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 and even manufacturing headaches can be avoided.

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 are 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 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 design.

For more complicated layouts; 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 design. Especially if the board size has to be kept to a minimum.

2. 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.

3. 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 design. 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.

2. 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 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.

3. 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.

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 recommended footprint shows more GND copper on the component side, but 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.

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.

 

3. Panelise
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.

1. Lack of planning

With PCB layouts, preparation is the most important part of the job.  The amount of time spent preparing will affect the success of the design.  Selecting the right PCB design software is the most important; each having advantages, disadvantages and limitations.

Each PCB is unique it it’s own way.

Some areas of the design are more important than the rest, for example power supplies, impedance signals, DDR, address and data bus.  If these areas are not completed before the rest of design, precious time is lost and considerable effort is then spent reworking the layout.

Setting up rules and constraints are there to guide from the placement stage till the gerbers are completed.  When a PCB is planned correctly, the rest of the design will become a much simpler process.

 

2. Constraint rules

There isn’t anything more powerful than the human mind, unfortunately it is not perfect!

There are lots of things to think about with a PCB design, and it is easy to get lost with all of the information.  By using the tools available from PCB software, constraints can be implemented; spacing, keepouts, length matching, propagation delays.

Once these rules have been implemented, the designer can focus on other areas of the layout.

 

3. Poor Communication

As PCBs become more complex, the communication between engineer and PCB designer is essential.  By eliminating any placement or routing issues early on can save on costly reworks.

It is very important for the engineer to review the circuit board as often as possible.  Using on-line meeting tools can allow the engineer to inspect the board in real time and discuss potential issues.

By setting out clear objectives and agreeing on them from the start of the layout can give the designer a better understanding of what you want to achieve and can shorten time-to-market.

 

4. Using ineffective layout techniques

PCB layouts are becoming more complex thanks to advancements in electronic technology.  Problems such as electrical noise, crosstalk, impedance mismatch, timing issues, ESD – all need careful consideration.
Practical PCB design rules, board stackup, PWR & GND planes, decoupling capacitors, faraday shields – these are valuable when used correctly.
Reference designs provides the optimum solution to meet requirements for complex layouts.  Some of their suggestion may be difficult to achieve, but they do give some insight on how the PCB should be designed correctly.

 

5. Forgetting to backup data

Backup completed designs, no brainer.  Hundreds of hours are invested in most designs, eventually all designs will have to be modified.

Obsolete components and new technology will demand the board is updated.

If the original files have been lost, the whole project will have to start again or be scrapped.  The use of a cloud is a cheap and easy solution to backing up data.

 

6. Becoming a One man island

Any experienced PCB designer may look on a completed design and see perfection.  It is easy to get “tunnel vision”; concentrating on one area of the board and missing a detail on another.  A colleague not as involved in the project can be more impartial and provide an objective and invaluable insight.

Regular design reviews can help detect future errors and allow individuals to share experiences and knowledge.

Despite increasing levels of semiconductor integration and readily available systems-on-chips for many applications, in addition to the increasing availability of highly-featured development boards, electronics often still require a custom PCB. Even for “one-off” developments, the humble PCB still performs an important role. It’s a physical platform for a design, and the most flexible for pulling an electronics system together. In this article, we outline ten best practices of PCB design, most of which have stayed consistent for 25 years. These rules are in no particular order, can generally be applied to any PCB design project, and should prove as a useful guide both to veteran design engineers as well as makers alike…

1. Use the right grid
Find a grid spacing that suits as many of your components as possible and use it throughout. Although multiple grids may seem appealing, a little additional thought at the early stages of the layout can avoid spacing difficulties and will maximize board use. Many devices are available in different package sizes, so use that to your advantage. Furthermore, as the polygon is an important shape when adding copper to your board, and boards with multiple grids will often produce polygon-fill discrepancies, not standardizing on one grid can make your life tougher than necessary.

2. Keep trace lengths as short and direct as possible
This rule applies even if it means going back over parts of the layout again to optimize track lengths. This applies particularly in analogue and high-speed digital circuitry where impedance and parasitic effects will always play a part in limiting your system performance.

3. Whenever possible, use a power plane to manage the distribution of power lines and ground.
Using pours on the power plane is a quick and easy option in most PCB design software. It applies plenty of copper to common connections and helps ensure power flows as effectively as possible with minimal impedance or voltage drop, and that ground return paths are adequate. If possible, run multiple supply lines in the same area of the board and remember that if the ground plane is run over a large section of one layer, it can have a positive impact on cross-talk between lines running above it on an adjacent layer.

4. Group related components and test points together
Place the discrete components needed for an opamp close to that device so the bypass capacitors and resistors are co-located with it. This helps with the track lengths in Rule #2, and it also makes testing and fault-finding easier.

5. Panelise your PCB by replicating the board you need several times on a larger board
Using a size which best suits the equipment used by your manufacturer will improve the cost of prototypes and manufacturing. Start by laying out the board as one panel. Ask your board house what size panel they prefer. Then, after your design rules have been corrected, do your best to step and repeat your design multiple times within the preferred panel size.

6. Consolidate your component values
As a designer, you will have picked some discrete components that could be a higher or lower value and work just the same. Consolidating on a smaller range of standard values makes the BOM simpler and probably less expensive. It also makes stock decisions easier in the long run if you have a range of PCBs based on your preferred device values.

7. Design rule check (DRC) as often as you can
The DRC function on PCB software takes a little time, but checking as you go can save hours on more complex designs, and it is a good habit to adopt. Every layout decision is important, but the DRC keeps the most important ones top-of-mind.

8. Use the silkscreen wisely
The silkscreen can be used to portray a wealth of useful information to the board builder, as well as the service or test engineer, installer, or device operator. Clear labels depicting functions and test points are obvious, but orientation of components and connectors should also be considered wherever possible. Even if annotation ends up under your components following board assembly, it’s still a good practice. Full use of silk screening on both sides of the board streamlines production and can reduce re-work.

9. Decoupling caps are not optional
Do not try and optimize your design by avoiding decoupling power lines and trusting the absolute limits of component data sheets. Capacitors are inexpensive and robust; take the time to fit them in wherever possible and remember Rule #6 – use a range of standard values to keep the inventory neat.

10. Generate your own PCB manufacturing data and verify it before sending it out to be fabricated
Most board houses will be happy to do this for you, but if you output your own Gerber files first and use a free viewer to verify it looks as you envisioned, then you can avoid misunderstanding. You may even catch an error inadvertently included before it’s set forever in fiberglass, resin, and copper.

As circuit designs are more widely shared, and reference designs are relied upon more and more by in-house teams, we believe it’s important that basic rules like these remain in printed circuit design. Keeping sight of the basics means developers retain the flexibility to add value to their products and extract the most from every board they make. Finally, anyone new to board design will accelerate their learning-curve and confidence when the basics are “designed in.”

DL Designs can produce from low budget, mechanically constrained to high speed SI; all developed in the UK. From concept to a full turn key build and test operation.

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