Hi, Jason,
The resistivity setting will depend on a few factors:
1. The total area (both sides) of boards you have in the machine during any
given cycle.
2. The cleanliness level you're trying to achieve for the boards.
3. The volume of water in your rinse cycle.
4. How good your machine is at correlating actual contamination levels to
what it measures or reads.
If you multiply your total board area by the cleanliness level you require,
you will get the maximum salt equivalent total weight of contamination
allowable.
That amount of contamination divided by the volume of water will have some
direct relationship to the resistivity delta of the water, which I guess
you can either look up in tables or work out for yourself by
experimentation with known volumes of water at known resistivity to which
you add salt in precisely known increments and chart the resistivity drop
against cumulative increments. Look up the total allowable contamination
weight form your first calculation and compare it with your chart - that
should give you an equivalent resistivity.
Personally, I do not trust cleaning machines that give you a "cleanliness"
readout for a couple of reasons:
1. It gives you a very subjective result - the best job it can give you, no
matter how good or how bad, is always "clean". Why? Because it's using the
same water to tell you so that it's been using for rinsing the boards. If
the machine can remove no more contamination, the resistivity will be high
(i.e. board = clean), but you have no idea how much contamination the
machine has not been able to remove. There is no alcohol in the rinse water
to dissolve other contaminants that are trapping remaining ionics.
2. The reading a cleaning machine gives you is an average cleanliness of
all the boards in the machine during that cycle. It's likely that the
machine does not clean entirely evenly - boards in the corners of the
machine probably don't get the same pattern of cleaning water or rinse
water that a board in the middle of the machine does, so cleanliness
results will vary. Although your average reasult may show that your batch
of boards are clean enough, it is only an average. You will thus wind up
with boards that are cleaner than average and others that are dirtier - how
much cleaner and how much dirtier, you cannot tell.
Therefore to again have a better idea on what resistivity figure to set you
machine to, you will need an independant ionic cleanliness tester to test
several batches of boards using an alcohol/DI Water mix. Your tester will
also have to be checked out so that you have a correlation chart for it as
well (actual salt weight vs machine reading). Test each board individually
and map results against the board's position in the machine. Examine the
variance of the results and see how many boards are dirtier than average
and where they are located in the machine. From this analysis, you should
be able to determine what resistivity value you should plug into your
cleaner such that all your boards come out at least as clean as you want.
Hope this doesn't give you a bad weekend.
Peter
Wilson Jason <[log in to unmask]> 16/04/2003 06:34 PM
Sent by: TechNet <[log in to unmask]>
Please respond to "TechNet E-Mail Forum."; Please respond to Wilson Jason
To: [log in to unmask]
cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: Re: [TN] Cleaning Question - Rinse water resistivity?
Hi,
We do use DI water in the rinse.
We use AAT Corp Emulsonator 9700. It has the feature of monitoring the
rinse cycle's resistivity, and comparing that to a predetermined value.
When the resistivity reaches the predetermined value the rinse stops (if
the time has expired).
Thus I am not sure what to enter for the predetermined rinse restivity
value. Does anyone have a similar machine? We use an RMA type flux.
Thanks in advance for any help,
Jason
>>> [log in to unmask] 04/15/03 04:31PM >>>
Jason,
I would recommend that you use deionized water in your cleaning for
the
same reason that soft water is better for cleaning clothes. DI water
is a
hungrier solvent. I would recommend using 2 megohm-cm resistivity
water
(0.5 uS/cm conductivity) as a minimum. When you get below 2 megohm,
you
get in RO purity water, better than tap water, but not as good a
cleaner.
I also recommend 140-150F for temperature.
A 10 minute rinse is about normal.
Doug Pauls
Rockwell Collins
Jason Wilson
<wilson.jason@NORTHROP To:
[log in to unmask]
GRUMMAN.CA> cc:
Sent by: TechNet Subject: [TN]
Cleaning Question - Rinse water resistivity?
<[log in to unmask]>
04/15/2003 01:39 PM
Please respond to
"TechNet E-Mail
Forum."; Please
respond to Jason
Wilson
Hi Everyone,
I am a new member to the list and I have a question on cleaning.
Thanks in
advance for any help you can provide. It is much appreciated.
What should the value be for rinse water resistivity? The cleaner we
have
is a AAT Corp Emulsonator 9700.
We use an RMA flux (both in wave solder, and in solder paste). The
solvent
we use is EC7R.
Also, we currently rinse for 10 min (+ time to reach resistivity).
Does
this seem low? We are having trouble with board cleanliness.
Once again, thanks for any help provided.
Jason
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