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DESIGNING and MAKING A PCB
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Using CAD |
Advantages of using CAD |
Basic Procedure |
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Double sided Boards
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Making a PCB |
Tip's for Designing PCB's |
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Designing a PCB |
Assembling a PCB |
Soldering |
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Soldering by Hand
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Through Hole Assembly |
Surface Mount Assembly |
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Common Soldering Faults |
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Designing and making a PCB is perhaps the most important part
of a successful Electronics project. If the PCB is faulty or badly designed your
circuit is unlikely to work and any modifications will be obvious to an
examiner.
Using CAD
Because we have access to a network of PC's and CAD software
to help us develop PCB designs, the process we use to create the PCB design in
school is very similar to the design process
used in industry.
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| In the design stage a CAD package (such
as PCB Wizard above) is used to create a PCB photographic mask, which is
used in the photo-eching process to print the circuit onto a copper clad
board (a plastic board with a thin sheet of copper glued onto it),
before it is etched to remove the unwanted copper. |
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Advantages of Using CAD
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CAD (computer aided design) techniques have many advantages
over other methods of producing the PCB mask.
1. Changes are easily made;
2. Direct plotting or printing onto boards is possible
3. Production of photographic masks is made easier;
4. Component layout drawings and silk screens can be
produced at the same time;
5. Double sided boards can be produced accurately;
6. Auto-routing can be used to simplify the design of the
track layout.
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| Basic
PCB Design
The basic
procedure is quite simple. Firstly the components are placed in their
likely positions. Then one by one the tracks are routed on the screen.
It is important to work from a circuit diagram and cross off each track
as it is laid down. When the final PCB has been checked and a
satisfactory solution has been produced it can be printed.
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It is important to indicate
which is the component side so that the PCB mask can be placed the
correct way up when printing. |
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Having produced the PCB mask the next stage is to print
this onto the surface of the copper clad board.
This is achieved using a photo-sensitive varnish. It is
possible to buy this in a spray, but it is easier and more convenient to buy
ready coated board. The only difference between the way this is achieved in
school and the methods used in an industrial environment is the scale of
production.
In school PCB's are usually prototype boards made singly or
in small numbers. In industry the numbers of boards produced is much greater and
instead of printing the mask onto a piece of board just big enough to contain
the PCB, much larger boards are printed containing multiple copies of the mask.
Often the optimum number and position of the masks is determined automatically
using a computer program.
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| Double
Sided Boards
The
same program can be
used to drill the holes automatically before the
circuit is etched.
Quite often circuits used in industry
are double sided (tracks and pads are drawn on both sides). The tracks
on the top and bottom surfaces have to be connected together if the
circuit is to work. This used to be done at the assembly stage when a
wire was passed through connecting holes and soldered top and bottom. |
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Today, it is achieved by
'plating through' at the PCB making stage. Plating through is a
technique where the tracks on the top and bottom sides are
electro-plated with a tin/lead alloy so that the inside of holes are
also plated. When passed through an oven (hot air levelling) the tin
lead alloy fuses to join the top and bottom surfaces everywhere there is
a hole. |
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We can use double sided boards in school if we are careful to
align the top and bottom PCB masks, but most of the time circuits can be
produced using single sided boards which are cheaper and easier to use.
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Making
A PCB
The basic method for making a PCB
from the PCB mask is outlined below:
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| 1. Peel the black
protective plastic off the photo-resist board. Place the PCB mask the
directly onto the photo-resist making sure that the mask is the correct
way round. |
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2. Hold them together and
carefully place them into the UV box with the PCB mask against the
glass. Set the timer for about 3-4 minutes and switch on. Wait untill
the timer finishes. |
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board from the UV box, place it into a solution of sodium hydroxide.
After a short while, the etch-resist should be dissolved, revealing the
track pattern. Wash in water. |
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4. Place the circuit board
in the bubble-etch tank. The etching will take about 15-20 mins. Check
the condition of the board at regular intervals. When the circuit has
been etched remove from the tank and wash in water. |
| 5. Dry and clean the printed
circuit board and drill the holes needed for the
components and wires. |
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Tips for Designing PCB Artwork
| 1. Try to take the shortest
route between pads. |
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2. Keep tracks aligned
neatly. |
| 3. Use components to bridge
tracks wherever possible to avoid using wire links. |
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4. Route tracks between IC
pads only where absolutely necessary. |
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Designing a PCB
If you have designed a circuit using crocodile clips, you
should be able to use this to import the components into PCB Wizard. If not you
will have to select them manually (find out how to do this if you are not sure).
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You should then have a diagram similar to the one shown right. It
is important to save each stage as a different PCB file ie PCB-1, PCB-2 etc.
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The next stage involves making the Printed Circuit Board as
small as possible. Each component is highlighted and dragged to another
position. At this time it is possible to rotate components as well in order to
try to disentangle the 'nets' (the thin green lines joining components
together). In your final PCB these lines will form the tracks between pads
and they must only touch other pads and tracks to which they are should be
connected.
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made the PCB as compact and as tidy as possible you should try to 'autoroute'
the tracks to see how many nets you will need to route by hand. At this
stage you should check the PCB against your circuit diagram to make sure
all the components and tracks that you need are in position.
