Best PCB Layout Practices for 2018
One of the first steps that PCB design experts recommend when taking up a PCB layout design is to start with a high quality PCB design software. Make sure the software package comes bundled with a good library of component parts, and allows you to add to it. It should allow you to manage your layer structure easily, and to place and route a complex multilayer board design. Among the necessary features, it should come with a strong but flexible built-in DRC and allow you to conduct a DFM check. Overall, the design of the software package must be intuitive to enable a short learning curve.
Set up the Library for Multilayer Designs
Designing multilayer boards requires a different library configuration than necessary for single or even double layer boards. It is important to set up the following three areas for handling multilayer board design:
Pad Shapes: Designers differentiate the first pin of a through-hole IC with a differently shaped pad for easy orientation. However, this is necessary only on the topmost layer while on the inner layers all pads can retain the same shape. For libraries not set up for multilayer configuration, the pad shapes may have a mismatch.
Drawing Marks: Designers place different marks on various layers to identify them during fabrication and assembly. Therefore, when setting up the software package for multilayer boards, the designer must save the corresponding logos, tables, and views to the library. Additionally, standardizing them for the organization will avoid confusion.
Negative Planes: When creating power and ground planes, multilayer PCB layouts use negative image plane layers. These layers require additional clearances around pad and footprint shapes for drilled holes. Therefore, pads and footprint shapes for multilayer design must contain these additional clearances for the negative planes. If you are not careful with these clearances, they will ultimately create shorts.
Understanding the Fabrication Shop Requirements
It is important for a PCB designer to work closely with a fabrication shop and understand their requirements, so that it is possible to fabricate the ultimate product without issues. Multilayer PCB designs offer several benefits over single and double-layer boards. Chief among them are space saving and increasing the design density. Multilayer boards also allow better control over signal integrity, but to achieve that it is necessary to make sure the fabrication shop is able to manufacture the multilayer design before you start.
Fabrications shops will have their own limitations based on their level of board technology. For instance, they may be set up to manufacture boards up to a certain layer count, or they may be able to make vias, traces, and spacing widths only to a certain dimension. Exceeding those limitations may mean looking for a better fabricator, thereby increasing fabrication costs, time, and effort, or not being able to get the board fabricated.
Best Layout Practices for Basic Multilayer PCB Design
Reduce Crosstalk: It is important to guard against crosstalk from the beginning. Preferably, route the signals on adjacent layers so they are at 90° to each other—this helps to reduce broadside crosstalk problems.
Use Ground and Power Plane Layers: Distribute power and ground layers evenly throughout the stack. This will prevent ground loops, ground bounce, and help with creating microstrip structures for managing signal integrity.
Use Special Vias: Using special vias such as micro-vias, buried and blind vias opens up more routing channels for the designer. Check if the Printed Circuit Board CAD software allows using land-less vias and via-in-pad, as these are now becoming commonplace for packages such as BGA and other fine-pitch IC packages.
Use IPC-2223: Using a common point of reference makes it easier for both the designer and the fabricator. Communicating in a common language for documentation reduces errors and misunderstandings while avoiding expensive delays.
Use Modern File Formats: Rather than delivering files to your fabricator in Gerber format use a modern file format such as the ODB++ or one that meets IPC-2581 standards, as these formats identify specific layer types and the result is unambiguous documentation.
Best Layout Practices for Rigid-Flex PCB Design
No Corner Bending: Always place copper traces at right angles to the flexible circuit bend to avoid bending them at the corners. If this is unavoidable, use conical radius bends.
Curved Traces are better: 45° hard corners and right angle traces increase stress on copper traces when bending—using curved traces is a better option.
No Abruptly Changing Trace Widths: Any abrupt change in the width of a trace can weaken it. As a trace approaches a pad or via, prefer to use teardrop patterns to change its width gradually.
Using Hatched Polygons: Planes using solid copper pour in flex boards offer heavy stresses when bent. Using hatched polygons such as hexagons makes the plane more flexible.
Stagger Flex Traces: For traces running over one another in the same direction on either side of a layer creates uneven tension between the layers. Staggering the traces eliminates the stress.
Best Layout Practices for Industrial PCB Design
Designing for Industrial environments requires PCB designers to demonstrate not only functionality of the PCB, but also its reliability to work under harsh conditions. This is especially true for applications with expensive downtimes.
Use Proper Grounding:PCB layout for industrial applications must carefully segregate power ground, analog ground, and digital ground. This is essential for reliable performance of the PCB in harsh electrical environment. Connecting these various grounds to a suitable single point is also important.
Maintain Signal Integrity: Harsh electrical environments affect communication, analog, and digital both. This can be detrimental to the performance producing erroneous data. Although a proper selection of cables and other installations can offset this largely, PCB designers and manufacturer must follow sound design practices to maintain signal integrity.
Heat Management: Industrial environments can be very hot, and PCBs generating their own heat may easily cross their safe operating temperatures. Use thermal vias and other heat management techniques such as heat sinks to remove heat from the PCB and its enclosure.
Group Components: Prevent interference by grouping components based on their function in the circuit. For instance, analog circuits separated from their digital counterparts, and power circuits in their own area prevents coupling of interference. PCB design experts advise arranging the schematics into modules to plan better components placement.
Design for Moisture Control: Some industrial environments may be moist and humid. Moisture buildup on the PCB could damage circuits and components. PCB design experts suggest designing the PCB layout with a view to applying a layer of conformal coating on the assembly. The PCB designer needs to factor in the additional heat buildup and the necessity to remove it. It may be necessary for the PCB design to incorporate an intelligent circuitry to detect humidity and turn on a heater integrated into the system.
Using a good CAD software package for PCB layout does not guarantee an optimum output. Although CAD software packages incorporate auto-routers to help the designer, it is best to review the output after auto-routing and rectify the quirks the software may have introduced.