One day while learning to fly, my instructor suddenly turned our airplane sharply to avoid another plane as we were approaching the airport. At the same time, I noticed a similar problem with a different airplane on the other side and tried to turn sharply in the other direction. With dual controls in the airplane, we had a bit of a navigational tug-a-war between us until we sorted it out. After settling on a common direction, we easily avoided the other planes. I was amazed, however, at how quickly the vast open sky can run out of safe space in which to fly.
When you are designing a printed circuit board, you too can quickly run out of room in which to design. What started as a vast expanse of empty circuit board soon gets filled up with your components and mechanical parts. Yet, you still need room to route all of your power and signal traces. It is tempting to try to squeeze together your PCB pad-to-pad spacing, but that can lead to some serious manufacturing problems down the road. Let’s take a look at those spacings, and how to best optimize them for manufacturing.
Circuit Board Design and PCB Pad-to-Pad Spacing
Of course, you must get all of your parts on the board, and you probably have already done a space study to ensure your parts will fit. If for some reason the parts don’t fit due to mistakes in package types or sizing, added features, or reduced board size, then you will have to rethink the design and come up with some ways to reduce the circuitry. But at this point in the design, there’s probably enough room for the parts. The concern now is coming up with the optimum component placement for the best design functionality.
Sensitive components may need a specific amount of spacing in relation to other components to reduce the chance of signal interference. Differing areas of circuitry, such as analog versus digital, will also need precise spacing between them. In addition, parts that run hot will need space for cooling, and you will need to allow yourself enough room for routing. On the flip side, components that are part of high-speed signal paths, power converters, or other sensitive circuits must be placed close together.
All of these needs can cause pad-to-pad spacing problems for you if you aren’t careful. Some designers will shrink their pads to create more room around them, or they will squeeze them together to save space or reduce signal length. Both of these conditions can create problems during manufacturing. To avoid this, you need to follow the industry-standard rules for component footprint creation and parts placement. Let’s look at some of the problems that can arise from inadequate pad-to-pad spacing if the rules aren’t followed.
Potential Manufacturing Problems with Inadequate Spacing
It is often tempting to place parts too close together or to purposely create PCB CAD footprints with shrunken pads to accommodate tight placing requirements. This is usually done with the best intentions in mind, such as improving the signal performance in high speed or RF circuits, but it can harm the manufacturability of the board:
- Solder bridging: Pads that are too close together are susceptible to the solder bridging between them. This is often seen in fine pitch components where the solder mask between pads isn’t sufficient to prevent this problem. The best thing that you can do to prevent this is to follow the IPC specifications when creating your land pattern footprints.
- Shorts to adjacent metal: Similar to solder bridging, pads that are too close to adjacent metal are also susceptible to solder flowing between them. The prevention here is to obey the DFM spacing rules when placing components close together.
- Difficulties with soldering processes: Components and pads that are too close to each other can cause problems during the soldering process. Large components may shadow smaller components during wave solder, which results in poor solder joints in the smaller components. Pads that are created too large could cause components to float out of position during solder reflow, while pads that are too small could result in a bad solder joint. Once again, following the IPC specifications for pad sizes and the DFM minimum placement rules are the best prevention against these problems.
- Difficulties with PCB rework and testing: Parts that are placed too close to each other can cause problems during rework as technicians try to solder and desolder components to their pads. The close proximity of pads can also make it more difficult for probes to reach the correct component pin for testing. Additionally, parts that are too close together are physically more difficult to access for rework and test technicians.
Although unique circumstances may demand that you place your parts too close together, or create your pads smaller than desired, those are the exceptions and not the rules. Modifications like these shouldn’t be done without first consulting your PCB contract manufacturer to make sure that they can be worked with. In fact, your CM may have some better solutions and workarounds for you so that you don’t have to violate the DFM rules in your PCB design.
How You Can Avoid Manufacturing Problems
There are several things that you can do to avoid assembly problems and ensure your printed circuit board design is easily manufacturable:
- Use the features in your PCB CAD tools to build correct PCB land patterns and pad sizes in your CAD tools. Many of these tools already have footprint and pad building wizards within them that can do much of the work for you, and they are often are tied to the IPC land pattern specifications.
- Use component vendor and third-party component library parts when possible. Some of these are already connected to the more popular design tools, making it easy to download the parts into your design.
- Follow industry standard DFM rules for placing components.
The single best thing that you can do, however, to help yourself design a printed circuit board that is easily manufacturable, is to work together early with your contract manufacturer. This way, you can get a full understanding of the placement requirements your design will need to be optimally assembled.
