Try it!!!! (Was - Re: One More PCB Dishwasher Question)

From: ajp166 <ajp166_at_bellatlantic.net>
Date: Sat Dec 29 14:05:26 2001

From: Bob Shannon <bshannon_at_tiac.net>

>TXCO's come from the factory with special caps you place over them while
>they go through the soldering oven (modern boards don't get wave
soldered anymore).


Older ones were effectively sealed units with a removable screw opening
that
was sealed with an O-ring. Those stand dishwaser just fine. Of course
many
computers never have anything like a TCXO on them.

>Failing to place these protective caps over the TXCO's when they go
through the
>wash cycle leads to a drifting oscillator that very often fails soon
after its in
>the field.


This is to be expected with any sealed part or temperature sensitive
part.
Then again a reflow oven is far hotter than a dishwasher.

>As the focus of the list is older machines, we need to keep things in
mild like
>paper roll caps, etc. Advising that running old boards through the
dishwasher,
>while sometimes safe, is not an absolutely safe thing to do.


Actually caps like that are likely to have failed from age by now.
Systems
that used them are likely quite old and not of the easily dishwashed
contruction
for mechanical reasons other than components used.

>I do agree that most often, semi-modern boards will survive the process,
but there
>are many components that will not. These components used to be much
more common
>than they are today. But as the discussion relates to this older
technology, any
>reccomendation to run the boards through a dishwasher should address the
very real
>risks.


Modern as in PDP-8/11/ and vax series, flip chip and similar are
certainly
cleanable this way and likely were in the factory back then too. Older
modular constuction of a more hand wired era may be not suitable. Then
again most of the DEC wirewrapped backplanes would likely survive a
dishwash but, it may be ill advised as they are mechanically fragile as
those that have worked with them know.

>How hot was the water? I don't know, its not something I can easily
control. Is
>is possible that the 'dishwashers' used for this function commercially
have been
>altered, and/or are connected to a lower temprature source of water? I
do know
>that dishes come out a bit too hot to handel unless you open the door
and allow
>them to cool.


Generally domestic hotwater never exceeds 160f due to scalding risks for
the users. Some dishwasers have reheaters to compensate for low domestic
water temps but they still only shoot for 160ish (F) max, and often that
can
be turned off by using the economy cycle. The bake dry cycle should be
avoided if there is one (or too warm). One thing we ar not talking about
is
temps near boiling (212f) or water that hot.

>I do know its a heck of a lot hotter than any bath, after all, there is
a heater
>element inside the dishwasher.


Usually for dry cycle, sometimes powered to compensate for low domestic
water temps. Econco cyle turns if off more often than no. bath water is
maybe
105-115f (Very hot!) FYI. People are susceptable to harm with water over
130f.

>I'm not sure its a temprature issue, as some have assumed. A dishwasher
may have
>very powerful waterjets and a lot of vibration. The dammage may be
mechanical,
>possibly a bonding wire detachment.


No, internal bonds for the parts can take that shock and likely 10X that
all day.
External bonds??? We are talking soldered boards not wire wrap or really
old
MIL spotwelded.

>But I'd like to point out once again that there are a good number of
components
>that will be dammaged by water. Some of these have been listed in posts
here
>already. If we accept that some components cannot be washed in this
way, how can
>anyone defend a blanket statement that using a dishwasher on a board
will be safe
>for that board?


Most of the components are of the "open" contruction and not suitable or
the problem
of assuring they will dry needs addressing. The average printed circuit
construction
used in computers often does not contain them or they are designed to
allow for that
kind of cleaning.

Parts I worry about and see:

    Pots (variable resistors of enclosed design)
    small relays of non hermetic design
    DIP switches (may need replacing anyway)
    Power upplies in general, (other than potted units).

>All those tiny little pulse transfromers on your core memory sense
amplifiers, do
>you know those are able to withstand this treatment? Many are not fully
>encapsulated, and would not be safe to treat this way. Some components
(like
>crystals, not oscillators, just quartz crystals) cannot even be soldered
safely,
>and are socketed for this reason. Is it s good idea to run these
through your
>dishwasher?


Soldering is high stress compared to 160f water. Also of they are
socketed
then by all means unplug them first then was the board. Most quartz
crystals
however are hermetic and can withstand significant amounts of heat.
Again
oen design parts have to be evaluated, most tolerate wetting well if
properly
dried before use.

A core memory sense board fo the PDP-8e/f/m design and era tolerate this
very well, then again they are of modern design. I have done it to
several with
at least one comming out working where it didnt' before! Cleaning prior
to
troubleshoot was to make life easier in that case but instead removed
whatever debrie causing the inital problem. Something from the PB250 era
would be more suspect, mostly due to a multitude of other reasons.

I've done it as well to the PDP-8/f front pannel (rotary switch and many
lamp
sockets) with excellent results. It's still working well over two+ years
later.

>If your sure no components will be effected, go ahead and try your
dishwasher. If
>your not absolutely sure, or if replacement parts are hard to get, don't
take the
>risk, and use a little IPA and some elbow grease to clean your boards.


That first half is fair advice, be sure first. The second half is faulty
however as
there are just as many parts that will not tolerate IPA for extended
times or
the residue that may be left behind if not adaquately rinsed.


It's fair to use caution but, to be a nelly maid over it is usually not
warrented.

Allison
Received on Sat Dec 29 2001 - 14:05:26 GMT