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
About PCBWAY
Since 2003 PCBWAY has been the leading PCB quick turn manufacturer specializing in both Prototype and Production quantities, Initially produced single-sided and double-sided printed circuit boards for the consumer electronics market. PCBWAY is ranked among the top 4 board fabricators in asia and is well-known for its expedited turn time capabilities and its reliable best on-time shipping record.
Today, we have over 450 operators with high modern facilities to manufacture multi-layer PCB up to 12 layers. Backing up with a group of professional engineers, and well established quality system. PCBWay has grown to become a major PCB manufacturer in Asia to serve in diverse customers base such as electronics appliance, communication, educational electronics, power supplies, Automationsetc.
Our mission is to become one of leading PCB manufacturer that provide in high quality product with total customer satisfaction.
