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.
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.
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.
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.
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.
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.
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.
Click here to see our guide on Switch Mode Power Supplies
Check out our video on high speed design
See the new blog from our apprentice
When using components such as switch mode power supplies (SMPS), the PCB layout is critical.
Faults in the PCB layout cause a number of problems including switching jitter. Poor output voltage regulation and possible failure with the PCB design.
Issues like this can be avoided, saving money and time on scrapped circuit boards and PCB modifications.
When putting together a PCB layout the best approach is to review datasheets for devices such as SMPS. A respectable manufacturer such as Texas Instruments will offer guidelines and reference designs to follow for your PCB layout.
Usually a layout guide will show a PCB design with all of the parts, copper tracks and vias arranged for an optimum performance. It will show lots of space with no other components interfering. This does not happen in the real world, most of the time there is not enough room to keep parts and tracks away from critical areas.
The best way is to follow the guide and bend the rules only when it’s absolutely necessary to do so.
Below is a part of the schematic that show the SMPS and the related components. The second diagram is the recommended PCB layout from the manufacturer.
The decoupling caps (input and output) must be close to the regulator. The most important aspect of SMPS is to reduce high current loops. The recommended layout shows a GND plane under the component, flooding over other components connecting to GND. There is also a skinny blue line representing a track on the bottom side of the board. This is the feedback signal, the track is on the bottom side to reduce interference and must not be in the high current path.
The PCB layout must follow this guide for it to function correctly.
Here is the schematic and PCB layout to highlight the key areas, orange for input, purple for output and burgundy is feedback.
The PCB layout shows the input signal running from a plane (using vias) through the decoupling caps through the inductor and into the module (REG1 highlighted in yellow) via more caps. The output signal runs through decoupling caps and away through the inductor. The GND signals connect to the GND plane through vias at the module and at the decoupling caps.
The burgundy feedback signal is routed from pin 4 of the module, through the resistor and capacitor and connecting to the output signal via C16.
Here is the recommended footprint and PCB design as a comparison. The drawing has been rotated to the same orientation as the PCB design.
The recommended footprint shows more GND copper on the component side. This is not that practical on the PCB layout due to a lack of space and the input signal is connected to other pins on the module.
Click here to see our guide on designing a good PCB Layout
Check out our video on high speed design
See the new blog from our apprentice
1. Smarter design
Planning the PCB layout and assembly process is one of the most smart and cost effective ways of saving money.
The strategical engineering of these boards can result in using fewer and more cost effective parts which will significantly lower the cost of each PCB.
In the long run, this will enable your company to reduce costs and deliver high quality PCB layouts.
2. Use manufacturers reference designs
Engineers designs can look good on paper but when it comes to design, costs could impact on overall cost and reduce profit. Costs can increase during the manufacturing stage.
One of the cost effective ways is to refer to manufacturers notes. The information provided can save effort, time and money for the design to be produced.
A decent chip manufacturer will provide a schematic, BOM, gerber and assembly drawings and reference designs.
The reference designs are created by the manufacturer to show how the design is made to their specification.
Placing multiple PCBs on the same panel allows all of them to processed at the same time, instead of separately. Not only are boards manufactured like this, they are assembled and shipped on a single panel. The more PCBs on one panel, the more cost effective it becomes.
4. Get the Manufactures involved
Providing the manufacturers with any information during the design process can pay dividends when it comes to releasing the production files.
Board stack up, clearance issues, materials and special requirements – these could cause problems for the manufacturer and these could be costly if told in the last-minute.
By communicating and agreeing with in advance this would give the manufacturer time to resolve any issues and even offer an alternative solution.
You could have time to find an alternate manufacturer if your demands are not met.
5. Using the same assembly house for prototyping and mass production
Once the boards have been designed and manufactured, the assembly process can begin.
If there are no design changes between prototyping and productions it would be practical to use the same assembler for both and to start mass production as soon as possible.
Material costs are fairly low, significantly lower when bought in bulk. The main costs are time spent assembling the PCB.
It takes time to review the design and resolve any potential problems.
It takes even more time to add the parts into the pick and place machine. And it takes time for the assemblers to learn what’s required for the project. If this is achieved during the prototype, the mass production run will go much faster.