1 Abstract A set of guidelines for circuit layout to consider is presented, as it relates to structural concepts. Namely, shock and vibration environments. The information is targeted to electrically-minded engineers that have little experience in structural analysis and test. It aids understanding the trades that accompany part placement beyond common electrical and thermal considerations.

Good Vibrations in Electronics Structural Considerations for Electronics Systems Design STI Electronics, Inc. Jason Tynes, Manufacturing Engineer

3 Outline Overview Why should I care? Mode Shapes Profile Comparisons How to Use this Information for Design Test and Verification Methods

4 Overview What is Vibration? Random Vibration – motion that cannot be precisely predicted i.e. Vibrating seat in moving vehicle What is Shock Physical Shock – sudden acceleration that can typically be predicted i.e. Impacts/drops Vibration and shocks stimulate system We are typically concerned with the response – not stimuli Response – how the system reacts to a specific stimulus Local stresses and deflections

5 Who Cares???? Why would a non-mechanical person care about vibration anyway? Concurrent Circuit Design Flow

6 Who Cares???? Communication of Keep Out and High Stress Areas typically occurs after board outline has been transmitted, if communicated at all Layout is typically underway and driven by schematic requirements long before structural input is available Structural Analysts check/verify stress margins are acceptable

7 So… Why do I Care Again? This could be your BGA Says the Structural Analyst: The board survived. Was that your BGA???

8 Mode Shapes The shape and frequency of the board during natural, sinusoidal movement Recall simple harmonic motion 1 Dimensional Medium (Line) Mode shape #7 requires 49X the energy of #1 in order to achieve similar amplitudes Conversely, mode shape #7 exhibits 1/49 th the amplitude of #1 with similar energy Constant Amplitude Less Energy Much More Energy

9 Complex Shapes with 2D Medium 1 st 2 nd 3 rd 4 th Mode shapes generated for representative PCB Using FEA tool Standard PCB Material Properties Amplitude/Displacement causes stress Higher Frequency  Lower Amplitude  Lower stresses For Constant Energy Most Stressful Least Stressful

10 ShapeFrequency Shape 489 Hz2441 Hz 950 Hz2513 Hz 1397 Hz3774 Hz 1440 Hz4622 Hz 2243 Hz4841 Hz More Complex Shapes when Constrained Mode shapes when restrained Most relevant More complex Usually only insightful to about 2,000 Hz Reduce first 4 or 5 modes to most likely to cause damage

11 ShapeFrequency 489 Hz 950 Hz 1397 Hz 1440 Hz 2243 Hz Mode Shape Reduction Which Mode(s) are more likely to be excited in the environment that hardware is to be used? Depends on the environment For Profile below, Frequencies Between 300 and 1000Hz are Energetic 489 and 950 Hz modes to be excited 489 Most susceptible due to increased amplitude

12 How to Use Info Showing 489 Hz Mode Avoid placing large footprint and/or massive components in high stress areas Avoid positioning mission-critical pins within high-damage areas Bad Best Better Mission Critical Pins of Grid-Array Component Best Bad

13 How to Use Info Everything so far can be done at your computer workstation Free/Cheap CAD Software Available Online (Cubify Design Shown Here) Free/Cheap FEA Software Available Online (LISA- Finite Element Technologies Shown Here) Testing/Verification cannot be performed on a computer workstation Correlation between computer model/analysis predictions and real response is critical Supports Predictions on Design Margin Enables Improved Model/Analysis Practices Shaker Table and Real Hardware Required Testing Required to Instill Confidence in Design Choices and Analysis

14 Testing and Verification Modal Survey / Ping Test Identify Frequencies Information Used to Refine FEA Model’s Material Properties and Update Predictions Hardware Suspended in as Close to Free-Free Condition as Possible Free to Translate Free to Rotate Simplest Solution  Rubber Bands Accelerometer Attached Near Expected Location of Maximum Displacement in 1 st Mode Used to Measure Motion in Frequency Domain Gently Tap the Board Using a Material Softer than the Board Eraser End of Pencil is Ideal Board Responds by Displaying All Mode Shapes Simultaneously Accelerometer captures response in frequency domain Shows Amplification and Attenuation vs. Frequency

15 Testing and Verification Shock and Vibration Testing Shaker Table Used to Produce Vibrations and/or Shocks that Meet Environmental Specification Control Accelerometer Allows Motion to be Automatically Monitored and Corrected / Controlled to Specified Limits Representative Hardware Mounted to Shaker Table Using Fixtures to Mimic Fielded Installation Lightweight Response Accelerometers Attached to Precise Locations on CCA to Measure Response

16 Conclusion Have expectations for electronics environmental exposure Part placement and even orientation can be the difference between success and failure of fielded electronics Get to know your mechanical analysts, including the structural variety Test to make sure your assumptions are legitimate

17 Thank You Questions??? Jason Tynes (256) STI Electronics, Inc. 261 Palmer Rd Madison, AL