Plastic Package Reliability
C. Glenn Shirley
Integrated Circuits Design and Test Laboratory
Electrical and Computer Engineering
Portland State University
Portland, Oregon
http://web.cecs.pdx.edu/~cgshirl/
© C. Glenn Shirley
Outline
•
•
•
•
•
•
•
Plastic Packages
Stress and Test Flows
Thermal Mechanisms
Moisture Mechanisms
Thermo-mechanical Mechanisms
Moisture-mechanical Mechanisms
Technology Update
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
2
Plastic-Encapsulated Microcircuits
• The course covers plastic-encapsulated microcircuits.
• Molding compound (MC) in PEMs comes in direct
contact with the die and chip connections.
Single Layer
Lead Frame
Expoxy Based Molding
Compound
Copper Heat Slug
Tape/Adhesive
Dielectric
Metal Heat Sink Plate
Ag - Epoxy Adhesive
Ag - Epoxy Adhesive
BT Resin
Multilayer PCB
Solder Balls
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1.27mm Grid Array
Plastic Package Reliability
© C. Glenn Shirley
Epoxy Based Molding
Compound
Solder Balls
3
Plastic-Encapsulated Microcircuits
• Die is mounted on a lead frame (A42 or Cu).
Alloy 42
• Bonds are made by.
Fe 58% Ni 42% alloy
– Wirebond: Au or Al; mostly Au.
– TAB: Tape-Automated Bonding.
– C4: Controlled Collapse Chip Connect.
with CTE matching Si.
• Assembly is encapsulated in molding compound.
– Molding compound is in direct contact with die, wire
bonds, etc.
• External leads are trimmed and solder plated.
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Molding Compounds
• MC is thermoset (curing) epoxy, typically novolac.
– Cures at ~ 170-180C.
– Silica “filler” controls CTE and increases thermal
conductivity.
– MCs are (now) free of ionic contaminants.
• Glass Transition ~ 140C.
• Moisture properties of MC:
– Permeable to moisture.
– Absorbs moisture. “Hygroscopic.”
Note distinction between thermoset and thermoplastic.
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Molding Compound Properties
• At the glass transition (> 140 C)..
MC strength decreases
CTE increases
L. T. Manzione “Plastic Packaging of Microelectronic Devices,” Van Nostrand Reinhold 1990.
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Molding Compound Properties, ct’d
• Molding compound strongly absorbs water.
– Saturation uptake is proportional to RH, and independent
of temperature.
– Rate of uptake depends strongly on temperature.
L. T. Manzione “Plastic Packaging of Microelectronic
Devices,” Van Nostrand Reinhold 1990.
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Outline
•
•
•
•
•
•
•
Plastic Packages
Stress and Test Flows
Thermal Mechanisms
Moisture Mechanisms
Thermo-mechanical Mechanisms
Moisture-mechanical Mechanisms
Technology Update
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
8
Life of an Integrated Circuit
Shipping
OEM/ODM
Assembly
Storage
End-user Environment
Temp
Assembly
DT
Shipping Shock
Temperature
Cycle
Bend
Reflow
Handling
Temp, RH
Power Cycle
Bent Pins, Singulation
Temp
Desktop
DT
BAM!
Shipping Shock
Mobile PC
Temperature
Cycle
Bend
Reflow
Handling
Temp, RH
Bent Pins, Singulation
User Drop Power Cycle
& Vibe
DT
Shipping Shock
Handheld
8/2-4/2011
Temperature
Cycle
Bend
Reflow
Handling
Plastic Package Reliability
Source:© Eric
Monroe, 2003
C. Glenn Shirley
Temp, RH
BAM!
User Drop
& Vibe
Keypad
press
9
Slide by Scott C. Johnson.
Stress/Test Flow
From: JEP150 “Stress-Test-Driven Qualification of and
Failure Mechanisms Associated with Assembled Solid
State Surface-Mount Components” (JEDEC)
Simulate shipping, storing, and
OEM board mounting process.
Preconditioning
8/2-4/2011
Environmental Stress
Simulate in-service end use
conditions.
Electrical Test, Failure
Analysis
Determine pass/fail. Diagnose
failures.
Plastic Package Reliability
© C. Glenn Shirley
10
Industry Reliability Standards
• International
– ISO International Standard Organization
– IEC International Electrotechnical Commission
• Europe
– CEN, CENELEC Comite Européen de Normalisation
Électrotechnique
• Japan
– JIS Japanese Industrial Standard, EIAJ
• US
– MIL (US Department of Defense), EIA/JEDEC
For US commercial products JEDEC standards have mostly superseded MIL standards.
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Preconditioning
Preconditioning
Environmental Stress
Electrical Test, Failure
Analysis
• Preconditioning simulates board assembly.
• Specified by
– JESD22-A113F Preconditioning of Nonhermetic Surface
Mount Devices Prior to Reliability Testing.
– IPC/JEDEC J-STD-020D.01 Moisture/Reflow Sensitivity
Classification for Nonhermetic Solid State Surface Mount
Devices
http://www.jedec.org/
Free downloads, registration required.
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Preconditioning
Environmental Stress
Electrical Test, Failure
Analysis
Preconditioning Flow
JESD22-A113F
8/2-4/2011
Slide 50
IPC/JEDEC J-STD-020D.01
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Preconditioning
Environmental Test
Environmental Stress
Electrical Test, Failure
Analysis
• Stress-based testing (traditional).
To keep things simple, we’ll
follow this approach.
– Standards: JESD47, MIL-STD-883.
– Pro:
• Well-established standards. Lots of historical data. Good for
comparisons.
• Little information about mechanism or use is required.
– Con:
• Overstress: May foreclose a technology.
• Understress: Misses a mechanism.
– May not accurately reflect a use environment.
• Knowledge-based testing.
– Risk assessment of Use and Mechanism guides test.
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Preconditioning
Knowledge-Based Test
Environmental Stress
Electrical Test, Failure
Analysis
• Knowledge-Based Testing
– Stress depends on knowledge of
use conditions and mechanisms.
– Risk assessment, using methods
Use Condition
Mechanism
Risk Assessment
Stresses
• JEDEC: JESD94, JEP143, JEP148
• Sematech: “Understanding and Developing Knowledge-based
Qualifications of Silicon Devices” #04024492A-TR
– Pro:
• Stresses fit the product and its use. May make a product feasible.
– Con:
• Easy to miss mechanisms in new technologies and overlook use
conditions in new applications.
• Tempting to misapply to “uprate” devices, relax requirements.
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Preconditioning
Environmental Stress
Electrical Test, Failure
Analysis
JESD47 Example
Note: TS (JESD22-A106) and AC (Steam) (JESD22-A102)
are not recommended. Very different from use.
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Preconditioning
Example Stress Flow
Environmental Stress
Electrical Test, Failure
Analysis
ELFR
Early life failure rate. Assumptions
168 hrs
168 h is equivalent to early life requirement
JESD22-A108
SS computed from goal. eg. 3x611 = 1833
Focus of this course.
Preconditioning.
JESD22-A113
PC
(Lots x Units)
3 x 77
3 x 25
3 x 25
3 x 25
HTOL
HTSL (Bake)
TC
THB or HAST
High temperature
operating life.
168 -1000 hrs
JESD22-A108
1000 hrs
JESD22-A103
700 cycles
3 cycles/hr
233 hrs
JESD22-A104
1000 hrs (85/85)
JESD22-A101
96 hrs (130/85)
JESD22-A110
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Condition B or G
(C is too severe)
Plastic Package Reliability
© C. Glenn Shirley
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Sampling
• JESD47 sample requirements are minimal.
– Single “snapshot” is a crude validation of the reliability of
the product.
– Small SS does not generate failures to give clues to process
weaknesses.
• Risks.
– Qualification hinges on single failures.
• Moral hazard to “invalidate” a failure is high.
– Lot-to-lot variation, excursions.
• Incoming materials, fab lots, assembly lots, test lots.
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Preconditioning
Sampling, ct’d
Environmental Stress
Electrical Test, Failure
Analysis
• Number of lots covers risks of machine-to machine,
day-to-day, etc. variation.
• Often minimum SS to validate a goal is chosen.
– Pro: Saves $, and there are no failures to explain.
– Con: Pass/fail of the qual is at the mercy of a single failure.
• Verrry tempting to invalidate a failure.
– Con: No mechanism learning.
(Accept/Reject  0/1)
• eg. To validate 500 DPM at ELFR using minimum SS at
60% confidence, 1833 units are required.
– 500 DPM is a typical goal (see ITRS)
Useful “mental furniture”
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Plastic Package Reliability
© C. Glenn Shirley
 ln(1  cl )
D
 ln(1  0.6)
1833 
500  106
SS 
19
Preconditioning
Sampling, ct’d
Environmental Stress
Electrical Test, Failure
Analysis
• For environmental stress, 77 is a typical SS, why?
SS 
 ln(1  cl )  ln(1  0.9)  ln(.1) 100 230.26
;


