Homebrew PCBs

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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