Functional requirements when placing PCB components
Manufacturing and accessibility requirements for PCB component placement
Advanced PCB design tools to help with component placement
Visual cues like this give placement guidance when designing to specific design constraints
PCB layout has steadily become more complex over the years. Additional circuitry is needed as functional requirements increase, while usable board space is reduced to accommodate smaller devices. High-speed circuitry requires precision in its layout to avoid EMI and other signal integrity problems while generating more heat that has to be dissipated through the design. All of this lands squarely on the shoulders of the PCB layout designer, who constantly has to come up with new and innovative ways to fit 10 pounds of treasure into a 5-pound sack.
One of the first steps in ensuring a PCB layout will contain all of the required components and perform flawlessly is to perfect the component placement of the board. This is more complex than it may seem, as the PCB designer has to balance the performance needs of the placement with the manufacturing requirements. At the same time, parts need to be placed where they are accessible for testing and upgrades without causing collateral damage to the rest of the board. To help, we have come up with some basic PCB component placement guidelines. We’ll start with a circuit board’s functional requirements first.
Functional Requirements When Placing PCB Components
For the best PCB performance, components need to be placed where they will help serve the functional needs of the circuitry. Although the best results would be achieved by placing each component pin where it can connect directly with the next component pin of a circuit, this is impossible with high-density layouts. For each circuit that can be placed with its pins close together, the next circuit will end up with its pins separated. This placement conundrum puts designers squarely into the position of having to manage multiple compromises in their layouts to achieve the best overall PCB performance.
The first step to creating the best component placement for performance starts with floor planning the layout of your parts. These functional partitions allow you to diagram out where the digital, analog, RF, and power areas of the board need to be located before you start juggling components. The key here is to keep these areas isolated to prevent one area’s noise from affecting another area. For the best signal and power integrity results in your design, you will also want to arrange your circuitry areas to avoid using a split ground plane.
With the floor plan of different circuitry areas completed, you can begin component placement. Here are some general placement guidelines for digital parts to keep in mind:
- Place fixed components, such as connectors, first and build the rest of the placement out from them.
- Don’t allow components from one area of circuitry to infringe into another area—except for transitional components like analog to digital converters.
- When placing dense areas of circuitry, keep in mind that you will need space for escape routing and bus routing on the internal areas. Don’t choke your routing channels.
- Remember that the vias used for escape and bus routing could potentially block your signal return paths on the reference plane, so plan ahead to maintain clear return paths.
- Place the components of high-speed signals close together so that the routing can follow the signal path laid out in the schematic. You need these connections to be short and direct.
You will also want to follow the same general guidelines when placing the analog and power sections of your design. Power supply components also have some additional requirements that should be adhered to for good power integrity:
- High-speed components that use a lot of power (for example, processor devices) need an ample amount of decoupling capacitors connected to their supply pins. This can be a real challenge with large pin-count parts in BGA packages, but these caps must be placed as close to their pins as possible to manage ground bounce and other power integrity issues.
- Place the components of a power supply as close together as possible. This will enable you to keep the routing between pins short and wide, which is essential for reducing inductance.
- Keep power supply components on the same layer of the board to eliminate the need for vias, which also create undesirable inductance.
It is also important to place components in such a way as to manage the heat generated by their operation. Hot running components, such as processors, should be located towards the center of the design to better dissipate their heat throughout the board. Individual power supplies should not be located next to each other. It is also important to be mindful of the airflow across the board so as to not block it from providing cooling. This may require relocating a taller component that is blocking the airflow. Also, allow room in your placement for heat sinks and other thermal dissipation devices and tactics.
With all of these functional PCB component placement guidelines, you would think that would be enough for any designer. But, there is more—now we need to evaluate the placement with an eye towards manufacturing.
Component placement on a circuit board
PCB Component Placement Guidelines for Manufacturing
Printed circuit boards are assembled using automated assembly processes that include wave soldering, which is primarily for thru-hole components, and solder reflow for surface mount parts. Each of these systems has requirements that PCB layout designers need to be aware of to ensure the highest manufacturing yields when the boards are in full production.
Wave soldering systems move a circuit board through a molten wave of solder using a conveyor belt. The wave process forces solder up through the plated thru-holes in the board, forming a solid connection to the pins inserted in them. This process is quick and takes less setup time than others, but does require that components are placed according to some basic guidelines:
- Provide extra clearance from SMT parts on the back of the board that can’t be wave soldered to any nearby thru-hole pins. This will ensure there is enough room to mask these SMT parts off without hindering the wave from reaching the thru-hole pin.