 76.7
2
D
3 10
3
3
– So, 0/1 accept/reject validates, to 90% confidence, a
failure rate less than 3% at the accelerated condition.
• If the stress corresponds to the lifetime (eg. 7 years)
then the average wearout failure rate is
3 102
FR 
 109  489 Fits
7  365  24
– This falls into the range of ITRS goals.
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Preconditioning
Environmental Stress
Electrical Test, Failure
Analysis
Reliability Goals from ITRS 2009
http://www.itrs.net/reports.html
Infant Mortality
Wear-out
Constant fail rate
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Example Data
• Additional attributes
enhance value of
data.
–
–
–
–
–
Device type
Date code
Package type
Readouts
Failure analysis
Maxim product reliability report RR-1H
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Preconditioning
Test and Failure Analyis
Environmental Stress
Electrical Test, Failure
Analysis
48 HOUR TEST WINDOW
• Moisture-related tests
(85/85, HAST, Steam)
have specific test/FA
reqt’s.
Functional Test
Pass
Fail (unvalidated failure)
Validate Using Functional Test
Invalid Failure
Pass
(Censor sample or
return to stress)
Fail
Diagnostic Functional Test
Including raster scan, electrical analysis
Pass
for electrical location of defect
– Test must be done while
unit contains moisture.
Fail by Invalid Mechanism
All Other Failures
(eg. package defect
in die-related certification)
• Within 48 hr.
VALID FAILURE
– Units must not be “wet”.
• Wet = liquid water.
Bake + Diagnostic Functional Test
Fail
Irreversible Damage
Pass
Reversible
(eg. corrosion)
• Diagnostic Bake.
• Risk decision before
destructive analysis.
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Return to Stress
(eg. leakage path thu moisture films)
Non-Destructive F/A
(visual, X-Ray, CSAM)
Risk Decision
(based on likely mechanism)
Chemical Decapsulation
(to observe mechanical defects
such as cracks)
Plastic Package Reliability
© C. Glenn Shirley
Mechanical Decapsulation
(to observe chemical defects
such as residues)
23
HAST versus 85/85
• Moisture MUST be non-condensing. (RH < 100%).
• Both require about < 1% fail. Typical SS ~ 100.
• 130/85 HAST duration is 10x less than 85/85
duration. We’ll see how this was justified.
6 week bottleneck in
• 85/85 (JESD22-A101)
information turns.
• HAST (JESD22-A110)
10x less time
8/2-4/2011
33.3  14.2
 1.34 atmospheres  Pressure vessel required.
14.2
Plastic Package Reliability
© C. Glenn Shirley
1 atm = 14.2 psi
24
Example: Comparison of HAST Stds
Intermittent Bias
Test and Failure Analysis Window
From Sony Quality and Reliability Handbook
8/2-4/2011
http://www.sony.net/Products/SC-HP/tec/catalog/qr.html
Plastic Package Reliability
© C. Glenn Shirley
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HAST System
• Large vessel (24” dia) requires forced convection.
C. G. Shirley, “A New Generation HAST System”, Non-proprietary Report to Intel, Despatch Industries, and MicroInstrument Corp. describing learnings during development of NG HAST. December, 1994.
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HAST System
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HAST System
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HAST Development Team
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HAST Ramp Up Requirements
• Ramp Up: Avoid condensation on load.
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HAST Ramp-Down Requirements
• Mechanical pressure relief must be slow (3 h).
• Vent when pressure reaches 1 atm.
• Hold RH at test value.
– Units must retain moisture acquired during test.
~ 3.5 atm
1 atm
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Vapor Pressure and Relative Humidity
Log scale
10
4
Coexistence of liquid and vapor
Q = 0.42 eV
2
1
.4
Pressure
(Atm) .2
Liquid Water
.1
Water Vapor
.04
.02
.01
200
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160
120 100 80
60
40
20
Temperature in deg C (Arrhenius Scale)
Plastic Package Reliability
© C. Glenn Shirley
0
Linear in 1/T (K),
with ticks placed at
T (C).
32
Vapor Pressure and Relative Humidity
Actual water vapor pressure at temperature T
H
Saturated water vapor pressure at temperature T
or
PH2O  H  Psat (T )
Important.
What is Psat?
klM
 lM 
 QP 
  P0 exp
 QP 
Psat (T )  P0 exp
 0.42 eV
 RT 
 kT 
R
Where
l = 2262.6 joule/gm (latent heat of vaporization)
M = 18.015 gm/mole, R = 8.32 joules/(mole K), k = 8.617x10-5 eV/K
This is a fair approximation. The main benefit is physical insight.
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Vapor Pressure and Relative Humidity
An accurate formula for Psat (in Pascals) is
 3

