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School of Electrical, Electronic and Computer Engineering
N Pillay
ENEL3CA/DA: Electronic/Computer Design 1Lecture 5:
PCB Philosophy and Design using Ultiboard
Outline Some Philosophy What is a PCB? The PCB Structure, Fabrication Important tips
Design using Ultiboard Footprint editing “In-place editor” Creating footprints A detailed summary Practice project
What is a PCB?
•A printed circuit board orprinted wiring board is usedextensively in electronics and isa connected organization ofelectrical components on aboard to execute a specificfunction.•The main two purposes are tomount components and providethe means of electricalconnection between thecomponents.
What is a PCB In the early era of vacuum tubes and even later electronic
equipment was constructed by hand wiring and point-to-point soldering.
The equipment was large, awkward and bulky. As smaller components were developed and modular design
became popular inter-component wiring and assembly came about – the PCB
PCB Structure
• The PCB starts out with a non-conducting sheet of material,normally a fibre glass epoxy laminate material.•The board is coated with a thin layer of Cu. About 0.2mmthick. A double-sided board requires a copper layer on eitherside.
What is a PCB Essential components of the PCB: The Base (thin board of insulating material, rigid or flexible) The Conductors (high purity Cu)
What is a via?? (Unfilled or Filled)
Single sided board
Manufacture: “print and etch”, “die-cut” Number of jumpers inverse proportion to size of board
What is a PCB Double-sided board Plated-through-hole (PTH) NPTH
What is a PCB NPTH
Eyelets also commonly used
What is a PCB Multi-layer boards are very popular As many as 50 layers has been used
What is a PCB Comparison of technologies Market drivers for PCBs
Some Common Solder techniques oCVD - using conjugated polymers
Wave method
Solder paste – reflow method
Hand soldering
FabricationAt the simplest level: Starting with a double-clad copper fibre glass epoxy laminate An etch-resistant material is deposited on the impurity-free
copper layer – this etch resist is the image of the circuit. Board is dipped into an etchant. Etch resist is removed. Board can be finalized with air blowing, hot oil, solder mask,
silkscreen, solder dip, etc. Another option – use photo resist and UV light source. Many modernized techniques used in conjunction with pick-
and-place machines.
The PCB structure, fabrication
Fabrication: CNC Processing
Fabrication Single-sided board
Fabrication PTH double-sided
Important tips Creating an electronic product at the system-level
Design elements for the PCB designer
Important tips Performance parameters
Important tips How to choose the width of track? Remember it depends on the current and temperature rise
requirement. For external tracks we use
For internal tracks we use
Important tips
Important tips Choosing the conductorwidth
Important tips Nomographs Conductor resistance Conductor thickness Conductor width
Important tips Capacitance consideration Conductors on opposite sides form a capacitor
Important tips Inductance Even at 10kHz, high frequency components of rectangular
shaped signals can cause problems.
Important tips Standards – IPC – IPC-2221-”Generic Standard on Printed
Board Design”. Always start with a detailed, debugged and accurate
schematic diagram. Choose a metric for the design of the PCB – keep the
enclosure/packaging in mind. Millimeter (mm) vs.Thou (mil). Try to use as many wide tracks as possible. Wide tracks have
low impedance. Use “necking”, to fit tracks between pads and places allowing
small clearance.
Important tips Tracks that are acceptable and unacceptable
Important tips Bunching
Important tips Recommended and not recommended
Important tips Try to use either round or oval pads, use rectangular pads if
needed. Use round pads for through-hole components, eg. Resistors,
capacitors, etc. Use rectangular pads for suface-mount components. Use rectangular pads for pin 1 on IC footprints. Try as much as possible to minimize the number of vias
possible. Use polygons as much as possible especially on the ground
path – lower impedance.
Important tips Break design into functional blocks if possible and organize
components accordingly. Good practice to route inputs from the left to outputs on the
right. Babak’s Test of Futility. Check design rules and connectivity. Check manually with the schematic. Use chamfers and tear drops as much as possible. Try to avoid acid traps. Use test points in your PCB design. Follow all manufacturer constraints. Don’t cut too close to
these.
Important tips Swapping of gates and parts and pins
The Schematic
Ultiboard Transfer
Drag the components
Aside When generating your schematic try not to use virtual
components. If virtual components are used then you will need to insert
footprints manually. This is not too difficult either but takes alittle more time.
Footprint editing
Creating new footprints
Detailed Summary Design the schematic and check extensively Transfer to Ultiboard using the Transfer tab. It is also possible to export from Multisim to many other
common PCB Tools. Use a track witdh of 0.8mm, use a clearance of 0.5mm.
(These constraints can be relaxed – depending on the technology) Adjust board size Use theTools > Board wizard Choose between single layer, double layer, multi-layer
Detailed Summary Specify form-factor for board Drag components onto board Try to group similar components into groups or sub-systems Keep connectors at the edges of the board Always think about the packaging solution – The board must
be tailored to suit this. Double-click footprints to edit the pads, etc. Use Tools > Component Wizard to create new footprints, or
simply use the footprint of a similar component and edit bydouble clicking.
Detailed Summary Carefully place components by looking at the ratsnest and try
to minimize big bunches of wires travelling around the board Change the orientation of components to avoid tangled lines Don’t worry about the size of the board too much, it can be
changed after arranging components by accessing it’sProperties
Good to do the power first and the ground Can be put onto a separate layer to protect the other
sensitive analog and digital circuitry from EMI. Use bottom for rest of connections, i.e. signal lines, etc.
Detailed Summary Choose the other copper layer Hide or take out the original copper layer Continue with routing You can also check the Netlist and DRC check or the Connectivity
check. Connect all the traces and run a final check. All rules must be
followed. Check ERC tab. If you are stuck and cannot route further use vias. Try to minimize
these. The PCB can be finally optimised within your list of constraints
and must be checked extensively for bugs. Include test points on your PCB.
Detailed Summary Also possible to use Autorouting where some routing
algorithm is used to connect up all components. Some algorithms take long others are quicker. We normally settle for manual routing – gives the designer
most control. Another approach to use the autoroute and to manually
check and change in accordance. What is a via? What is a net/netlist? Avoid right angles, t-junctions and acute angle routing – can
lead to acid traps and a possible decrease in signal integrity.
Class Practice Project Each student is required to complete the full PCB for the
Multisim sample circuit (C:\Program Files\ElectronicsWorkbench\EWB9\samples\FrequencyDivider.ms9).
Remember to include test points and to follow all the designrules and constraints in the guide.