PCB Fabrication

A printed circuit board (PCB) mechanically supports and electrically connects electronic components using conductive tracks, pads and other features etched from copper sheets laminated onto a non-conductive substrate. PCB's can be single sided (one copper layer), double sided (two copper layers) or multi-layer. Conductor on different layers are connected with plated-through holes called vias. Advanced PCB's may contain components - capacitors, resistors or active devices - embedded in the substrate.

DFM - From Gerber files to PCB production data
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. The engineer can also check and measure individual areas where he wishes. 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.

Phototools for PCB image transfer

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. A laser writes this onto the film. The exposed film is automatically developed and unloaded for the operator. The films are ready now for the PCB fabrication process.

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.

Innerlayer imaging for multilayer PCB
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.

We print the panels in a clean room to make sure that no dust gets onto the surface where it could cause a short or open circuit on the finished PCB.

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 and in the panel. 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. The clean room uses yellow lighting as the photoresist is sensitive to UV light.

Outside the yellow room the panel is sprayed with a powerful alkali solution which removes the unhardened resist (etching). The panel is pressure-washed and dried. The copper pattern we want is now covered by the hardened resist. The operator checks a sample of the panels to make sure that the copper surface is clean and all the unwanted resist has been removed. You can now see in the blue resist what will be the copper on our inner layer panel.

Etching inner layer in PCB production
We remove the unwanted copper using a powerful alkaline solution to dissolve (or etch away) the exposed copper from the inner layer. The process is carefully controlled to ensure that the finished conductor widths are exactly as designed. But designers should be aware that thicker copper foils need wider spaces between the tracks. The operator checks carefully that all the unwanted copper has been etched away.


Next we strip off the blue photoresist which protected the copper image. So now we have the exact pattern required. The operator checks that all the photo-resist has been removed. You can see that Eurocircuits put several different designs on one production panel. That way we can make small numbers of PCBs cost-effectively.

Registration and inspection of inner layers in PCB production
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. The operator loads the core into the optical punch which lines up the registration targets in the copper pattern and punches the registration holes.

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. Any errors are displayed on screen.

PCB multilayer fabrication - lay-up and bond
The outer layers of our multilayer consist of sheets of glass cloth pre-impregnated with uncured epoxy resin (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 builds up 3 panels on the baseplate in the same way. Then he rolls the heavy stack under a press which lowers down the steel top plate. He pins the stack together and rolls the finished stack out of the clean room into a rack.

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. Our board has 4 layers but complex PCBs for defence, avionic and telecommunications applications can have more than 50. These may include sub-assemblies of cores, prepregs and foils drilled and plated before being assembled into the final 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. He unloads each of the panels from the stack, removing the aluminium press plates used to ensure a smooth copper finish. The copper foil is now bonded in place to form the outer layers of the PCB.

Drilling printed circuit boards
X-ray drill of reference holes
Now we drill the holes for leaded components and the via holes that link the copper layers together. First we use an X-ray drill to locate targets in the copper of the inner layers. The machine drills registration holes to ensure that we will drill precisely through the centre of the inner layer pads.

Prepare the stacks for drillng
To set up the drill the operator first puts a panel of exit material on the drill bed. This stops the drill tearing the copper foil as it comes through the PCB. Then he loads one or more PCB panels, and a sheet of aluminium entry foil.

Drilling the hole
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. To give you an idea of the size, the diameter of a human hair is about 150 microns. Drill change is fully automatic. The machine selects the drill to use from the drill rack, checks that it is the correct size, and then loads it into the drill head.

Once all the holes are drilled the operator unloads the panels from the drilling machine and discards the entry and exit material.

Cut-off excess resin
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 operator loads the panel onto the bed of the machine and selects the correct program with the X y co-ordinates of the path for the cutter to follow. The drilling machine uses the points of the drill but the profiling machine uses the specially patterned shank of the tool. The cutter mills out the final profile for the production panel. The drilled panel is now ready for plating.

The first step in the plating process is the chemical deposition of a very thin layer of copper on the hole walls. The operator clamps the production panels into the jigs. The line is fully computer controlled and the panels are carried through a series of chemical and rinsing baths by the overhead crane. 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). This is a multi-stage process as you see from the video with washing steps between the stages. 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, the photoresist, which is hot-rolled onto the copper using a cut-sheet laminator. The laminated panels are collected by an automatic rack. The clean room uses yellow lighting as the photoresist is sensitive to UV light.

The bed of the printer has registration pins matching the holes in the phototools and the panel. The operator loads the first film onto the pins, then the laminated panel and finally the second film. The pins ensure that the top and bottom layers are precisely aligned. The printer uses powerful UV lamps to harden the photoresist. So the photomask is clear where we want the resist to harden and black where we don’t want resist.

The Mylar film which protected the photoresist is now removed and the imaged panel is conveyored out of the clean room and through a developer which removes the unhardened resist. For inner layers the copper pattern we want was covered by the resist. For outer layers it is exposed ready to be plated. The operator now checks the panels to make sure that the copper surface is clean and all the unwanted resist has been removed.

