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Eurocircuits are European manufacturers
of standard technology printed circuit boards (or PCBs)
We're specialise in providing prototype
and small batch PCBs for designers,
product development departments,
niche market electronics companies,
universities and research establishments.
We have made this film
to help electronics engineering students
understand how a PCB is made.
In this film we will show the manufacture of a 4 layer multilayer.
This covers all the different processes used in PCB production.
The board designer
has prepared his layout on a Computer Aided Design
or CAD system.
Each CAD system uses its own internal data format
so the PCB industry
has developed a standard output format
to transfer the layout data to the manufacturer.
This is Extended Gerber or RS274X.
The Gerber files define the copper tracking layers
(4 in the job we are following)
as well as the soldermasks and component notations.
First we check that data meets our manufacturing requirements.
These checks are mostly done automatically.
We check the track widths,
the space between tracks,
the pads around the holes,
the smallest hole size etc.
Once the data is verified as good
he will output all the tool files needed to drive the machines
that will make and test the PCB.
We use laser photoplotters
in a temperature and humidity-controlled darkroom
to make the films we will use later to image the PCBs.
The photoplotter takes the board data
and converts it into a pixel image.
We have generated one film or phototool per PCB layer.
Now the films are registered with each other
so that the different layers of the PCB will be perfectly aligned.
We do this by punching precise registration holes in each sheet of film.
The operator puts the film on the table of the punch
and then micro-adjusts the table
until the targets on the film are exactly lined up
with the targets on the film punch.
She then punches each sheet of film
with the registration holes
which will fit onto the registration pins in our imaging equipment.
To produce the inner layers of our multilayer PCB,
we start with a panel of laminate.
Laminate is an epoxy resin and glass-fibre core
with copper foil pre-bonded onto each side.
The first step is to clean the copper.
The cleaned panel is coated with a layer of photosensitive film,
the photoresist.
The bed of the printer has registration pins
matching the holes in the phototools.
The operator loads the first film onto the pins,
then the coated panel then the second film.
The pins ensure that the top and bottom layers are precisely aligned.
The printer uses powerful UV lamps
which harden the photoresist
through the clear film to define the copper pattern.
Under the black areas the resist remains unhardened.
Outside the yellow room
the panel is sprayed with a powerful alkali solution
which removes the unhardened resist.
The copper pattern we want is now covered by the hardened resist.
You can now see in the blue resist
what will be the copper on our inner layer panel.
We remove the unwanted copper
using a powerful alkaline solution
to dissolve (or etch away)
the exposed copper
The process is carefully controlled
to ensure that the finished conductor widths are exactly as designed.
Next we strip off the blue photoresist which protected the copper image.
So now we have the exact pattern required.
The inner core of our multilayer is now complete.
Next we punch the registration holes
we will use to align the inner layers to the outer layers.
We won't be able to correct any mistakes
on the inner layers once we have assembled the multilayer
so we now give the panel a complete machine inspection.
The automatic optical inspection system
scans the board in broad strips
and compares it with the digital image
generated from the original design data.
The outer layers of our multilayer
consist of sheets of glass cloth
pre-impregnated with uncured epoxy resin
or prepreg
and a thin copper foil.
The lay-up operator has already placed
a copper foil and 2 sheets of prepreg
on the heavy steel baseplate.
Now he places the pre-treated core carefully over the alignment pins.
Then he adds 2 more sheets of prepreg,
another copper foil and an aluminium press plate.
He then rolls the heavy stack under a press
which lowers down the steel top plate.
The press operator collects 3 stacks on a loader
and loads them into the bonding press.
This press uses heated press plates
and pressure to bond the layers of the PCB together.
The heat melts and cures the epoxy resin in the prepreg
while the pressure bonds the PCB together.
The process is computer controlled
to build up the heat and the pressure correctly,
hold it and then to cool the press down.
In this way we ensure a permanent bond
that will last the lifetime of the PCB.
Once the cycle is completed
the press operator unloads the press
and carefully rolls the heavy stacks into the clean room.
Here the lay-up operator de-pins the stack and removes the top plate.
The copper foil is now bonded in place to form the outer layers of the PCB.
Now we drill the holes for leaded components
and the via holes that link the copper layers together.
The operator first puts a panel of exit material on the drill bed.
Then he loads one or more PCB panels,
and a sheet of aluminium entry foil.
The drilling machine is computer-controlled.
The operator selects the right drill program.
This tells the machine which drill to use and the X Y co-ordinates of the holes.
Our drills use air-driven spindles
which can rotate up to 150,000 revolutions per minute.
High speed drilling ensures clean hole walls
to provide a secure base for good plating on the hole walls.
Drilling is a slow process as each hole must be drilled individually.
So depending on the drill size
we drill a stack of one to three PCB panels together.
