Impact of Spacecraft Vibrations on Printed Circuit Board Assemblies
Managing Vibrations and Thermal Control in Space Vehicles
Spacecrafts face unique challenges when it comes to vibration control and thermal management during launch and flight. In this article, we'll explore the sources of vibration in space vehicles and the techniques used for thermal control.
Vibration Sources in Space Vehicles
Vibrations in spacecrafts can be traced back to mechanical, structural, and operational factors. Engine and propulsion system vibrations stem from rotating components such as turbine rotors, gears, and combustion events in rocket engines. Mechanical vibrations from hardware, on the other hand, can result from loose fittings, misfires, imbalances, or gear-driven components.
Structural vibrations induced by launch loads occur due to aerodynamic forces, stage separation, and mechanical excitations during launch. External excitation sources like aerodynamic buffeting, dynamic pressure fluctuations, and environmental factors can also induce vibrations. Lastly, payload vibrations may arise in sensors or instruments due to coupling with the spacecraft structure or internal mechanisms.
To mitigate these vibrations, anti-vibration frames can be employed, although they tend to increase cost. Mechanical waves causing vibration in PCBs can be managed by installing interconnections near bulky parts to prevent damage. Sealing agents used in component assembly act as shock absorbers, reducing vibrations on the board. Constricting circuit boards around the edges can also help prevent vibrations.
Thermal Control in Space Vehicles
Heat balance between the vehicle and the surrounding environment influences a spacecraft's surface temperature in Earth's orbit. To maintain thermal balance, radiators, thermal blankets, surface coatings, and heaters are used. Conduction and radiation are the only two modes of heat transfer to get rid of excess heat in space vehicles.
In the design of PCBs for space vehicles, components should have specific values, and trimpots, variable capacitors, or variable inductors should not be used. Critical nets need to be routed on the same layer, or a minimum via should be used along with a second redundant track and via to prevent open circuit failure of a single interconnection. A copper-clad laminate structure should be used for better peel strength.
Proper soldering is essential for establishing reliable connections in PCB assembly. The conductive patterns and dielectric materials must have wide conductor dimensions to avoid slivers and be peelable. Solder mask is not recommended for circuit boards installed in space vehicles due to issues such as poor adhesion, lower performance regarding outgassing, and a high risk of selective tin-lead stripping.
Conformal coatings should be applied to external layers of electronic components to prevent Electrostatic Discharge (ESD) and Paschen discharge during launch. The structure of the board, including layers, copper distribution, and component placement, should be symmetrical to avoid twists and wrap. Flight model PCBs require through-hole and buried vias, but blind vias are not acceptable.
Lastly, it's worth mentioning that the IPC Class 3 Design Guide provides valuable information on IPC guidelines for manufacturing defects, IPC standards for assembly processes, common differences between the classes, and IPC documents to set the level of acceptance criteria. De-rating, a process used in space vehicles where the current carrying capacity of tracks is reduced by 66% to minimize failure rate in severe working conditions, is also an essential aspect of thermal management in spacecrafts.
By understanding and addressing both vibration sources and thermal control, engineers can ensure the robustness and operational reliability of spacecrafts, paving the way for successful missions.
[1] NASA, Vibration Testing [2] NASA, Vibration Testing [3] NASA, Vibration Testing [4] NASA, Vibration Testing [5] NASA, Vibration Testing
The material selector in space-and-astronomy must consider the importance of anti-vibration frames for spacecrafts to attenuate vibrations originating from mechanical, structural, and operational factors. In the design of PCBs for these vehicles, scientists must adhere to guidelines provided by NASA's IPC Class 3 Design Guide to ensure thermal control through proper soldering, conformal coatings, and specific component placements, such as the use of through-hole and buried vias to prevent open circuit failure.
