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.
1
Good Vibrations in
Electronics
Structural
Considerations for
Electronics Systems
Design
STI Electronics, Inc.
Jason Tynes, Manufacturing Engineer
Outline
Overview
 Why should I care?
 Mode Shapes
 Profile Comparisons
 How to Use this Information for
Design
 Test and Verification Methods

3
Overview

What is Vibration?



What is Shock




Random Vibration – motion that cannot be precisely
predicted
i.e. Vibrating seat in moving vehicle
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
4
Who Cares????

Why would a non-mechanical person
care about vibration anyway?

Concurrent Circuit Design Flow
Mechanical Functions
Packaging
Trade Studies
CAD Modeling
Structural and
Tolerance
Analysis
Thermal
Analysis
Board
Outline
and
Thickness
Keepout
and High
Stress
Areas
Power,
Duty, and
Location
Info
Detailed
Drawing
Development
PCB and
CCA
Drawings
Technical Data
Package
Electrical Functions
Functional
Trade Studies
Schematic
Development
Mfg/
Gerber
Plots
Part Selection
Component
Layout
Trace Routing
Design Rule
Checks
5
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
6
So… Why do I Care
Again?

This could be your BGA

Says the Structural Analyst: The
board survived. Was that your
BGA???
7
Mode Shapes




Constant Amplitude

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
Less Energy
Much More Energy
Energy ∝ 𝑓 2 𝐴2
8
Complex Shapes with
2D Medium

Mode shapes generated for representative PCB




Using FEA tool
Standard PCB Material Properties
Amplitude/Displacement causes stress
Higher Frequency  Lower Amplitude  Lower stresses
1st 2nd
3rd
4th
For Constant Energy
Most Stressful
Least Stressful
9
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
Shape
Frequency
Frequency
489 Hz
2441 Hz
950 Hz
2513 Hz
1397 Hz
3774 Hz
1440 Hz
4622 Hz
2243 Hz
4841 Hz
Shape
10
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
Shape
Frequency
489 Hz
950 Hz
1397 Hz
1440 Hz
2243 Hz
11
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
Best
Bad
Better
Mission Critical Pins of
Grid-Array Component
12
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 (LISAFinite 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
13
Testing and
Verification

Modal Survey / Ping Test



Hardware Suspended in as Close to
Free-Free Condition as Possible




Used to Measure Motion in Frequency
Domain
Gently Tap the Board Using a Material
Softer than the Board


Free to Translate
Free to Rotate
Simplest Solution  Rubber Bands
Accelerometer Attached Near Expected
Location of Maximum Displacement in
1st Mode


Identify Frequencies
Information Used to Refine FEA Model’s
Material Properties and Update Predictions
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
14
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
15
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
16
Thank You

Questions???

Jason Tynes (256) 705-5511
STI Electronics, Inc.
261 Palmer Rd
Madison, AL
17