PCB
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PCB etching with ferric chloride
i'm having a problem in pcb etching using ferric chloride. i have an assignment to determine the dissolving rate of the ferric chloride and the
copper, and about the possibility in determining the time of the etching process, and also about the saturation rate of ferric chloride. can anybody
please help me ?
JR
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chemoleo
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A kinetics perspective
Unfortunately there are many factors coming into it.
For one thing, the surface texture of the copper sheat. If it is rough, etching will work faster (greater surface area). Temperature is an issue, and
pH.
Then the molar excess of FeCl3 vs Cu.
Normally the FeCl3 is in vast excess, so there won't be significant depletion of FeCl3. Therefore you can assume that the rate of Cu dissolution
is linear, until all the copper is gone (i.e.: 1 mole Cu, and 300 mole FeCl3. After the reaction it is ~0 mole Cu and 299 mole FeCl3. So essentially
the amount of FeCl3 changes by 1/300th, which is negligible). The actual value of the rate (i.e. 0.5 g Cu(dissolved) per minute) depends on the
factors above, and of course on the substrate itself (FeCl3) - i.e. etching with Na2S2O8 has a different rate, even if all the other conditions are
exactly the same.
If the molar quantities are about identical, then the reaction will essentially still come to completion, but the actual rate of reaction (while the
reaction is proceeding) will vary, in an exponential decay type of function/equation (i.e. 1-e^(-kt)).
However, because the equilibrium Cu + 2 FeCl3 --> CuCl2 + 2 FeCl2 lies VERY far to the right (and under conditions of FeCl3 or Cu excess), a plot
of Cu (undissolved) versus time will essentially look like ' \_ ', i.e. where the bent is the stoichiomtry point (no Cu left if FeCl3
excess, no FeCl3 left if Cu in excess). The gradient m of the linear section depends on the substrate (Na2S2O8, HNO3, whatever), if all other
conditions are the same.
Do correct me if anyone thinks otherwise - I am only extrapolating from my kinetics knowledge.
[Edited on 6-9-2004 by chemoleo]
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Twospoons
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Is this a theoretical exercise or is there some practical result expected? From my own experience with etching PCBs the most crucial speed factors
(given excess FeCl3) are temperature, and agitation of the etch bath. Poor agitation in particular causes very uneven etching.
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JohnWW
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This subject, etching of Cu, either on PCBs or Cu plaques, with FeCl3 and alternatively HNO3, has been fairly thoroughly thrashed out on the Yahoo
group thechemistrycluster@yahoogroups.com over the last few days. Join it, and see the recent posts on this thread.
I came to the conclusion on that group that such dissolution of Cu metal by FeCl3, which becomes reduced to Fe(II), is possible only in the presence
also of a large excess of HCl which results in the formation of FeCl4- and CuCl4--. It also appears that the FeCl3, as a Lewis acid, may well not be
the actual oxidant, but a catalyst for the real oxidant, either O2 from the air, or oxidants like NO3- added to commercial etch preparations, with the
Fe(II) produced being re-oxidized to Fe(III).
John W.
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Twospoons
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I used to make up my FeCl3 solution from the solid hexahydrate. No added acids, no precipitates, and it etches the copper off PCB laminate just
beautifully.
I don't use it anymore - too messy - ammonium persulphate is much nicer to use.
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PCB
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pcb etching with ferric chloride (cont.)
i'm very grateful for the responses from other members. but i'm still now quite confused. if i'm not mistaken i have gotten the
chemistry equation for the pcb etching process using ferric chloride is something like :
FeCl3 + 3 H2O --> Fe(OH)3 + 3 HCl,
then the process continue into 3 steps, which is :
FeCl3 + Cu --> FeCl2 + CuCl
FeCl3 + CuCl --> FeCl2 + CuCl2
CuCl2 + Cu --> 2 CuCl
are these equation correct ? because i'm an undergraduate student majoring in electrical engineering, and i'm quite not familiar qith
chemistry. i'm now trying to make my thesis involving the pcb etching process with ferric chloride. so, can anybody help to explain me about how
to determine the etching process time, the dissolving n saturation rate of the ferric chloride ? thank u.
JR
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JohnWW
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No, it came out on the Yahoo group thechemistrycluster@yahoogroups.com that the reaction can occur only in the presence of a strong acid, particularly
HCl which forms FeCl4- and CuCl4- ion (and intermediate -OH and -H20 complexed species), and which prevents the hydrolysis of FeCl3 (which can be
crystallized as the hexahydrate only in the presence of a strong acid) to insoluble Fe(OH)3 which would be unreactive.