1
n
Psat (T )  1000  exp  an  x , x 
 n 0

273  T ( C)
a0  16.033225, a1  35151386
.
 103 ,
a2  2.9085058  105 , a3  5.0972361 106
which is accurate to better than 0.15% in the range 5 C< T < 240 C.
1 atm = 101325 pascals
This is an accurate formula.
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© C. Glenn Shirley
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Vapor Pressure and Relative Humidity
• Relative humidity at “hot” die in steady state.
– Partial pressure of water vapor is the same everywhere:
PH2O (die)  PH2O (ambient )
– So RH at die is given by:
H(die)  Psat (Tambient  DTja )  H(ambient )  Psat (Tambient )
– Where the ratio, h is defined as:
H(die)  h  H(ambient )
Psat (Tambient )
h
Psat (Tambient  DTja )
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Vapor Pressure and Relative Humidity
1.00
0
2
0.80
4
0.784
6
0.60
8
R
10
0.40
15
20
30
40
0.20
0.00
0
20
40
Curves labelled with Tja
60
80
100 120
T(ambient)
140
160
180
200
Example: At 20/85 and Tja = 4 C, the die is at 24/(0.784x85) = 24/67.
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Outline
•
•
•
•
•
•
•
Plastic Packages
Stress and Test Flows
Thermal Mechanisms
Moisture Mechanisms
Thermo-mechanical Mechanisms
Moisture-mechanical Mechanisms
Technology Update
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
37
Focus Topic: Acceleration
• Acceleration between two stresses is the ratio of
times (or cycles) to achieve the same effect.
• The “same effect” could be the same fraction failing.
– eg. The ratio of median (not mean!) times to failure in
different stresses is the Acceleration Factor (AF).
• AF is proportional to 1/MTTF
AF (2 |1) 
 Q
MTTF1
 exp 
MTTF2
 k B
 1 1  
  
 T1 T2  
Thermal
AF (2 |1) 
 Q
MTTF1
 exp 
MTTF2
 k B
 1 1  
    exp C V2  V1 
 T1 T2  
Thermal and Voltage
 Q  1 1  
MTTF1  a  bV2   RH 2 
AF (2 |1) 


 exp      Moisture ("Peck")
MTTF2  a  bV1   RH1 
 k B  T1 T2  
m
MCTF1  DT2 
AF (2 |1) 


MCTF2  DT1 
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m
Thermal Cycle ("Coffin-Manson")
Plastic Package Reliability
© C. Glenn Shirley
38
Thermal Mechanisms
ELFR
Early life failure rate.
168 hrs
JESD22-A108
PC
Preconditioning.
JESD22-A113
HTOL
HTSL (Bake)
TC
THB or HAST
High temperature
operating life.
168 -1000 hrs
JESD22-A108
1000 hrs
JESD22-A103
700 cycles
3 cycles/hr
233 hrs
JESD22-A104
1000 hrs (85/85)
JESD22-A101
96 hrs (130/85)
JESD22-A110
8/2-4/2011
Condition B or G
(C is too severe)
Plastic Package Reliability
© C. Glenn Shirley
39
Gold-Aluminum Bond Failure
• Gold and Aluminum interdiffuse.
– Intermetallic phases such as AuAl2 (“Purple Plague”) form.
– Imbalance in atomic flux causes Kirkendall voiding.
– Bromine flame retardant is a catalyst.
• Kirkendall voids lead to
– Bond weakening - detected by wire pull test.
– Resistance changes in bond - detected by Kelvin
measurement of bond resistance.
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Thermal (Ordinary) Purple Plague
Cross-section of gold ball bond on aluminum pad
after 200 hours at 160C
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© C. Glenn Shirley
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Gold-Aluminum Bond Failure
Log scale
1 year (!)
(8760h)
1E4
1E3
100
Hours
10
1
0.1
Source: S. Ahmad, Intel
380 340 300 260
220 200 180 160
Temperature (deg C, Arrhenius Scale)
Linear in 1/T
(K), with ticks
placed at T (C).
Arrhenius plot of time to 10% of wire pull failure.
Activation energy (Q) = 1.17 eV.
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Gold-Aluminum Bond Failure
• Kelvin resistance
measurements.
Log{ Resistance Change (milliohms)}
1.4
• Resistance increase of
1.2
Au bonds to Al pads vs
bake time.
1.0
• Bake at 200 C.
• Various levels of Br
flame-retardant in
molding compound.
• Br catalyzes Au-Al
intermetallic growth.
• Br flame retardants
are being phased out
today.
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2.7
1.45
0.8
0.5
0.8
0.6
Weight % Bromine
0.4
Source: S. Ahmad, et al. “Effect of Bromine in
Molding Compounds on Gold-Aluminum Bonds,”
IEEE CHMT-9 p379 (1986)
0.2
0
20
40
60
80
100
Bake Time, hours
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120
140
Thermal Degradation of Lead Finish
• Only an issue for copper lead frames (not Alloy 42).
• Cu3Sn or Cu6Sn5 inter-metallic phases grow at the
interface between solder or tin plating.
• Activation energy (Q) for inter-metallic phase growth
is 0.74 eV.
• If inter-metallic phase grows to surface of solder or
tin plate, solder wetting will not occur.
• Main effect is to limit the number of dry-out bakes of
surface mount plastic components.
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Thermal Degradation of Lead Finish
Solder
Solder
Lead
Cu6Sn5 intermetallic
Copper
Post-plating solder plate
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Copper
Post burn-in solder plate showing
copper-tin intermetallic
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© C. Glenn Shirley
45
Thermal Degradation of Lead Finish
X-section of solder-plated lead
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X-section of solder-coated lead
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Outline
•
•
•
•
•
•
•
Plastic Packages
Stress and Test Flows
Thermal Mechanisms
Moisture Mechanisms
Thermo-mechanical Mechanisms
Moisture-mechanical Mechanisms
Technology Update
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
47
Example Stress Flow
ELFR
Early life failure rate.
168 hrs
JESD22-A108
PC
Preconditioning.
JESD22-A113
HTOL
HTSL (Bake)
TC
THB or HAST
High temperature
operating life.
168 -1000 hrs
JESD22-A108
1000 hrs
JESD22-A103
700 cycles
3 cycles/hr
233 hrs
JESD22-A104
1000 hrs (85/85)
JESD22-A101
96 hrs (130/85)
JESD22-A110
8/2-4/2011
Condition B or G
(C is too severe)
Plastic Package Reliability
© C. Glenn Shirley
48
H2O Diffusion/Absorption in MC
MODEL
Surface exposed to ambient.
Zero flux or "die" surface
2L
Surface exposed to ambient.
ACTUAL
Plastic Molding Compound (MC)
L
A
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H2O Diffusion/Absorption in MC
Diffusion Coefficient:
Saturation Coefficient:
 Q 
D  D0 exp   d 
 kT 
See slide 13.
D0  4.7 105 m 2 / sec Qd  0.50 eV
Q 
S  S0 exp  s 
 kT 
S0  2.76 104 mole/m3 Pa Qs  0.40 eV
Source: Kitano, et al IRPS 1988
Henry’s Law:
Msat  PS  HPsat S
See slide 33.
H2O vapor pressure.
Key Observation: Saturated moisture content of molding compound
is nearly independent of temperature, and is proportional to RH.
M sat
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 (Qs  Qp ) 
 HP0 S0 exp 

