PRINTING CIRCUIT

BOARDS

Overview

Printing a PCB is a three stage process,

1. Exposing the blank PCB
2. Developing the exposed PCB
3. Etching the exposed copper to leave the tracks that form the circuit.

At the heart of this process is the pre-sensitised blank PCB.  It is a lamination of fibreglass sheet, copper and a UV light sensitive film.

Preparation

1  The developing solution should be prepared at least five minutes before developing any boards.  This ensures the castic soda is completely dissolved before developing.  The exposed PCB may be destroyed if any castic soda remains un-dissolved.  Mix at a rate of 1.5 teaspoons (7.5 ml) of castic soda to 1 litre of water.

2  Etching takes place when the etching solution is at or above 60 degrees celcius.  The solution of Ammonium persulphate and water should be prepared and heated prior to the commencement of the printing process.  Mix the etching solution at a rate of 300 grams to 1 litre of water.

Exposing the PCB

You will Need

• Blank pre-sensitised PCB cut to approximate dimenssions
• Circuit Board Positive
• A source of UV light (Exposure Chamber or sunlight)
• A Timer (to measure 60 - 70 seconds)
• Exposure Frame
• PCB Transparency

Transparency

Note the initials CdS must be legible when exposing the PCB.  This ensures the PCB is not reversed (therefore useless)

Exposure Frame

The Process

IN SUBDUED LIGHT- tear off the plastic protective  cover  from  the blank PCB.   Place  it  resist side up in the exposure frame and place the artwork on top.  You should be able to read the initials "CdS" on the  transparency.   Place  the  top sheet of glass in position and expose the frame to direct, bright sunlight (or exposure chamber) for 60 -70 seconds.

Developing the PCB

You will Need

• Water
• Sodium Hydroxide - NaOH (Castic Soda)
• An exposed PCB

The Process

Remove the PCB from the frame and submerge in mixed developer.  Gently swish the developer around.   You  will  see  a  misty  blue  trace arising  from  the  PCB.   The manufacturer calls it "smoke" rising in the liquid from the board.  Leave it in  the  developer  until  the  track  is quite  clearly  visible  and  the copper surround bright.  Take it out and wash it in water.

Developing the PCB

Etching the PCB

You will Need

• Water
• Ammonium Persulphate
• An etching tank

Heat the etching compound to around 60 degrees C, and place the PCB into the chemical.  Airate gently until the copper around the  track  has been  completely etched away.  This will take 5 - 20 minutes, but keep the etchant hot - it won't work cold.  Remove the PCB and wash it in water

Etching the PCB

The chemical reaction is :

Examination and drilling

Inspect the tracks for fine breaks and test the continuity of  each track with an electronic circuit tester or ohm-meter.

Drill  all locations with a 0.8mm or 1.0mm drill.

The PCB is now ready for kit assembly.

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