3D Localization by Lock-in Thermography - eufanet

3D LOCALIZATION BY
LOCK-IN THERMOGRAPHY
EUFANET – Toulouse – November 2011
Shermin Danaie - Master1 EEATS at Joseph Fourier University Grenoble
Quentin Saulnier – STMicroelectronics Grenoble – Failure Analysis
Arnaud Loubaresse – STEricsson Grenoble – Failure Analysis

Purpose
Increasing functionalities in
mobile phone
System In Package (SIP)
NEW CHALLENGES:
∙ Defect localization in 3D (X,Y,Z) mainly when dices are
stacked
∙ Non destructive methods
ONE SOLUTION:
∙ Use lock-in thermography for Z location of a defect
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Outline
• Lock-in thermography principle
• Thermography equipment
• Trial outline and analysis cases
• Modeling for simple cases
• Z measurements with Hamamatsu software
• Conclusion & outlook
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LOCK-IN THERMOGRAPHY
PRINCIPLE

Lock-in thermography principle
•Two images (0° & -90°) are converted as below:
Amplitude:
Phase shift:
∙ Phase shift is about propagation time of the wave between
defect and sample surface
∙ Z localization can be theorically found
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THERMOGRAPHY EQUIPMENT

Thermography equipment
∙ Themos 1000 from Hamamatsu is used:
∙ InSb detector, max efficiency between 3.7µm to 5.2µm
∙ Examples of images for the 3 infrared lenses (Germanium):
∙ CSP product, silicon thickness around 350µm
0.8X, FOV 12mm*9mm
4X, FOV 2.4mm*1.8mm 15X, FOV 0.64mm*0.48mm
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TRIAL OUTLINE & ANALYSIS
CASES

Trial outline
•REF unit reverse biased Diode connected to GND is polarized in
direct.
•Reference value for phase shift measurement.
•Comparison of phase shift values between REF reverse biased and
FAIL units.
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Analysis case: back to back device
Problem description and aim of analysis:
bumps
Digital die
wire bonding
Analog die
∙ Case 1: Short-circuit on digital signal
∙ Case 2: Short-circuit on analog signal
TRIAL OUTLINE:
∙ Z localization of defect by Lock-in thermography
∙ Physical characterization of defect
∙ Effect of each layers on phase shift
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Case 1
REF reverse biased, digital die:
Frequency
(Hz) 10 5 1
Phase ( ) 180 124 60
Failing unit:
Frequency
(Hz) 10 5 1
Phase ( ) 188 131 68
Hypothesis: Defect is located at digital die level
Themos Localization Package Layout X-ray Image
Defect is on wire bonding but Z is very close to die
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REF reverse biased, analog die:
Frequency
(Hz) 10 5 1
Phase ( ) 229 172 88
Case 2 (1)
Failing unit:
Hypothesis: Defect is located at analog die level
Frequency
(Hz) 10 5 1
Phase ( ) 240 180 100
Effect of each material on phase shift (F lock-in = 10Hz)
100° (thickness: around 140µm)
50° (Process ~10µm)
0° (thickness: silicon 320µm,
glue)
Conclusion:
• Process and resin have a great influence on phase shift
• Silicon and glue have a low influence on phase shift
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Case 2 (2)
Checking the hypothesis:
Thermography of analog
die after polishing
OBIRCH image
Optical microscope
∙ Analog die is broken
∙ Remark:
• Phase shift of thermal signal at defect level is not null, it’s
around 35°
• This is called Initial Phase Shift
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MODELING FOR SIMPLE CASES

Thermal wave propagation inside a solid
• For isotropic and homogeneous material, Phase shift of thermal
wave between defect (z=0) and surface (z=l)
z=l
Where
µ (in mm) is called thermal diffusion length, and is depending on:
• Thermal conductivity (λ en W/m*K)
• Thermal capacity (cp en J/g*K)
• Density (ρ en g/cm³)
• Lock-in frequency (f en Hz)
z=0
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Phase shift (°)
Phase shift vs Z localization
φ(f lock-in) for resin and silicon:
Silicon
Resin
Silicium
Résine
Frequency (Hz)
• As seen during trials:
• Resin has a great influence on phase shift
• Silicon has a low influence on phase shift
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Checking modeling for Case 1
• Localization of defect is close to resin thickness: Z=140µm
• Resin is the only material on top of defect
λ (W/m*K) cp (J/g*K) ρ (g/cm³) μ (mm)
Theorical
phase shift (°)
at f=1Hz
Experimental
phase shift
(°)
at f=1Hz
0,98 1,43 2.03 0.32 25 68
But initial phase shift has to be take into consideration, it’s around
35°.
Theorical: Φ=25° Z=140µm
Trial: Φ=68-35=33° Z=184µm
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Z MEASUREMENTS WITH
HAMAMATSU SOFTWARE

Z measurements with Hamamatsu software
• Need also to know characteristics below:
• Thermal conductivity (λ en W/m*K), Thermal capacity (cp en
J/g*K) & Density (ρ en g/cm³)
Wire bonded device with 450µm of resin
• Works fine for simple cases
• Trials performed on 3D
packages but accuracy is not
good enough
Measured:
448µm
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CONCLUSION & OUTLOOK

Conclusion
• Knowledge on:
• Lock-in frequency influence on measurements
• Low frequencies are suitable for Z localization while high
frequencies are better for X/Y localization
• Material influence on phase shift
• Z localization improvement:
• Relation between Z and phase shift is proven
• Experimental outline is in place
• Modeling and Z measurements are working for simple cases
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Outlook
• To developed:
• What about Z localization on others 3D packages like face to
face packages
• Need to work on package description, means to know very
well Thermal conductivity, Thermal capacity & Density (ρ en
g/cm³)
• Create a database with material characteristic
• Keep strong collaboration with Hamamatsu
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THANK YOU