kT


Plastic Package Reliability
© C. Glenn Shirley
Qs  Qp  0.02 eV
Nearly zero!
50
H2O Diffusion/Absorption in MC
1.00
0.80
Total Weight Gain (M)
(C-Cinit)/(Ceq-Cinit)
or
0.60
(M-Minit)/(Meq-Minit)
0.40
Concentration at Die Surface (C)
0.20
0.8481
0.00
0.00
0.20
0.40
0.60
0.80
Fourier Number = Dt/L^2
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
51
1.00
Response to Step-Function Stress
Typical Use Saturation Times
1K
Hours
Package Moisture
Time Constant
DIP, PLCC (50 mils)
400
200
100
(time to 90% saturation)
PQFP (37 mils)
t (sat ) 
20
10
TSOP (12 mils)
4
 Qd 
L2
0.8481 exp

D0
 kTmc 
L
2
1
220 180
140
Typical 130/85 HAST Saturation
Times
8/2-4/2011
130
100
60
40
20
T (deg C)
Normal Operating Range
Plastic Package Reliability
© C. Glenn Shirley
52
Cyclical Stress
One-Dimensional Diffusion Equation Solutions
8 hours on, 16 hours off cycling for PDIP
Moles/m3
Moles/m3
540
460
380
0
Cj
20 40
50
60
Hours
520
0 440
360
mils
280
0
T(ambient) = 100 C
t(sat) = 28 hours
0
mils
20 40
50
60
Hours
T(ambient) = 60 C
t(sat) = 153 hours
Moisture concentration at the die is constant if Period << t(sat)
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
53
Peck’s Acceleration Model
• Fundamental environmental parameters are T, H and
V, at the site of the failure mechanism.
– If the die is the site, this is denoted by “j”.
• A frequently used acceleration model is due to Peck
AF  (a  b V )  H mj  exp( Q / kT j )
• Find a, b, m, Q from experiments with steady-state
stress and negligible power dissipation.
• Typically a is small or zero: Bias is required.
• Requires H > 0 for acceleration: Moisture is required.
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
54
Moisture: MM Tape Leakage
Leadframe
Ground Plane
Power Plane
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
55
Moisture: MM Tape Leakage
MM Leakage vs 156/85 Biased HAST Time
1000
156/85 HAST of MM Tape Candidates
0.15
800
"A"
0.1
600
Leakage
mA
400
1/MTTF
(hrs)
0.05
200
"B"
50
0
10
20
30
40
Biased HAST Stress Time (HR)
0
1
2
3
4
5
Bias (Volts)
6
7
8
Experimental Tape Data:
Acceleration factor is proportional to bias.
Tape m
Q eV
“A”
>12
0.74
AF 
“B”
5
0.77
Constant  V  H m exp(Q / kT )
Source: C. Hong, Intel, 1991
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
56
Moisture: Internal Metal Migration
• TAB Inter-lead Leakage/Shorts
– Accelerated by voltage, temperature and humidity
– Seen as early as 20 hrs 156/85 HAST
– Highly dependent on materials & process
+
Copper dendrites after 40 hours of biased 156/85 HAST
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
57
Lead-Stabilizing Tape Leakage
•
•
•
•
•
A vendor process excursion.
Leakage observed after 336 hours of steam.
Re-activated by 48 hours at 70C/100% RH
No leakage seen between leads not crossed by tape
Rapid decay for leads crossing end of tape
– Tape dries from exterior inwards
Die
8/2-4/2011
Lead Stabilizing Tape
Plastic Package Reliability
© C. Glenn Shirley
Tape provides mechanical stability
to long leads during wirebond.
58
Lead-Stabilizing Tape Leakage
1E-5
Pins Crossed by Tape
1E-6
1E-7
Leakage
Current (A) 1E-8
Pin 2-3
Pin 3 -4
1E-9
1E-10
Pins NOT crossed by tape
1E-12
1E-13
10
8/2-4/2011
100
1000
Recovery Time (Minutes)
Plastic Package Reliability
© C. Glenn Shirley
Source: S. Maston, Intel
59
Aluminum Bond Pad Corrosion
AF 
Fe++
Lead Corrosion
Microgap
Ni++
Constant  V  H m exp(Q / kT )
Cl-
Moisture
Source
m
Peck (a)
2.66
Hallberg&Peck (b) 3.0
Moisture
Q (eV)
0.79
0.9
(a) IRPS, 1986; (b) IRPS, 1991.
Pins
1
Source: P.R. Engel, T. Corbett, and W. Baerg, “A
New Failure Mechanism of Bond Pad Corrosion in
Plastic-Encapsulated IC’s Under Temperature,
Humidity and Bias Stress” Proc. 33rd Electronic
Components Conference, 1983.
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
Shortest path has highest
failure rate.
60
Passivation in Plastic Packages
• Passivation is the final layer on the die.
• Passivation has two main functions:
– Moisture Barrier
•
•
•
•
Molding compound is not a moisture barrier.
Silicon oxides are not good moisture barriers.
PECVD silicon nitride or silicon oxynitride film is a good barrier.
Film must be thick enough to avoid pinholes, coverage defects.
– Mechanical Protection
• Silicon nitride films are brittle.
• Polyimide compliant film protects silicon nitride.
• Polyimide can react with moisture (depending on formulation).
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
61
Polyimide/Au Bond Failure
• Bonds overlapping passivation don’t necessarily
violate design rules.
• But can activate polyimide-related “purple plague”
failure mechanisms in combination with moisture.
• Acceleration modeling showed no field jeopardy.
Polyimide
Metal Bond Pad
PECVD Oxynitride/Nitride
Other films
Silicon
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
62
Wire Bond Pull Test
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
63
Moisture-Related Gold Bond Degrad’n
Effect of 80 hours of 156/85 HAST vs 156/0 Bake
and Centered vs Off-Centered Bonds on Wire Pull Test Data
99.99
99.9
99
90
HAST
OFF-CENTER
70
50
%30
10
HAST
CENTERED
1
BAKE
CENTERED
0.1
BAKE
OFF-CENTER
0.01
0
2
4
6
8
10
12
Pull Force (gm)
14
16
18
20
Source: G. Shirley and M. Shell, IRPS, 1993
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
64
Moisture-Related Gold Bond Degrad’n
Wire Pull Strength of Polyimide vs No Polyimide and Centered
vs Off-Centered Bonds after 40 hours of 156/85 HAST
99.99
99.9
99
90
70
50
%30
POLYIMIDE
OFF-CENTER
10
1
0.1
0.01
0
NO-POLYIMIDE
CENTERED: SQUARE
OFF-CENTER: TRIANGLE
8
10
12
14
16
18
20
Pull Force (gm)
POLYIMIDE
CENTERED
2
4
6
Source: G. Shirley and M. Shell-DeGuzman, IRPS, 1993
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
65
Moisture-Related Purple Plague
Cross-section of gold ball bond on aluminum pad
after 80 hours at 156C/85%RH
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
66
Moisture-Related Gold Bond Degrad’n
121/100
135/85
b  112.7 gm
17hr 156/85 + x 156/65
156/85
a0  113
.  1010 (gm - hrs) -1
100
m  0.98; Q  115
. eV
80
F50
F50 
(gm)
40
1
(at ) 2  1 / b 2
a  a0  h m  exp( Q / kT )
FP  F50  exp(   Z P )
20
20
40
100
200
Hours
400
1E3
2E3
  0.17
Source: G. Shirley and M. Shell-DeGuzman, IRPS, 1993
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
67
Circuit Failure Due to Passiv’n Defects
10m
Site of failing bit. SRAM after HAST stress.
Courtesy M. Shew, Intel
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
68
Circuit Failure Due to Passiv’n Defects
2m
2m
Etch-decorated cross-section of passivation. Note growth seams.
