Tom,
There are several things for you to consider when thinking about DI water.

>
>  I am a small contract manufacturer located in San Jose.  We use OA flux
>  and water soluble solderpaste for wave-solder and reflow, repectively.

In my opinion, a 100% soluble water soluble flux exists only in the minds of
marketing types.    The question is - have I removed enough of the flux so
that I have no problems?  It gets back to the question of how clean is clean,
or how clean is clean enough?

> I currently use city water for cleaning(approximately 500 gal/day).  I have
>  not run into any contamination problems on the PCBs from this process
>  yet.   Is there a need to switch to DI?

I would suggest that you convert your wash sections to at least RO quality
water, with DI as preferable.  We get several dozen projects each year of
bare boards and assemblies that fail from electrochemical mechanisms
(electrical leakage, corrosion, metal migration).  Many can be traced back to
cleaning with tap water during fabrication or assembly.  Most manufacturers
use tap water for the obvious reason - it's cheap.  As I often explain to
customers, the difference between a $20,000 circuit board and your favorite
pair of jeans is not much, from a cleanliness standpoint.  If you want your
jeans clean, then you clean in softened water, not tap water.  The tap water
has all kinds of impurities that interfere with solvating the soils and
interfere with the effectiveness of any detergents you may add.  It is the
same for your circuit boards and assemblies.  Tap water is a poor cleaner.
 It solvates poorly, has problems with surface tension, and fights any
saponifier you put in it.  Most city water treatment plants add both chlorine
and sulfate to water to kill bacteria. I know of one manufacturer who went to
great lengths to specify halide free solder pastes and fluxes, only to put on
10 times the flux amount from the tap water.

Another responder indicated that if your customer is happy now, why sweat it.
 I think that if you are wise,  you will work to improve the quality of your
board or assembly.  By doing so, you may avoid problems when your customer
calls for a design more sensitive to contamination, or when they change to a
cleanliness spec based on ion chromatography.  In addition, giving your
customer an assembly that is markedly cleaner, you help increase the
reliability of the product.  Maybe your customer has a problem with
intermittent electrical leakage they can't pinpoint.  If your product was
cleaner, that nagging problem might go away.

>  If so, what are some of the pros
>  and cons of using DI water?( cost, effectiveness)  Also, if I do need to
>  change, what is the best way to setup a system?  Our current in-line
>  cleaner consists of two wash chambers, rinse chamber, and a final rinse
>  chamber that can utilize different source of water from the previous.

In my view, 500 gallons per day is not high volume.  You can get some quotes
from your local water treatment professionals (e.g. ours is Culligan), who
can arrange to set up 4-6 industrial DI columns and then periodically change
/ rotate them depending on your need.  If cash is a real problem, then look
into cleaning up you water to RO levels rather than DI, with perhaps a good
DI water rinse.  I would NOT recommend plumbing in softened water (lots of
chloride from the salt), at least without some final filter to remove the
salt residues.

I would also recommend the use of a small amount (3-5%) of an effective
saponifier in your first stage wash.  It will help you in a number of ways.
 I have a few formulations that I prefer.  Get with me off-line if you want
details (no commercials here).

I hope this was helpful.  Let me know if you have more questions.

Doug Pauls
Contamination Studies Labs
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