Next we electroplate the boards with copper. The operator loads the panels onto the flight bars. He checks all the clamps to ensure a good electrical connection. The panels themselves act as cathodes for electroplating and we can plate the hole walls thanks to the conductive carbon layer already deposited there. The operator starts the automated plating line. The copper surface of the panels is cleaned and activated in a number of baths and then electroplated. The whole process is computer controlled to ensure that each set or flight of panels stays in each bath exactly the right amount of time. You can see the copper anodes in their bags.

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. So if we start with a typical 17.5 micron copper foil it will be 40 – 42 microns after processing.

The baths are designed to produce an even copper thickness across the panel. Modern chemical solutions also have good “throwing power” to produce an even thickness of copper right through the hole.

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.

When plating is completed the flight of panels is returned to the operator and he unloads and stacks the plated panels. He then uses non-destructive testing to check a sample of each flight to ensure that the copper and tin plating is the correct thickness.

We have now plated the panel with 25 microns of copper through the hole and an additional 25 – 30 microns on the tracks and pads. 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. But designers should be aware that thicker copper foils need wider spaces between the tracks.

Finally we strip off the thin tin coating which protected the copper image. So now you can see that only designed copper pattern remains.

As the boards emerge from the line they are stacked automatically.

Most boards have a epoxy-ink soldermask printed onto each side to protect the copper surface and prevent solder shorting between components during assembly.

The panels are first cleaned and brushed to remove any surface tarnish and then are conveyored into the yellow room.

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 double action ensures that the ink completely encapsulates the copper tracking, typically now 35 – 40 microns higher than the surface of the panel.

The panels are now racked and put through a conveyorised drier which hardens the resist just enough to allow it to be printed (“tack-dried”). The operator checks for a complete and even coating.

Next the coated panels are imaged. For this we use a two drawer UV printer. The operator mounts the phototool films on the machine and then places the panel onto the registration pins. She checks that the film and the copper layer are precisely aligned. Mask alignment will be better than 50 microns. As with the etch and plating resistsused 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. For this we use a conveyorised oven in the same way in which the boards were previously tack-dried. 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.

Under the EU Reduction of Hazardous Substances (or “RoHS”) legislation we cannot not use lead in our finishes, so we offer gold over nickel as you see, chemical silver using a similar process to deposit a sterling silver finish, or lead-free hot-air levelling. For this the panel is lowered into a bath of molten tin. As it is lifted from the bath hot air jets blast the surplus molten metal from the panel to leave an overall coating of tin about 2 microns thick.

For edge-connectors we electroplate hard gold. First the operator puts protective tape on the board above the connectors. Then he mounts the panel on a horizontal electroplating bath. Electroplated gold is needed for edge connectors on printed circuit boards which will require repeated insertion and removal.. Electroless gold gives good solderability but is too soft to withstand repeated abrasion. For this you require a hard electroplated gold. For edge connectors we electro-plate 1 – 1.5 microns of gold over 4 – 5 microns of plated nickel. So if your PCB will be repeatedly inserted into a connector, you should specify hard gold plate on the edge connectors.

When the edge connectors have been plated we will remove the tape.

Most PCBs have a component legend to show which component goes where.

Today we use ink-jet printers 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. Ink-jet printing needs no set-up. Previously we lost time preparing and cleaning silk-screens for each legend printing. That is why the legend is often called the silk-screen.

Now we finally cure both the epoxy ink soldermask and the legend. Once this took 90 minutes in a batch oven. Now it takes less than 10 minutes using a 5 stage conveyorised oven.

At the end of the PCB production process we electrically test every multilayer PCB against the original board data. 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.

The flying probe testers in our quality control department are easy to set up as they don’t need a test fixture but testing every net is slow. 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%.

For double sided boards electrical test is an option.

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. The cutter follows the path defined in the original tool file. Next the milling head automatically picks up a 2 mm cutter, checks the diameter and mills around each PCB. The brush around the milling head ensures that all the dust produced is collected by the vacuum system.

The circuits are held in place by small bridges of material. We will drill through these and then remove each separate PCB from the production panel. This panel has also been V-scored.

Final inspection

In the last step of the process ( Final inspection - quality control ) a team of sharp-eyed inspectors give each PCB a final careful check-over. Here an inspector checks a customer’s assembly panel. She looks for any cosmetic defects like scratches. Then she measures the panel against the mechanical drawing, checking hole diameters with a tapered probe.

If everything is OK, she prints out the production release note.

After inspection the PCBs are vacuum-sealed to keep out dirt and moisture. Then they are bubble-wrapped, securely boxed, sealed and shipped off to the customers throughout Europe by overnight courier. The process is complete. We have followed our PCB from the customer’s original design to the physical circuit board on its way to him to be assembled into his finished product.

About PCBWAY