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
69
Circuit Failure Due to Passiv’n Defects
SRAM VOLTAGE THRESHOLD MAP FOR CELL PULLUP TRANSISTOR
(Baseline threshold is 0.89 V. Passivation is 0.6 m nitride, no polyimide.)
Vthreshold
Vthreshold
Row
Row
Column
Column
2 bits recover after further 2 hr
bake at 150 C
After 120 h 156/85. 4
failed bits with Vt > 2.5 V
Source: C. Hong, Intel
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
70
Acceleration Model Fit of HAST Data
SRAM HAST and 85/85 Bit Failures (No Polyimide)
90
85/85, standby
140/85, standby
140/85, no bias
156/85, standby
156/85, active
70
50
%
30
156/85, no bias
Model: 85/85 standby
Model: 140/85 standby
Model: 140/85 no bias
Model: 156/85 standby
Model: 156/85 active
Model: 156/85 no bias
10
1
0.5
30
100
Hours
1E3
1E4
AF  Constant  ( a  bV )  H m  exp( Q / kT )
a  0.24 b  014
.
m  4.64 Q  0.79 eV
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
Notes:
• “Standby” = 5.5V bias, low power
• “Active” = 5.5V bias, high power
• 156/85: non-standard, limit of
pressure vessel.
• Source: C. G. Shirley, C. Hong. Intel
71
Peck Model Parameters
Mechanism
MM Tape A
MM Tape B
Single Bit SRAM
Corrosion, THB (early Peck)
Corrosion, THB (later Peck)
Bond Shear
Q(eV)
0.74
0.77
0.79
0.79
0.90
1.15
m
12
5
4.6
2.66
3
0.98
Q/m
0.06
0.15
0.17
0.30
0.30
1.17
Q/m <
0.42eV?
Yes
Yes
Yes
Yes
Yes
No
Hours of
130/85 
1kh 85/85 Reference
69
2
62
2
57
3
57
4
39
5
16
6
Yes/No: Increasing power dissipation at die, slows/accelerates the moisture mechanism.
1.
2.
3.
4.
5.
6.
8/2-4/2011
Kitano, A. Nishimura, S. Kawai, K. Nishi, "Analysis of Package Cracking During Reflow Soldering Process," Proc.
26th Ann. Int'l Reliability Physics Symposium, pp90-95 (1988)
S. J. Huber, J. T. McCullen, C. G. Shirley. ECTC Package Rel. Course, May 1993. Package tape leakage
acceleration data courtesy C. Hong.
G. Shirley and C. Hong, "Optimal Acceleration of Cyclic THB Tests for Plastic-Packaged Devices," in Proc. 29th
Ann. Int'l Reliability Physics Symposium, pp12-21 (1991)
S. Peck, "Comprehensive Model for Humidity Testing Correlation," in Proc. 24th Ann. Int'l Reliability Physics
Symposium, pp44-50 (1986).
Hallberg and D. S. Peck, "Recent Humidity Accelerations, A Base for Testing Standards," Quality and Reliability
Engineering International, Vol. 7 pp169-180 (1991)
G. Shirley and M. Shell-DeGuzman, "Moisture-Induced Gold Ball Bond Degradation of Polyimide-Passivated
Devices in Plastic Packages," in 31st Ann. Int'l Reliability Physics Symposium, pp217-226 (1993).
Plastic Package Reliability
© C. Glenn Shirley
72
HAST versus 85/85, ct’d
• For all mechanisms surveyed, 1000 hours of 85/85 is
equivalent to < 96 hr of 85/85.
• For packages < 10 mils covering die, moisture
saturation occurs within 10 h at 130/85.
• For Tj-Ta > 10C, most mechanisms (with Q/m < 0.42
eV) can be more accelerated with cyclical bias.
• 85/85
– JESD22-A101
• HAST
– JESD22-A110
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
73
Steady Power Dissipation
• Superimpose log(H) vs 1/T contour plots of
– Peck model for THB acceleration factor.
– Partial pressure of water vapor, Psat.
• Contours are straight lines:
– Peck model: Iso-acceleration contours with slope
proportional to Q/m.
– Psat: Isobars with slope proportional to Qp = 0.42 eV.
• Depends on: In steady state, the partial pressure of
H2O is the same in the ambient and at the die.
Reference: C. G. Shirley, “THB Reliability Models and Life Prediction for IntermittentlyPowered Non-Hermetic Components”, IRPS 1994
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
74
Steady Power Dissipation
Iso-acceleration contours for example mechanism
(m = 4.6, Q = 0.8 eV) superimposed on water vapor pressure isobars.
5 atm 4 3 2
100
100K
1 atm
0.1 atm
0.01atm
1K
X
Typical Climate
RH (%)
at Die
Hj
Increasing steady-state
dissipation (X to Y).
Follows isobar.
100
(log scale)
10
40
1
Y
30
180
8/2-4/2011
140
100 80 60 40
Tj at Die (deg C)
Relative slope
Q/m < 0.42 eV: Deceleration
Q/m > 0.42 eV: Acceleration.
0.01
20
0
(Arrhenius (1/TK) Scale, marked with TC)
Plastic Package Reliability
© C. Glenn Shirley
75
Example: Comparison of HAST Stds
Non-steady-state requirements
From Sony Quality and Reliability Handbook
8/2-4/2011
http://www.sony.net/Products/SC-HP/tec/catalog/qr.html
Plastic Package Reliability
© C. Glenn Shirley
76
Time-Varying Power Dissipation
• Most moisture-related mechanisms..
– Have slow or zero rates at zero bias (V=0). AF  V  H  exp(Q / kT )
– Have Q/m < 0.42 eV so, in steady-state, depressed die
humidity due to power dissipation slows the rate.
m
j
• There is an optimum duty cycle which maximizes the
effective acceleration.
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
77
j
JEDEC 85/85 and HAST Req’ts
• Tj-Ta 10C: 100% duty cycle.
• Tj-Ta > 10C, 50% duty cycle.
Effective
Acceleration
Factor
Assumptions:
• 85/85
• Peck Model m = 4.64, Q = 0.79 eV
• AF = 0 for V = 0.
• MC thickness 50 mils
• Kitano et. al MC properties.
Effective
Acceleration
Factor
Tj – Ta = 20C
G. Shirley and C. Hong, "Optimal Acceleration of Cyclic THB Tests for Plastic-Packaged
Devices," in Proc. 29th Ann. Int'l Reliability Physics Symposium, pp12-21 (1991)
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
78
Outline
•
•
•
•
•
•
•
Plastic Packages
Stress and Test Flows
Thermal Mechanisms
Moisture Mechanisms
Thermo-mechanical Mechanisms
Moisture-mechanical Mechanisms
Technology Update
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
79
Thermomechanical Mechanisms
ELFR
Early life failure rate.
168 hrs
JESD22-A108
PC
Preconditioning.
JESD22-A113
HTOL
HTSL (Bake)
TC
THB or HAST
High temperature
operating life.
168 -1000 hrs
JESD22-A108
1000 hrs
JESD22-A103
700 cycles
3 cycles/hr
233 hrs
JESD22-A104
1000 hrs (85/85)
JESD22-A101
96 hrs (130/85)
JESD22-A110
8/2-4/2011
Condition B or G
(C is too severe)
Plastic Package Reliability
© C. Glenn Shirley
80
Key Material Properties
• Material properties which drive temperature cyclinginduced failure mechanisms.
Material
Thermal Coeffic’t
of Expansion
(ppm/°C) “TCE”
Young's
Modulus
(GPa)
Thermal
Conductivity
(W/m °C)
Copper
Alloy 42
Silicon
Molding Compound
17
5
3
21
119
145
131
18
Alumina
6.5
25
25
15-17
11
25
PC Board
8/2-4/2011
Match!
Plastic Package Reliability
© C. Glenn Shirley
398
15 Low!
157
0.6
81
Cracking Due to Temperature Cycle
Bond and TFC damage
Void & cracks
Shear
Stress
Crack
With crack
Without crack
Zero
Normal
Stress
Die Edge
With crack
Zero
Without crack
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
82
Crack Propagation in Test Conditions
