Microcomputers notes

1
CPE495 Computer Engineering Design I
Introduction to PCB Design
Emil Jovanov Electrical and Computer Engineering
The University of Alabama in Huntsville
2
Schematic Capture and PCB Design

• ExpressPCB schematic capture and PCB Design
• http//www.expresspcb.com/ ExpressPCB software
• Component selection
• Through hole
• SMT
• Design for Test
• Test points, grounds, etc.
• Spare components

3
ExpressSCH

• ExpressPCB schematic capture
• Quick Start Guide
• examples
• New Schematic
• Component Manager
• Custom Components
• Wiring
• Standard NET lists
• ExpressPCB integrated support
• Sheets and Ports
• Checking for Errors

4
ExpressPCB

• ExpressPCB PCB design
• Quick Start Guide
• examples
• Layers
• Traces
• Linking the Schematic and PCB
• Grid
• Rules for Component Placement
• Ground plane
• Component Manager
• Custom components
• Ordering boards

5
PCB Design

• Component floor plan
• Placing components on one side only / both sides
• Snap-to-grid
• Turn-off for unusual pin spacing
• Components that need to be in specific locations
• Daughter board
• Minimize trace lengths
• Component orientation
• Polarized part orientation
• Mark the positive leads / pin1 (silk or text on
  the layer)
• Leave enough room for traces in the beginning
• Test points and design for test
• Print out a copy of the layout and check the
  design with real parts

6
PCB Design Power and Ground(s)

• Wide traces to common rails
• 0.05 - 0.1 (GND) if possible
• Suggested Power rail along the front edge,
  Ground along the rear edge of the board
• Ground(s)
• Analog and digital ground
• Design issues
• System separation
• Ground problems
• Optical isolation
• Double diode
• Examples

7
PCB Design Signals

• Wide traces for higher current signals
• 0.01 300 mA
• Good for low current digital and analog signals
• 0.05 2 A
• 0.1 4 A
• Make at least 10 mils spacing
• Larger voltages larger gaps
• Vias (plated-through hole)
• Vertical traces on one side, horizontal on the
  other
• Directions vertical, horizontal, 45?
• Avoid sharp, right angle turns (use two bends)
• Place text to determine what is the top layer
• Project, author,etc.

8
Analog Ground Example
AnalogGround
9
WISE 2.1 Example
10
Custom components

• Custom components in SCH and PCB
• Libraries
• Sharing the same libraries
• Team work

11
Verification

• Print and check component layout
• Double check the routing of every signal
• Use highlighter
• Check for missing vias
• Print separate layers
• check for traces with dead-ends (not connected)
• Check for traces that cross each other
• Check for metal components (heat sinks, crystals,
  etc.) that can cause shorts
• Ordering boards