Practical Electronics

PCB Design

Griffith School of Engineering

Many companies have PCB design departments.

Proper PCB design is an INTEGRAL part of design.

PCB layout may break operation and performance of design

(eg high speed digital, low level analog and RF).

Note: ALL PCB traces have R,L&C

Designing PCB is like painting a picture artistic

If it looks good itll work is an old catch phrase.

PCB Design

(Ref: D.L.Jones

www.alternatezone.com)

First create SCHEMATIC

(Garbage IN - Garbage OUT!)

GOOD PRACTICE for SCHEMATIC

Signals inputs on left, outputs on right

Bypass capacitors next to components

Add notes on schematic (Pin requires guard track to GND)

PCB Design

PCB LAYOUT STANDARDS IPC 2221 Generic Standard on Printed Board Design

UNITS Most components are IMPERIAL (Using METRIC will make layout harder)

1 thou = 1 inch/1000 (=0.0254 mm) (Old 1 mil = 1 milli-inch =1 thou) 1 mm = 40 thou

Use mm for mechanical i.e. hole size and board dimensions Newer surface mount components are metric.

100 thou is a base reference point as many components relate to this unit eg DIL packages.

PCB Design - Layout

WORKING TO GRIDS

RULE: LAYOUT BOARD TO FIXED GRID Called snap grid Use coarse grid eg 100 thou for through hole components, 50 thou for tracks This keeps components NEAT and SYMMETRICAL BAD PCB design recognized by tracks not lining up with pad centres

GOOD PCB LAYOUT PRACTICE

Start with coarse grid ~50 thou, then 25 thou and 10 thou later for finer routing Two types of grids snap grid and visible grid on screen Helps for aligning tracks Can have two grids set to different units Many designers use 100 thou visible grid (Others electrical grid cursor snaps to electrical object and component grid component movement (use multiples of snap grid))

PCB Design - Layout

WORKING FROM THE TOP

PCB ALWAYS DONE LOOKING FROM TOP OF BOARD

(Bottom view only for checking purposes)

Text on bottom mirror image view

PCB Design - Layout

TRACKS No standard for track size Depends on electrical requirement, routing space and clearance Normally a mix of sizes Bigger tracks have less DC resistance and lower inductance Easier to manufacture and etch Easier to inspect and rework Lower limit set by PCB manufacturer IPC standard 4 thou lower limit Typical PCB using laser printer 10 thou track/10 thou space Use largest track/space DO NOT PUSH THE LIMITS As start 25 thou signal tracks, 50 thou power/gnd tracks (10-15 thou between pads - keep tracks thick reducing size called necking)

PCB Design - Layout

TRACK WIDTH is determined by CURRENT and maximum TEMPERATURE.

Every track has resistance and dissipates heat.

Thickness of copper (ounces per square foot) 1oz/ft2 most common (others 0.5,2,4 oz/ft2)

Rule of thumb 10C rise is safe For 1oz/ft2 1 amp & 10C rise for 10 thou track

Track inductance

A great improvement may be obtained by running opposite current paths (outgoing & returning) close to each other, where the mutual coupling will cancel.

This is easily achieved by running power rails one above the other on opposite sides of the PCB.

PCB Design - Layout

PADS

Pad sizes, shapes and dimensions depend on component Usually in library Important parameters PAD/HOLE ratio Usually >1.8 allows for alignment errors, top to bottom Pads for leaded components round >70 thou (1.7mm) DIL oval (90 thou x 60 thou), Pin 1 rectangular Surface mount rectangular In general use circular and oval

With pads on ground plane use thermal relief to enable enough heat for good solder joint.

PCB Design - Layout

VIAS Vias connect tracks from one side of board to the other

Commercial Plated-Through-Hole (PTH) Practically no difference between VIA and PAD However treat differently Dont use PAD for VIA Holes for VIAs usually smaller (0.5-0.7mm) Using VIAs is called stitching layers

Different types of vias: (1) Through hole. (2) Blind via. (3) Buried via. The grey and green layers are non-conducting, while the thin orange layers and vias are conductive.