• Tensile Test of Notched Samples
– Measure crack growth rate for sinusoidal load:
Crack
Load
a
Load
Notch
– Sample geometry and load determine stress intensity
factor, K.
– Plot crack growth rate da/dN versus K on log-log plot to
determine Coffin-Manson exponent, m:
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
83
Crack Propagation in Test Conditions
1E-1
Encapsulant A
1E-2
Slope of lines on log-log plot
Crack Growth Rate
da/dN
1E-3
mm/cycle
1E-4
25C
da
m
 Const  DK 
dN
150C
-55C
m  20
1E-5
Source: A. Nishimura, et. al. “Life Estimation for IC
Packages Under Temperature Cycling Based on
Fracture Mechanics,” IEEE Trans. CHMT, Vol. 10,
p637 (1987).
1E-6
0.5
1
2
3
Stress Intensity Factor Amplitude
K (MPa m)
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
84
Crack Propagation in Package
• The rate of crack propagation is also given by
da
m
 Const  DK 
dN
• But in plastic packages under temperature cycling,
Important
the stress concentration factor is
DK  Const  ( moldingcompound   silicon )  Tmin  Tneutral 
• α is the TCE of MC.
DT in temperature cycling-driven models is the temperature difference
between the neutral (usually cure) temperature, and the minimum
temperature of the cycle. Tmax is less important.
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
85
Package Cracking and Delamination..
..damages Wires, Bonds, and Passivation Films.
Wire Shear
Normal (tensile)
Crack
Shear
Silicon
Au
Substrate Damage
Thin-Film Cracking
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
86
Bond Damage: Wires and Ball Bonds
• Cracks can intersect wires, TAB leads.
• Bonds can be sheared at the bond/pad interface
• Shear and tensile normal stress can break wires at
their necks.
• Substrate cracks induced during bonding can
propagate and cause “cratering” or “chip-out”.
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
87
Wire Damage
(Open)
Wires sheared by wire crack
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
88
Bond Damage
Sheared Bond
20 m
Unfailed Bond
Die Corner
Ball bonds in plastic package after temperature cycle.
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
89
Bond Damage
Necking Damage
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
90
Bond Damage
Necking fracture
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
91
Bond Damage
Delamination induced down bond fail after temperature cycle
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
92
Bond Damage
Cratering damage on bond pads
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
93
Bond Damage
Die
Bond shear at die corners after temperature cycle
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
94
Bond Damage and Delamination
A8
Intermetallic fracture at bond due to
shear displacement.
A7
Pulse-echo acoustic image of mold compound/ die
interface in four devices. Delamination is shown in
black. White boxes added to show locations of low
bond wire pull strength results.
8/2-4/2011
44 PLCC devices that failed after solder
reflow and 1000 cycles (-40A9
to 125C)
Source: T.M.Moore, R.G. McKenna
and S.J. Kelsall, IRPS 1991, 160-166.
Plastic Package Reliability
© C. Glenn Shirley
95
Bond Damage (TAB)
Ti barrier
Cu Lead
Cu Lead
Au bump
Au bump
Etch
Al pad
Crater
Substrate Crack
Barrier crack and Au/Al intermetallic
Silicon
TAB cratering and diffusion barrier damage revealed by wet etch.
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
96
Bond Damage (TAB)
TAB bonds Au/Al intermetallic formed at cracks in Ti barrier
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
97
Bond Damage (TAB)
Crater under TAB bonds
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
98
Thin-Film Cracking (TFC)
Wire Shear
Normal (tensile)
Crack
Shear
Silicon
Au
Substrate Damage
Thin-Film Cracking
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
99
TFC - Plastic Conforms to Die Surface
Die Surface
Replica in Plastic
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
100
TFC – Effect on Thin Films
Die Corner
Aluminum
Polysilicon
Cracks
A
B
Die center
A
Shear stress applied to passivation
Aluminum
Passivation
Crack
Crack
B
PSG
SiO2
Channel
8/2-4/2011
Polysilicon
Plastic Package Reliability
© C. Glenn Shirley
101
Thin-Film Cracking (TFC)
1 micron
Source: K. Hayes, Intel
Passivation delamination crack propagates into substrate.
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
102
Test Chip
• Thin film cracking can be detected electrically by test
structures in the corner of the die.
– Sensitive to opens and to shorts.
• Buss width is varied to determine design rule.
Die edge (saw cut).
TFC sensor
Edge ring
TFD
TFC (20 um bus)
TFC (40 um bus)
Metal 5
Metal 4
Via 4
BOND PADS
ATC
Minimum
Minimum
X
Minimum
TFC (10 um bus)
TFC (100 um bus)
100 microns
X = 10, 20, 40, 100 microns
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
103
TFC – Effect of Buss Width
Factors Affecting TFC: Buss Width Effect
Polysilicon
meanders under
buss edge are
vulnerable to
crack-induced
opens.
No Contacts
17 m contacts
Buss Widths
3 m contacts
21 m
7m
105 m
Source: Shirley & Blish, “Thin Film Cracking and Wire Ball Shear...,” IRPS 1987.
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
104
TFC – Effect of Buss Width, ct’d
Factors Affecting TFC: Buss Width Effect
70
50
30
% Fail
105 micron bus, no slots or contacts
10
5
2
1
Busses with slots and/or contacts
.1
10
100
Cycles
1000
Narrow buss, or contacts, stabilizes buss, reduces incidence of TFC.
Leads to buss width design rules, and buss slotting in die corners.
8/2-4/2011
Plastic Package Reliability
© C. Glenn Shirley
105
TFC – Effect of Temperature Cycle
• Drivers: T/C conditions, and number of cycles.
• Mimimum T/C temperature, not amplitude, is key
aspect of stress.
– Stress depends on difference between cure temperature
(neutral stress) and minimum stress temperature.
95
80
60
Same amplitude!
-65 C to 150C
-40 C to 85 C
0 C to 125 C
125 C Amplitude
Source: C. F. Dunn and J. W. McPherson, “TemperatureCycling Acceleration Factors for Aluminum Metallization
Failure in VLSI Applications,” IRPS, 1990.
40
Cum %
Fail 20
10
5
2
1
8/2-4/2011
10
100
Cycles
1000
Plastic Package Reliability
© C. Glenn Shirley
106
TFC – Effect of Passivation Thickness
• Fraction of PDIPpackaged SRAM failing.
100
Source: A. Cassens, Intel
80
• Post 1K cycle of T/C C.
• No Polyimide die coat.
• Thicker passivation is
more robust.
60
%
Failing 40
20
0
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0.5 0.6 0.7 0.8 0.9 1.0 1.1
Total Passivation Thickness in microns
Plastic Package Reliability
© C. Glenn Shirley
107
TFC – Effect of Compliant Overcoat
