From: firstname.lastname@example.org (Bob Wilson)
Subject: Re: Etching!
Date: Fri, 25 Oct 2002 04:03:08 -0000
Organization: Your Organization
X-Newsreader: WinVN 0.99.9 (Released Version) (x86 32bit)
References: <3DB0977E.747EAFBB@earthlink.net> <3DB7523D.BC943100@rica.net>
In article <3DB7523D.BC943100@rica.net>, email@example.com says...
>> I am new to etching. Got a few questions...What does shining UV light
>> through a transparency (containing a circuit diagram) do to the PCB?
>> It is to my understanding that the lines on the transparency will
>> block out the UV light and prevent the light from contacting the
>> copper (thereby forming lines), but what about the rest of the copper
>> that is exposed? Is there a chemical reaction?
>> If there is a chemical reaction, what is the purpose of using NaOH?
>> Does it adhere to the shadowed lines on the circuit board, thereby
>> protecting those lines from reaction to the acid applied onto the PCB
>> in the proceding step?
>> Or...is the tranparency somehow glued to the PCB, irradiated with UV,
>> then NaOH applied to it to remove the transparency??
>> Please help!
>> Is the use of HCl the industry standard for etching purposes?
>I buy presenstized boards that are coated with a positive resist.
>This is a blue polymer coating that looses its resistance to
>dissolving in NaOH when it is exposed to UV. After the exposure and
>removal of the parts that had covered copper I intend ot remove, I
>etch the board in ferric chloride solution. This leaves copper hiding
>under the unexposed resist film, and removes any unprotected copper.
>Then I drill and solder the board.
To answer the OP's question, there are 2 general kinds of resists. The first
(and oldest) is negative-working resist as pioneered by Kodak (with their
KPR line of resists). They are polymerized by UV light, hence wherever light
hits the resist, it chemically hardens it. Washing the exposed resist in a
blend of aromatic solvents, washes away the unexposed (and unpolymerized)
areas, whereas the exposed areas have been crosslinked by exposure, and are
resistant to the solvent. This type of resist therefore requres negative
artwork (Black background with clear lines and pads).
The second type of resist is positive-working. Shipley's line of positive
resists (especially their original AZ-111) were the most popular. UV
exposure chemically alters the resist, in such a way that the exposed areas
can be broken down by an alkaline water-based solution (Ordinarly sodium
hydroxide is commonly used). The unexposed resist is unaffected by this
solution. Thus, artwork needs to be "positive" (clear background with black
lines and pads).
Negative resists were a pain to work with. The unexposed resist that went
into solution in the aromatic "developer" tended to form a film on the bare
copper areas when the remains of the solvent evaporated, which caused spotty
etching at times. This is why it was common to develop these resists in a
vapour degreaser (which always had fresh, clean solvent condensing on the
board). They were also very prone to pinholing, and (worse) the pinholes
were always in the area where the polymerization was incomplete, or where
there was dust in the way of the UV (i.e. right in the middle of traces and
Positive resists are far better. Since the clear areas of the artwork are
not on the traces, and since the resist does not depend on polymerization to
remain on the PCB, traces and pads usually have essentially no pinholing.
Dust shows up as a small fleck of copper in the areas where no copper should
be, but because the copper "fleck" that results is so small, a little
overetching usually elimninates it completely.
The NaOH you refer to is the so-called "developer" that breaks down the
exposed positive resist. Obviously, since NaOH is highly soluable in water,
and since it has no reaction with copper itself, it does not remain in the
PCB after rinsing. Its purpose is simply to get rid of the exposed resist,
leaving the unexposed resist to "resist" the action of the etchant, when the
board is etched.
I don't understand your question about what the UV is supposed to do to the
"rest of the copper". Metals are not affected in any way by UV. The purpose
of the UV is to expose the resist, as I explained above.
The artwork is pressed against the resist-coated laminate prior to exposure.
It is obviously not glued! Typically, a vacuum frame is used to ensure
intimate contact between artwork (or more correctly, "Phototool") and the
HCl is (also rather obviously) not used AT ALL for etching, since it has no
effect whatsoever on copper. As you probably learned in grade 10 science,
any metal that lies below hydrogen on the electromotive scale (e.g. copper,
silver, gold, lead, and others), cannot be attacked by common
(non-oxydizing) acids such as HCl. If you ever tried putting a piece of
copper in HCl, you would have seen that the only effect is that the oxides
on the copper were removed, but the copper was not attacked at all.
Typical etchants for copper that are commonly used are Ferric Chloride
solution, Ammonium Persuplphate solution, and ammonia-based proprietary
etchants. Less common ones include Cupric Chloride solution.
If you are really interested in this, why don't you do something radical,
like take out a book from the library on the subject? You will find that
this will give you a far better background than asking somewhat disconnected
questions on the subject.