In any movie today that features an earthquake as a plot point, you know that there is going to be a lot of mayhem and destruction. To highlight this, the film makers will often show close-ups of metal girders twisting, bolts popping, and solid structures shaking until they collapse. Even if it is all due to Hollywood magic, it is still serves as an example of vibration fatigue at its worse. The truth is though that most everything around us is subjected to some sort of vibration, even if it isn’t obvious. The motion of our cars produces stress, and thermal expansion creates moans and groans in the buildings we live and work in. These effects of vibration must be accounted for and designed into the structures and devices we use every day, even in our circuit boards.
While a lot of circuit boards will sit in one place without too much in the way of motion, others are used in applications that are subjected to a large range of motion. These devices can be anything from small toys to complex spacecraft. And while some boards may not actually travel anywhere, they are still subject to vibrations from the stress of manufacturing, thermal changes, and even getting slapped around by frustrated users. To counter this PCB designers need to understand the basics of vibration fatigue in the designs they create, and how best to counter these affects. Here are some ideas that should help.
Environmental Stress and Vibration Fatigue
It has been estimated that as much as 20 percent of printed circuit board failures are due to vibration and shock. Although these figures were first quoted by the air force, many other industries have reported similar findings. This underscores the importance of designing circuit boards to better withstand the stress of random vibration fatigue. It becomes even more important for boards that are going to be used in environments that are more prone to vibration and shock such as aerospace applications.
Although the core materials of circuit boards, such as FR-4, do fairly well under the stress of vibration and shock, the same can’t be said of the electronic components soldered to them. As vibrations cause a board to bend, the leads on its components can break due to their stiffness and stretching. Solder is also vulnerable to vibration stress and can break causing leads to disconnect from the board. Even small amounts of vibration over a long period of time will cause fatigue in the component leads and soldered connections.
Without proper design practices, these solder joints could break due to vibration fatigue
PCB Manufacturing Stresses that Can Lead to Vibration Fatigue
Another contributing factor to vibration fatigue failure is the stress that the board is under during manufacturing. Component leads and soldered joints all are susceptible to thermal shock, and good printed circuit board design for manufacturing (DFM) processes are essential to help counter those effects. Once such example of this is designing the pads on a PCB to enable a component lead to correctly solder to it.
Pads that are badly designed can cause the solder to not fillet correctly to surface mount leads, or wick solder away from where it is needed in thru-hole pads. These conditions can result in parts that have poor solder connections. On large thermal pads for instance, solder wicking through uncovered vias can prevent the grounding pin of a device from getting a good solder connection. This part may make it through manufacturing and test, but will be prone to an intermittent or total failure at some point in the field due to vibrations wearing down the already thin solder connection.
A 3D PCB design CAD system will help you manage your component pads and footprints
What Can You Do to Help Prevent Vibration Fatigue?
The first step in combating vibration fatigue is to design for reliability (DFR). This is a process for ensuring the reliability of a PCB during the design phase before boards are built. Part of this process will be incorporating good DFM practices in the design. Your PCB manufacturer can help you with creating the correct pad and footprint sizes for your parts as well as providing you with the design rules that will allow you to design to the relevant IPC class. Another part of DFR is using simulation tools to predict where failures in your design may occur so that you can make changes accordingly.
New technologies are being introduced every day to help with the challenges of designing for vibration fatigue, and conducting random vibration fatigue analysis. In the meantime though, it is still a common practice to submit new designs to physical vibration and shock testing. By applying much higher vibration and shock stresses on a product then they would actually encounter during their normal operation, failures can be quickly forced. This highly accelerated life cycle testing, or HALT, is an important part of new product development to identify potential failures due to vibration in order to ensure that production builds of the circuit board will function reliably.
Another step that you can take in your battle against vibration fatigue is to use the most advanced PCB design tools available to you. With the right tools you can easily create, modify, and manage the pads and footprint shapes being used on your design, as well as create advanced design rules and constraints for error free manufacturing. Allegro PCB Designer is the kind of system that you need to do this. With advanced part building tools to help you create your physical layout libraries as well as the most comprehensive design rules in the industry, you will be well on your way to controlling vibration fatigue.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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