• SRAM in PDIP
• Temperature Cycle Condition C
• Polyimide Overcoat
200 cycles
500 cycles 1000 cycles
No Polyimide
0/450
13/450
101/437
Polyimide
0/450
0/450
0/450
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108
Theory of TFC
• Shear stress applied to die surface by MC
– Is maximum at die corners
– Zero at die center.
(At die Surface)
Die
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109
Theory of TFC, ct’d
• Buss width effect: Okikawa et. al.
• Passivation thickness effect: Edwards et al.
• TFC occurs when and where
2
t
 ( Passivation Surface)  K  E   
 L
•
•
•
•
K = dimensionless constant
E = Young’s modulus of passivation
t = Passivation thickness
L = Buss width
Thicker passivation, and/or narrower busses implies less TFC.
Sources: Okikawa, et al. ISTFA, Oct. 1983. Edwards, et al. IEEE-CHMT-12, p 618, 1987
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110
Outline
•
•
•
•
•
•
•
Plastic Packages
Stress and Test Flows
Thermal Mechanisms
Moisture Mechanisms
Thermo-mechanical Mechanisms
Moisture-mechanical Mechanisms
Technology Update
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111
Thin Film Delamination
• Saw cut exposes thin film edges to moisture..
Polyimide
Metal Bond Pad
PECVD Oxynitride/Nitride
Other films
Silicon
• And shear stress tends to peel films.
Shear stress
Thin film delamination.
Moisture
Thin films
Substrate (Si)
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112
Test Chip
• “Edge rings” are lateral moisture barrier.
• Effectiveness of edge rings can be tested electrically
by a TFD sensor on a test chip.
Die edge (saw cut).
Edge ring
Cross section of TFD sensor
TFD
TFC (20 um bus)
TFC (40 um bus)
thin film#7
BOND PADS
ATC
TFC (10 um bus)
TFC (100 um bus)
thin film#6
thin film#5
thin film#4
thin film#3
thin film#2
thin film#1
substrate
interconnect
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Plastic Package Reliability
© C. Glenn Shirley
via
113
Thin-Film Delamination
Edge ring
TFD Sensor
Source: C. Hong
Intel
Delamination at die edge after 168 hours of steam.
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© C. Glenn Shirley
114
Thin-Film Delamination
Edge Ring
Crack breaks
continuity of
TFD sensor
10 m
Source: C. Hong
Intel
Delamination at die edge after 168 hours of steam.
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115
Popcorn Mechanism
ELFR
Early life failure rate.
168 hrs
JESD22-A108
PC
Preconditioning.
JESD22-A113
HTOL
HTSL (Bake)
TC
THB or HAST
High temperature
operating life.
168 -1000 hrs
JESD22-A108
1000 hrs
JESD22-A103
700 cycles
3 cycles/hr
233 hrs
JESD22-A104
1000 hrs (85/85)
JESD22-A101
96 hrs (130/85)
JESD22-A110
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Condition B or G
(C is too severe)
Plastic Package Reliability
© C. Glenn Shirley
116
Popcorn Mechanism
a
t
Moisture Absorbtion
During Storage
Moisture Vaporization
During Solder
Pressure Dome
Bond Damage
Delamination Void
Pressure in Void = P
Plastic Stress Fracture
Package Crack
Collapsed Voids
Plastic package cracking due to “popcorn” effect during solder reflow
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117
Popcorn Damage
Plastic package cracking due to “popcorn” effect during solder reflow
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© C. Glenn Shirley
118
Popcorn Damage
B-Scan
line
A8
SEM of cross section
Pulse-echo acoustic image through
back of 68PLCC that developed popcorn
A7 cracks during solder reflow
Source: T.M.Moore, R.G. McKenna and S.J. Kelsall,
in “Characterization of Integrated Circuit Packaging
Materials”, Butterworth-Heinemann, 79-96, 1993.
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Plastic Package Reliability
© C. Glenn Shirley
A9
Acoustic B-scan
119
Popcorn Damage
Pulse-echo acoustic
image through top
(delamination in black)
Acoustic time-of-flight
image indicating
package crack
132 lead PQFP which was damaged
during solder reflow.
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Real-time x-ray image showing
deformation in wires where
they intersect the crack
Source: T.M.Moore, R.G. McKenna and S.J. Kelsall,
in “Characterization of Integrated Circuit Packaging
Materials”, Butterwoth-Heinemann, 79-96, 1993.
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© C. Glenn Shirley
120
Factors Affecting Popcorning
•
•
•
•
•
Peak temperature reached during soldering.
Moisture content of molding compound.
Dimensions of die paddle.
Thickness of molding compound under paddle.
Adhesion of molding compound to die and/or lead
frame.
• Mold compound formulation.
Not on this list: Pre-existing voids in the plastic package.
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121
Popcorn Model
• A crack propagates to the surface when maximum
bending stress max exceeds a fracture stress
characteristic of the molding compound
 max   crit (Treflow )
• crit depends on MC formulation, and on
temperature (see next slide).
Source: I. Fukuzawa, et. al. “Moisture Resistance Degradation of Plastic LSIs by
Reflow Solder Process,” IRPS, 1988
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122
Popcorn Model, crit
100
Source: Kitano, et al.
IRPS, 1988
80
Molding Compound
Strength (MPa) 60
40
20
0
50
100
150
200
Temperature (deg C)
250
crit is proportional to molding compound strength.
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123
Popcorn Model, max
• Maximum bending stress occurs at center of long
edge of die and is given by:
2
 max
–
–
–
–
a
 6 K   P
t
a is the length of the die edge.
t is the thickness of the molding compound over the die.
K is a geometrical factor (K = 0.05 for square pad).
P is the internal pressure. Depends on
• Moisture content of molding compound.
– Depends in turn on RH and T of previous soak ambient.
• Peak temperature during reflow.
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124
Popcorn Model, Internal Pressure
Impermeable
Surface
Cavity
Molding Compound
Ambient
m ( x, t )
Before
Shock (T0)
After
Shock (T1)
Time = t
 (t )
x = -l
x=w
m ( x , t )  Concentration Profile in Molding Cpd.
 (t )  Moisture Concentration in Cavity
Pcav (t )  R  T1   (t )  Cavity Pressure
m 0  H0  Psat (T0 )  S (T0 )  Initial Moisture Conc. In Mold. Cpd.
For t  0 the boundary condition at x  0 is:
Pcav (t ) 
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m (0, t )
S (T1 )
Plastic Package Reliability
© C. Glenn Shirley
Henry’s Law
125
Popcorn Model, Internal Pressure
 T1 S1   2  c  exp( n2 f ) sin[ n (1   )]
m ( , f )
 1
 1  
m0
( n2  c 2  c) sin  n
 T0 S 0  n1
 S1    2  ( n2  c 2 ) exp( n2 f ) sin[ n (1   )] 

