sherlock 3.0 modeling vibration and shock · o does not consider printed circuit board bending...
TRANSCRIPT
© 2004 - 2007© 2004 - 2010
Modeling
Vibration and
ShockNathan Blattau, Ph.D.
© 2004 - 2007© 2004 - 2010
o Nathan Blattau, Ph.D.
Senior Vice President of DfR Solutions, has been involved in the
packaging and reliability of electronic equipment for more than ten
years. His specialties include best practices in design for reliability,
robustness of Pb-free, failure analysis, accelerated test plan
development, finite element analysis, solder joint reliability,
fracture, and fatigue mechanics of materials.
© 2004 - 2007© 2004 - 2010
Vibration Fatigue
o Vibration fatigue is due to mechanical stress induced by vibration
o Millions of cycles to failure
o Small changes in stress have large impacts on time to failure
o According to U.S Air-Force statistics 20 percent of all failures observed in electronic equipment are due to vibration problems
Steinberg D.S. Vibration analysis for
electronic equipment.
John Wiley & Sons, 2000.
© 2004 - 2007© 2004 - 2010
o Failures under vibration could be
o Low cycle fatigue (LCF)
o High cycle fatigue (HCF)
o LCF is driven by inelastic strain (Coffin-Manson)
o This is not typical of field environments
o Failures would occur in seconds to minutes due to the cyclic rate experienced during vibration
o Vibration Fatigue is typically considered to be high cycle fatigue
o Failures above 100,000 cycles
o Elastic deformation behavior
o Predictions are usually done using the Basquinequation
Vibration Fatigue
( ) b
f
f
e NE
2σ
ε =
-0.05 < b < -0.12; 8 < -1/b < 20
Fatigue of Structures and Materials,
J. Schijve, Springer, 2001
© 2004 - 2007© 2004 - 2010
o Failure sites may occur in the lead or solder (or even PCB traces)
o Usually in the bulk materials
o Failures occurring at other locations, typically indicate a much higher stress application (such as shock)o Intermetallic fracture
o Laminate cracking
o Component body
Vibration Fatigue – Failure Sites
SnPb SMT 2512 fatigue crackSAC SMT 2512 fatigue crack
Solder fracture Lead fracture
© 2004 - 2007© 2004 - 2010
Vibration fatigue
o Presence of preexisting cracks can provide and initiation site
Well defined
crack path
Shrinkage crack provided
initiation site
Shrinkage Crack
© 2004 - 2007© 2004 - 2010
Modeling Vibration - Loads
o Single frequency o Random vibration is a continuous spectrum of frequencies
MIL-STD-810GAN INTRODUCTION TO RANDOM VIBRATION – Tom Irvine
© 2004 - 2007© 2004 - 2010
Modeling Vibration - Loads
Harmonic
Steinberg D.S. Vibration analysis
for electronic equipment.
John Wiley & Sons, 2000.
Random
MIL-STD-810G Figure 514.6C-1
US Highway truck vibration
exposure
1 hour is equivalent to 1000 miles
© 2004 - 2007© 2004 - 2010
Modeling Vibration - Loads
o Exposure to vibration loads can result in highly variable results
o Vibration loads can vary by orders of magnitude (e.g., 0.001 g2/Hz to 1 g2/Hz)
o Time to failure is very sensitive to vibration loads (tf ∝ W4)
o Very broad range of vibration environments
o MIL-STD-810 lists 3 manufacturing categories, 8 transportation categories, 12 operational categories, and 2 supplemental categories
© 2004 - 2007© 2004 - 2010
o The board displacement during vibration is modeled as a single degree of freedom system (spring, mass) using an estimate (or measured) of the natural frequency (Steinberg).
o Calculation of maximum deflection (Z0)
o PSD is the power spectral density (g2/Hz)
o fn is the natural frequency of the CCA
o Gin is the acceleration in g
o Q is transmissibility(assumed to be square root of natural frequency)
Vibration Modeling - Steinberg
20
PSD2
38.9
n
n
f
QfZ
⋅⋅⋅×=
π
Random
20
8.9
n
in
f
QGZ
××= Harmonic
Steinberg D.S. Vibration analysis for electronic equipment.
John Wiley & Sons, 2000.
© 2004 - 2007© 2004 - 2010
Vibration Modeling – Steinberg
Lchr
BZ c
00022.0=
o Calculate critical displacement, this is the displacement value at which the component can survive 10 to 20 million cycles (harmonic, random)o B is length of PCB parallel to component
o c is a component packaging constanto 1 to 2.25
o h is PCB thickness
o r is a relative position factoro 1.0 when component at center of PCB
o L is component length
Steinberg D.S. Vibration analysis for electronic equipment.
John Wiley & Sons, 2000.
© 2004 - 2007© 2004 - 2010
o Life calculationo Nc is 10 or 20 million cycles
o Several assumptionso CCA is simply supported on all four edges
o More realistic support conditions, such as standoffs or wedge locks, can result in a lower or higher displacements
o Chassis natural frequency differs from the CCA natural frequency by at least factor of two (octave)o Prevents coupling
o Does not consider printed circuit board bending (components can have zero deflection but still be subjected to large amounts of bending)
Vibration Prediction - Steinberg
4.6
0
0
=
Z
ZNN c
c
Steinberg D.S. Vibration analysis for electronic equipment.
