程序代写代做代考 29/08/2013

29/08/2013
Embedded Systems Design ELEN90066
Lecture 10 Advanced PCB design
David Jahshan
Notes from last lecture
• Marking Sheet – A few notes
• Buttons Two Configurations = does it work?
• You can use pull down for you soft power push button
• HowtorouteyourPCB
– Make sure your PCB is easy to read
– Take time laying out your components
– Try to have a solid ground plane
– Use 45 degree routing
– Tent Vias under your components and near smd runs – Thicker Tracks for VCC and higher current lines
PCB minimums • SetbytherulesofyourPCB
• Rulewizardensuresthatyoudonotcreate something that can not be manufactured
• Ourrules
– 10mil minimum track width
• Thicker the better
– 10mil minimum clearance
– 6mil minimum annular ring
– 40mil vias 20mil hole
– 36mil minimum text height 8mil text thickness
Advanced PCB design
􏹧 Multilayer PCBs
􏹧 PCB capacitors, inductors and antennas 􏹧 Reflections and proper termination
􏹧 Cancelling far end crosstalk
􏹧 Track length matching
Multilayer PCBs
􏹧 4 and 6 layer PCBs are commonly used in high speed electronics
􏹧 Typical layer stack for a 4 layer board is centre layers are VCC and GND, outer layers are signals.
􏹧 When multiple voltages required, the internal plane can be broken up for different voltage regions.
􏹧 To set up layer stacks use Design → Layer Stack Manager in Altium Designer
High Speed Multilayer Layout
􏹧 When laying out a multilayer PCB it is important to ensure shortest possible return path for fastest signals
􏹧 Start component layout with highest frequencies. Make these are as short as possible.
􏹧 Ensure that there is a via close to the return path and make sure you have plenty of decoupling caps at the freq required (low enough ESR)
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29/08/2013
PCB capacitors, inductors and antennas
􏹧 Sometimes it is easier to construct an inductor, capacitor or antenna on the PCB.
􏹧 Taking into account the PCBs dielectric, accurate devices can be constructed on the PCB without increasing the cost of the device.
􏹧 Values can vary due to variance in substrate thickness, dielectric used, thickness of soldermask etc.
􏹧 Antennas on devices such as 802.11 are often printed straight onto PCB to save money
Terminations
􏹧 A signal travelling down an unterminated wire will reflect doubling the voltage on the way back
􏹧 By matching the impedance of your track with the input impedance of the device, you can eliminate these reflections. (often with the help of a termination resistor)
􏹧 The impedance of the track can be controlled by the thickness of the track. (Also dependant on the PCB specs)
Cancelling far end crosstalk
􏹧 Cross talk occurs when a high frequency signal induces cross talk on a parallel track.
􏹧 Consists of two components, capacitive coupling which induces a positive spike that propagates away from the wave front
􏹧 Inductive coupling induces a negative spike propagating forward from the wave front, positive spike propagating backwards
􏹧 By picking mutual capacitance and inductance values carefully, it is possible to cancel the forward propagating wave
􏹧 By source terminating the signal you can ensure the backwards propagating wave does not reflect back to the receiver
Track length matching
􏹧 Signals do not arrive at their destination instantaneously
􏹧 When high precision timing is required, having accurate track length is essential
􏹧 By adding some bends into the track, PCB track length can be matched to ensure signals arrive at the same time
􏹧 Tools → Interac􏹨ve Length Tuning in Al􏹨um designer and tools either side
Pin Swapping
• To optimise your PCB layout sometimes you can move pins around
• PCB design does not need to be a one way process. You can start laying out your PCB, then go back and change schematic to simplify layout
• Pin swapping can be useful when laying out RAM, uC and FPGAs.
• Altium designer supports pins swapping : Tools → Pin/Part Swapping
High Currents
􏹧 Large currents cause track heating depending on the cross sectional area and length of the track and the location of the track (inside layer or outside layer)
􏹧 When it is impractical to make the track any wider you can remove soldermask off the track and add solder to thicken the cross section
􏹧 To open the soldermask just draw lines on soldermask layer. (soldermask layer is a negative layer)
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Designing for manufacturability
􏹧 Huge area, couple of lectures in itself
􏹧 Try to keep all components on one side
􏹧 Try to minimise processes (either solder re flow or wave soldering)
􏹧 Add test points for easy testing of product before shipping (Ground pin for probe)
􏹧 Follow manufacturers suggestions about location of components for particular soldering processes
Autoroute? & PCB as heatsink.
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