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© imec 2015
PHYSICS-OF-FAILURE BASED RELIABILITY-BY-DESIGN
GEERT WILLEMS – BART VANDEVELDE
IMEC-CEDM
29 JUNE 2015
PLOT MEETING
© imec 2015
CONTENT
1. Need for Design-for-Reliability innovation
The Physics-of-Failure approach
2. Physics-of-Failure based failure prediction
3. Physics-of-Failure based mitigation
4. Need for Design-for-Reliability innovation
Product Design-for-Reliability
5. Physics-of-Failure practice
6. The DfR projects
© imec 2015 3
1. NEED FOR DFR INNOVATION: POF
Definition of reliability:Probability that a product will perform its required function under stated conditions for a specific period of time.
“cEDM definition of reliability”:Reliability is the ability of the product to maintain it’s Quality under stated conditions for a specified period of time.
Quality definition• The properties of the product – whatever they may be –
agree to or exceed specifications or expectations.
• A non-quality issue is any property of the product that does not satisfy specifications or expectations.
© imec 2015 4
1. NEED FOR DFR INNOVATION: POF
Number of failures as a function of time or number of cycles:
The Bathtub Curve. (Ref: MIL-HDBK-338B)
Random failures
h(t)=f(t)/R(t): hazard or instantaneous failure rate.
Probability of failure (f(t)) at time t when no failure (R(t)) took place prior to t.
© imec 2015
1. NEED FOR DFR INNOVATION: POF
The traditional view on product reliability: no wear-out
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USE PERIOD
Time
Failu
re r
ate
Early
FailureUseful
Life Deco
mm
isionin
g
Wear-out
Ref: IEC 61163-1
Constant failure rate assessment using e.g. Fides, MIL-HDBK-217, ...
Assumption: testing 10 items 10000h = testing 1000 items 100h
Machinery, automotive, EEE, ...
© imec 2015
1. NEED FOR DFR INNOVATION: POF
▸ Wear-out mitigation or quantification.
Increasing failure rate:
▸ Cannot rely on constant failure rate models.
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Time
Failu
re r
ate
USE PERIOD
Early
Failure
Useful
Life
Deco
mm
isionin
g
Wear
out
Wear-out enters use period: electronics!
Wear-out:
10 items x 10000h
≠1000 items x 100h
© imec 2015
1. NEED FOR DFR INNOVATION: POF
How to quantify f(t), h(t), R(t)? Testing?
▸ To get meaningful results for f(t), testing to failure is
required.
▸ Long lifetime (>5 years) (too) long testing times
▸ Accelerated testing?
- What to test? Each new product? A test structure?
- How to accelerate? T, RH, mechanical, fast, slow, high, low,...?
- How to relate to operational conditions?
What is the acceleration factor?
- What is the impact of a design change?
▸ Cost? Time-to-market?
Is there an alternative to product testing?
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© imec 2015
1. NEED FOR DFR INNOVATION: POF
Physics-of-Failure (Wikipedia)
Physics of Failure is a technique under the practice of Design for Reliability that
leverages the knowledge and understanding of the processes and mechanisms that
induce failure to predict reliability and improve product performance.
Other definitions of Physics of Failure include:
A science-based approach to reliability that uses modeling and simulation to
design-in reliability. It helps to understand system performance and reduce
decision risk during design and after the equipment is fielded. This approach
models the root causes of failure such as fatigue, fracture, wear, and corrosion.
An approach to the design and development of reliable product to prevent failure,
based on the knowledge of root cause failure mechanisms. The Physics of Failure
(PoF) concept is based on the understanding of the relationships between
requirements and the physical characteristics of the product and their variation in
the manufacturing processes, and the reaction of product elements and materials
to loads (stressors) and interaction under loads and their influence on the fitness
for use with respect to the use conditions and time.
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© imec 2015
1. NEED FOR DFR INNOVATION: POF
Physics-of-Failure based
Quantification versus Mitigation
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Connection failure
Quantifiable with PoF:
Thermo-mechanical stress
Insulation failure
PoF:
How to avoid
© imec 2015
2. FAILURE PREDICTION
Step 1: Determine the “load”
Interconnection failures▸ Solder fatigue: plastic and creep deformation
▸ Via/track Cu fatigue: plastic deformation
▸ Solder-Ni interface failure: stress level & stress rate
Insulation failures▸ SIR, electromigration, CAF:
Applied voltage, moisture, ionic contamination.
▸ Corrosion: moisture, ionic contamination
▸ Creep corrosion: SO2, Cl2
▸ Whisker: internal film stress, temperature.