In addition, I am of the opinion that the Fe(III) , after reduction to Fe(II), is oxidized again by either air or oxidants such as NO3- in commercial
etch preparations, and so is more accurately a catalyst.
John W.
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unionised
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Whatever the Yahoo group say, I have etched copper with FeCl3 solns without added acid, so has twospoons and I dare say plenty of other folks here.
Why do you gainsay something that is quite common experience?
Given that etchant stops working after a while, I don't see how it can be a catalyst.
The copper starts off as the metal and ends up as the chloride. The chloride is used up.
Ferric ions are hydrated in solution and that hydrated species is quite a strong acid so all aqueous solutions of FeCl3 are acid. If you try to
neutralise them the iron falls out of solution.
In answer to the original question, get a piece of copper sheet, weigh it, place in etchant for a while, take it out, dry it and weigh it again.
You might do well to etch the sheet before you start weighing it because the surface will have slightly different properties to the bulk metal.
You need to do this at different temperatures, concentrations and rates of agitation etc. to get the full story.
[Edited on 7-9-2004 by unionised]
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chemoleo
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Oh boy....
JohnWW - excuse me, but ... stop trying to prove things without evidence!!!
By repeatedly insisting on the same thing again and again you don't make anything 'righter' or 'wronger'!
Plus it's not the first time you are doing this (my I remind you of the putative SCl4 in the SCl2 thread that should be formed, according to your
valued opinion?)!
There is NO evidence whatsoever that FeCl3 as an etchant is used in combination with HCl.
In fact I have this for you:
http://www.produktinfo.conrad.com/datenblaetter/550000-57499...
It's in german, but the point is clear - it even STATES the chemical composition of the etchant, which is FeCl3 * 6H2O.
In fact, it is delivered in yellow/orange pellets, SOLID which already makes it unlikely that it contains free HCl (besides the fact that it makes no
mention of HCl on the data sheet).
Also, where did you get this theory of conversion to Fe(OH)3 come from? I used this etchant many times (from the company conrad.de as above), and
never ever did I notice a precipitate. In fact, the solid etchant melts at 37 deg C (crystal water), and does liberate HCL upon DECOMPOSITION!! Just
like Aluminium chloride hexahydrate, or whatever! The HCl that MAY be liberated in solution of FeCl3, if anything, then is just as part of an
equilibrium! Sure you may get all sorts of complex ions, hydrated or not, but the essence of the story remains: Cu + 2 FeCl3 --> CuCl2(aq) + 2
FeCl2.
The fact that the iron III chloride solution is acidic only shows that things are at equilibrium, just like with any other weak base and strong acid.
Also, where does your theory of HCl dissolving Cu come from? It doesn't dissolve it. In the presence of oxygen, it dissolves Cu VERY slowly,
limited by the rate of dissolution of oxygen. And you want to tell me that oxygen is the sole cause of dissolution, while the copper is dissolved in a
matter of 30 minutes, in reality ??? What magic!
Your yahoo group discussion is no evidence either, as you seem to dominate the discussion anyway.
Reusing your FeCl2 solution by oxidation with oxygen? Good luck. Better invest in an oxygen tank, and a few days of bubbling. H2O2 works better.
Hardly a catalyst huh?
Besides - a catalyst's definition is that it can be extracted after the reaction as the original. Definitely not the case here (FeCl2).
Please take more care of your postings in the future. This is a place where critical thinking is desired, and applied when deserved. Noone owns the
ultimate truth, and neither do you, JohnWW. All theory is USELESS if experiment says otherwise!
Anyway.. PCB:
Dissolving FeCl3 hexahydrate in deionised water will give a dark solution, with no precipitate. So FeCl3 + 3 H2O --> Fe(OH)3 + 3 HCl, doesn't
hold.
The reaction FeCl3 + Cu --> FeCl2 + CuCl is, if anything intermediary, as the CuCl is itself insoluble, and would prevent further dissolution of
Cu that is underneath. Hence the mechanism of the reaction is possibly via a two-step mechanism (FeCl3 + Cu --> FeCl2 + CuCl , then FeCl3 + CuCl
--> FeCl2 + CuCl2), or it occurs at once. It depends on the respective rate constants of conversion for each copper chloride, whether there is any
significant quantity of CuCl at any given moment. I doubt it's the case.
Why does it work?
(1) 2Fe(3+) + Cu -> 2Fe(2+) + Cu(2+) : --> Standard E cell = 0.43 V
(2) Fe(3+) + Cu -> Fe(2+) + Cu(+) : --> Standard E cell = 0.25 V
What remains to be checked is the conversion potential from Cu(+) to Cu(2+). It would be interesting to see whether thats the exactly the
difference between (1) and (2).
