Printed Circuit BoardsPCBs

Design Manufacture & Test

Mike Doyle

December 2016

• In the last unit you saw the process of producing simple electronic devices.

• Starting with the design stage – drawing up a circuit diagram.

• Moving to software simulation.

• Then on to prototyping (breadboard & vero-board)

• Ultimately on to a Printed Circuit Board (PCB)

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• But, what is a Printed Circuit Board [PCB]?

and

• How are they made?

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• PCBs are often, but not always, made from glass reinforced plastic with copper tracks in the place of wires.

• Some are made using paper.

• Components are fixed in position by drilling holes through the board, locating the components and then soldering them in place as you have seen.

• The copper tracks link the components together forming the electrical circuit.

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• PCB Manufacture is a ‘Staged Process’

• That is to say there are a number of stages that must be completed in order to produce the finished artefact.

• In its simplest for the process list could be as follows:-

• Circuit design – drawing the electronic circuit usually with CAD

• PCB Artwork production – again usually with a CAD package

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• PCB Artwork transfer to copper clad board

• PCB Etching – where unwanted copper is removed – usually chemicals

• PCB Holes drilling – holes are drilled ready to accept components

• PCB component soldering – permanent connection of components

• PCB Testing – ensuring the original design/performance has been met

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• Circuit design – draw the electronic circuit - usually done with CAD

• So far you have used a number of simulation packages:-

• Yenka , Falstad and Circuit Wizard

• PCB Artwork production – again is usually done with a CAD package

• There are many packages:

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• Eagle CAD, Protel, Proteus and many more…

• You will be using Circuit Wizard.

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• Lay out your circuit using the circuit diagram capture software element of Circuit Wizard.

• Simulate the circuit to make sure it performs as required.

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• Define the board size

• Place the components

• Interconnect components with tracks

• Strategically place useful text

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• Create the Artwork for the PCB

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• Transferring the artwork to the PCB material is again a staged process.

• First stage is to print out the PCB track artwork

• This must be in fine detail

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• The raw PCB board is basically copper clad, either single, double sided or multi-layer boards

• You will use single sided.

• The board has a special ‘Photoresist’ included on its surface to which we offer up the PCB artwork or ‘MASK’

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• Lets take a look at the step-by-step process.

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• The artwork or MASK is affixed/overlaid on either or both of the photoresist surfaces of the board.

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• The PBC is then exposed to UV light for a specific time, based on the level/strength of the UV light and the photoresist chemical make-up or even the age of the photoresist.

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• Once exposed, the areas exposed are either rendered soft or hard depending on the process type. Two types exist positive or negative processing.

• In the case - here the photoresist that is exposed to UV softens leaving behind the still hard mask shadow.

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• This, just like the old fashioned film process is ‘developed’ to produce an image.

• This process removes the soft photoresist leaving behind the hard photoresist on top of the copper.

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• The next step is to spray, wash or some other way agitate the PCB in the presence of an ‘Etchant’

• Etchants are generally NASTY chemicals – after all their job is to eat away the exposed copper.

• Etchants include Acids or strong Alkalis – Hydrochloric acid, and Ferrous Oxide.

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• MASK REMOVAL

• After etching has taken place, usually a timed process, making sure the chemicals etch down but not sideways. Why?

• The Mask needs to be removed – solvents such as acetone [nail varnish remover] are used.

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• COMPLETED PCB

• This leaves us with a fully etched board.

• This is by no means the last process as there are as yet no holes in the board.

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• DRILLING OF HOLES

• This step is not only for pin through hole PCBs as many surface mount PCBs require holes as well.

• Can be done automatically, based on the CAD data produced –or manually.

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• PCB POPULATION

• This can be a manual or automatic process for both Through Hole [Board Stuffing] and Surface Mount [On-sertion]

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• There are also many other processes that can be involved but these tend to be for the more complex ‘Multi Layered’ boards.

• Processes might include:

• Interlayers

• Through hole plating

• Inter layer vias

• To name but a few.

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• INSPECTION

• This is an extremely important stage where the board is either manually examined by a human or automatically by a machine vision system in a high quantity production system.

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• ELECTRICAL TESTS

• There would of course always be electrical testing required, purely from a Quality point of view.

• This can be an auto-test system but can also involve manual testing where machines can not perform.

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• BED OF NAILS TESTER

• Although not actually nails, merely that they look like nails, these can be used in high production environments but require re-programming for new circuits.

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• Each of the “nails” contacts the board in a specific place and either measures current, voltage, capacitance, inductance or some other aspect of the circuit usually under computer control.

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• TEST PROBES “NAILS”

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• Simplistic bed of nails testing is also possible

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• Once the automated or manual testing is complete and has passed all tests then the new board is considered to be ‘ready for use’.

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• Now it is over to you to start your own design process.

• For this you will need a PC with Design Wizard installed on it.

• An assessment sheet with the design circuit.

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