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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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 → Interacve Length Tuning in Alum 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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