The schematic design is done! What is the next big thing?
Once the schematic design is complete, it is time to pass it to the next phase of the design stage to continue with the PCB layout design. All the efforts and know-how invested during the schematic design phase will be significantly compromised if the PCB layout design is not done correctly. Component placement is one of the foundational integrals of the PCB layout design process. It is more like placing the foundation of a house. There must be good planning and envisioning of what the final PCB should look like to have a board that is -Right the first time!
PCB floor planning
Component placement requires the PCB layout designer to envision various aspects of the design steps in advance. Before starting the placement of the component, the following important aspects of the design phases should be well thought out. As hard as it may be, these seven aspects of the design technics will need to be envisioned before and during the component placement stage.
- Smooth routing (continuity of the circuitry flow)
- Functionality (components that have similar functions)
- Signal integrity
- Design for manufacturability (DFM)
- Design for testability (DFT)
- First time right!
- Design for usability.
Starting with a block diagram will really help to plan out the PCB floor planning to an excellent kickstart.
The size of the board is the decisive factor in how the component placement is going to be done. The bigger the board size the easier it is to plan out the component placement.
When components are placed further apart, it gives room for routing and via-fan out. Being able to place all the components on the same side of the board will cut the assembly cost by a great deal. Placing parts on both sides of the PCB will be considered as an option when there is not enough space to place all the components on one side of the board. This will be the case in high-density interconnect (HDI) design and miniaturized board designs.
Board size
The first thing we must do is determine the board size. The board size is typically provided by the mechanical designer. A drawing exchange format (DXF) is a typical file format that the mechanical designer uses to communicate with the PCB layout designer. The DXF will contain board cutout, mounting hole locations, certain connector locations, switches, or LED (Light Emitting Diodes) locations, and any components that will need to be placed on a specific spot on the board. Also, heatsinks, sensors, and antennas can be part of the information that the Mechanical designer may include in the board outline. We can call or refer to these parts as fixed parts. It is always good to clearly communicate with the mechanical designer or anyone involved in this level of the design stage. It is this kind of clear communication that will allow the designers to determine and make sure that the board outline and the placement of any critical component are lined up with the enclosure, faceplate or housing of the final product.
“Design is really an act of communication, which means having a deep understanding of the person with whom the designer is communicating.” Donald A. Norman
Once the PCB layout designer takes care of the placement of the fixed parts, it is particularly important to communicate back to the mechanical designer. The best means of communication back to the mechanical designer may be using STEP model of the board along with the critical components which can be exported and handed over to the mechanical designer to double-check and verify the accuracy of the board size as well as the placement and clearance of the fixed components.
Design rules
Before getting into the full-scale component/footprint placement mode, it is critical to set up the design rules. Referring to and following IPC standards is key. How tight or relaxed the design rule should be is mainly determined by the size of the board and the number of components. Of course, there are various other factors that govern the placement of the components. Unless the board is exceedingly small and the component count is exceptionally high, it is always good to keep the design rule to be on the high end. The more the parts are spread the easier it is to route the board. Also, when it comes to assembly, it is much easier and less expensive to assemble a board with components placed spread out versus components that are close or touching each other due to lack of space. If the design is determined to be a high-density interconnect (HDI), then it is highly recommended to start by communicating with the contract manufacturer (CM) / assembly house at an early stage. Finding the capability of the contract manufacturer (CM) will help to fine-tune the design rules check (DRC) to that of the CM’s capability. It is always good to find out the minimum and maximum capability of the various CMs (Contract Manufacturer’s) design rules. This will allow the procurement team to have a pool of CMs to choose from. Once the design rules are set, it is good to make sure that the auto design rule check is turned on. Most or almost all layout design tools have the option to turn on and off the on-line DRC mode. Using online DRC while placing components from the start will save so much time. It is sometimes annoying to be forced not to temporarily place components very close to another component just to clear space for rearranging other component placement. But that should not be a reason to completely turn off online DRC. Even if online DRC is turned off for this purpose, it is very important to turn it on back and resume the component placement with online DRC turned on.
A component placement done with design for manufacturability (DFM) in mind will give peace of mind to the entire team, including purchasing and the product development team.
We are ready to start placing the components once the design rule is set up. Always use the highest component placement grid that is possible. Again, this is also determined by the board size and component size (the bigger the board size the higher the grid should be). The same thing with the components; the bigger the components, the higher the grid should be). But regardless of what; it is always advised to never place components without setting up a reasonable grid system based on an increment of 0.05mm (about 0 in). Using a grid system will also help ease routing among other things.
Placing component by functionality
Effective communication with the schematic designer is particularly important. Understanding the flow of the schematic even before starting the component placement will save a lot of unnecessary effort. Are there any critical components that need special attention? Are there components that are expected to get extremely hot? Is this a Radio Frequency (RF) design, analog design, digital design, and or mixed-signal design? Grouping components by their functionality can be a good starting point at the high level. This can be done outside of the board outline till it is determined what to place where. This will be a good time to color all the power (PWR) and ground (GND) nets as this will display on the ratsnest during the component placement.
It is always challenging to figure out where to start the component placement, especially when there are quite a lot of components in a design. How do we determine where to start the component placement? To quote Desmond Tutu, “There is only one way to eat an elephant: a bite at a time.” Where do we even start that one bite? Start with components such as connectors, switches, LEDs (Light Emitting Diodes), and or any component that is determined to be placed at a certain location due to mechanical constraints. Mounting holes should also be placed in the early component placement stage. It is then an excellent strategy to continue placing components that are associated to the already placed components. It is at this point using the schematic as a guide will begin. The schematic is the source of guidance. It is also an innovative idea to first group components that need to be grouped together such as power supply, digital circuitry, RF circuitry, and analog circuitry together. By this time, the designer should determine if components will be placed on both sides of the boards or not. Using both sides of the boards for component placement will increase the cost of the assembly but it certainly helps to make the best of the space. Using both sides of the board for component placement will allow decoupling capacitors to be placed close to the power pins. If the board is a dense board, using both sides of the board for component placement will allow a smooth routing.
It is unusual to expect placing components once and to be done with component placement. In a normal scenario, the PCB layout person will go back and forth placing, rotating, flipping, and moving components repeatedly. The only components that will be placed once and be kept untouched are the fixed components that the Mechanical Engineer provided with the board outline. Component placement is a time-consuming step of the process. As the saying goes – PCB design is 90% placement and 10% routing. Indeed, 90 percent of the PCB layout design is invested in component placement. If that is not the case, the routing process will be much more difficult and time-consuming.
Top side placement
Bottom side placement
PCB layout component placement requires the ability to visualize the various processes and steps simultaneously and throughout the component placement process.
The following are important points to keep in mind while placing the components
- Smooth routing space is required especially for critical routing nets.
- Keeping enough space to fan out surface mount devices
- Having enough space for copper pour as needed for power and ground.
- Enough space for placing silkscreen for each component. This is less important on the list.
- It is important to orient pin one marking for the IC’s the same direction whenever possible.
- Keep components that are expected to dissipate a lot of heat away from other components and make sure there is enough space to place heat sink as well as place enough copper area on the board.
Component placement is time-consuming and requires a good understanding of the various disciplines that are associated with the flow of placing components. It is good to start with a simple design if doing the placement for the first time or without any experience.
The fact that the software tools are becoming capable of viewing the board in 3D has made it much easier to visualize the board as if one is holding an assembled board.
