> Leslie,

It really is time to disspell the myth that you require ANY free acid at all. The only "benefits" you receive
from having free acid present are a lower etch factor, wasted chemicals and unstable etchant. The only reason
that you "require" free acid is because the normality probe in those type of systems will not work below .5N
or so. This means that the ORP must be kept artificially low to maintain enough cuprous chloride (which does
not etch) in the system to act as a buffer against a reaction with all that acid, releasing chlorine. By
using light transmission to control the etchant, you are looking at the phase change between cupric hydroxide
and no cupric hydroxide as far as controlling acid, which happens to be at zero normal. To bring back the
speed you lose, the etchant is brought up to a level above 30 ounces per gallon (30-34) so that you place
more cupric chloride in contact with more copper at one time. As an example, etchant set up at 2.5N, 18
oz/gal will etch about the same speed or lower than 0N, 32 oz/gal. The difference between the two is an etch
factor of 2:1 with 2.5N acid vs. an etch factor of 6.7:1 at 0N.

At zero normal with a sodium buffer, the benefits are higher etch factor, rock stable chemistry, rock stable
etch rate with no concern for board spacing, no chlorine smell from the etchant, lowest cost, lowest scrap
rate and the highest value spent etchant.

How about you contact me off-line for a full discussion about this.

e-mail: [log in to unmask]
tel. 626-256-6557

Best regards,

Phil Culpovich

> ------------------------------
>
> Date:    Fri, 6 Nov 1998 08:25:16 -0500
> From:    "Leslie O. Connally" <[log in to unmask]>
> Subject: Re: FAB: Cupric Chloride etching
>
> Hi Josh,
>
>         I'm not sure what the definition of an average inner layer etch line
> is, but I can provide some information.
>         The parameters to track are as follows, but not limited to those alone:
>         1) Normality -- We run a normality of 2.6 here, but if you are looking
> for a better etch factor or fine line improvement, I would suggest a
> normality of 1 - This will etch slower but provide greater quality.
>         2) Oxidation/Reduction Potential (ORP) - for the most stable etch rate
> 420 to 460 millivolts.
>         3) Copper content - Usually controlled by Baume' or specific gravity.
>            This can vary somewhat but about 20g/liter or a Baume' of about 32
>         4) Conveyor speed - Equipment dependent
>         5) Etch rate or weight loss from a fixed size coupon. Evaluate top to
> bottom variability.
>         6) Spray pressures. - again equipment Dependent (est. about 30 PSIG)
>
> I hope this is of some help, If you want to pursue further, contact me off
> line.
> Regards,
> Les
>

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