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© imec 2015
2. FAILURE PREDICTION
Step 2: Quantify the “load” – solder joints
Macroscopic forces due to:▸ Thermal cycling, CTE differences, dimensions, build-up
▸ Bending: vibration - shock
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PCB
component
© imec 2015
2. FAILURE PREDICTION
Step 2: Quantify the “load” – solder joints
Derive stresses and
(cyclic) strains
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Strain
Stress
Inelastic Energy density
Inelastic strain
time
tem
p
0oC
Hysteresis loop in cyclic testing
100oC
© imec 2015
2. FAILURE PREDICTION
Step 3: Lifetime prediction - Wöhler
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1
5
2
10
20
30
40 50
60 70
80
90 95
99
100 1000 10000
56 QFN Board Level Reliability
8x8 mm sq., 0.25 mm Pad Length
%
P
a
c
k
a
g
e
s
F
a
i
l
e
d
Number of Thermal Cycles
407.3513 7.0287 0.902 32/1
445.0654 6.5313 0.928 32/4
812.3991 4.6738 0.881 32/0
Eta Beta r^2 n/s
W/rr
0 to 100 C
-40 to 125 C
-40 to 125 C (45 deg)
Physics determining the acceleration factor
© imec 2015
2. FAILURE PREDICTION
Solder joint lifetime depends on▸ Component body Materials
CTE mismatch - stiffness
▸ Component build-up
Stiffness - Warpage
▸ Terminal shape – material - configuration
Force transfer - flexibility - deformation
▸ PCB materials and build-up
CTE mismatch – stiffness
▸ PBA fixation and component population
▸ Tmin, Tmax, dwell time, vibration
PoF required to correctly assess all reliability
determining factors14
© imec 2015
3. FAILURE MITIGATION
Not everything is quantifiable
Use PoF to determine a mitigation strategy:
Science based understanding of failure mechanism,
relationships and reaction to loads.
Example: corrosion - SIR
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© imec 2015
2Me −> 2Me++ + 4e-
3. FAILURE MITIGATION
Corrosion – SIR
Three elements needed:
▸ Closed electric path
- Electrical circuitry
- Ionic contamination
▸ Moisture: cathodic reaction
▸ Electrical potential
- Externally applied
- Galvanic couple
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© imec 2015
3. FAILURE MITIGATION
Closed electric path:
▸ No ionic contamination: cleanliness
▸ Encapsulate: no clean flux residu, coating, ...
▸ Block/isolate return path: solder mask, coating, ...
Moisture: hard to avoid
▸ Dry, hermetic, vacuum environment
▸ Not coating!
Electric potential
▸ Avoid large electric fields especially DC
▸ Be aware of galvanic couples
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© imec 2015
4. NEED FOR DFR INNOVATION: PRODUCT DFR
▸ Experience based DfR rules and (accelerated) testing
▸ Iterative
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Concept Design Production Operation
Experience basedDfR rules
Registration of failures
Traditional approach
Test
fai
lure
s
Prototype
Experience based(accelerated)
Lifetime testing
© imec 2015
4. NEED FOR DFR INNOVATION: PRODUCT DFR
Experience based DfR rules and (accelerated) testing are obsolete
because changes in electronic materials and package types of the
last decade have significantly changed the way electronic
interconnections fail and have a major (mainly negative) impact on
the product lifetime:
- Lead-free solder
- Low CTE mold compounds (packages)
- Low CTEz and high T260/T288 PCB laminates
- Lead-free terminal and PCB finishes
- Zero stand-off components
- Copper wire bonds
- ...
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© imec 2015
4. NEED FOR DFR INNOVATION: PRODUCT DFR
RELIABILITY - BY - DESIGN
▸ PoF based load level determination and modification by design
▸ Strength: qualified technologies, parts, materials, processes.
