PCB Design Rule

7/28/2019 PCB Design Rule

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Many PCB designers like to think of PCB layouts as

works of art

It is, however, quite difficult to try and teach

PCB design It is like trying to teach someone how

to paint a picture.

apart from that PCB design is a highly creative

and individualprocess.


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These standards are controlled by the former

Institute for Interconnecting and Packaging

Electronic Circuits IPC(www.ipc.org).

The major document that covers PCB design is

IPC-2221, Generic Standard on Printed Board

Design


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you MUSThave a complete and accurate

schematic diagram.

Place notes in Schematic.

i.e. this track should be as short as possible (ASPS-

rule)

putting bypass capacitors next to the component

Your schematic really should be drawn with the

PCB design in mind.


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Imperial and Metric

you should always use imperial units (i.e.

inches)

A thou ( /a/), also known as a mil, is the

verbalized abbreviation for "thousandth(s) of

an inch

Whatever you have to do in PCB design youll

need to become an expert at imperial to

metric conversion, and vice-versa.
http://en.wikipedia.org/wiki/Wikipedia:IPA_for_Englishhttp://en.wikipedia.org/wiki/Wikipedia:IPA_for_Englishhttp://en.wikipedia.org/wiki/Wikipedia:IPA_for_Englishhttp://en.wikipedia.org/wiki/Wikipedia:IPA_for_Englishhttp://en.wikipedia.org/wiki/Wikipedia:IPA_for_Englishhttp://en.wikipedia.org/wiki/Wikipedia:IPA_for_English


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Imperial and Metric

100 mils = 0.1 inch = 2.54 mm

50 mils = 0.05 inch = 1.27 mm

1 pitch = 1 inch

i.e. 0.1 pitch pin spacing


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Imperial and Metric

Use mils (thous) for tracks, pads, spacings and grids,

which are most of your basic design and layout

requirements.

Only use mm for mechanical and manufacturing

type requirements like hole sizes and board

dimensions.


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4. Snap Grids

Lay out your board on a fixed grid

Standard placement grid: 100 mils

For finer routing and placement: 50 25 20 10 5

Dont use anything else, youll regret it.


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5. Visible Grid

Most common: 100 mils visible grid


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6. Component Grid

Snap grids your PCB designs will be

one step closer to being neat and

professional.


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7. Tracks

Track size depend on:

Electrical requirements

Routing spaceClearance

Your own personal preference

The bigger the track width, the better


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7. Tracks

The bigger the track

width, the better!

keep tracks as big aspossible, and then to

change to a thinner track

only when required to

meet clearance

requirements.


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7. Tracks

The thickness of the copper on the PCB: is nominally

specified in ounces per square foot, with 1oz copper

being the most common. Other thicknesses like0.5oz, 2oz and 4oz.

Required track width based on the current and the

maximum temperature rise.


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7. Track width vs. Current


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8. Pads

The pad should be at least 1.8 times the diameter of

the hole, or at least 0.5mm larger.

Pin 1 of the chip sould always be a different pad

shape, usually rectangular (square).

Most SM components use rectangular pads (R,C,),although surface mount SO package ICs should use

oval (oblong) pads. Again, withpin 1 being

rectangular.


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8. Pads


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9. Vias

Vias connect the tracks from one side of

your board to another, by way of a hole in

your board.

Holes in vias are usually a fair bit smaller

than component pads, with 0.5-0.7mm

being typical.

Use vias as less as possible!


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10. Polygons

Copper Pour, Copper Area

Solid vs. Hatched

Make sure youplace polygons after you

have placed all of your tracks and pads.


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11. Clearances

At least 15 mils is a good clearance limit

for basic through hole designs.

10 or 8 mils clearance for SMD layout.

Be careful with HV on board.

(i.e. 315mils (8mm) for 240V tracks and signal tracks)


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11. Clearances


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Set 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

blockswhere possible.Identify layout critical tracks on your circuit and route them

first.

Place and route each building block separately, off the board.