Anyway, from this info we can't really say whether the mechanism proceeds via CuCl or directly to CuCl2.
The equation 'CuCl2 + Cu --> 2 CuCl ' is unlikely as the FeCl3 is normally in excess, making sure that CuCl is immediately converted to
CuCl2 immediately. So you needn't worry about it.
Regarding 'how to determine the etching process time, the dissolving n saturation rate of the ferric chloride ?' : Unionised said that
already. Test things at all conditions, and make reaction curves of Cu dissolved (measured by weight) versus time, at a given set of conditions. Then
you will get the plot described above. How about you do a few neat experiments, and then get back to us? You can obviously post the results too.
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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JohnWW
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The fact that aqueous solutions of FeCl3 are fairly strongly acid, without any further acid being added, IS evidence of the extensive hydrolysis to
Fe(OH)3 and intermediate species, about which I complain!!! It may not be visible unless and until the hydroxide colloid particles (which practically
cannot react with Cu) reach macroscopic size. At the very least, this hydrolysis MUST reduce the rate of reaction of Fe(III), as the medium of
oxidation, in the etching process, unless its formation is prevented by the addition of acid to shift the equilibrium. Besides, for the Cu to go into
solution, there has to be a source of anions.
As for those "standard cell potentials" you quote, and of which I am amply aware, they are for "STANDARD" cell conditions, which
certainly are far from the etching conditions here. It is easy to fall into such traps, when applying data from "standard" conditions to
non-standard situations, noting again that Fe is above both H and Cu in the electrochemical series.
I have tried searching on Google with this search string: etch OR etching "ferric chloride" acid copper hydrochloric OR sulfuric. Several of
the 606 results definitely refer to acids, particularly HCl, and other additives including oxidants, being added to the FeCl3 solution, either after
dissolution of solid FeCl3 or in a commercially supplied liquid solution for etching, e.g. http://www.phibro-tech.com/03040301.asp and http://www.metprep.co.uk/history6.html .
There is nothing more for me to say on this matter; I know what the facts are. Please stop engaging in hubris and character assassination, and
selective use of information, Chemoleo, for having been beaten to the point. Are you jealous?
John W.
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unicorn
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My 2 cents guys:
Like unionised, I have etched copper PCB's with home made etchant, made from solid FeCl3 with no need for added acid.
An aqueous solution of FeCl3 will do it.
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Twospoons
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John, neither of those links appear to pertain to the discussion at hand. One is a recycler - dealing with spent etchant ( hence the description of
'dark brown/green' ), the other decribes Fe(III) nitrate etchant for revealing grain structure and cracks in copper and copper alloys.
It is true that commercially prepared FeCl3 solutions are often acidified to enhance their performance, but it is also true that plain old FeCl3 will
work all on its own. I speak from hands-on experience.
FeCl3 used to be supplied in anhydrous form, as well as the hydrate, but this went out of favour as the dissolution was highly exothermic.
PCB, I agree with Chemoleo - you need a set of controlled experiments
[Edited on 8-9-2004 by Twospoons]
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chemoleo
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No, I am not jealous, why should I be?
I can be wrong as anyone, just like you.
Except I have not as much a problem admitting it.
So ... let's examine this:
You state the problem of hydrolysis, of FeCl3 to the hydroxide and HCl. Again, it's an EQUILIBRIUM. Of course there will be a tiny amount of HCl,
and Fe(OH)3. BUT - Fe(OH)3 will be REDISSOLVED by nascent HCl, according to
FeCl3 + 3 H2O <--> Fe(OH)3 + 3HCl.
This equilibrium lies FAR to the left, else visible amounts of Fe(OH)3 would be visible (let's ignore invisible colloids for the moment - and as
you should know (if you ever basified a solution of Fe(3+), Fe(OH)3 precipitation can be seen after the tiniest addition of base)
Hence no acid addition is needed as LONG AS the equlibrium lies far enough to the left (which is the case here). Quite unlike the process of lead
dissolution in CuAc2, where the equilibrium situation is such that VISIBLE amounts of lead oxy -hydrates/acetates/carbonates are formed. Hence an
excess of HAc is advisable.
(Edit: Sure, the Fe(OH)3 may convert to Fe2O3 /Fe2O3 * H2O with time - which is not much soluble in HCl - and hence at least partially outside the
equlibrium. But this is hardly the issue, what FeCl3 in solution (degassed, deionised) does after standing for several days/weeks. During the time
scale of the reaction, no Fe-based precipitate can be seen)
As to standard potentials with respect to Pt - why are these far from those conditions? Pt only serves as a vehicle to calibrate the
potentials against. Regardless - I don't mean to debate the accuracy of these values - the only thing they show is that this reaction is
favourable, EVEN in the absence of O2 (as you claim this is necessary for the reaction).
