Hi Edward,
In answer to your question about HCl and H2SO4, there's really no problem
using either acid to baseline weight gain and even acid resistance.  And
yes, you will see a difference with each acid.  We perform weight gains
using H2SO4 and measure the gain in mg/cm2.  Other people use HCl and
record it in microinches.  I think it's pretty much a matter of preferance.
 Each is repeatable and will give consistant results although HCl will
normally show a higher number.  Personally I prefer to use H2SO4.  I
believe it to be more accurate since the HCl does strip copper as well as
the oxide.

After performing a weight gain on one coupon we also perform acid
resistance tests on another to find out how well the DMAB did its job.  The
only thing I would caution you about is to use the same acid on this test
as you did the weight gain.  Running a gain using HCl and a resistance test
using H2SO4 won't yield  comparable results.  I didn't see any mention of
the acid resistance test in your letter but it's a good idea to perform
this test at least daily.  It confirms the effectiveness of the reduction.
It's also the best way to determine (and maintain) proper hold times
between oxide and lay-up and to determine bake cycles.  Reduced oxide can
re-oxidize due to excessive hold times, staging atmosphere, excessive
temperature in bake or simply because of a poor reduction in DMAB.  The
acid resistance test is fast and quantifies the reduction.  Normal
expectations on a 25 to 35 mg/cm2 oxide coating would be to see less than
.09 loss during resistance test.  More than that can hint at possible pink
ring.  If it gets up towards .15 or more you have a real problem with the
coating thickness or reduction process.

And commonly we use 10% HCl by volume as we do with most all "indicator"
type tests.  I believe that to be the normal accepted method especially
when doing a visual test such as the one you described.  That method, using
HCl to see if attack occurs on reduced oxide, is indicative at best.  It's
not really accurate and certainly isn't quantitative.  I've seen panels
pass that test yet fail the acid resistance test and in the end showed
areas of pink ring.

Good luck.

----------
> From: Eltek Ltd. - Process Engineering <[log in to unmask]>
> To: [log in to unmask]
> Subject: [TN] Black Oxide
> Date: Tuesday, January 20, 1998 1:10 AM
>
> Hi technetters !
> We are changing now our "  old thick black oxide "  ( 0.7 mg/sq cm )
> followed by DMAB reduction to thin oxide ( 0.35 mg /sq cm ) followed by
DMAB
> reducer .Following my  previous question regarding  "conducting reduced
> black oxide" some additional points arosed :
> 1 ) the thickness is being measured by stripping the oxide from dummy
panel
> ( both reduced and not-reduced ) . The differences between " reduced "
and "
> not reduced " oxide are reasonable , but  we have different results with
> H2SO4 stripping and HCL stripping . Both stripping solution are fresh and
> the ammount of dissolved copper is even not visible . Which method is "
> common practice " and why the results are different .
> 2 ) We are checking the quality of reducing process by 10% HCl for very
long
> time . Just now we are not sure about proper definition of " 10 % HCl " :
it
> is 10 % by volume of commercial HCl ( 35 % ) or 10 % by weight of pure
HCl (
> 1: 3 dilution of commercial HCl ) . What is the common practice and
expected
> results ?
> Best regards
> Szpruch Edward
> Eltek Ltd - Israel
> Tel  972 3 9395050
> Fax 972 3 9309581
> E-mail :  [log in to unmask]
>
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