone who had the opposite problem. He designed a CMOS
circuit which worked fine when forst contructed, but stopped when given
anti-environment protection or was potted. He'd inadvertantly relied on
the normal leakage across a PCB to hold the unused inputs of a CMOS gate at
a particular level. Remove the leakage current and the gate stops working
properly. Solution: add the pullup resistor that should have been there in
the first place.
--
Pete Peter Turnbull
Network Manager
University of York