Source: http://en.wikipedia.org/wiki/File:Via_Types.svg

Source: http://en.wikipedia.org/wiki/File:Bga_und_via_IMGP4531_wp.jpg

PCB Design - Layout

POLYGONS

Polygons available in many packages Fills in area around pads and tracks Useful for ground planes

Can be solid fill or hatched copper tracks (solid preferred)

PCB Design - Layout

CLEARANCES Important too small hairline shorts Good clearance ~15 thou for through hole (~10 thou for SMT) For 240V power legal requirements (>>8mm separation) Table: IPC multilayer PCB standard clearances (including altitude)

PCB Design - Layout

COMPONENT PLACEMENT and DESIGN

Old saying PCB Design 90% placement, 10% routing

Placement is VERY IMPORTANT

Makes layout easier and gives best electrical performance

BUT NO ABSOLUTE RIGHT WAY FOR PLACEMENT

PCB Design - Layout

Few BASIC RULES for PCB layout

Set your snap grid, visible grid and default track/pad sizes Throw down all the components onto the board Divide and place your components into functional building blocks Identify layout critical tracks and route them first Place and route each building block separately, off the board Move completed blocks into position Route the remaining signal and power connections General tidy up Design Rule Check Get someone to check it

To check board print schematic Using highlighter pen compare net on paper and PCB layout At end all connections highlighted

PCB Design - Layout

BASIC ROUTING

Keep NETS as short as possible Tracks should only have angles of 45 (rarely 90) Enable electrical grid centres pads and tracks Take track to center of pad Use single tracks between nodes Only take 1 track between 100 thou pads Lay down power and ground first and keep them close Aim for symmetry

For NON-PTH

Do not place VIAs under components Use component legs to reduce VIAs

PCB Design - Layout

PCB Design - Layout

FINAL STEPS

Keep mounting holes well clear of component etc Minimise number of hole sizes (cheaper) Double check component hole sizes (common problem too small) Use draft mode to see track ends IDENTIFICATION

Finally add your signature/symbol of identification SINGLE SIDED DESIGN

Cheaper to manufacture More challenging Component placement is critical Minimise jumper links

PCB Design - Layout

SILK SCREEN

Component layer C1, R1 etc Put designators in correct place with polarization markers SOLDER MASK

Thin polymer coating (often green) which surrounds pads to prevent bridging (Silkscreened epoxy, liquid or dry film photoimageable solder mask with heat cure)

Essential for surface mount and fine pitch devices Covers all but PADs and VIAs Gap is called Mask Expansion keep small Solder mask over bare copper SMOBC Can have mask over VIAs called tenting

A screenprint with a photographically produced stencil. The ink will be printed where the stencil does not cover the substrate.

PCB Design - Layout

MECHANICAL LAYER

Provides outline of board

KEEPOUT

Defines areas of board that you dont want routed

LAYER ALIGNMENT

Tolerances for each layer Holes, Pads and VIAs may not correctly align

PCB Design - Layout

GOOD GROUNDING

Fundamental to operation of many circuits More copper in ground path lower impedance Run separate ground paths (consider frequency) GOOD BYPASSING

Active components draw switching current Bypassing uses capacitor placed across power rails close to component to smooth

Typical values 1nF-1F Sometimes many capacitors Use 1 capacitor per IC

VGND

VCC

Vsig

Power rail fluctuations can lead to ground bounce and false triggering

Poor Bypass/Decoupling

PCB Design - Layout

Hi impedance guarding

Placing a guard around high impedance inputs reduces possible effect of surface leakage.

With a guard between input, the input isolation is improved, crosstalk is reduced and resistance between inputs is increased.

