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Printed Circuit Board (PCB) What Is It?

Printed Circuit Board - with out the PCB and automation your computing experience would be non existent!

The PCB is the base for all modern electronic devices.

The actual board is made out of a thin non conducting material such as fiberglass or hard plastic, then has copper bonded to each side of the board. When the circuit is made on the board the copper is etched away with a solution that dissolves the copper that isn't covered by a special coating.

A PCB can be one layer or it can be multiple layers.

A single layer would be the PCB in your car's remote control for locking/unlocking the doors or some other small device.

The more complex the device the more layers the PCB will have.

Some extremely complex devices have over fifteen layers, your computer motherboard will have from three to ten depending on how old it is and the embedded features on the PCB.

So how do they make a Printed Circuit Board, and with more than one layer?

First comes the design of the over all circuitry of the device. When a trace has to pass where another trace is present then the solution to keeping the traces separate is by doing a bridge, and that bridge is a layer.

A few terms:

  • PCB - Printed Circuit Board is a thin fiberglass base with a thin copper sheet bonded to one or both both sides.
    • Only single layered PCB has circuit traces/pads on both sides, layered PCB will only have the circuit/traces on one side.
  • Trace - this is a thin copper wire that is bonded to the PCB.
  • Pass through - these are small copper barrels that allow a component and/or trace to go from one layer to another.
  • Pad - this is a small area of copper large enough for a component to be attached (soldered) to the PCB.

The step after design is layout, the layout procedure isn't complicated but it is lengthily. A technician will draw the circuit on the raw PCB. The material that is used to draw the layout is chemically resistant to the etching solution. This 'ink' will keep the etching solution from eroding away the copper underneath the ink, leaving a thin line of copper called a trace, a pad, or a round pad for drilling to insert a pass through. (In large production runs this is all done by robots with dies where the layout is stamped on the PCB.)

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The next step is the etching of the PCB, normally done in a solution that will dissolve the copper that isn't protected by the ink. The process will take from half an hour to over an hour depending on how thick the bonded copper layer is.

After etching the PCB is cleaned, this removes any residue copper and etching solution, it also removes the ink that kept the copper from being etched away.

We now have a single layer PCB, to get multiple layers each layer is laid out, etched, and cleaned.

The next step is called masking, this is printing the information that the designers want on the top and bottom layers for the finished PCB. For hand made devices you will see individual component values on the PCB. Robot made PCB masks are not as verbose and only the minimal information is printed on the PCB.

For normal devices the PCB layers are pressed together with heat applied, this makes a strong bond between the layers.

Note: For devices where the PCB will be exposed to extreme environments such as there is a large change in temperature, high vibration, or large G forces the layers are bonded together with a glue that will not transfer electrons and will dissipate heat. (This makes the devices more expensive because of the added materials and processing time).

Once all the layers are bonded together the next step is drilling, each area where a pass through is present a hole has to be drilled through the layers, the hole is the diameter of the pass through.

Pass troughs come in different sizes, such as the pass through is for a small diameter wire of a diode, or a larger diameter wire for a capacitor, or even larger for a power supply connector.

Next the Printed Circuit Board gets the pass troughs pressed in the holes, these pass troughs are not soldered, the tight fit will insure that the trace or pad will make a good connection.

The next step is inspection either by robot or by hand.

Next the top and bottom layers get a sealant applied, this will keep the traces from lifting and provide some insulation from shorting out. It is a protective coating only, over time heat will degrade this sealant, too much heat and the traces will lift off the Printed Circuit Board!

The last step is testing the circuitry for continuity in all the traces, normally this is done in a jig that has contacts where the trace would have a component.

The Printed Circuit Board is now ready for the individual components, sockets, or other parts to be solder on to make the device.

The only down side to making a Printed Circuit Board in layers is that if a interior trace (between the layers) burns through or a layer separates it is not repairable.

If you want to experiment with design, layout, etching and building your own devices you can buy experimental kits that has all the ingredients to make your own devices, I used these kits while in college for the labs.



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