How to design a good PCB layout

Information

It’s important to have everything necessary to make the perfect PCB layout.

A good engineer would foresee any complications that may crop up; problems with sourcing datasheets, lack of available space on the PCB layout and even manufacturing headaches can be avoided.

Three types of information are crucial for a successful PCB layout. DXF or mechanical drawing, BOM (parts list) and a schematic.

A layout guide provided by the design engineer and necessary reference designs are also useful for a successful PCB layout.

Mechanics

Today’s PCB designs are very complicated with limited space. Some projects may involve multiple PCB layouts fitted together in a box.

The best way to ensure the PCBs are fitted together correctly without complications is with a CAD package such as solid works.

CAD drawings can be imported onto a PCB design. This saves a lot of time, the parts can be placed on the imported locations.

This is the most accurate and cost effective way of placing components in specified areas.

 

Design Rules

This is a crucial stage and makes up the backbone of any PCB layout.

If this isn’t completed accurately, there could be major flaws with the functionality of the circuit board.

The rules basically control any clearance and track thickness on the PCB layout.

For more complicated PCB designs; rules for impedance controlled signals, extra clearances for noisy tracks, relative propagation delays and constraint regions can also be implemented.

 

 

Component stage

It is crucial to start and maintain a parts library during the PCB layout stage.

Most of the PCB design errors come from incorrect footprints.

Using a standard footprint name such as IPC can help design engineers and colleagues select the correct part without recreating the same footprint over again.

Part generators are useful tools to create parts.

Part sizes are entered into the generator, the software calculates the optimum size for pads, including the paste (sometimes this is reduced by 30% to prevent excessive solder shorting pads together).

Other useful information such as component and placement outlines, component heights and keepouts are added to simplify and aid the PCB layout.

All components must be double checked before added to the library and updating the PCB design.

A library with reliable components can be used to save time and money.

 

 

 

Placement stage

Each PCB layout is unique, there are many questions that need to be asked to make sure the design meets the customer or engineer’s needs. If these queries are not resolved, it could end up with the placement reworked multiple times.

  1. Is there enough space to fit all of the parts onto the board?

Some engineers may not take into consideration the space required for parts and signals on the PCB layout. Especially if the board size has to be kept to a minimum.

  1. Can all of the parts fit on one side on the board?

Assembly costs is increased if surface mount components are fitted to both sides. Through-hole components are mostly fitted by hand, these can be placed on either side, provided they don’t impede and mechanical constraints.

  1. What sort of PCB layout is this?

Sensitive impedance controlled signals, antennas, modules – these all have restrictions that prevent copper, vias and components from being fitted in certain areas. Is it a high power board? Large copper areas, multiple vias, GND return, feedback signals these all take up valuable space.

Some of the components (connectors) have be positioned in an exact location. Associated components can be placed nearby, outside the board edge from now. Important nets should be assigned colours so they stand out from other nets.

The schematic will have to be followed closely during the placement and routing stage of the PCB layout. Placing around the ICs into groups is a good place to start. The parts should be positioned close to each other in case there isn’t much space on the board. Try to visualise how the PCB layout is routed to visualise how much space is required.

When placing critical components such as SMPS, view the part’s datasheet. This could show the best way to place and route this area.

 

 

Tracking \ Routing stage

As with the placement stage, crucial questions must be asked before routing can begin.

  1. Are there any impedance controlled signals?

Impedance controlled signals can be controlled by adjusting the track width, the gap between signals (if diff pairs are involved), the distance from the routing layer to the adjacent layer, the track thickness. A ground strip of copper (co-planer) can be added to help achieve impedance but this has to be checked by the design engineer. Impedance calculators can be accessed online.

  1. Is there any high current required on the PCB?

Some PCB layouts will have a connector supplying power to the PCB. The power signal will run from the connector through fuses, regulators and inductors; sending other power signals to different parts of the layout.

The schematic will specify the amount of current required for each power supply area. A PCB layout calculator can measure how much copper is required to carry the necessary current. Other factors involved is the copper thickness (standard is 1oz, ½ oz, 2oz, 4oz+ is also used) The thicker the copper the more current can be carried. Inner layers are suppressed , less current can be carried on these layers.

  1. Are there any areas to avoid?

Some areas of a circuit board are especially noisy such as power supplies. Signals can be affected by this noise and should avoid these areas.

The routing stage tends to be the most time consuming part of PCB design.

To save time, it’s best to minimise reworking the tracking.

As mentioned above, impedance and high current areas will take up space on the board.

These must be completed first, it’s easier to redraw a 0.2mm track than a 3mm in a busy area!

Visualise how much space tracks will take.

What tracks will run together, what tracks will be routed on each layer? If a plan is made it could save up to 30% of time taken for tracking on the PCB layout.

More PCB designs use multiple layers, it’s not uncommon to use 10+ layers. Standard vias will affect all layers, it’s important to get thee added to the PCB layout in case they get missed. This is quite easy on a PCB with 10-20,000 signals.

 

Checks

The DRC (Design Rule Check) and connectivity checks work with the design rules to constantly analyse the PCB layout as it is routed. When the PCB design is completed, any errors must be fixed before gerber files are completed.

 

 

Check List

PCB layouts can be complicated. It’s easy to be bogged down with lots of details and forgetting to perform tasks, especially whilst juggling multiply PCB designs. Introducing a check list is a good way to ensure all bases are covered. It also allows you to go back and look through the design and review any oversights.

 

 

Follow these guidelines and you’ll complete the PCB layout faster, cheaper and a better functioning circuit board.

 

 

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