Hello Max
I have been attempting a similar process for producing
double sided plated through holes, and I can not agree with
you more about it being a long and involved system than
first meets the eye.
I am far from the stage of being able to make a reliable
board in relatively short time. As you probably know there
is a ∗LOT∗ to learn especially if your mixing and analyzing
your own chemistry.
To answer you question on tin plating resist compatible
enchants then I should tell you what I've found. I am using
electrolytic tin as the metal resists primarily for low
toxicity of the plating solution, compared to standard
fluroborate tin/lead solder plating baths. The tin bath is
stannous sulfate/sulfuric acid/brighteners which I assume is
what you are using.
According to the popular Coombs' PCB handbook. I read that
FeCl3 etchants attack all common plated metals except gold,
so I didn't even try etching with this. I also read that
CuCl2 etching attacks tin and solder plating resists and
again never attempted using such an etchant.
The Coombs' PCB handbook stated several etchants that were
use for tin plated boards. Two of these included persulfate
etching and hydrogen peroxide/sulfuric acid. I extensively
tested the ammonium persulfate compatibility with tin plated
boards and found that the tin is slowly etched during the
period of time it took for the copper to etch. The results
were random and some areas it would etch completely through
while other areas just made the tin black and discolored. It
was not reliable enough. The Coombs' book mentioned
phosphoric acid (20-50g/l) as an additive to the persulfate
etchant , but I only found it to improve the tin resistant
slightly in bubble etching and the tin was black and awful
looking after etching.
http://www.thinktink.com/ is a company selling lab
equipment for producing PCBs. One process includes making
tin only pattern plated boards and etching with H2O2/H2SO4.
I sent them a letter asking about the results of the etching
and they said the tin is black and discolored after etching,
and must be plated quite thick( > 10 um) for it to work. I
wasn't happy with that answer and so didn't bother to try
H2O2/H2SO4.
Another problem with tin plate corrosion , even if it
doesn't completely corrode through, is that the
electrochemical reaction between the copper and tin metals
can cause a large variation in the etch rate at vicinity of
the tin/copper interface. Spray etching will solve the
uneven etch rate problem due to electrochemical reactions.
More research lead me the the ammonical alkaline etchants.
The etchant that was most interesting to me was sulfate
based ammoniac etchant. This etchant contains ammonia (for a
pH > 10)/ copper sulfate (1.0 Molar) and ammonium sulfate
(0.5 Molar). The problem with this etchant is the ammonia
must be very high for appreciable etching rates at room
temperature and therefore can only be used in a closed
container. It also must be stored sealed when not used for
long periods or slow loss of ammonia will occur. Even tin
plating thickness of just 1 um are completely untouched by
this etchant, and retain its bright mirror finish, even
after long periods in solution (10 times longer than etch
copper time). This etchant can be directly regenerated by
electricity using stainless steel electrodes. The anodes are
permanently installed in the tank which are used during
electrolytic regeneration. See,
http://www.elo-chem.com for
commercial etching machines using this etching chemistry.
The most common etchant for tin plating resists used in PCB
shops is the ammonical chloride. This chloride etchant is
much faster the sulfate version and does not need such high
pH (<8.3) and ammonia can be reduced to a level where it
doesn't fume. Some quick experiments with this etchant
showed it to have only a minimal effect on the tin, it
therefore may not be good enough for bubbles etchants
because the electrochemical effect. I have not tired this
and so cannot be certain. The higher the pH the lower
electrochemical potential difference bweteen copper and tin.
Adam
Max Davies wrote:
> Hi everyone.
>
> A newbie to this group, I am (perhaps over-ambitiously) attempting
> to set up a micro through-hole plating line at home. It's a lot
> more involved than I ever thought it would be, but I now have most
> of it working fine. ...Except what should be the easy bit - the
> final stage - the etching!
>
>
> To give a brief description of what I do...
>
> STAGE 1: Starting with plain, copper-clad board, I do the drilling.
> STAGE 2: Apply photopolymer laminate, then expose & develop. (This
> is positive photopolymer, so the copper which will ultimately become
> tracks is exposed to the air on developing the pattern)
> STAGE 3: Go through a 6-stage process to electroless-plate the
> entire thing (including the holes & edges). The copper cover it puts
> on is pretty thin, but uniform at 1-2 microns.
> STAGE 4: Electroplate the copper - this gives reasonable thickness
> to the copper applied in stage 3 - I aim for 25 microns.
> STAGE 5: Electroplate with tin to 10 microns. This
> (theoretically!) protects the copper from etchant.
> STAGE 6: Remove remaining resist. Then etch. This should leave
> the tin-plated areas untouched, everything else should be zapped.
>
>
> But alas no! Stage 6 fails, because both tin ∗and∗ copper are
> etched, leaving me with a fine, blank piece of fibreglass! It
> matters not whether I use Ferric Chloride or Sulphuric/Peroxide
> etchant - they both destroy what has been so lovingly created!! So
> I reckon there must be something awry with my tin plating.
>
> It's a total impasse, and nothing I do, from increasing/decreasing
> current to re-formulating the tin plating bath according to
> manufacturer's instructions seems to change things.
>
> Does anyone else use a similar process, or have any clue about what
> might be the problem here?
>
> Max.
>
>
>
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