Pcb designing

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basic pcb designing

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  • 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 tech