INTRODUCTION TO PCB

INTRODUCTION TO PCB

Printed Circuit Board

Circuit connects components through board

Made of alternating layers of conducting and insulating materials

Why PCB?

Features

Surprisingly Affordable

Highly reliable

Compact

Drawbacks

Requires more layout than other board types

Higher initial cost than wire wrap or point-to-point construction

Aprinted circuit board (PCB)mechanically supports and electrically connectselectronic componentsusingconductivetracks, pads and other featuresetchedfrom copper sheetslaminatedonto a non-conductivesubstrate.

PCB's can besingle sided(one copper layer),double sided(two copper layers) ormulti-layer.

A PCB populated with electronic components is called a printed circuit assembly(PCA),printed circuit board assemblyorPCB assembly(PCBA)

Introduction to a Printed Circuit Board

What PCB is made of

A basic PCB starts with a copper-clad fiberglass material or thin copper sheets attached to either side of the board. It consist of :-

Copper Foil

Copper Plating

Solder Flow

Solder Mask

Trace

Slots and Cut-outs

PCB characteristics

Through-hole technology

The first PCBs used through-hole technology, mounting electronic components byleadsinserted through holes on one side of the board and soldered onto copper traces on the other side. Boards may be single-sided, with an un plated component side, or more compact double-sided boards, with components soldered on both sides

Surface-mount technology

Surface-mount technology emerged in the 1960s, gained momentum in the early 1980s and became widely used by the mid-1990s. Components were mechanically redesigned to have small metal tabs or end caps that could be soldered directly onto the PCB surface, instead of wire leads to pass through holes

Etching process

Chemical etch

Chemical etchingis usually done with ammonium per sulphate or ferric chloride.

For PTH (plated-through holes), additional steps ofelectroless depositionare done after the holes are drilled, then copper is electroplated to build up the thickness, the boards are screened, and plated with tin/lead. The tin/lead becomes the resist leaving the bare copper to be etched away.

Cont.

Laser/Plasma Etch

A new process creating new standards and the demise of the chemical process is plasma etch. In addition to no etch-back, this process also eliminates imaging, or film error using a direct imaging process, which transfers the layer image directly to the material.

Design Assembly

Manual Assembly

Hole preparation is normally done in the manufacturing process that plates/or coats the exposed copper areas with solder. This prepares the surface of pads, as well as the hole itself, with a fine coat of solder that will make soldering easier by helping the solder adhere to the pad and hole wall and helping the solder to "wick" through the hole.

Auto Assembly

Auto assembly is an automated process where most of the component on a board are placed and soldered to the board by machine. Auto assembly requires additional information from the designer, in addition to the standard Assembly information provided for manual assembly.

Assembly Limitation

The number of parts that can be assembled in one pass

Double-sided assembly

Overall panel/board size

Component size

Component types (surface mount or thru-hole)

Routing

Manual Route

Route the following types of nets first:

Most difficult

Most complex

Tight fitting nets first

Very high current (primarily external)

Very high voltage (primarily internal)

Auto Route

Manually route those items shown in "manual routing " first, if necessary.

Manually clean up paths.

Miter right angle corners.

Run DRC /design rules to ensure clearances are met.

Check annular ring.

Parts of a PCB

Components

Pads

Traces

Vias

Top Metal Layer

Bottom Metal Layer

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Components

Components are the actual devices used in the circuit.

This includes input/output connections.

I/O ports, including power supply connections, are also important in the PCB design.

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Pads

Location that components connect to.

You will solder components to the pads on the PCB.

Pads will connect to traces.

Pads have an inner diameter and outer diameter.

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Traces

Traces connect pads together.

Traces are essentially the wiring of the PCB.

Equivalent to wire for conducting signals

Traces sometimes connect to vias.

High current traces should be wide.

Signal traces usually narrower than power or ground traces

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