School of Electrical, Electronic and Computer Engineering

N Pillay

ENEL3CA/DA: Electronic/Computer Design 1Lecture 5:

PCB Philosophy and Design using Ultiboard

Outline Some Philosophy What is a PCB? The PCB Structure, Fabrication Important tips

Design using Ultiboard Footprint editing “In-place editor” Creating footprints A detailed summary Practice project

What is a PCB?

•A printed circuit board orprinted wiring board is usedextensively in electronics and isa connected organization ofelectrical components on aboard to execute a specificfunction.•The main two purposes are tomount components and providethe means of electricalconnection between thecomponents.

What is a PCB In the early era of vacuum tubes and even later electronic

equipment was constructed by hand wiring and point-to-point soldering.

The equipment was large, awkward and bulky. As smaller components were developed and modular design

became popular inter-component wiring and assembly came about – the PCB

PCB Structure

• The PCB starts out with a non-conducting sheet of material,normally a fibre glass epoxy laminate material.•The board is coated with a thin layer of Cu. About 0.2mmthick. A double-sided board requires a copper layer on eitherside.

What is a PCB Essential components of the PCB: The Base (thin board of insulating material, rigid or flexible) The Conductors (high purity Cu)

What is a via?? (Unfilled or Filled)

Single sided board

Manufacture: “print and etch”, “die-cut” Number of jumpers inverse proportion to size of board

What is a PCB Double-sided board Plated-through-hole (PTH) NPTH

What is a PCB NPTH

Eyelets also commonly used

What is a PCB Multi-layer boards are very popular As many as 50 layers has been used

What is a PCB Comparison of technologies Market drivers for PCBs

Some Common Solder techniques oCVD - using conjugated polymers

Wave method

Solder paste – reflow method

Hand soldering

FabricationAt the simplest level: Starting with a double-clad copper fibre glass epoxy laminate An etch-resistant material is deposited on the impurity-free

copper layer – this etch resist is the image of the circuit. Board is dipped into an etchant. Etch resist is removed. Board can be finalized with air blowing, hot oil, solder mask,

silkscreen, solder dip, etc. Another option – use photo resist and UV light source. Many modernized techniques used in conjunction with pick-

and-place machines.

The PCB structure, fabrication

Fabrication: CNC Processing

Fabrication Single-sided board

Fabrication PTH double-sided

Important tips Creating an electronic product at the system-level

Design elements for the PCB designer

Important tips Performance parameters

Important tips How to choose the width of track? Remember it depends on the current and temperature rise

requirement. For external tracks we use

For internal tracks we use

Important tips

Important tips Choosing the conductorwidth

Important tips Nomographs Conductor resistance Conductor thickness Conductor width

Important tips Capacitance consideration Conductors on opposite sides form a capacitor

Important tips Inductance Even at 10kHz, high frequency components of rectangular

shaped signals can cause problems.

Important tips Standards – IPC – IPC-2221-”Generic Standard on Printed

Board Design”. Always start with a detailed, debugged and accurate

schematic diagram. Choose a metric for the design of the PCB – keep the

enclosure/packaging in mind. Millimeter (mm) vs.Thou (mil). Try to use as many wide tracks as possible. Wide tracks have

low impedance. Use “necking”, to fit tracks between pads and places allowing

small clearance.

Important tips Tracks that are acceptable and unacceptable

Important tips Bunching

Important tips Recommended and not recommended

Important tips Try to use either round or oval pads, use rectangular pads if

needed. Use round pads for through-hole components, eg. Resistors,

capacitors, etc. Use rectangular pads for suface-mount components. Use rectangular pads for pin 1 on IC footprints. Try as much as possible to minimize the number of vias

possible. Use polygons as much as possible especially on the ground

path – lower impedance.

Important tips Break design into functional blocks if possible and organize

components accordingly. Good practice to route inputs from the left to outputs on the

right. Babak’s Test of Futility. Check design rules and connectivity. Check manually with the schematic. Use chamfers and tear drops as much as possible. Try to avoid acid traps. Use test points in your PCB design. Follow all manufacturer constraints. Don’t cut too close to

these.

Important tips Swapping of gates and parts and pins

The Schematic

Ultiboard Transfer

Drag the components

Aside When generating your schematic try not to use virtual

components. If virtual components are used then you will need to insert

footprints manually. This is not too difficult either but takes alittle more time.

Footprint editing

Creating new footprints

Detailed Summary Design the schematic and check extensively Transfer to Ultiboard using the Transfer tab. It is also possible to export from Multisim to many other

common PCB Tools. Use a track witdh of 0.8mm, use a clearance of 0.5mm.

(These constraints can be relaxed – depending on the technology) Adjust board size Use theTools > Board wizard Choose between single layer, double layer, multi-layer

Detailed Summary Specify form-factor for board Drag components onto board Try to group similar components into groups or sub-systems Keep connectors at the edges of the board Always think about the packaging solution – The board must

be tailored to suit this. Double-click footprints to edit the pads, etc. Use Tools > Component Wizard to create new footprints, or

simply use the footprint of a similar component and edit bydouble clicking.

Detailed Summary Carefully place components by looking at the ratsnest and try

to minimize big bunches of wires travelling around the board Change the orientation of components to avoid tangled lines Don’t worry about the size of the board too much, it can be

changed after arranging components by accessing it’sProperties

Good to do the power first and the ground Can be put onto a separate layer to protect the other

sensitive analog and digital circuitry from EMI. Use bottom for rest of connections, i.e. signal lines, etc.

Detailed Summary Choose the other copper layer Hide or take out the original copper layer Continue with routing You can also check the Netlist and DRC check or the Connectivity

check. Connect all the traces and run a final check. All rules must be

followed. Check ERC tab. If you are stuck and cannot route further use vias. Try to minimize

these. The PCB can be finally optimised within your list of constraints

and must be checked extensively for bugs. Include test points on your PCB.

Detailed Summary Also possible to use Autorouting where some routing

algorithm is used to connect up all components. Some algorithms take long others are quicker. We normally settle for manual routing – gives the designer

most control. Another approach to use the autoroute and to manually

check and change in accordance. What is a via? What is a net/netlist? Avoid right angles, t-junctions and acute angle routing – can

lead to acid traps and a possible decrease in signal integrity.

Class Practice Project Each student is required to complete the full PCB for the

Multisim sample circuit (C:\Program Files\ElectronicsWorkbench\EWB9\samples\FrequencyDivider.ms9).

Remember to include test points and to follow all the designrules and constraints in the guide.