Subject: Etching Brass (again)

From: "J. Eroe Jr." <[email-masked]>
Date: 7 Jun 1995 05:07:04 GMT

I am building a brass model locomotive. It is about to be finished , but 
there are some missing items. I would like to put frames to the windows.
It is made so on the original too, but it has another purpose as well:
the frames can hide the not-quite-straight filing of the windows.
As a second step it would be nice to model the rivets of the original.

First I tried to solder the frames from brass strips of 0.5x0.1 mm.
I made a jig to keep the strips parallel and in right angle. 
The soldering job was however a nightmare: I needed 15 minutes for one 
window. The model has 32 of such windows. And the result is also not 
quite satisfying: the frame is not really flat and the corners with 
the soldering are not too nice. These frames are pretty senstive too, 
they break easily.

Thus I decided to make them by etching. As electronics engineer I have 
some idea and even some experience with photoetching Printed Circuit 
Boards (PCB). The present job however seems to be different:

1. The PCB has a copper layer not brass.

2. The PCB has a layer thickness of 35 micrometers (0.035 mm) - this is 
practically a foil. The thinnest brass sheet available (and usable for
modelling) is 0.1 mm, almost three times thicker.

3. The PCB will be etched from one side, the other is laminated to the 
epoxy. The brass sheet can be etched on both sides.

4. The electronics application needs "black & white" etching: where the 
layer will be etched, it should be etched fully away, where it is covered,
it should fully remain. With brass we need sometimes to remove only a 
thin layer.

Thus I made some experiments to find out the possibilities. I draw the 
frames on the brass sheet by a water-resistant pen and made a etching 
bath in an Aspirin box. I used ammonium-persulfat, as the dealer of 
the electronic supply shop had promised better results (at least for 
copper): he said, the ammonium-persulfat etches better the edges and 
it does not need to be heated. One important point was for me, that I 
remembered: the ferro-chlorid can put ugly spots on your clothes 
and everywhere it drops. I put the prepared brass sheets into the bath 
and I moved around regularly (by hand). The frames are really etched, 
the result however is not very nice: the straight parts are in the center
region thinner than the needed 0.5 mm, but at the corners there is surplus
material remaining: the inner part of the corners is actually round.

I made experiments to model rivets too. I painted one side of a thicker 
(0.5mm) brass sheet black, on the other side I made small spots with the 
water-resistant pen and made the same etching procedure as before. 
The result: well, the "rivets" are clearly there. But the surface around 
is not quite flat. It is rough, grainy, has a strong "texture", it is like 
a wall surface or like a stone. I have no idea why. Thus my questions:

- My experiments show that etching brass CAN be made with the same 
procedure as making PCB. But are really no differences there ?

- The increased thickness can be a reason for under-etching. Can it be 
made better ?

- Is it necessary to etch from both sides (the two shapes should match 
very exactly !), or can even a such thick material (0.1mm) be etched 
only from one side ?

- Was it the right choice to use ammonium-persulphat or is the ferro-
chlorid better for brass ?

- Should the ammonium-persulphat be kept hot too ?

- I have the impression that most of my problems originate in the 
limited movement of the sheets while etching. Would it be better to use 
a motor for it, or just a small pump ?

- When using pump makes it sense to direct the flow in right angle to 
the surface to be etched ?

- Instead of pumping the fluid would it make sense to pump air into the 
bath and the bubbles can move the fluid too ?

- I used a water-resistant pen for my experiments. Gives the photo-
procedure much better results ? (here in Europe there is a well known 
product: the "Positive 20". It is a photo-resist fluid in spray-can.)

- What can be the reason for the textured surface ? The insufficient 
moving or perhaps the brass quality ??

---

From: [email-masked] (WRitt)
Date: 7 Jun 1995 19:23:39 -0400

Contact [email-masked] he has been photo-etching brass
and nickel silver for 2 1/2 years.

Bill Rittner
Manchester, CT
USA