If there is one thing that a rainy day is good for, it is the time-honored tradition of putting together a jigsaw puzzle. Puzzles can be great for gathering friends and family together to work toward a solution. There is, however, one thing you learn quickly when working on a jigsaw puzzle—if you try to put a piece in where it doesn’t belong; you are going to have problems. There aren’t a lot of rules to puzzle building, but trying to fit a piece in that isn’t supposed to be there is the one rule that you can’t break.
Laying out a printed circuit board has often been compared to putting together a puzzle because the components must fit together in a certain way. The parts must be close enough to their associated circuitry to work electrically, positioned optimally for thermal and mechanical considerations, and placed correctly to ensure manufacturability.
In the same way that you can’t pound a puzzle piece into the wrong place and expect the puzzle to be turn out right, PCB components must be placed correctly for the board to be manufactured adequately. Therefore, the best thing you can do is follow some basic PCB component placement guidelines to help ensure the success of your layout.
The Problem With Ignoring PCB Component Placement Guidelines
PCB components that are not placed correctly in a layout can cause problems when the board is being assembled by a contract manufacturer. Incorrectly placed components may not solder correctly, which can result in a host of reliability problems. For example, shorts may be introduced, causing failure of the circuitry. And, the board may have difficulty going through some or all of the automated manufacturing processes.
If not addressed prior to production, component placement issues can lead to delays, or even stop the board from being manufactured. In addition to the immediate manufacturing delays, there could also be a ripple effect that creates larger issues for your project, including:
- Time to market for the PCB or the product that it is used in could be delayed.
- Quality issues could result in reduced confidence in the design.
- There could be an inability to push the design forward into high-volume manufacturing.
- More resources could be required to support design changes.
- PCB or project funding could be jeopardized.
For designs that have not been optimized during the design phase, finding and resolving the issues that can cause immediate manufacturing problems will require more engineering time on both the part of the designer and the CM. Yet, these corrections are necessary to improve yields, manufacturing cycle times, and reduce quality issues with your PCB.
On the other hand, following good component placement guidelines while designing your PCB will help avoid these manufacturing problems and streamline your PCB’s production process.
Basic PCB Component Placement Guidelines to Consider
The majority of component placement problems that affect manufacturing are when components are placed too close to the edge of the board. This potentially can result in damage to the board from different sources, such as automated assembly equipment during a volume manufacturing run. Here are some other placement considerations that you should keep in mind:
- Component spacing: Components placed too close together could cause problems for automated pick-and-place machines during assembly and for the fixtures used for testing.
- Missing solder mask between pads: This can lead to solder bridging during assembly, which can cause electrical shorts between pins.
- Incorrect surface mount pad sizes: When discreet SMT pad sizes are not the correct shape or are imbalanced due to differing trace sizes, one pad could reflow faster than the other. This can cause the part to lift up on one end in an effect called “tombstoning.”
- Component orientation and location for wave soldering: Discrete components should be placed so that both pins enter the solder simultaneously when the board goes through the wave. Larger parts should also not precede smaller parts to avoid shadowing, which could cause insufficient solder joints between component pins and their metal pads on the board.
Although not directly part of component placement, you should also consider the stackup requirements of the board. Board layer stackups are driven by the signal integrity and impedance needs of your design. These needs will affect how you place the components for high-speed designs. Once your high-speed board layer stackup requirements are determined, you can choose the board fabrication materials that will be needed up front instead of having to retrofit board materials to match your design after it has been completed.
Component placement to optimize production volumes should also extend beyond the manufacturing of your single board to the final assembly of the entire system. Connectors on your board that have other PCB components placed too close to them may be difficult to access during final system assembly.
Also, connectors on your board that aren’t placed optimally for other system connections can result in unnecessary long wiring configurations for the full system. Not only can this affect the final system assembly with the schedule and cost overruns, but it can also have an adverse effect on the overall signal performance of the system.
Partner with Your CM to Design Your Board for Manufacturability
The key to avoiding these manufacturing problems is to work together with your CM for the most optimum component placement on your PCB design. This means partnering with a CM that is knowledgeable and skilled in their design for manufacturing (DFM) capabilities.
Your CM should work with you during the design phase of your project to ensure that you are creating a PCB that is both manufacturable and functional. They should also review your completed design to identify areas of DFM concern. The CM should have the ability to work directly with your design and resolve the issues themselves while providing you with a full set of updated documentation. Partnering with a CM with this level of expertise is one of the best things you can do to ensure the successful manufacturing of your PCB assembly.
