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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

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Outline Overview Why should I care? Mode Shapes Profile Comparisons How to Use this Information for

Design Test and Verification Methods

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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

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Who Cares???? Why would a non-mechanical person care

about vibration anyway?

Concurrent Circuit Design Flow

Electrical Functions

Mechanical Functions

Packaging Trade Studies CAD Modeling

Board Outline

and Thickness

Functional Trade Studies

Schematic Development Part Selection Component

Layout Trace Routing Design Rule Checks

Structural and Tolerance Analysis

Thermal Analysis

Technical DataPackage

Mfg/Gerber Plots

PCB and CCA

Drawings

Power, Duty, and Location

Info

Keepout and High

Stress Areas

Detailed Drawing

Development

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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

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So… Why do I Care Again? This could be your BGA

Says the Structural Analyst: The board survived. Was that your BGA???

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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 1/49th the amplitude of #1 with similar energy

Energy∝ 𝑓 2𝐴2

Consta

nt A

mplitu

de

Less Energy

Much More Energy

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Complex Shapes with 2D Medium

1st 2nd

3rd 4th

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 EnergyMost StressfulLeast Stressful

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Shape Frequency Frequency Shape

489 Hz 2441 Hz

950 Hz 2513 Hz

1397 Hz 3774 Hz

1440 Hz 4622 Hz

2243 Hz 4841 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

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Shape Frequency

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

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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

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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

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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 1st 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

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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

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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

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Thank You Questions???

Jason Tynes (256) 705-5511

STI Electronics, Inc.261 Palmer RdMadison, AL