Concepts of Printed Circuit Design

Unit 1: Introduction to PCB Technology SAQ Answers

Self - Assessment Questions Answers

Question 1

Class 1. General products, including consumer items, computers and peripherals, and some military systems
Class 2. Dedicated service products, including communication equipment, business machinery, industrial controls, instruments, and military systems, where extended life and reliable service is needed
Class 3. High-reliability products, including equipment and systems where continuous performance or performance on demand is essential

Question 2

The key elements of a simple bare PCB and the materials used are:

resin (epoxy resin)
reinforcement (glass fibre mat)
interconnection track (copper foil)

Question 3

This stage patterns the copper on the board surfaces by a photo-reactive method (i.e. using UV light), in order to form the circuit connectivity. This is accomplished by first applying a coating of photo resist to the surfaces in question, prior to exposure to UV light through an artwork mask. The mask is formed by a photoplotter and provides a connection pattern for one copper layer.

Question 4

Through-Hole Mount Technology (THT)

The technology is characterised by:

Wire component terminals
Component mounting by insertion of wire terminals through the holes in PCB followed by terminal soldering to “printed” conductors on PCB
Interconnections by flat “printed” conductors on PCB
Wave soldering is normally used
Components are physically larger
Difficult to automate process
Straightforward to repair/rework

Surface Mount Technology (SMT)

The technology is characterised by:

Terminals: (a) short stamped “legs”, (b) metallised ends of chip components
Component mounting by placing on PCB surface followed by terminal soldering to “printed” conductors on PCB
Interconnections by flat “printed” conductors on PCB
Reflow soldering is normally used
Components are physically smaller
Process readily automated
Difficult to repair/rework

Question 5

The three main sections within the PCB CAD toolset are:

Design Entry (or Schematic Capture) Tools

Design entry tools enable a design to be partitioned into a number of manageable blocks that define a hierarchy from system level down to component level. System libraries allow the design to be ‘built’ on the schematic entry screen and the interconnections established. Design properties that may be used by downstream tools may also be added at this stage.

Design Verification (or Simulation) Tools

Implicit in all circuits of whatever complexity is the potential for design errors. Design verification tools enable the functionality, performance and testability of a design to be evaluated prior to fabrication and assembly. This is essential in ensuring that the design will work ‘first time’ and will remain robust and resilient in service under all operating conditions.

Design Layout Tools

Defining circuit structures at the layout level is particularly complex. A detailed knowledge of device topologies and associated process technologies is required. Design layout tools incorporate a set of process related design rules to ensure that the required structures can be successfully manufactured within the limits of the intended process technology. This stage centres on laying out the physical component shapes on the surface of the PCA and then routing the connections between them.

01.02.05 RA