- Smaller discrete components that can be wave soldered should be oriented with their pins perpendicular to the direction of travel through the wave, so both pins will be soldered at the same time.
- Smaller SMT components shouldn’t be placed directly behind larger components going through the wave to prevent shadowing from creating a good solder joint.
- Thru-hole connectors should be oriented so their pins travel perpendicular to the wave to get the best solder coverage.
- Avoid placing tall components on the back of the board that won’t fit the height restriction of the wave.
Surface mount parts are held in place on a circuit board using solder paste and then run through the solder reflow oven. The heat melts the solder paste, which then hardens and forms a solid solder joint. Solder reflow doesn’t have as many placement requirements as wave soldering, but designers do need to ensure that their parts are positioned for the best manufacturing results. When smaller two-pin parts are unbalanced thermally, one pin will receive more heat than the other and their solder may melt at different speeds. This imbalance is usually due to differing amounts of metal in the pin connections and can result in the melted solder on one pad pulling the part off the other pad in an effect known as tombstoning.
After a circuit board is automatically assembled, there may be some manual rework required for parts that didn’t solder correctly or for parts that can’t be run through an automated assembly process. It is important for the PCB designer to allow room for a technician to access the board with rework tools such as tweezers, probes, and soldering irons. Parts that are placed under larger parts are very difficult for technicians to reach, and other parts may suffer collateral damage trying to rework them.
Automated Testing and Other Manufacturing Steps
Completed circuit boards typically go through an automated testing process to validate their assembly. Although this testing can be done manually, most production boards will be tested using either an in-circuit test (ICT) system or a flying probe machine. ICT systems use a fixture to probe test points on the circuit board simultaneously while flying probe machines test these same points individually. Either way, the circuit board needs test points designed into it to facilitate these processes, and the test points have their own placement requirements. The spacing values vary depending on the manufacturer, but designers should expect to adhere to the following requirements:
- Test point to test point spacing.
- Test point to component spacing.
- Test point to board edge spacing.
How the board will be arranged in its manufacturing panel will also have an effect on how the components should be placed. Some panels will separate the boards using a scoring tool, and components need to be placed with enough clearance to the edge so as not to interfere with that process. Other panels use break-out tabs, and components need to be kept back from the tabs to avoid breaking the solder joints and cracking the part casings when the boards are broken out of their panel. Another consideration to keep in mind is if the board will be conformally coated or not. It may be desirable to place some unique components near the edge of the board to avoid dipping them in coating fluid.
We’ve seen how PCB component placement is influenced by functional and manufacturing needs and requirements. Next, we’ll look at how parts should be placed to make them more accessible for technicians and field service.
Working with the different component placement clearances in a CAD constraint management system
Component Placement Recommendations for Accessibility
Circuit boards go through a test and debug stage, especially when they are first being developed. This level of testing usually requires access to the board for plugging in cables, probing connections, and attaching wire clips. In some cases, prototype boards will be built using sockets, so that devices can easily be swapped in and out during testing, which will eventually be left out of the production versions of the board. Layout designers need to place these parts where they can be accessed as easily as possible and give technicians the room they need to access the board.
Depending on the board, there will also be other parts that will require human interfacing. Wire harnesses and cables will have to be plugged in, switches set, batteries replaced, and other parts will need minor adjustments. PCB layouts that don’t take this into account may run the risk of going through a re-layout just to incorporate placement changes to provide access to a connector. Circuit boards used in a multi-board system also have to be able to connect to each other and the system controls. Component placement that doesn’t take into account the intra-system connections may also have to go through a redesign simply to gain access to a connector or two.
There is a lot to consider when laying out a circuit board that these PCB component placement guidelines should help with. Another helpful resource is using an advanced PCB design CAD system like Cadence’s Allegro PCB Editor to layout the board.
Allegro’s online DRC system shows a component placement error to be corrected
How Your PCB Design Tools Can Help Create the Best Component Placement
There are many different clearance values that you must work with to correctly place components to satisfy functional, manufacturing, and accessibility requirements. Cadence’s Constraint Manager allows you to set up electrical and manufacturing design rules for component placement, giving you useful design rule check information like you see in the picture above. Another useful feature for component placement within Allegro is the ability to view and check your layout in 3D. This capability gives you real-time feedback on your placement instead of waiting to build a prototype. You can also pull in other circuit board layouts to get a full system view of how your designs plug and play with other PCBs in the device.