John Wiley & Sons, 2000.
© 2004 - 2007© 2004 - 2010
FEA Based Vibration Predictions
o Finite Element Analysis can be used to capture more complex geometries, loadings and boundary conditions
o
Sherlock 3.0
© 2004 - 2007© 2004 - 2010
Mezzanine and Daughter cards
FEA Based Vibration Predictions
Sherlock 3.0
Sherlock 3.0
© 2004 - 2007© 2004 - 2010
o Loading can be applied to the model
directly from the specification
o Vibration is applied to the structure
through the standoffs/mount points
FEA Modeling Loads
© 2004 - 2007© 2004 - 2010
o Determining the response of the structure to a vibration load is commonly done
using a Modal Dynamic Analysis
o It is necessary to do a modal analysis before conducting this analysis
o Determines the eigenvalues and eigenmodes (natural frequencies)
o Calculates the stiffness and mass matrices
FEA Vibration Simulation
© 2004 - 2007© 2004 - 2010
o During vibration the board strain is proportional to the solder or lead strains and
therefore can be used to make time to failure predictions
o This requires converting the cycles to failure displacement equations (Steinberg)
to use strain
o The strain for the components is now pulled from the FEA results
o The critical strain for the package types is a function of package style,
size, lead geometry
Sherlock - FEA Failure Prediction
n
ccNN
=
0
0
ε
ε
Sherlock 3.0
Lcc
ζε =
ζ is analogous to 0.00022B but modified for strainc is a component packaging functionL is component length
© 2004 - 2007© 2004 - 2010
o Example, vibration test coupon
o SMC (DO-214AB) diodes
o 0.062” FR-4, 7” x 3.5” pcb with four corner standoffs
o Harmonic vibration
single frequency
90 mil peak to peak displacement
Sherlock - FEA Failure Prediction
© 2004 - 2007© 2004 - 2010
o Time to failure
Sherlock - FEA Failure Prediction
0
500
1000
1500
2000
2500
3000
D33 D3 D4 D8 D2
Time to Failure (minutes)
Reference Designator
Predicted
Experimental
© 2004 - 2007© 2004 - 2010
o Vibration test coupon
o SMC (DO-214AB) diodes
o 0.062” FR-4, 7” x 3.5” pcb modified with an additional standoffs
o Harmonic vibration
single frequency
Same loads that generated
90 mil peak to peak displacement
Sherlock - FEA Modifications
© 2004 - 2007© 2004 - 2010
o Vibration test coupon
o SMC (DO-214AB) diodes
o Displacement reduced from 1.1 mm peak (90 mils peak to peak) to
0.029 mm peak (2.3 mils p to p)
o Failures no longer occur
Sherlock - FEA Modifications
© 2004 - 2007© 2004 - 2010
o Initially driven by experiences during shipping and transportation
o Increasing importance with use of portable electronic deviceso A surprising concern for
portable medical deviceso Floor transitions (1 to 5 inch
‘drop’)o Environmental definitions
o Height or G levelso Surface (e.g., concrete)o Orientation (corner or face; all
orientations or worst-case)o Number of drops
Mechanical Shock
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Mechanical Shock (JEDEC)
JESD22-B110A, Subassembly Mechanical Shock
© 2004 - 2007© 2004 - 2010
Mechanical Shock Failures
o Failures related to mechanical shock typically cause:
o Pad cratering (A,G)
o Intermetallic fracture (B, F)
o This is an overstress failure (not fatigue)
o Random failure distribution
© 2004 - 2007© 2004 - 2010
Shock Prediction
o Sherlock implements Shock based upon a critical board level strain (similar to vibration)
o Either the design is robust with regards to the expected shock environment or it is not
o Additional work being initiated to investigate corner staking patterns and material influences
Shock Life
y = 1998.8x-0.39
y = 833.05x-0.23
200
250
300
350
400
450
500
550
600
1 10 100 1000
Drops to Failure
Adju
ste
d G
Le
vel
Staked
Unstaked
Power (Staked)
Power (Unstaked)
© 2004 - 2007© 2004 - 2010
Shock Simulations
o There are techniques that use simple spring mass approximation to predict the board deflection during a shock event
o FEA simulations are usually transient dynamic
o Sherlock utilizes an implicit transient dynamic simulation
o Shock pulse is transmitted through the mounting points into the board
o The resulting board strains are extracted from the FEA results and used to predict robustness under shock conditions
© 2004 - 2007© 2004 - 2010
CPU Card with DC/DC Converter
o 50G shock pulse
o Results in 12 mm deflection (severe)
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Shock Failure Predictions
o Excessive bending strains
Sherlock scoring on deformed plot
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o Two additional mounting points added mid-span
o Deflection drops from 12 mm to 1.65 mm
Model Modification Shock
Still some component failures
more support is needed
© 2004 - 2007© 2004 - 2010
o Board mounted to a chassis plate
Model Modification Shock
Presence of chassis reduces
board bending
© 2004 - 2007© 2004 - 2010
o Finite element based solutions to shock and vibration issues are necessary to adequately capture the complex mounting configurations and response of circuit card assemblies
o Displacement only techniques may miss critical board bending issues associated with shock and vibration
Conclusions
Thank You!