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Making of a Printed Circuit Board (PCB)
PRINTED CIRCUIT BOARDS - Background
Printed circuit boards (PCB's) are laminates. This means that they are made from two or more sheets of material stuck together; often copper and fibreglass. Unwanted areas of the copper are etched away to form conductive lands or tracks which replace the wires carrying the electric currents in other forms of construction. Some parts of the side with copper tracks is coated with solder resist (usually green in colour) to prevent solder sticking to those areas where it is not required. This avoids unwanted solder bridges between tracks. Sometimes the boards are double-sided with copper tracks on both sides. Tracks on one side can be joined to tracks on the other by means of wire links. Plated through holes are available which do the same thing but these make the PCB more expensive. Components are stuffed into the board by hand or by pick and place machines. Soldering is done by hand or by flow wave soldering where the PCB passes over a wave of molten solder. Most recent PCB's use surface mount techniques where components are on the same side of the board as the tracks. Components are stuck to the board with adhesive and the solder caused to flow by heating the board in a hot gas or by some other technique. When fitting components ensure that they are orientated correctly and lay flat on the board unless otherwise stated. When the board is assembled avoid flexing it which may crack tracks. Avoid touching the board which may cause contamination due to dirty fingers or damage due to static electricity carried on your body. It is best to handle PCB's by holding them by two edges only, between thumb and forefinger.
Making a PCB
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The design layout of the PCB is done on the
computer using CAD; the program is EasyPC or EPCPROX.
The layout is printed out on a transparent A4 size
sheet called acetate, which is especially used for the purpose. This is
done in the same way as printing out a word document. Care must be taken
to ensure the circuit layout will be to scale and won't be too big to fit
on the sheet. The layer to be printed out must be defined and pad holes
must be set to 'Avoid' so as black dots and not rings are printed to
indicate holes to be drilled.
The base material is FR4 epoxy all woven glass
laminate, thickness 1.6mm with copper foil cladding 1 oz per sq. ft.
The surface resistance is 100,000 Megohms
Photo-resist is positive working sensitive to
ultra violet light with a developed image of blue/green tint.
Supplier : Farnell Electronic Components LTd , Cat
Ref : 149-063 |
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The copper-clad laminate board consists of a layer of
copper, covered over by a layer of green resin called photo-resist. The
protective black plastic tape, that protects the copper laminate from scratches,
is removed to reveal green positive photo-resist covering the copper.
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The printout mask of the image (on acetate) is put
over the photo-resist face down, so a mirror image of the circuit layout
can be seen over the photo-resist side of the laminated board. On single
sided boards this is important because the PCB is designed from looking
down from the component side, but the tracks are on the opposite side of
the laminated board on the copper side, therefore a mirror image of the
PCB layout must be seen. With double sided PCB manufacture, the board is
put between two sheets of acetate.
One sheet having the design for the top layer and
the other sheet having the design for the bottom layer.
Both sheets are taped together or stapled at the
edges ensuring alignment of the printouts. Double-sided copper coated
laminate is put between the sheets of acetate so that the two images are
lined up on the laminate. With single a sided board, the acetate is placed
over the photo-resist side. In each case the laminate and acetate are
enclosed under ultraviolet light and agitated for 2 to 8 minutes. |
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Developing
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A solution of Liquid photo-resist Developer
concentrate is mixed in a beaker with 1 part developer to 9 parts
water, total 500mls and poured into a basin.
A beeper will sound when the 2 minutes are up, the
board is taken out of the UV enclosure, and (the acetate is not required
any more). The green photo-resist that was exposed will appear a lighter
colour and the darker imprint of the PCB can be seen when examined
closely. The board is put into the solution and the liquid is flowed over
and back on the board. The lighter photo-resist will flow away showing
copper and the PCB layout will be revealed. It will be necessary to wash
the board under tap water and clean with tissue paper to ensure no traces
of photo-resist remain on the copper, otherwise etching would be
difficult.
A PCB marker pen can be used to correct any errors
such as breaks in the track at this stage.
A
video of the developing process can be viewed by clicking here. |
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Etching
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Following inspection to satisfaction the board is
etched in etchant as follows:
The etching tank is about half the size of the
household water tank in the attic. The tank consists of two compartments.
One compartment is 2/3 the size of the other compartment. The larger
compartment consisted of a thermostatically controlled heater element.
Covering the heater is a protective grill mesh. This grill mesh filters
the waste copper away from the heater element .The tank would be filled to
the level of the filter with Ferric Chloride Hexahydrate solution, about 5
litres. The solution is made up of etchent granules dissolved in water. An
electric motor spins a shaft enclosed in a tubular barrel 18 inches long
containing holes perforated on the circumference from top to bottom. The
motor and tube are vertical from the top of the tank so as when switched
on, the motor spins the tube and stirs the solution. The effect is to
suction of the solution into the barrel and to spray the board being
etched.
The solution is heated to 50
degree Celsius before use; a light on the control panel will extinguish
when the temperature is reached. |
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The board held in a clamp the copper side facing the center of the tank so
as to gets the full force of the spray. On the control panel the timer is
set to between 3 to 10 minutes, depending on the quality of solution and
size of the board. A 'start etch button' is depressed and the display will
count down, a beeper will sound when the timer has reached zero, all
exposed copper should be disolved away from the board.
The smaller compartment is plumbed into the water
main and is continually flushed, and is used for washing the board after
etching is complete. For double-sided boards, the process is repeated on
the opposite side of the board. |
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Stripping
(optional)
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After etching the positive resist 9 (photo-resist)
maybe left on the copper to act as protection. Solder is readily achieved
through the resist. This green photo-resist can be removed using a tube of
photo resist stripper, (like shoe polish) and the PCB washed clean under
tap water and dried using tissue paper. |
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Tin-plating
(optional)
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This is done to provide a nice finish and to
protect the copper from oxidization; also soldering will appear neater and
will flow better.
A solution made up of fine tin powder mixed with
water is poured into a basin. The copper is cleaned to a shiny finish be
rubbing using a rubber supplied with the kit. The board is placed in the
solution for 10 minutes. The board should then have a silver finish. |
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Drilling
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After cutting the PCB to size around the perimeter
using the guillotine, drilling using a 0.9-mm drill can now be performed
in the workshop.
The board is now ready to stuff with components |
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Contributed
by Seamus Rooney
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