Printed Circuit Board Design Flow

A printed circuit board (PCB) mechanically supports and electrically connects electronic components using conductive tracks, pads and other features etched from copper sheets laminated onto a non-conductive substrate. PCB's can be single sided (one copper layer), double sided (two copper layers) or multi-layer. Conductor on different layers are connected with plated-through holes called vias. Advanced PCB's may contain components - capacitors, resistors or active devices - embedded in the substrate.

A design flow is a rough guide for turning a concept into a real, live working system

Inspiration(Concept)
“An air-deployable motion sensor with 10 meter range and 6 month lifetime.”

Implementation(Working System)

Starting with the end in mind: a printed circuit board

The cross-section of a PCB shows its layered construction

A practical PCB design flow that is action-oriented and artifact-focused

Brainstorming
1.Goal: generate as many ideas as possible!
2.Use the “needs” as the rough guide
3.Do not (yet) be limited by constraints or formal requirements
4.Ideally, brainstorm in a group so diversity of perspectives emerge

Brainstorming example: energy metering in sensor networks
1.Need: measure the energy consumed by a mote
2.Brainstorm
3.Resulting design concepts
-Single-chip battery “fuel gauge”
-High-side sense resistor + signal processing
-Low-side sense resistor + signal processing
-Pulse-frequency modulated switching regulator

Requirements and constraints address the myriad of important details that the system must satisfy
1.Requirements address:
-Functionality
-Performance
-Usability
-Reliability
-Maintainability
-Budgetary
2.Requirements may be at odds!
3.Use correlation matrix to sort things out in this case

Evaluation
1.Goal: identify best candidates to take forward
2.Use requirements and constraints as the metric
3.Get buy-in from stakeholders on decisions
4.Also consider
-Time-to-market
-Economics
--Non-recurring engineering (NRE) costs
--Unit cost
-Familiarity
-Second-source options
5.If none of the candidates pass, two options
-Go back to brainstorming
-Adjust the requirements (hard to change needs though)

Evaluation example: energy metering in sensor networks

Accuracy / linearity are really important for an instrument


Sometimes a single experiment or figure says a lot

Design I
1.Translate a concept into a block diagram
2.Translate a block diagram into components
3.Top-down
-Start at a high-level and recursively decompose
-Clearly define subsystem functionality
-Clearly define subsystem interfaces
4.Bottom-up
-Start with building blocks and increasing integrate
-Add “glue logic” between building blocks to create
5.Combination
-Good for complex designs with high-risk subsystems

Design II
1.Design can be difficult
2.Many important decisions must be made
-Analog or digital sensing?
-3.3V or 5.0V power supply?
-Single-chip or discrete parts?
3.Many tradeoffs must be analyzed
-Higher resolution or lower power?
-Higher bit-rate or longer range, given the same power?
4.Decisions may be coupled and far-ranging
5.One change can ripple through the entire design
-Avoid such designs, if possible
-Difficult in complex, highly-optimized designs

Design example: energy metering in sensor networks

Schematic capture turns a block diagram into a detail design
1.Parts selection
-In library?
--Yes: great, just use it! (BUT VERIFY FIRST!)
--No: must create a schematic symbol.
-In stock?
--Yes: great, can use it!
--No: pick a different park (VERIFY LEADTIME)
-Under budget?
-Right voltage? Beware: 1.8V, 3.3V, 5.0V
2.Rough floorplanning
3.Place the parts
4.Connect the parts
5.Layout guidelines (e.g. 50 ohm traces, etc.)

The schematic captures the logical circuit design

Layout is the process of transforming a schematic (netlist) into a set of Gerber and drill files suitable for manufacturing
1.Input: schematic (or netlist)

2.Uses: part libraries

3.Outputs
-Gerbers photoplots (top, bottom, middle layers)
--Copper
--Soldermask
--Silkscreen
-NC drill files
--Aperture
--X-Y locations
-Manufacturing Drawings
--Part name & locations
--Pick & place file

4.Actions
-Create parts
-Define board outline
-Floorplanning
-Define layers
-Parts placement
-Manual routing (ground/supply planes, RF signals, etc.)
-Auto-routing (non-critical signals)
-Design rule check (DRC)

Layout constraints can affect the board size, component placement, and layer selection
1.Constraints are requirements that limit the design space (this can be a very good thing)
2.Examples
-The humidity sensor must be exposed
-The circuit must conform to a given footprint
-The system must operate from a 3V power supply
3.Some constraints are hard to satisfy yet easy to relax…if you communicate well with others. Passive/aggressive is always a bad a idea here!
4.Advice: the requirement “make it as small as possible” is not a constraint. Rather, it is a recipe for a highly-coupled, painful design.

Layout: board house capabilities, external constraints, and regulatory standards all affect the board layout

Floorplanning captures the desired part locations

The auto-router places tracks on the board, saving time

Layout tips
1.Teaching layout is a bit like teaching painting
2.Suppy/Ground planes
-Use a ground plane (or ground pour) if possible
-Use a star topology for distributing power
-Split analog and digital grounds if needed
-Use thick power lines if no supply planes
-Place bypass capacitors close to all ICs
3.Layers
-Two is cheap

There are lots of design flows in the literature but they are awfully general