Move completed building blocks into position on your main

board, route the remaining signal and power connectionsbetween blocks.

Do a general tidy upof the board.

Do a Design Rule Check.

Get someone to check it

12. Component Placement


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Your components should be neatly lined up.

i.e. ICs in the same direction, resistors in neat columns,

polarised capacitors all around the same way, and connectorson the edge of the board.

Digital and analogjust do not mix, and will need to be

physically and electrically separated.

High frequency and high current circuits do not mix with

low frequency and low current sensitive circuits.



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If you have placed your componentswisely, 90% of your work will be done.


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13. Basic RoutingSome basic routing rules:

Keep nets as short as possible.

Tracks should only have angles of45 degrees.

Snake your tracks around the board, dont just go

point to point.

Always take your track to the center of the pad,

dont make your track and pad just touch.


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Only take one track between 100 mils pads unlessabsolutely necessary.

Only one large and very dense designs should you

consider two tracks between pads.

For high currents, use multiple vias when going

between layers.


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Neck down between pads where possible.If your power and ground tracks are deemed to be

critical, then lay them down first.

Also, make yourpower tracks as BIG as possible.

Keep power and ground tracks running in closeproximity to each otherif possible, dont send them

in opposite directions around the board.


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13. Basic Routing


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13. Basic Routing


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13. Basic Routing

Some basic routing rules:

Dont leave any unconnected copper fills (also called

dead copper), ground them or take them out.


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14. 2-Layer Routing

Do not place vias under components.

Try and use through hole component legs to connecttop tracks to bottom tracks. This minimises the

number of vias.

Use as less vias as possible

SMDs are always better than leads components!


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15. Finishing Touches

If you have thin tracks

(


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Check that you have any required mounting

holes on the board. Keep mounting holes well

clear of any components or tracks. Allow roomfor any washers and screws.


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Minimise the number of hole sizes. Extra hole sizes

cost you money.

Double check for correct hole sizes on all yourcomponents.

Nothing is more annoying than getting your

perfectly laid out board back from the manufacturer,

only to find that a component wont fit in the holes!This is a very common problem, dont get caught

out.


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Minimise the number of hole sizes. Extra hole sizes

cost you money.

Ensure that all your vias are identical, with the

same pad and hole sizes. Remember yourpad to

hole ratio.


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Check that there is adequate physical

distance between all your components.

Watch out for components with exposed

metal that can make electrical contact

with other components, or exposed

tracks and pads.

Add teardrops to all your pads and vias.


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16.Silk Screen Layer

Silk Screen Layer (SSTOP, SSBOT): contains your

component outlines, designators (C1, R1 etc), and free

text.

Make sure that you keep all your component

designators the same text size, and oriented in the

same direction.

Make sure that you add a component overlay that

reflects the actual size of your component.


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16.Silk Screen Layer

Ensure that all polarised components are marked,

and that pin 1 is identified.

Ensure that no part of the silkscreen overlaps a bare

pad.

As a general rule, dont put component values onthe silkscreen, just the component designator

(but)


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17. Solder Mask

Solder Mask Layer (SMTOP, SMBOT):

The solder mask typically covers everything exceptpads and vias.

The gap it leaves between the pad and the solder

mask is known as the mask expansion.


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18. Other Layers

The mechanical layer is used to provide an outline

for your board, and other manufacturing instructions.

Keepout

The keepout layer generally defines areas on your

board that you dont want auto or manually routed.

This can include clearance areas around mounting holepads or high voltage components for instance.


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Auto Tool

Auto Route

Real PCB designers dont auto route!

Artificial intelligence and neural based technology are

some of the marketing buzz words used, but no matterhow smart an auto router is, it simply cannot replace

a good human PCB designer.

Auto Placement

Professional PCB designers do not use Auto Placement

tools. Dont rely on the Auto Place feature to select the

most optimum layout for you. It will neverwork!


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References

[1] PCB Design Tutorials David L. Jones

[2] wikipedia.org


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Documents

PCB Design Rule