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The tracks were then selected (choosing select all
from the edit menu) and then increased in thickness using the edit properties
menu.Having checked that the PCB is correct against the original
circuit it could be printed as it is, but there are a number of things we can do
to improve it.
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The first of these is to block in pads and tracks that are
joined together. This saves on etching time and materials and makes it easier
for components to be soldered, because there is a much greater area of copper to
solder to.
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The final stage is to provide some fixing points for the PCB
and somewhere to write your name and other details as shown below. |
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Your PCB is now ready for printing. You will need to print a copy of your PCB in: |
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Normal
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Real World
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and Artwork views.
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The normal and real world views are used to help you assemble
your circuit (they show which components go where) and the artwork is used to
print the actual PCB onto copper board.
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You
should keep a copy of your PCB at each stage, as well as a copy of the
Normal, Real World and Artwork views for the development section of your
folder.
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Make
sure you explain what changes were made at each stage and why. Try to show
how you checked your PCB for mistakes before showing the three views of
your final circuit.
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Assembling
a circuit |
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Mounting
Components
There
are two ways of mounting components on a PCB. Traditionally, holes for the
components are drilled through the PCB, the components are inserted and the
connections are soldered on the underside of the PCB between the component
lead and the PCB pad.
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In
recent years, there has been a move, particularly in industry, towards a
technique called surface mounting of components. For this method of assembly
components are initially glued into place directly onto the copper pads.
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For
surface mounting most common components have been re-designed without wire
leads to enable them to be made smaller. This means that more components can
be packed into a smaller space
Surface
mount capacitors and resistors look like small black 'bricks' with silver
end caps.
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Surface
mounting is preferred to through-hole assembly in industry, largely because
circuits can be made smaller and because this method of assembly is much
cheaper for mass produced circuits.
It
avoids the need to drill holes for each component, as well as cutting and
pre-forming a large number of component leads. Automatic component placement
machines which 'pick and place' components are easier to develop and use
with surface mount components than similar pick and place machines for
through-hole assembly.
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In
school we would find it quite difficult to use surface mount components
because they are so small, but it is possible to use surface mounting
techniques with conventional through-hole components. Apart from resistors,
diodes and capacitors, a lot of components have only been adapted slightly
for surface mounting in industry. Many circuit designers particularly at the
prototype stage surface mount conventional components. The most obvious
advantage is that the components can be assembled in almost exactly the
position shown in a circuit diagram or on a PCB component placement diagram.
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It
is important to decide at the PCB design stage whether to use through-hole
or surface mount assembly techniques because the PCB mask must be placed the
correct way round when the PCB is printed.
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Soldering
In
school we have to solder components by hand, because there is no
alternative. In industry however, hand soldering is rarely used on a
production line because it is so time consuming and expensive.
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When
through-hole assembly is used components are usually pre-formed and placed
in position using an automatic or semi-automatic component placement system.
The assembled boards are then flow-soldered by being passed through a wave
soldering machine.
A
wave soldering machine is simply a wave of molten solder over which the
assembled board is passed. The board and exposed pads and leads are sprayed
with a flux before being placed on a conveyor system which passes the bottom
of the board across the wave. All the exposed joints are soldered in one
pass. Areas of the board which do not require soldering are protected by
'masking' with a heat resistant varnish.
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With
the introduction of surface mount components manufacturers are now employing
a process called re-flow soldering. With this process both the surface mount
components and the PCB are pre-tinned (at the manufacturing stage) so that
when heated in a re-flow oven the solder melts and 're-flows' to complete
the soldered joint.
Some
hand soldering is still needed to correct errors or perhaps to solder any
through-hole components which have been used on a surface mount board which
has been re-flowed.
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Soldering
by hand
Correct
soldering is crucial to the success of any electronics project. Over recent
years there has been a trend in industry to reduce the dependency on
soldering flying leads by using press fit connectors. However many
components will need to be soldered and it is important that the process for
achieving a good soldered joint is understood.
There
are five simple rules which will avoid most of the common problems.
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will not join to dirty materials. Both the pad and component leads need
to be clean.
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| Solder
will not join to cold materials. Both the lead and pad must be heated
equally. Dirt or lack of heat can both cause 'dry joints'.
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| Wait
about 5 seconds for the pad and lead to heat up, then apply a small
amount of multi-core solder to the joint.
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soon as the solder flows into the joint take the solder and iron away.
Too much heat can damage components.
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too much solder it is easy to bridge tracks. Bridging can also occur if
leads are not trimmed with wire cutters.
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Through-hole assembly
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Insert the
components through the correct holes in the PCB so that they stick
out of the track side. |
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Hold the components by
the lead with small pliers. This helps to get rid of any excess heat. |
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Use a soldering iron
with a clean, hot tip. Hold the tip against the copper pad and the
lead of the component. |
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After about five
seconds, bring the solder into contact and allow a small amount to
melt onto the track and lead |
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Remove the solder then
the iron, taking care not to move the component until the solder
cools. |
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component is secure and trim back the lead as short as possible.
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Surface
Mount Assembly
If surface mounting - pre-form and trim
the component before soldering the joint as explained
above.
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Common
SOLDERING Faults
(and how to avoid them)
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