We can drill holes down to 100 microns in diameter.
During bonding
excess resin from the prepreg is squeezed to the edge of the panel
outside the image area.
This excess is now cut off on a computer controlled profiling machine.
The first step in the plating process is the chemical deposition
of a very thin layer of copper on the hole walls.
Almost all PCBs with 2 or more copper layers
use plated through holes to connect the conductors between the layers.
A good connection needs about 25 microns of copper on the walls of the holes.
This thickness must be electroplated,
but the walls of the holes are non-conductive glass cloth and resin.
So the first step is to deposit a conductive layer over the hole walls.
We use electroless copper,
that is we deposit chemically
a layer of copper about 1 micron thick over the walls of the hole
and incidentally across the whole panel.
We pre-treat the panel,
then we seed the hole wall with micro-particles of palladium,
and finally deposit the copper.
We image the outer layers in a clean room
to make sure that no dust gets onto the panel surface
where it could cause a short or open circuit on the finished PCB.
The panel is first coated with a layer of photosensitive film,
which is hot-rolled onto the copper using a cut-sheet laminator.
For inner layers the copper pattern we want was covered by the resist.
For outer layers it is exposed ready to be plated.
The imaged panel is conveyored out of the clean room
and through a developer which removes the unhardened resist.
Next we electroplate the boards with copper.
The panels themselves act as cathodes and we can plate the hole walls
thanks to the conductive carbon layer already deposited there.
The copper surface of the panels is cleaned
and activated in a number of baths
and then electroplated.
To ensure good conductivity through the holes
we need to plate an average of 25 microns of copper on the hole walls.
This means that we also plate 25 -- 30 microns on the surface tracks.
Once we have plated the copper onto the board
we then plate a thin layer of tin.
This we will use in the next step of the process
when we etch off the unwanted copper foil.
The copper is covered with a thin layer of tin as an etch resist.
Now we will remove the unwanted copper foil from the surface.
We do this on a single continuous process line.
The first step is to dissolve
and wash off the resist which covers the unwanted copper.
Then we remove the unwanted copper
using a powerful alkaline solution to etch away the exposed copper.
The process is carefully controlled to ensure that as we etch down
we don't etch sideways as well.
This means that the finished conductor widths are exactly as designed.
Finally we strip off the thin tin coating
which protected the copper image.
So now you can see that only designed copper pattern remains.
Most boards have a epoxy-ink soldermask
printed onto each side to protect the copper surface
and prevent solder shorting between components during assembly.
Each panel is given a final clean to remove any dust from the surface
and loaded into the vertical coater.
The coating machine simultaneously covers both sides
of the panel with the epoxy soldermask ink.
The panels are now racked
and put through a conveyorised drier
which hardens the resist
just enough to allow it to be printed or "tack-dried".
Next the coated panels are imaged.
As with the etch and plating resists used earlier in the process,
the UV lamps in the machine harden the ink where the film is clear,
that is where we need soldermask on the finished board.
The imaged panels are put on a conveyor out of the clean room
and into the developer
which strips off the unhardened and unwanted resist.
Later the required resist will be further hardened or "cured"
to provide a robust and permanent coat.
But first the operator checks the alignment of the soldermask
on the panel and makes sure
that there are no traces of ink on the pads or through the holes.
Even slight traces will compromise the solderability of the finished PCB.
The copper component pads and holes have been left clear of soldermask.
Now we apply a solderable surface finish
to protect the copper
until the components are soldered onto the board.
On this line we chemically deposit first nickel onto the copper
and then a thin coating of gold over the nickel.
This is a chemical process needing no electrical connections.
The line is fully automated,
moving the panels through a series of tanks
which clean and sensitise the copper surface
and then deposit about 5 microns of nickel
and a tenth of a micron of gold.
Most PCBs have a component legend
to show which component goes where.
Today we use ink-jet projecters
to image the legends direct from the board digital data.
Like a conventional paper printer
the ink-jet printer sprays minute droplets of ink
onto the panel to generate the image.
If a legend is needed on the second side
the ink is tack-dried on a conveyorised heater
and the printing process is repeated.
Now we finally cure both the epoxy ink soldermask and the legend.
Using a flying probe tester
we check each net to ensure that it is complete
no open circuits
and does not short to any other net.
A faster test method is the Acceler8.
This uses 4000 tiny probes like a brush.
It builds an electronic map of the PCB from a pre-tested good board.
Then it compares each board to be tested with its map.
This cuts test times by 90%.
The final manufacturing stage is to profile the PCBs
and cut them out of the production panel.
For this we use a computer-controlled milling machine or "router".
First the machine mills out any small slots
or internal cut-outs.
Next the milling head automatically picks up a 2 mm cutter,
checks the diameter and mills around each PCB.
The circuits are held in place by small bridges of material.
We will drill through these