‘Component Technology and Design Parameters’
Printed Circuit Boards (PCB’s) are in everything that we use nowadays and they replaced the old vacuum tubes. Nowadays ‘Solid State’ devices with transistors, diodes, resistors and capacitors have replaced the need for these old vacuum tubes. PCB’s can now be found in virtually everything that we use from computers, essentially a PCB with additional components to jet airplanes… Vacuum tube, Wikipedia (2008)
An old vacuum tube
PCB Layout Program
The design of the PCB (Printed circuit board) can then be made once you decide what components you need and the job it needs to do. PCB’s can then support any of the electrical applications on the inventions. The PCB’s use conductive pathways to achieve this. Usually they are traced or etched from copper sheets then laminated onto non conductive substrate. They are also called PWB’s or etched wiring boards. With all the other components on the board they are called a PWA (Printed circuit assembly) or else a PCBA (printed circuit board assembly) The PCB’s are usually very good and reliable as well as being strong. They do need a lot of work in the beginning and can cost because of this. They can be wire wrapped or else of a point to point construction. For high volume making they are of course a lot cheaper. The PCB’s have to follow ICP guidelines.
A 1983 spectrum computer board
PCB Layout Program
They are made by etching onto the board. So a blank PCB is made the unwantened copper is removed after putting on a mask. This then leaves the coppertraces. Some are made by adding traces to the bare boards. This is called electroplating. Electroplating uses electric current to reduce some of the material on the board and to also coat the board with thin layers of metal. The three methods used for removing copper are, silk screen printing, photoengraving and PCB milling.
Silk screen printing allows writing to be made in the PCB. Photoengraving uses a photo mask and chemicals to etch copperfoil from the substrate; this is done with a photoplotter. The PCB milling uses a two to three axis milling system to mill away the copper foil from the substrate; this then can extract files from PCB design software and add then to the board. The holes on the boards are drilled using tungsten carbide bits.
In the old days they plated the copper with solder but now it has changed to reduce the lead due to EU controls. They also use OSP (organic surface protectant) and IAG (Immerson silver), immersion tin, electroless nickel and immersion gold, also direct gold. Some connectors on the edges of the boards are gold plated.
To test the boards they do a bare board test so that they test each circuit connection as shown on their netlist. In high production they use a bed of nails tester or a fixture as well as a rigid needle adapter this connects to the copper holes to aid in testing. They use a computer to do the test and it sends electric through each contact point. For other boards they use a flying probe or flying grid tester that uses moving heads to contact with the holes.
Then the components can be added this is called a printed circuit assembly (PCA).
There are two ways to do this either through hole construction or surface mount construction. The component leads are electrically and mechanically put onto the baord with molten metal solder. The board when ready made is tested again using a visual test and automated optical inspection, this is to check quality control and they have to follow strict JEDEC guidelines for everything. Other tests are analogue signature analysis and power off testing. With the power on they do an in circuit test to measure voltage and a functional test so that the PCB does what it is designed to do. They also put on extra pads to do these tests. If the board fails the tester may desolder and replace the components.
The PCB can be a single layer or multi layer depending on the number of wiring circuits needed. The multilayer PCB includes many layers o copper with each layer separated by a dielectric material. Multi layer circuit boards are used more in electronic devices to connect to electronic components. Such as joining an integrated circuit to another one. PCB’s in computers are mounted within a plastic or sheet metal housing. There are also double sided PCB’s. Multisim and CircuitLogix are analog and digital simulators and are used to simulate PCB’s and check there affectivness, latency, and design. Printed circuit board, Wikipedia (2008)
CircuitLogix screen shot.
Simulation languages started with GPSS in the 60’ and now they use SIMSCRIPT. SIMSCRIPT III, (2008)
CAD Drawing
Solid modelling CAD is used in engineering departments and Solid Modelling software creates a 3D model of the electronic components. CAD, Wikipedia (2008)
Injection molding is one aspect of casing and housing design and has been used for many years.
A molding machine
Plastic injection molding
Now engineering companies can actually grow plastic in their factories from CAD designs to create complex and elaborate designs such as the product above. This allows the designers to create more elaborate products than the traditional molding methods. Injection molding, Wikipedia (2008)
A pick and place machine
Lazer guided PCB assembly
Companies using robots and lazer guided automated assembly lines are saving time and money due to an increase in productivity. This is also achieved via networking of robots and factories so that managers can access and fix any problems in the factory from anywhere in the world. The arguments are that people are better at creative judgments but that repetitive processes can be dome better by machine as this does not require that judgment. In some cases if a person is operating a machine it might be 30% inefficient compared to robots 5% as the robot is always waiting for the part and the human user is not. I think the automated lazer guided PCB assembly line is good in that it can produce PCB’s a lot quicker than humans, however some creative judgments are needed by human controllers to maximise the output and to increase productivity. Magus Electronics, (2008)
Bibliography
Vacuum tube, Wikipedia (2008) http://en.wikipedia.org/wiki/Vacuum_tube Date accessed 12.05.08
CAD, Wikipedia (2008) http://en.wikipedia.org/wiki/Computer-aided_design Date accessed 18.05.08
SIMSCRIPT III, (2008) http://www.caciasl.com/products/simscript.cfm Date ACCESSED 19.05.08
Printed circuit board, Wikipedia (2008) http://en.wikipedia.org/wiki/Printed_circuit_board
Date accessed 19.05.08
Injection molding, Wikipedia (2008) http://en.wikipedia.org/wiki/Injection_molding Date accessed 19.05.08
Magus Electronics, (2008) http://www.maguselectronics.co.uk/magprod.htm Date accessed 19.05.08