Printed circuit board making at KU
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Printed circuit board making
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The following pictures present the process of making printed circuit
boards by using simple methods. The method starts from printing
films, exposing PCBs with light-sensitive coating to UV, developing,
and finally etching. The whole process takes less than an hour.
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The first phase in making a printed circuit board is to
expose the uv-sensitive coating of the pcb with ultraviolet
radiation at places where copper needs to be removed.
This is accomplished by using a film, and UV exposer.
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A film is printed with ordinary laser printer. This film is
used for exposing the pcb coating. The dark "ink" of the
print on film does protect the coating from UV while other
parts of the coating are exposed to UV. It is necessary to
print the same image to film twice to get enough thickness.
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The unit shown left is used for exposing the coatings. The unit
is constructed of an old electronics cabinet with door. Inside
is coated with aluminium tape.
There is a shelf for two glass plates at the middle of the cabinet.
The pcb and film is placed in between the glass plates.
It is possible as well to expose two-sided pcb:s by
using film at both sides.
There is a ceramic socket for uv exposure lamp both under and over
the shelf. At the bottom of cabinet, under aluminium shield, are
current-limiting chokes for the lamps. There is a limit switch to prevent
accidental exposure when door is open.
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The UV exposure lamps are ordinary mercury vapour lamps
with the outer glass envelope, which has the fluorescent
material, removed. There is a arc-tube with three electrodes,
including starter electrode which is connected through
a resistor.
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The exposed coating must be etched away from the surface
of the pcb. This is done with a water solution of NaOH.
The exact consentration of the solution is not of importance,
but about 10 grams of NaOH per litre of water works fine.
Surprisingly, the quality of water seems to be quite critical.
The local tap water did not give good results, while distilled
water always gives good results with same NaOH. Thus distilled
water is always used to make the developer solution. Technical
grade NaOH is used.
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The developing setup is made of a canister by cutting it open
and clearly marking the usage of it. This is the best container
I could find for this usage - large enough area for PCBs,
cheap and polyethylene plastic.
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Normally half a litre of distilled water is measured and poured
to container, and 5 grams of NaOH is added. The measurement of
NaOH is done with a digital laboratory scale. The scale has 10mg
accuracy, but for this application the amount of 5.0g +- 0.1g is
accurate enough. Water is measured with a 500ml volume measurement
flask which has 5ml resolution.
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The NaOH dissolves to water, and the water is mixed with plastic
tool to get even concentration. After this, PCB is immersed to
developing solution, and solution gently agitated by rocking the
container side to side. The unexposed parts of coating get a
dark blue or green colour, and copper is shown at exposed
parts after coating is removed by developer. After this is
complete, PCB is lifted with pliers from developer, and
immersed to water bath.
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The etching is done using ferric chloride dissolved in
water. This solution actually etches most metal alloys,
including aisi304 stainless steel, when it is hot. In this
setup, it is warmed to about 50C get a higher etching rate of
copper. Ferric chloride is quite safe to handle, and
it can be stored practically indefinitely in the tank, so
it is much preferred compared to either short-lived
(ammoniumpersulfate etc.) or more strict safety protocol
requiring (nitric acid or hydrocloric acid with hydrogen
peroxide etc.) etchants.
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The etching setup is composed of two aquarium air pumps, a plastic
tank with air conduits leading to bottom of it for bubbling, a heater
in the tank, a thermometer on the tank acrylic lid, a wooden base for
fixing the pumps and the tank, and a plastic container for all the other
parts in order to contain spills of etchant in case of an accident or
a failure of the tank.
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The rod at the middle of wooden base is used to keep the
air hoses, from pumps to air conduits of tank, so that they are always
above the top of the tank etching solution - otherwise backflow of etching
solution to pumps would occur.
Air pumps are simple membrane pumps
with magnet attached to membrane and moved by iron-cored electromagnet
driven with 50Hz AC.
All electrical cables are fixed to the wooden base and then
takes away from the setup to an extension cord. The extension
cord is properly protected from any possible spill of etchant.
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The tank heater consists of a test-tube like glass envelope
with a coil of resistance wire wrapped on a ceramic form.
The temperature of the solution is regulated by a bi-metallic
thermostat switching on and off current to heater. The temperature
setpoint is adjustable from white knob. The heater is fixed to
tank wall at one side of the tank.
The temperature of the etchant is measured with a traditional
coloured alcohol in capillar tube thermometer. The thermometer
is attached to a hole in lid, so that the sensing bulb at the
end of thermometer is properly immersed in etching solution.
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The etching procedure is started by switching on heater
and exposing/developing pcb while the temperature of
etching solution is rising towards 50C. After that,
pcb is attached to holder, and immersed to tank so
that that holder keeps pcb at proper depth. After
etching is ready, pcb is lifted from etchant by holding
the holder, and immersed to water. The etchant is washed
away in water bath.
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The following pictures show some circuitboards manufactured
with the above described pieces of equipment.
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68HC11f1 in PLCC68 with 32k SRAM, 32k EEPROM, rs232, 10cm*7cm
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(made in 1997)
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Range-switching electrometer, 7cm*5cm
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16bit 2 channel AD converter with optoisoled interface, 5cm*4cm
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12bit DA converter with optoisoled interface and 0-8V 5A powerstage, 9cm*6cm
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Thermocouple vacuum gauge, 6cm*5cm
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Analog hot filament ionization vacuum gauge prototype, several circuitboards, assembled in 19" rack case
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Copyright 2001, Kristian Ukkonen, all pictures and text.
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