DJ Delorie wrote:
>
>
>
> More random thoughts...
>
> I was thinking about plated-through holes and how big fabs do them.
> You drill, plate the holes, ∗then∗ etch. They use a tin electroplate
> as the mask. Can we do something similar?
>
> The idea:
>
> Drill and electroplate the pcb.
>
> Use a standard photomask, but in negative - expose where you want
> copper to stay. Develop. Not sure if TT would stand up to the tin
> plating.
>
> Immerse in "Liquid Tin" to tin-plate the copper that's going to
> remain.
>
> Strip the mask and etch the board.
Spent many hours experimenting with tin etch resists. Ultimate finding
was you ∗need∗ a alkaline ammonia etchant. Ammonium persulfate will
attack tin enough for it to be unusable for a etch resists. Even the
copper sulfate byproduct of this etchant will attack tin so some degree.
This is especially true for very thin (< 2um) tin platings as you get
with electroless tin.
Unfortunately alkaline etchants demand more process control compared to
the hobby etchants. Ammonia alkaline etchants are best suited in
production where they run continuously and automated.
Simplest alkaline etchant I've come across is your everyday
ammonia/ammonium chloride at fairly low pH (7.5~8). This has minimal
ammonia fuming yet still fast. The drawback (as with all alkalies) is
you need to add three ingredients to keep stuff working throughout its
life.
1) concentrate aqua ammonia to keep pH > 7
Use pH paper strips, or your nose :).
2) Ammonium chloride is added to keep chloride/copper molar ratio at
about 2:1.
This is much more difficult to test and only way I know is to analyze
both copper and chloride using titration techniques. i.e you need a mini
laboratory.
3) Water to keep copper density around 100~140 g/l
Could be done with a hydrometer, but exact concentration is determined
in (2) above.
If TT can etch resist then it will work as plating resist too.
Problem is when copper deposit overlay edges of the toner. You will
always have some trapped toner.
Adam