At 16/06/2003 10:15, Robert Gillat wrote:

Hi,
A colleague of mine used to produce artwork on film using a laser printer.
The right sort of film was essential to ensure dimensional stability with
the heat of the laser printer. This yielded black tracks on a transparent
film. He then used a positive etch process to remove the photoresist which
had been exposed to light. It was a very simple procedure. I always
understood that this was the standard method, no need for negatives etc. I
notice that products are available that improve the density and definition
of a standard paper or film print using a contact method (Reprophane film).



That is EXACTLY the process I have been using for the past eight years for all (100+ boards) my hobby projects.... Works like a charm!
Positive sensitive PCBs are readily available and inexpensive (at least in the Netherlands :-).
Exposure is possible with a large number of UV sources (skin tanning devices, mercury vapour lamps or even direct sunlight); for small boards it is even possible to use a strong (at least 350 Watts) incandescent lamp at short range. Be sure to test the optimum exposure time for your setup (light source, film type and contrast, distance, photo resist type and age!).
Good contact between the film and board is essential, especially with long exposure times.
After exposure I develop in a solution of caustic soda (2 teaspoons in 1 liter water) at room temperature, rinse thoroughly and test with a drop of diluted etchant (FeCl3) to see if all unwanted photoresist is gone. That way you can always develop some more after rinsing. Hell, I even re-exposed and then re-developed some boards that came out under developed....
Etching can be done with what you have available: Ammoniumpersulphate, FeCl3, even nitric acid (20% or so). Be careful and wear appropriate protection, these are ALL really MEAN chemicals.....
For the hasty people: things go better when heated above room temperature. Etching a 10 by 16 cm eurocard in FeCl3 at 60 C takes less than three minutes in a fresh, strong solution (I lost two boards this way because of a telephone call: the stayed in too long and had no copper left at all).
DO NOT heat FeCl3 above 80 Celsius for it will produce a non-soluble deposit that interferes with the etching process.
DO NOT heat Ammoniumpersulphate above 60 Celsius for it will decompose.
DO NOT heat nitric acid at all, it is plenty reactive at roomtemperature....


The biggest problem I always have is the drilling: few drill stands are stable enough to use 18 mil tungsten drillbits without breaking them.

Good luck!

Leo



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