Dear joef:

I would assume you are monitoring the specific gravity as a quick
measure of total dissolved solids.  Because of the variablity of make-up
between different permanganate baths (sodium vs. potassium, the
concentration of sodium hydroxide, chemical vs. electrical regeneration,
etc.), I am leary of providing any acceptable TDS operating range.  Have
you asked your vendor for the TDS operating range specific to your
application?  Even when the bath is being maintained properly, including
continuous filtration, it has been our experience you will need to dump
and make-up a new bath periodically.  If your vendor does not have data,
I would begin measuring the TDS regularly, recording the result, and
establishing 
a dump cycle specific to your observations.

Regarding "gouging" experienced during etchback, I would focus on the
integrity of drilling and you solvent swell cycle.

Regards,
Ted Stern
Circuit Research Corp.

joef wrote:
> 
> 
>      How does one determine when (if) a sodium permanganate bath is spent??
> 
>      Problem: Our previously calibrated [Mil-class] etchback cycles for
>      various epoxies are now requiring 2-3 cycles; sections show too much
>      variability from .0005"-.0013"; lots of "gouging" into glass-rich
>      regions with lowest etchback at the copper interface.
> 
>      Conditions are as follows:
>      - All chemistries (solvent swell, permanganate, glass etch) are within
>        vendor specs.
>      - NaOH normality: 1.2-1.3
>      - NaMnO4 concentration: 60-70 g/l
>      - MnO3 conc. 9 -12 g/l
>      - S.G.: 1.13
>      - 180 gal tank with 500A continuous electrolytic regeneration
>      - swell temp: 175 degF,  permanganate temp: 185 degF
> 
>      Vendor's tech service won't touch this one - if you want to do
>      chemical etchback "you're on your own"...
> 
>      Any ideas out there before I dump this bath?
> 
>      Thanks, Joe Felts
>      PC World, Toronto

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