Electronic Circuit Board Design Reliability Lessons Learned


Download 12.09 Kb.
NameElectronic Circuit Board Design Reliability Lessons Learned
A typeLesson
manual-guide.com > manual > Lesson
Electronic Circuit Board Design Reliability Lessons Learned


By Tom Irvine

Email: tom@vibrationdata.com


July 22, 2015
_______________________________________________________________________


Figure 1. Typical Circuit Board

Introduction
Here are some design tips for reliability with respect to shock and vibration environments.
Circuit Board Mounting
Boards should be securely mounted in their enclosures using positive locking devices. The mounting and geometry should usually be designed to make the board as stiff as possible, to increase the natural frequency, and to minimize relative displacement.
Note that piece parts are sensitive to the board flexing which tends to have higher displacement amplitudes at lower natural frequencies. This deflection causes bending stresses in the solder joints and lead wires which may result in fatigue failure per Reference 1.
On the other hand, the box or enclosure should be mounted to the “next higher assembly” via isolators, if possible, to attenuate the shock and vibration energy flow into the circuit board.


Figure 2. Cracked solder Joints for Piece Part with “J leads”

Sockets
Sockets are often used to mount IC chips to circuit boards. Unfortunately, chips in socket mounts can easily detached from circuit boards that are exposed to shock and vibration.1 Or an IC may experience an intermittent open due to vibration even if it remains in the socket.
Ideally, IC chips would be soldered directly to circuit boards. The desire to allow for upgrading the CPU and RAM requires a socket approach, however. On the other hand, the furious rate of technological change would seem to make “upgrade-ability” of an existing system a moot point. The consumer must typically buy a whole new computer system to run the latest operating system and software.
Crystal Oscillators
Crystal oscillators can shatter when exposed to shock and vibration. Care should be taken to choose an oscillator that has been designed and tested to withstand shock and vibration.
Staking
Piece parts should staked to the circuit board with an RTV or epoxy compound to protect the part from detaching, as shown in Appendix A. A solder connection alone does not necessarily preclude detachment. This is particularly important for large components such as DC-to-DC converters.
Coating
Ideally, a layer of thick conformal coating should be applied to each board. This layer adds damping. It also protects against shorting due to stray solder balls. Unfortunately, the coat makes rework difficult.
Piece Part Locations
The highest deflection will usually occur at the center of the circuit board. This zone should be reserved for smaller, lighter parts. Heavier parts should be mounted along the outer perimeter.
Fasteners
Fasteners may come loose during vibration environments. Screws must be torqued properly during assembly. In addition, lockwashers should be used to maintain preload. A thread-locking adhesive may also be used.
A power screwdriver can induce shock pulses. Use a manual screwdriver if practical.

References


  1. D. Steinberg, Vibration Analysis for Electronic Equipment, Third Edition, Wiley, New York, 2000.

  2. H. McLean, HALT, HASS and HASA Explained, Accelerated Reliability Techniques, ASQ Quality Press, Milwaukee, Wisconsin, 2009.

APPENDIX A





Figure A-1. Crystal Oscillator Mounted on Circuit Board, Typical Mounting Method

The oscillator's structural response is similar to a cantilever beam. Resonant excitation may occur during shock and vibration. High stress levels may develop in the crystal and in the leads as a result.



Figure A-2. Crystal Oscillator Mounted on Circuit Board, Improved Mounting Method

The oscillator is bent over so that it is parallel to the board. Then it is staked down with an RTV or epoxy compound. Furthermore, a Mil-Spec oscillator should be used rather than a commercial unit.



1 The author personally experienced this problem many years ago with a numeric coprocessor chip in a 386 computer.




Share in:

Related:

Electronic Circuit Board Design Reliability Lessons Learned iconLessons Learned 8

Electronic Circuit Board Design Reliability Lessons Learned icon6 Lessons learned 30

Electronic Circuit Board Design Reliability Lessons Learned iconLessons learned note 29

Electronic Circuit Board Design Reliability Lessons Learned iconLessons learned from columbine 90

Electronic Circuit Board Design Reliability Lessons Learned iconLessons Learned from Continuous Emissions Monitoring Users Group (cemug) Conference

Electronic Circuit Board Design Reliability Lessons Learned iconE-t-a’s Programmable Electronic Circuit Breaker Improves

Electronic Circuit Board Design Reliability Lessons Learned iconElectromagnetic simulation is a new technology to yield high accuracy...

Electronic Circuit Board Design Reliability Lessons Learned iconFuture Technology Devices International (ftdi) is looking to hire...

Electronic Circuit Board Design Reliability Lessons Learned iconExpresspcb board (layout v0-3) Each interface board is half of an...

Electronic Circuit Board Design Reliability Lessons Learned iconBoard Support Package (bsp) for the Freescale mpc8536ds board

Electronic Circuit Board Design Reliability Lessons Learned icon21 cfr part 11; Electronic Records; Electronic Signatures

Electronic Circuit Board Design Reliability Lessons Learned iconElectronic Media Electronic Story Archive 1994 to

Electronic Circuit Board Design Reliability Lessons Learned iconUser Manual for electronic upload of vsat activation / Surrender...

Electronic Circuit Board Design Reliability Lessons Learned iconElectronic Warfare Operator Operated overland and ocean radar, infrared...

Electronic Circuit Board Design Reliability Lessons Learned iconAs an art, it involves the use of learned skills in

Electronic Circuit Board Design Reliability Lessons Learned iconFood safety and reliability ensured

Electronic Circuit Board Design Reliability Lessons Learned iconTechnical White Paper on Reliability (Cloud Data Center)

Electronic Circuit Board Design Reliability Lessons Learned iconIn the last chapter, we learned how to work with our customers to...

Electronic Circuit Board Design Reliability Lessons Learned iconChapter 4 Reliability of measures of Retinal Vascular Network Geometry...

Electronic Circuit Board Design Reliability Lessons Learned iconUser Interface Design Process Obstacles -usability -human Characteristics In Design




manual


When copying material provide a link © 2017
contacts
manual-guide.com
search