29/08/2013
Embedded Systems Design ELEN90066
Lecture 9 PCBs Do’s and Don’ts
David Jahshan
Notes from last lecture
• For good PCB design 80% of your time should be positioning parts, 20% on running the wires.
• Crystal is the highest frequency device in your system.
• Parallel Oscillator circuit layout
Example exam question
• Why would you not place parts under a component such as an LCD or battery holder?
• What order did you place your components and why.
• Draw the schematic for … and explain how it operates.
Marking Sheet
• We will quickly look through the marking sheet to make sure we are all on the same page.
• Please print out one of these and fill it out, and get a demonstrator to verify your mark.
Comments from previous years PCB
You have moved the location of name and student number
You did not label the LEDs for what they indicate
You did not label the push button switches
You should label which pin is positive
You should label where the + connectors are
Labels should be on overlay layers not on track layer. Move your labels from bottom layer to
bottom overlay, text depends on system fonts and they can accidentally grow and cause
shorts during manufacture.
Indicating which battery hole is + helps in assembly
Should not permit overlay to be over pads
Should avoid overlay over Vias
A number of designators are ambiguous to which component they refer to
If you changed Designator size to 36mil height 8mil width it would look a lot nicer
Text should all be facing the same way if possible
Comments PCB
Your components are a bit all over the place, you should group them more carefully
Your decoupling capacitors are far away from the microcontroller
Your buttons, JTAG connector or Battery are under the LCD and are not accessible,
place components around the LCD.
Your crystal is not located close enough to the uC
You should have thicker tracks for VCC, connection from battery to VCC and to LCD
(20mil +)
Thicker tracks to LCD would be better
You should avoid right angles in tracks
You should not have tracks at funny angles
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29/08/2013
Comments PCB
You should try to avoid exiting rectangular pads at 45 degrees, you should have a straight segment before the 45 degree section
You have a lot of tracks on the bottom surface
Your track layout is messy, there are lots of tracks not in the centre of pads
Your ground plane is very broken and does not provide short signal return loops
You don’t really have a ground plane
Having a ground plane around the crystal is enhanced if you put lots of vias to
connect to the other side
Your Ground Plane is not of the type hatched. When switching between version
fill sometimes breaks
Small VCC plane under IC
Examples
Comments PCB
You should tent vias underneath components
You should avoid vias near or on SMD pads, they will either bridge or siphon away solder
You have used 0603 Footprints for some Caps, Resistors, Diodes, you should use 0805.
You have a broken net
You have short circuits
Do not be afraid to start again • Tools -> Unroute -> All
Next Lecture • Advanced PCB layout
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