In the first column of this series, I talked about applications and challenges that PCBs face in harsh and extreme environments. This time, I'll dive deeper into PCB design considerations. Designing a PCB for a non-demanding environment can be a daunting task. There are many issues to consider, especially as you make certain decisions. But when I think about some of the challenges unique to extreme environments, I am amazed at the robust technology that is available. There are also negative effects of altitude, vibration, shock, and heat that plague the electronics systems that we rely on every day.
One challenge many PCB designers must resolve is the effects of air density that occur with a change in altitude. Without going into too much detail—I am a PCB engineer, not an atmospheric scientist—the density of the atmosphere becomes lighter as altitude increases. There is a region in the atmosphere between the lower atmosphere and the vacuum of space where electricity can arc at lower voltages. The breakdown voltage of the air in a thinner atmosphere is smaller. This allows electricity to arc across conductors at lower voltages than at sea level.
To make matters worse, charge builds up quicker at a point than it does on a rounder surface. The charges that build up can potentially act as arc points. There are some things we can do to mitigate this challenge. First, consult IPC-2221, Section 6, the part of the design specification that calculates how much electrical clearance your design will need.
Another way we can add mitigation to keep arcing at bay is to round out the corners in the design. Eliminate the 90° inside- and outside-corners on plane pours, surface mount pads, and the bends of traces. Engineers also can look at connection pins. Since these areas of the board are often associated with different voltage levels, there is more of a risk for arcing at these places as well. Sometimes we have unused pins on the connector that allow even greater clearance if we use every other pin.
Shock and Vibration Events
Some environments subject PCBs to extreme shock and vibration events. Extreme shock events subject PCBs to a high amplitude of movement force for a short time, whereas extreme vibration events subject PCBs to longer periods of movement with less amplitude. The movement force causes the board to oscillate about the center of mass. In turn, the oscillations stress the PCB materials and connection points. The materials will be stressed at the point where the pre-preg bonds with the copper. The hole barrels are internal connection points that could possibly crack, causing intermittent operation.
There are many small things we can do to mitigate shock and vibration. The key is to find the minimum amount of mass to add to the system while maintaining mechanical stability. Along with staking and potting compounds, what follows are some PCB design considerations.
First, we can design more mounting holes into the board, which means more mounting points to the chassis and more stability. This also will allow for more dampening devices to be mounted into the system.
Another aspect of the PCB is the stackup. In class, my favorite assignment was designing a board and stackup for a satellite application. For boards that will be subject to extreme shock or vibration events, it is critical to ensure the pre-preg has enough resin content to withstand these types of events. The pre-preg resin is the glue that holds the board together. During shock and vibration events, the PCB may flex and cause stress on the connection points. To mitigate as much stress as possible, we should consider using pre-pregs with over 50% resin content. Moreover, adding more than two sheets is another way to add bonding strength. This should be considered because signal integrity requirements do not always allow for changing dielectrics easily. What if you do not have a signal integrity requirement? If three sheets of pre-preg are good, then four sheets are better, right? Maybe. At a certain point, too much pre-preg will be detrimental to the layer-to-layer registration.
I would try to stay with three sheets at most, unless your PCB vendor says otherwise. If you are after more thickness but cannot add any more pre-preg, consider using unclad cores in the stackup. Unlike the pre-preg, an unclad core is already cured, so it does not add any resin to the press cycle. Layer-to-layer registration is not sacrificed.
Finally, there are the internal connection points. As the PCB oscillates, the hole barrels are also oscillating, stressing the plating in the holes. As the plating is stressed, it can crack. Some PCBs may work again after they are brought up to operating temperature, and some may experience catastrophic failure. Either way, it is not always possible to retrieve these PCBs in the field. Adding extra plating to the holes is just one more consideration to having a robust PCB. It will not work for every application.
Last is thermal management. For a non-demanding application PCB, passive cooling such as convection will be an adequate thermal management solution. Other PCBs may be connected to the chassis for heat transfer, and some may be cooled by other devices. PCBs that operate in extreme environments cannot always be cooled by these methods. We can design the PCB to have active cooling solutions.
An easy way to add heat mitigation is to increase the copper thickness of the PCB. Simply stated, thicker copper dissipates heat better. As a word of caution, make sure the electrical space in the PCB data will support manufacturing the PCB with thicker copper. Your PCB vendor will know how much space you will need for sure. They are always a good source for this type of information.
If simply increasing the thickness of the copper does not alleviate the thermal management issues, there is the option to add copper coin technology to the PCB. This technology is like building a discreet heat sink into the PCB. It is positioned directly under the component so that it is in contact with it. In some cases, the coin can be electrically connected to the component.
The advantage is greater heat transfer, rather than simply increasing the copper thickness. A disadvantage, however, is the number of copper coins that can be placed on one panel. This amount will vary from manufacturer to manufacturer, but be sure to consider that every time a coin is inserted into a PCB, a weak spot is created in the PCB material. Too many weak spots can cause warpage and unintended consequences. Consider an environment with shock and vibration events with many coins in one design.
Instead of designing the PCB with too many copper coins, consider metal core construction. Metal cores can be pressed into the stackup or pressed onto a layer to provide the ultimate in heat sinking ability. When planning for metal cores, dielectric thicknesses should be carefully considered. We want to use the thinnest dielectric possible between the component layer and the metal core, because thinner dielectrics have less heat resistance. (This advice goes directly against the recommendation I made for designing stackup to resist shock and vibration events. As the PCB gets more complex, we must consider each moving part of the “machine” in relation to other parts.)
Extreme environments subject PCBs to many new hazards that we must consider. Other challenges will amplify in magnitude. It is difficult to learn everything we can do to mitigate the effects extreme environments on our PCBs. Hopefully, adding a little here and there will help you design a more robust PCB.
The best advice I can provide is to always work with your PCB supplier as early as possible in the design phase. They know the design considerations and manufacturing capabilities to help design the PCB in the most efficient and sustainable way.
Ryan Miller is a field applications engineer at NCAB Group.