twb8899 wrote:
> Adam,
>
> In our process we used new drill bits for multilayers to mainly limit
> the amount of smearing. There could have been a requirement for this
> in the MIL-55110P specification as well. Some MIL requirements would
> require stuff like that. A new sharp bit helps big time on
> multilayers since you need a good sharp cut through the inner layer
> copper. These multilayer panels were so expensive that the bit cost
> was a small fraction of the overall cost.
Yep, I understand. Thakns
>
> When we went to the newer activated palladium catalyst in 1990 we no
> longer needed desmearing for commercial multilayers. We only did a
> slight etchback with straight technical grade (92.8%) sulfuric acid.
> Desmearing was still performed on military boards though because of
> the MIL specifications.
>
Wow, concentrated sulfuric acid !. Wasn't this a little dangerous
working with ? Was it heated or at room temperature ?
Conc. sulfuric is extremely hydroscopic and rips moisture out of the
air until the concentration reaches an equilibrium of about 30%. How
long did the tank last ? The acid is viscous like vegetable oil, dose
that cause a lot of the acid to stick to the PCB when you pull it out
? I imagine a lot of acid would be used in this part of the process
and this acid would use up a lot of alkaline for neutralization.
I've seen what this stuff does to organic materials, like wood, paper.
I had a pin drop of conc. sulfuric accidentally get on my finger
while pouring it from a bottle and it felt like someone jabbed me with
a hot soldering iron there. I was lucky to be right next to a water
tap so I was able to rinse it off in within a few seconds. I always
where a face mask and gloves now when handing the stuff, even a tiny
amount.