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.

Adam,

The sulfuric acid was like a thin syrup and was used at room
temperature. It would take on a slight red-brown color as the epoxy
level built up. We used a very narrow tall polyethylene tank with a
floating polyethylene cover to keep the air out. The next two tanks
were just a water dip. We would monitor the acid tank for proper
specific gravity and adjust it as needed with fresh acid. These tanks
never wore out. I built these tanks from the same polyethylene
material that the acid drums were made of.

The acid tank would always need additions to the operating level
because so much was being dragged out due to the high viscosity. We
would drain off the acidic water from the two static rinse tanks and
use it for pH adjustments in our waste treatment system and to
neutralize the caustic solutions when changing our cleaner
conditioner tanks and photoresist stripper solutions. Safety was a
big issue and the guy running this process wore a face shield, gloves
and plastic apron. The panels were always held by a racking system
and almost never touched so we had very few problems.

Tom

> 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.