[log in to unmask] wrote:
> 
> Our printed circuit manufacturing facility is currently investigating ways to
> reduce the amount of water used in its rinsing operations.  I understand that
> conductivity meters could be used to ascertain the amount of contamination
> within the rinse water; however, is there a predefined limitation as to the
> amount of contamination that would be considered acceptable before product
> quality would be jeopardized.  In other words are there any established
> guidelines to follow when attempting to regulate rinse water flow rates using
> conductivity as the basis for water quality.
> 
> Any assistance regarding water conservation would be greatly appreciated.____________________________________________________________________________

Conductivity meters will work to reduce water consumption.  I am not aware of published data which defines 
predetermined limits for rinse contamination although your chemistry supplier may be of assistance.  The limits 
would depend on the baths on either side of the rinse.  Ideally you should characterize each bath and determine 
the contamination level that can be tolerated by the subsequent process bath.  The water flow out of the rinse 
tank must be of sufficient volume to reduce the contaminate concentration below the level that the subsequent 
bath can tolerate while the work is in the rinse.  All of this can be pretty intimidating as a theoretical 
calculation.

The practical side is determine the amount of incoming contamination the subsequent bath can tolerate and run 
some experiments.  In the lab you can add increasing amounts of contamination to the rinse water, measure the 
conductivity and predetermine the setting for the meter.  Adjust the water flow on your rinse tank so 
the meter indicates clean before the work is removed.  Caution is recommended when attempting to use 
conductivity meters after acid or alkaline soak cleaners.  The darned surfactants don't ionize and therefore 
can't be detected by the meter.  You guessed it -- I've had foam in the microetch from acid cleaner drag out.

The most practical use for conductivity meters in reducing water consumption is in conjunction with multiple 
counter current cascading rinses.  The solenoid valve attached to the conductivity meter ususlly has enough 
pressure drop to cause it to act as a flow restrictor.  The double (triple is better if you can afford the 
space and expense) counter current rinse will exponentially reduce the water flow necessary to prevent 
contamination of the subsequent bath.  The conductivity meter then becomes an on/off device to stop water flow 
when the rinse is clean.  During normal operations, the rinse water will probably run almost constantly but at 
a greatly reduced rate because of the multiple rinses.

The water usage reductions from the use of cascading rinses and/or conductivity meters will be maximized 
through cycle time adjustments to incorporate a "hang time" when removing work from a bath (including a rinse) 
permitting the contiminated fluid to drain back into the tank.  Usually about 30 seconds is optimum.

Properly used a spray rinse before a dip rinse will reduce water usage.  You might also investigate some of the 
ion-exchange technology now being used to recycle rinse water.  Hope this wasn't too long.

Don Vischulis
[log in to unmask]