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© imec 2015
5. PHYSICS-OF-FAILURE IN PRACTICE
APPROACHES TO ASSESS RELIABILITY
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Analytical equations of
simplified structures
Experiments on test
structures IR measurement
CFD
spreadsheets
Simulations with advanced
tools (CFD, FEM)
© imec 2015
5. PHYSICS-OF-FAILURE IN PRACTICE
APPROACHES TO ASSESS RELIABILITY
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Time
<1 min
1 week
>1 month
Analytical equations of
simplified structures
Simulations with advanced
tools (CFD, FEM)
Experiments on test
structures
Level of acceptance
High
Medium
Low
© imec 2015
5. PHYSICS-OF-FAILURE IN PRACTICE
APPROACHES TO ASSESS RELIABILITY
23
Analytical equations of
simplified structures
Simulations with advanced
tools (CFD, FEM)
Experiments on test
structures
Output
Test sample fails after
1219 temperature cycles
Stress distribution shows
highest stresses in the
corner joints
The corner joint gets the
highest vertical tensile force,
due to the upward warpage
of the component
© imec 2015
5. PHYSICS-OF-FAILURE IN PRACTICE
CASE STUDIES
Bump fractures in flip chip assemblies after
solder assembly
Copper wire bond failures
Tilted assemblies of power LED’s
Reduced life time of QFN assemblies on
constrained PCB’s
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© imec 2015
5. PHYSICS-OF-FAILURE IN PRACTICE
SOLDERED FLIP CHIP ASSEMBLY
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center
No failures
All bumps
fractured
silicon
substrate
silicon
substrate
© imec 2015
5. PHYSICS-OF-FAILURE IN PRACTICE
SOLDERED FLIP CHIP ASSEMBLY
SAM study after solder assembly
All bumps were fractured
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© imec 2015
5. PHYSICS-OF-FAILURE IN PRACTICE
SOLDERED FLIP CHIP ASSEMBLY
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Fracture in BEOL
© imec 2015
5. PHYSICS-OF-FAILURE IN PRACTICE
SOLDERED FLIP CHIP ASSEMBLY
Analytical model calculates in each joint the shear
and normal forces and bending moments as function of
geometry, material properties and temperature loading
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PCB
component
© imec 2015
5. PHYSICS-OF-FAILURE IN PRACTICE
SOLDERED FLIP CHIP ASSEMBLY
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0.35 mm
Silicon: 169GPa, 2.6 ppm/°C
Substrate: 33GPa,
8 ppm/°C
(20 – 150°C)
center
2D-linear model single material
component
Application to practical case
© imec 2015
5. PHYSICS-OF-FAILURE IN PRACTICE
SOLDERED FLIP CHIP ASSEMBLY
30
The dominating parameter is the pitch, not the chip size !!!
Calculate bending
moment
(die size = 9 mm)
(die size = 13 mm)
(die size = 13 mm)
© imec 2015
5. PHYSICS-OF-FAILURE IN PRACTICE
CASE STUDIES
Bump fractures in flip chip assemblies after
solder assembly
Copper wire bond failures
Tilted assemblies of power LED’s
Reduced life time of QFN assemblies on
constrained PCB’s
31
© imec 2015
5. PHYSICS-OF-FAILURE IN PRACTICE
QFN ON CONSTRAINED PCB
FEM simulation to calculate the impact of the PCB
stiffness and its attachment to the casing.
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FEM of QFN assembly
2.4 mm 10 Cu layer board
casing
1.6 mm 4 Cu layer test board
Component supplier qualification
Box-build of OEM
© imec 2015
5. PHYSICS-OF-FAILURE IN PRACTICE
QFN ON CONSTRAINED PCB
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Step 1: Finite Element Model calculates the creep deformation per thermal cycle
max
min
Creep strain distribution
Step 2: Creep strain defines
the crack growth per cycle Step 3: Expected life time
𝐸𝑥𝑝𝑒𝑐𝑡𝑒𝑑 𝑙𝑖𝑓𝑒 𝑡𝑖𝑚𝑒
=𝐿𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑐𝑟𝑎𝑐𝑘 𝑓𝑟𝑜𝑛𝑡 𝑡𝑜 𝑓𝑎𝑖𝑙𝑢𝑟𝑒
𝐶𝑟𝑎𝑐𝑘 𝑙𝑒𝑛𝑔𝑡ℎ 𝑔𝑟𝑜𝑤𝑡ℎ 𝑝𝑒𝑟 𝑐𝑦𝑐𝑙𝑒
Most stressed joints
© imec 2015
6. THE DFR PROJECTS
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IWT O&O Rev-Up• Reliability testing
• Physics-of-Failure based
• Interconnection
• Surface Insulation Resistance
• “health monitoring”
ICON Compact• Physics-of-Failure based
reliability modeling
• Interconnection
• Selected components
• Time-dependent h(t) in
automotive et al. product
development.
VIS-traject InProVoL• DfR Guidelines
• DfR Tools
• Industrial implementation
• Consultancy
(See Prosperita) Start: 1/10/2015
© imec 2015
6. INPROVOL
INNOVATION TARGET
How to specify – design – produce – qualify an
intelligent product to assure quality of
operation for the given lifetime and mission
profile.
Project objectives
▸ Improved reliability
▸ Faster and lower cost product development
trajectory
▸ Reduced and predictable failure rate
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© imec 2015
6. INPROVOL
FUNDAMENTAL INNOVATION
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In product development
• PoF know-how• Models• Methods• Guidelines• Tools
• Design• Qualification• Supply chain
control
© imec 2015
6. INPROVOL
AD HOC CONSORTIUM
Become involved in InProVoL:
join the InProVoL ad hoc consortium
▸ Member of user committee: priority setting
▸ Use and early access to project results
▸ Contribution = cEDM partner/member fee
Contact: Geert Willems – Bart Cox
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