As to your links - the possibility that the etching may be faster in the presence of other oxidising agents doesn't exclude the fact that FeCl3
is PERFECTLY happy to do that job on its own.
So what are your 'facts'? 'Facts' that claim that Cu dissolution can be ONLY done in the presence of HCl, and vigorous oxygen
dissolution? 'Facts' that claim that FeCl3*6H2O on its own is NOT able to dissolve copper? 'Facts' that claim that FeCl3 led you
to believe (I quote) that it acts as a catalyst?
Maybe there isn't much left for you to say on this matter. I agree.
I don't agree that I am 'assassinating your character', I was merely pointing out that theory, as beautiful as you may think it is,
doesn't necessarily hold in reality. Particularly if it is completely sequestered from practical reality. And this, I think, is a criticism you
should be able to take - constructively - rather than accusing me of 'engaging in hubris'.
[Edited on 8-9-2004 by chemoleo]
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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PCB
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PCB etching with ferric chloride (cont..)
i'm very thankful for the help and answers from everybody. i have already intended to try an experience to get a result for the processing time n
the dissolving rate. however, does anybody know about the published lab source on this kind of experiment or maybe the official dissolving
characteristics chart or something like that ? because in order for me to complete my project, i have to include an official published experiment data
on ferric chloride etching. so, does anybody have a suggestion on how may i complete my report ? thank u.
JR
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chemoleo
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Probably the best thing is if you contact the electronic suppliers who sell FeCl3 and ask for product sheets. They will likely have it.
As to publications in journals - have a look in some publications 1880 to 1950... it's bound to have been done many decades ago.
Never Stop to Begin, and Never Begin to Stop...
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Twospoons
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I'd suggest also contacting one or two PCB manufacturers, since they're the ones who actually use the stuff!
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PCB
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PCB etching with ferric chloride (cont...)
dear members,
i've been taking all ur advices and answers. and i'm very thankful for it. i'm now planning on creating the etching devices for my
experiment. however, there are still some things i would like to ask to you guys.
1. if we use air bubbles for the etching process, do we still need to lift the PCB to the air, for oxidation or something maybe ? what is the purpose
of using air bubbles, is it for agitating or is it for oxidating ? and how much air bubbles do we need ?
2. how much agitation do we need for the etching solution ?
3. i was told that the optimal temperature for etching process is about 40 C. is it true ? what happens if it were done in a lower, or higher
temperature than that ?
4. what other factors influenced the etching process besides the temperature and agitation ? is there any references about these things that i can
read somewhere ?
thank u very much for all your attentions.
JR
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PCB
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PCB etching with ferric chloride (cont...)
dear members,
i've been taking all ur advices and answers. and i'm very thankful for it. i'm now planning on creating the etching devices for my
experiment. however, there are still some things i would like to ask to you guys.
1. if we use air bubbles for the etching process, do we still need to lift the PCB to the air, for oxidation or something maybe ? what is the purpose
of using air bubbles, is it for agitating or is it for oxidating ? and how much air bubbles do we need ?
2. how much agitation do we need for the etching solution ?
3. i was told that the optimal temperature for etching process is about 40 C. is it true ? what happens if it were done in a lower, or higher
temperature than that ?
4. what other factors influenced the etching process besides the temperature and agitation ? is there any references about these things that i can
read somewhere ?
thank u very much for all your attentions.
JR
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JohnWW
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I am sure this has been thrashed out enough before in a thread on this forum, and also the Yahoo group thechemistrycluster@yahoogroups.com . As well
as FeCl3, the better made-up etching solutions for Cu appear to contain added HCl to prevent hydrolysis of the Fe(III) ions and to enable the
formation of FeCl4- and CuCl2-- anions, and oxidants (which may possibly include air under some conditions) to enable the regeneration of reduced
Fe(II) to Fe(III) (which thereby effectively becomes a catalyst).
John W.
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chemoleo
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Phew...
PCB, first of all there is NO need to RE-post things in a new thread if you don't get an immediate reply. I merged the 'new' stuff with
the old thread.
Jesus Christ, JohnWW, you really can't help beating your own drum for the 10th time in the hope that people believe you??
What's up with you, who are you trying to convince, apart from yourself?