The guard must be taken to a low impedance reference.

a

b

c

Circuit (b)

PCB Design - Layout

HIGH FREQUENCY DESIGN

Consider parasitic inductance, capacitance and impedance Tracks can become transmission lines Creates reflections On FR4 EM travels 15cm in 1 nanosecond Must consider effect when length >/4 Note: 100MHz square wave has components in GHz region Ground planes important for correct operation Controls impedance and reduces EMI

DOUBLE-SIDED LOADING

Placing components on both sides of PCB can have benefits: Reduces board size For some packages- no room for bypass capacitors on top

PCB Design - Layout

AUTO-ROUTING

Real PCB designers dont auto-route!!

In reality often use auto-route

Can produce excellent results quickly

Especially for non-critical paths i.e. low frequency signals

Place critical tracks manually

PCB Design - Layout

SOLDERING

Must be considered in board layout Three soldering techniques hand, wave, reflow

Hand

Consider:

Access for iron Thermal relief for PADs Non-Plated-Through solder top VIAs

Wave

Suitable for surface mount and through hole

SM devices glued to board, through hole devices inserted Pass entire board over molten solder bath Ensure small devices not in shadow of large components

PCB Design - Layout

Reflow

Procedure for Surface Mount Components

Blank board coated with a mask of solder paste Components placed on board (adhesive) Placed in oven and baked (N2) Solder paste melts (reflows) on pads and leads

PCB Design - Layout

For Surface Mount (Double Layer Reflow)

PCB must have flat solder pads

Solder paste applied using stencil (both sides)

Flip

Components on 2nd side (NC machine) with adhesive (heat or UV cure)

Flip

Components placed on first side (Pick & Place NC machine)

Heated in oven solder surface tension aligns components to pads

Boards washed to remove flux residues

Pick and Place Robot (Wiki)

PCB Design - Layout

Solder Paste Stencils

Mastercut Technologies (Qld)

Lasercut and Chemical Milled Stencils

Accuracy

Positional tolerance typically

Main Surface Mount Advantages

The main advantages of SMT over the older through-hole technique are:

Smaller, lighter components on both sides of board SMT parts generally cost less than through-hole parts Fewer holes need to be drilled through abrasive boards Small errors in component placement are corrected automatically (the surface

tension of the molten solder pulls the component into alignment with the solder pads)

Lower lead resistance and inductance (leading to better performance for high frequency parts)

Fewer unwanted RF signal effects in SMT parts when compared to leaded parts, yielding better predictability of component characteristics

PCB Design - Layout

Main SMT disadvantages

The manufacturing processes for SMT are much more sophisticated than through-hole boards, raising the initial cost and time of setting up for production

Difficulty in manual handling due to the very small sizes and lead spacings of SMDs, making component-level repair of devices or manual prototype assembly extremely difficult, and often uneconomical.

PCB Design - Layout

Definitions

Track - copper conductors patterned on the board as interconnects between components.

Via - Inter-layer conductors connected through vertical holes.

Pad - A mounting or electrical contact point for a component or test point.

Solder mask - A solder resistant protective coating applied where solder will not be applied

Overlay/Legend/Silk screen - A component or text overlay added for identification or use during construction or fault finding.

(a) The pads and tracks should be as large as possible, and the separation distance between pads and tracks should be as large as possible

(b) Minimise the length of conductive tracks and the number of corners, bends and changes in track width

(c) Minimise the number of vias (d) Do not stack components on top of each other. (e) Provide circuit test points. (f) Ensure all logic inputs are tied high or low even if they are not used. (g) Use RC (analogue circuits) and C (digital circuits) for decoupling every active device on

the board. (h) Use standard parts or define new parts in the library before use.

Give every part a name and number. (i) Use the design rule checker. (j) Include target locators for alignment of top and bottom tracks. (k) Design grounding structure first and ensure that the width is adequate. (l) Include a set of mounting holes for your PCB. (m) Note that you are designing from the top of the board, so that any writing on the bottom

layer must be reversed.

PCB Design Guidelines Summary

Note that the cost of manufacture is affected by: Area of the board Number of holes Number of layers Number of cut-outs Number of units manufactured at one time The PCB material itself.

PCB Design Guidelines Summary