Working for a manufacturer of cleaning equipment which has a resistivity 
monitoring system I get this question quite often.  Many times from our 
own sales people looking to eliminate the need for a customer to perform 
separate ionic contamination testing.  As we make batch style equipment 
my comments relate only to that type of equipment, I don't have any 
experience with inline versions with resistivity monitoring.

The simple answer is, yes, you still need to perform a separate test 
with an approved method to ensure you are meeting the ionic 
contamination level in your specification.  All of the approved test 
methods use a DI/Alcohol mixture to ensure any residual flux 
(specifically rosin) is removed from the assembly.  Rosin is essentially 
insoluble in water, so the alcohol is needed to dissolve it.

I have encountered situations in our equipment where no cleaning 
chemical has been injected into the wash portion of the cleaning cycle 
and the resistivity monitoring during the rinse cycle says that the 
water is completely clean.  However when you open the chamber door at 
the end of the cycle, the boards are covered in white residue.  The 
rosin was never removed from the board because its not water soluble and 
the resistivity monitoring system can only detect substances which are 
dissolved in the water. When you place these boards in a DI/Alcohol 
tester the resistivity value drops immediately.

The resistivity monitor in the cleaner can assist as a guideline for how 
the cleaning process is running.  You can determine a relationship 
between the value you see in the cleaner and the value you get with the 
ionic tester.  However the correlation is very process and chemistry 
dependent. 

By looking through some test data we had from demos and sign offs I was 
able to determine in one of our machine models, with a specific cleaning 
chemistry, a water resistivity of 150 kOhm would pass the MIL-Spec for 
ionic contamination.  However the resistivity sensor in that model was 
not temperature compensated and so an equivalent resistivity in our 
newer equipment would be 270 kOhm since its system is temperature 
compensated to 25 degC.  Different cleaning chemistries also have 
differing percentages of ionic species in them, making the resistivity 
value for one type of chemistry and even percentage of chemistry 
different from another.

All of this boils down to the resistivity monitoring built in to the 
cleaners provides a good check for a stabilized cleaning process which 
has already been qualified to a specification.  The water resistivity 
might let you know that something is going wrong with your process, but 
it can't tell you the assemblies were being cleaned to begin with.

Hopefully this answers you questions, if not please let me know.  This 
all seems clear to me, but as the people I work with would tell you I 
don't always give the clearest explanations.

Steve Pence
Unit Design, Inc.


- bogert wrote:

> July 26, 2007
>
> I have a question to pose for Tech Net folks based on my limited 
> experience in this area.  If I have a cleaning machine that includes a 
> built in continuous test for rinse water or wash water resistivity, do 
> I need to then still do an independent ionic contamination test using 
> the standard DI Water/Alcohol Ionic Contamination test?  If so why?  
> What is the technical relationship between the two tests?  Can one 
> equate the cleaning machine Resistivity Test results to the J-STD-001 
> cleanliness limits or the results of the Ionic Contamination test?  If 
> so, how?  Have folks experience shown that one can pass the cleaning 
> machine Resistivity Test and still fail Ionic Contamination Test?  Why?
>
> Doug, Pauls, as the worlds (wow) expert in this area, what do you say 
> (It depends)???.
>
> ---------------------------------------------------
> Technet Mail List provided as a service by IPC using LISTSERV 15.0
> To unsubscribe, send a message to [log in to unmask] with following text in
> the BODY (NOT the subject field): SIGNOFF Technet
> To temporarily halt or (re-start) delivery of Technet send e-mail to 
> [log in to unmask]: SET Technet NOMAIL or (MAIL)
> To receive ONE mailing per day of all the posts: send e-mail to 
> [log in to unmask]: SET Technet Digest
> Search the archives of previous posts at: 
> http://listserv.ipc.org/archives
> Please visit IPC web site 
> http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional 
> information, or contact Keach Sasamori at [log in to unmask] or 
> 847-615-7100 ext.2815
> -----------------------------------------------------
>

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 15.0
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------