Is it so hard to accept that common etchants from common electronics suppliers are solely FeCl3 * 6H2O, that no reference of the necessity of
air/oxygenation is made (even though oxygen may marginally alter reaction rates), and at last that you won't have FeCl3 acting as a catalyst (I
just can't believe you keep going on about this), simply because you are running out of the chloride moiety, thereby REDUCING the amount of
reactant (which supposedly is your catalyst)? Not only that, catalysts decrease the activation energy of a particular reactant-product system. I
can't see that happening here.
Anyway... I am bored with this 
Neither my criticism nor your self-repetitions will help the questions at hand (to repeat - the determination of reaction rates in the etchant
solutions).
So let's bring this back on topic.
PCB:
As this is an assignment I'd do the following:
Reduce the number of factors coming into the system.
1. Regardless whether oxygen is re-oxidising the FeCl2 or not, do ALL your experiments in a Nitrogen/Argon flushed vessels. That way you eliminate the
re-oxidisation argument. If you want to study re-oxidisation, then do that in a separate set of experiments. Also, degas your FeCl3 solution.
2. Stirring/reaction vessel: Obviously the speed of the reaction will in part depend on the stirring rate. Increasing the stirring indefinitely will
not increase the reaction rate indefinitely, you are likely to reach a plateau (probably). If I was you, I would use one and the same container for
all the experiments, at a set stirring rate. Again, if you want to study the effect of mixing upon the reaction rate, then change the stirring rate
while keeping the other factors constant. For stirring just use a magnetic stirrer, or (if you want to waste your time) bubble the system with
different rates of N2/argon. Again, keep things simple. A normal 500 ml flask, flushed with N2, and a magnetic stirrer will do.
3. temperature : Place vessel into a waterbath, at a set temperature. 25 deg C (room temp) is desirable for starters. If you want to study temp.
effects, then vary it while keeping the other things constant. Do it at 25, 30, 35, 40, 45, 50. You will probably find some bell-shaped curve (if 40
deg is the max acitivty), else the reaction rate will just go up with temperature. Again, keep oxygen out of the system as it may introduce more error
with higher temperatures.
4. Other factors?? Well of course the concentration of FeCl3, its excess to Cu and so on. Then, of course the Cu itself. A Cu sheet will react more
slowly than Cu powder. You will have to decide for yourself what to use. Just use a normal sheet and accept the problems it might create (i.e. the
sheet's surface area will increase during the etching process, as the surface will turn from smooth metal to a rough rugged surface). I'd
mount the sheet in a certain position in the vessel (of certain, reproducible dimensions), so that the solution contacts it always the same way.
Alternativley, and much simpler, just use a flat container (sealed again) and put it on one of these slow shaker things, at a given setting.
5. Measure the weight of the copper at certain intervals, at a given set of conditions. observe how the weight changes with time, and plot a graph.
Make sure you have a vast molar excess of FeCl3 over Cu (>20 -fold, which translates to a 5% change in concentration - which is probably well
within the error of individual data points). Use thin copper sheets. Then start varying the temperature, and do the same thing again.
Factors such as stirring and the type of Cu (sheet, plated sheet, chunks, powder, atomised powder) always produce empirical results. You just have to
live with it.
Anyway, enough of this. Just get going with your experiments, and report back. Remember, keep it SIMPLE or you will have great trouble trying to
figure what's going on !
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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chemoleo
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By the way, yesterday I prepared a clean green solution of FeCl2 (straight from Sigma). I shook this up several times, to dissolve oxygen. I left this
at RT overnight.
Today, the solution is still clear green, and no brown precipitate can be found (which would be expected if oxidation occurred - there is no free HCl
to form FeCl3 - so one gets +III oxy/hydrates AND FeCl3 presumably).
Anyway, this goes to show that oxidation by air is most likely negligible, during the time course of the normal etching process (0.5 -->2 hrs)
[Edited on 29-9-2004 by chemoleo]
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chemoleo
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Two days later, the solution has now turned turbid, and dark yellow (brown precipitate)!! Indeed this confirms that oxidation is not an issue you need
to worry about, during the time course of the experiment. It also confirms that oxidation in solution does occur, albeit slowly, including the
formation of insoluble oxy-hydrates.
[Edited on 29-9-2004 by chemoleo]
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Twospoons
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1. Air bubbles are for agitation only, because this is simple, and gentle. I have never used air bubbles myself, preferring to gently rock the etch
bath. I have seen comercially available systems that use pumps, rather than air, for agitation.
2. You'll have to work that out for yourself, eh?
3. Go colder that 40 and it takes too long ,  hotter generally leads to
pin-holing, and under cutting (relating to manufacture of PCBs)
4. Cleanliness! The copper must be kept clean, bright, and free of grease (fingerprints). The etchant will not clean the copper for you - you have
to do this yourself.
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