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 design to avoid manufacturing headaches.
Three types of information are crucial for a successful PCB design. DXF or mechanical drawing, BOM (parts list) and a schematic.
A layout guide provided by the design engineer and necessary reference designs is also useful for a successful PCB layout.
Mechanics
Today’s PCB designs are very complicated with limited space. Some projects may involve multiple layouts fitted together in a box.
The best way to ensure the PCBs fitted together correctly without complications is with a CAD package such as solid works.
The PCB design import the CAD drawings. This saves a lot of time, we
place the parts 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.
Accurate placement is essential, there could be major flaws with the functionality of the circuit board.
The rules basically control any clearance and track thickness on the design.
For more complicated layouts; rules for impedance controlled signals, extra clearances for noisy tracks, implementation for relative propagation delays and constraint regions are necessary.
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.
Entering part sizes into the generator, the software calculates the optimum size for pads, including the paste (reducing this by 30% to prevent excessive solder shorting pads together).
Other useful information such as component and placement outlines, component heights and keepouts simplify and aid the PCB layout.
All components must be double checked before added to the library and updating the PCB design.
Time and money’s saved when using a library with reliable components.
Placement stage
Each PCB layout is unique, there are many questions that are asked to make sure the design meets the customer or engineer’s needs. Failing to resolve these queries, could end up with the placement reworked multiple times.
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 design. Especially if the board size has to be kept to a minimum.
Can all of the parts fit on one side on the board?
Fitting surface mount components to both sides increases assembly costs. Through-hole components are mostly fitted by hand, these are placed on either side, provided they don’t impede and mechanical constraints.
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.
The PCB designer needs to position the components (connectors) in the exact location. Placing the associated components nearby, outside the board edge for now. Assigning colours to important nets helps them stand out from other nets.
Following the schematic closely during the placement and routing stage of the design. Placing around the ICs into groups is a good place to start.
To prevent a shortage of space on the PCB, positioning the parts close to each other can help. Try to visualise how the tracking on the PCB layout to anticipate the available space.
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, asking crucial questions before routing can begin is a must.
Are there any impedance controlled signals?
Adjusting the track width, the gap between signals (if using diff pairs), the distance from the routing layer to the adjacent layer, the track thickness (height) affects the impedance controlled signals. Adding a ground strip of copper (co-planer) helps achieve impedance. The design engineer calculates these figures using software.
Is there any high current required on the PCB?
Some PCB designs 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 the required copper to carry the necessary current. Copper thickness is another factor (standard is 1oz, ½ oz, 2oz, 4oz+ is also used).
Thicker copper carries more copper. Inner layers carry less copper as they’re suppressed.
Are there any areas to avoid?
Some areas of a circuit board are especially noisy such as power supplies. Noise affects these signals 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.
Complete these 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 layers for routing? 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’re 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 during tracking. Upon completion of the PCB design, fixing the errors before completing the gerber files is crucial.
Check List
Complicated PCB layouts are more common. The numerous details can distract the PCB designer and lead to forgetting tasks, especially whilst juggling multiple PCB designs. Introducing a checklist is a good way to ensure no problems slip in. 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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