TSmith:

There are several ways to control the rinse water flow in your facility.
 Conductivity meters with their relay set point wired to a solenoid valve is
certainly the most accurate way, and it accounts for changes in your
production rate.  However, it is also the most expensive to install and
maintain (I have seen operators move the conductivity probe into the process
tank so the rinse flows constantly!).  Another option is to install flow
controls at each rinse.  This could be a metering valve / rotameter
combination or a simple in-line elastomer flow control.  These options are
cheaper, but they do not automatically account for changes in production
rate.   For manual lines you can install push buttons or foot valves with
time delay relays, so the water only flows when an operator is standing at
the tank.

Although several manufacturers have a feed water specification for what goes
into the tanks (i.e. DI water above a specific resistivity), I very rarely
see a facility which has a specification in the rinse tank.  That's too bad
because unless it's a spray rinse, the board doesn't come in contact with the
feed water, it comes in contact with the rinse water in the tank!

The quality required in each rinse tank will depend on each specific process
/ rinse tank combination.  Obviously a process like Entek will require a
cleaner rinse than the rinse after rack strip.  Remember that getting the
process solution rinsed off the board is required not only to clean the
board, but also to keep rinse water contaminants from building up in the
subsequent process.  For instance, chloride buildup in an acid copper plating
bath is sometimes due to drag in from a city water rinse before sulfuric dip.
 Contact the chemistry supplier of the process before and after the rinse
tank, and ask them about acceptable contamination levels.

Some processes actually require a certain amount of contaminants in the
rinse.  Suppliers of dry film sometimes will recommend against DI water in
the developer rinse, because they want some of the buffering effect
associated with contaminated water.

I do not know anyone who has actually taken the time to determine individual
quality requirements for each rinse tank.  Most facilities pick a controlled
flow rate out of the air, like 2 gpm, or a conductivity setpoint like 100 or
500 microsiemen/cm [uS/cm], and use that throughout the shop.  I can tell you
that of the ~100 PWB manufacturers I have surveyed, probably 90% of them have
rinse water conductivities between 150 and 750 uS/cm (when starting with DI
water).  That doesn't mean their right, it's just what the average figure is;
most of them are wasting water in one place or another.

Remember that when you determine the desired setpoint, you must account for
the conductivity of the incoming water; if the feed to the rinse tank is city
water, and your city water conductivity is 500 uS/cm, then all the flow in
the world won't bring that rinse down to 100 uS; plus, the city water
conductivity can change on you throughout the year.

Also, what you're really after is the concentration, or Total Dissolved
Solids (TDS), of what's in the tank; conductivity is not an accurate
measurement of the TDS when you are dealing with acidic and basic rinse
water.  A 10 mg/l solution of NaCl will have a conductivity of about 17
uS/cm; a 10 mg/l solution of H2SO4 will have a conductivity of about 70
uS/cm.  In general, neutral pH solutions have a lower conductivity to mg/l
ratio than acidic or basic solutions. 

Counterflow rinses were mentioned in the previous response by John Sharp.
 This is an excellent way to get the full use of your water when trying to
meet a conductivity setpoint.  In a well designed counterflow rinse, each
additional counterflow will follow an inverse logarithmic function of the
previous rinse.  If the first rinse is 1000 uS/cm, and the second is 100
uS/cm, then the third will be 10 uS/cm, the fourth 1 uS/cm, and so on.
 Notice there is diminishing returns with each additional rinse (the second
tank drops by 900 uS/cm, the third by 90 uS/cm, and the fourth by only 9).
 The Sheldahl example doesn't follow this because the water is so clean that
it is being contaminated by CO2 at the air / water interface (uS/cm is the
same as micromhos).  Most manufacturers stop at two counterflow rinses,
sometimes three.  Make sure the inlet and outlet of the tanks are such that
it forces the water in the tank to turn over (don't feed in the top and
overflow from the top).

Another water saving technique is dragout (stagnant) rinses.  These rinses
are simply dumped periodically when they get to 5-20% of the process bath
strength, or they are used to make up for evaporation loss if the process
tank is hot.  This is often done after permanganate.

One things for sure:  If you want to minimize your water use, first minimize
your dragout rate.  The lower the dragout, the less contaminated the rinse
will get, and the less water you will need to use.  If you need suggestions
for minimizing dragout, I can give you some, but it's too much info for this
e-mail response.

Know your dragout rate!  This is important information; if you don't know it,
you can estimate it by using the following conservative approximations for
vertical processing: Manual = 1.5 gal / 1000 ft2; Automatic = 1.25 gal / 1000
ft2 (this is surface square footage, i.e. count both sides; 18"x24" = 6 ft2).
 Dragout from horizontal equipment will vary greatly from one machine to the
next, and it really pays to maintain those rollers.

Dave Chew
Kinetico, Inc.
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(510)777-1410