In a message dated 96-11-06 22:10:45 EST, you write:

>I would like to know what will be the best way to clean sludge inside solder
>stripper chamber. we are using HNO3 based chemistry and we tried different
>way like HNO3 with HCl or Sodium hydroxide with sodium glyconate, still it
is
>difficult to remove all those sludge which not only short the bath life ,
but
>also cause salt stains on the copper surface, even mild microetch has
>difficult to remove those white salt spots. Any good suggestion? 

The sludge is largely Stannic Tin oxide which is not soluble in to much
except caustic, and HF.  It is likely that you should clean in two steps,
first use dilute Nitric to get all the Ferric that is in the stripper out,
then use Caustic Soda, and remember that the Stannic Tin oxide is ACIDIC, and
consumes Caustic, so you may have to use a lot, Try 10% of 50% material.

To check to see if you have enough, when you have the caustic in the sump,
turn it on, and heat it up as much as possible, and when it is at full
temperature, measure the pH.  The pH should be well over 12, if not, add more
caustic.  (Some pH meters do not read well over pH 12, and even in pure
caustic soda, which at 4% active, has a pH of 14 will only read pH = 12.4, or
so, an exact reading is exact to predict.)

This should do a real nice job.

The white spots are Stannic Tin oxide also, and likely will need a caustic
cleaner to remove, but then after this you should not need to, right??

By the way
>compare to Sn-Pb solder resist, The pure tin seems more easy generate
the>sludge.

You bet, the Lead part of the stripped resist is in solution, but pure Tin
all comes out as that White S**T, so you will see more in stripping a pure
Tin resist, than one that is only 63% Tin.

Are there other ways we can assist you with problems related to Tin
stripping?

Perhaps you need to look for a Tin stripper chemistry that keeps the Tin is
suspension, so that it all rinses out......(How am I doin' David B??)

Rudy Sedlak
RD Chemical Company

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