[TowerTalk] How do you get a good looking trace when etching?

[Jim Smith]

Back in the day when I was training high school electronics shop 
teachers we used a variety of methods to produce PC boards.  In general, 
the results were very good provided the student followed the instructions.

A.  Cleaning the board.
     This was vital.  To clean the board we'd put it under the cold 
water tap for a few moments, sprinkle Comet cleanser on it and scrub 
thoroughly with a Scotch cleaning pad  (Scotchbright?  I referred to it 
as a green scrubby).  Then, handling the board by the edges, rinse off 
the comet and examine the board for any missed spots, fingerprints, etc. 
  Then place the board Cu side down on a piece of paper towel and let it 
dry.

B.  Getting the resist pattern onto the board.
     A whole variety of methods, some better than others.

1)  Use a resist pen to manually apply the resist to the board.  Many 
students got lots of feathery edges, breaks in the traces etc so not 
good quality at all.  This was mostly due to pressing too hard on the 
resist pen (or using one which should have been in the garbage). 
Perfectly good enough for Grade 8, though.  Hopeless for boards with 
IC's on them as the minimum trace width was about 0.05" or more.

2)  Photo process.

a)  Use a CAD program such as Auto Sketch or ACAD Lite to draw the 
pattern. No auto-routing here.  The student decided where to put every 
trace and pad.  A wonderful exercise in schematic reading.  I believe we 
printed the result onto tracing paper with a laser printer.

b)  Get a photosensitive board.
     Buy it
       Don't know if we just had bad stock but had endless problems
     Make it
       Coat a clean board with photo resist using a decoupage brush.
       Hold it vertically briefly to drain off the excess
       Lay it flat in a dark, dust free environment to dry (like a 
cardboard box) and hope the bead of resist on the bottom edge will flow 
back over the board uniformly.
       The key to good results with the photo process is getting a 
reasonably uniform thickness of resist everywhere on the board.  This 
takes practice.

c)  Expose the board
     Put the artwork on an overhead projector
     Lay the board on top of the artwork
     Put a big textbook on top of the board
     Turn on the overhead projector for a suitable exposure time (which 
I've forgotten)

d)  Develop the board
     Don't remember much about this step.  I guess we put it in a 
developer bath.

3)  Thermal Transfer Process
     This was really slick.  Very easy to get really good results.

a)  Print the artwork onto special Thermal Transfer paper using HP 4L 
laser printer.
       Not all laser printers of the time were suitable.

b)  Place the artwork against the clean (uncoated) board with the 
printed side against the copper.

c)  Feed this combination into the input slot of the thermal transfer 
machine.

d)  Fish the board out of the water tray which it fell into when exiting 
the machine.

e)  Carefully peel off the thermal transfer paper.

f)  Admire the beautiful way in which the laser toner was transferred 
from the thermal transfer paper onto the copper.
     I had no trouble reliably running traces between 0.1" spaced IC 
pads with this process.

4)  Silk Screen
     Also capable of very good results if you use a proper silk screen 
exposure box.  Not cheap
     A definite learning curve


D.  Etching the board.

1.  Etchants I have used

a)  Ferric Chloride
     Typically sold as a liquid
     If handled carelessly makes a terrible mess which is virtually 
impossible to clean up.
     Stains clothes irreversibly
     Not allowed to pour it down the drain anymore
     Lasts a long time
     Works well

b)  Ammonium Persulfate
     I bought it in powder form
     Dissolve in water to make a batch of etchant
     No mess
     Bleaches clothes irreversibly
     Don't know current disposal requirements
     Batch lasts only a few hours
     Works well
     Requires simple ventilation

2)  Etching tank

a)  Configuration
     Boards are arranged side by side on an acrylic carrier which is 
submerged in the etchant.  The boards are vertical and there is space 
between them.

b)  Temperature
     Heat is provided by consumer grade aquarium heater.

c)  Turbulence
     You must make sure that all parts of the board are equally exposed 
to the etchant so that the rate at which the copper is dissolved is the 
same everywhere on the board.  There are many ways of doing this but 
turbulence is simple, cheap and works well enough.

     How do you get it?  Use an aquarium "bubbler" pump.


CONCLUSION

Every one of the methods described is capable of very good results in 
the hands of a reasonably skilled person, even the resist pen technique. 
  For example, for their first PC Board project my students were 
required to make a PC board key tag with appropriate label in copper, 
using a resist pen to hand draw the label.  The idea being that they 
would go through the entire process of making a PC board without it 
being a big deal if it didn't look very good.  I chose to label my 
sample "Garage" and to this day (30 years later) my garage key is on it. 
  I just had a look at it and there are no feathery or undercut edges.

However, some techniques were more problematic for students than others. 
  In particular, the photo process worked well for some and not others 
because not everybody was able to get a uniform coating of resist on the 
board in the time they had available.  (Several other courses competing 
for their time.)

It was a happy day for the students (and me) when I discovered the 
thermal transfer process.

Lest it be thought that I'm some kind of expert in this, when I started 
teaching I knew how PCB's were produced in industry but how do you do it 
in High School?  It took a lot of trying this and that to get to the 
point where my students could routinely expect that their PCB's would 
turn out well (and the same for their students).


73, Jim	VE7FO

On 2014-08-03 08:38, Roger (K8RI) on TT wrote:
> Over the years I've tried etching boards with various methods. I have
> never gotten a good looking trace. Whether the trace area is cleaned and
> painted on, or photo sensitive, the edges are always irregular.  They
> are not straight and taper to a thin edge. Just what you don't want.
>
> Temp, chemicals (Typically Ferric Chloride IIRC), chemical dilution,
> agitation: All according to directions with the same results.  I finally
> gave up.
>
>
> 73
>
> Roger (K8RI)
>
>
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