Homebrew PCBs

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.