  1  1  
2
2
( n  c  c) n
 S 0   n1

where c 
RT1S1
and  n are roots of  tan   c
Dt
x
and f  12 (Fourier Number);  
w
w
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Plastic Package Reliability
© C. Glenn Shirley
Ref. H. S. Carslaw and J.C. Jaeger,
“Conduction of Heat in Solids,”Oxford
2nd ed. (1986) pp128-129.
126
Popcorn Model, Internal Pressure
Water Concentration Profile
Cavity = 0.05 mm, Precond = 25/85, Tsolder = 215 C
Package Thickness = 0.60 mm
Cavity Water Vapor
Concentration
Moisture conc. in MC
>>
Moisture conc. in cavity.
0.1
350
250
150 Mole/m^3
50
1
Time 10
(sec)
100
1000
-0.05 0.15 0.35 0.55 (mm)
0 (Mold cpd/cavity interface)
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127
Popcorn Model, Internal Pressure
S (T0 )
Pcav 

l 0; w  ( or t 
0) H0  Psat (T0 ) 
S (T1 )
Delamination pressure exists even with no physical void.
Example:
•
Unit preconditioned in 85/85 for a long time,
then subjected to 215 C solder shock.
•
Saturation coefficient has activation energy of
0.4 eV. (eg. Kitano et. al.)
•
Steam table pressure at 85 C is 0.57 atm.

0.40 eV
1
 1



 0.85  0.57  exp

-5
 8.62  10 eV/ K  273  85 273  215 
Pcav
 15.3 Atmospheres
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Wow!!
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© C. Glenn Shirley
128
“Popcorn” Design Rules
400
Package Cracking
300
Pad Size, a
(mils) 200
No Package Cracking
100
a/t = 4.5
0
0
10
20 30 40 50 60 70
Plastic Thickness, t (mils)
80
Cracking sensitivity of PLCC packages after saturation in 85/85
followed by vapor-phase reflow soldering at 215 oC
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129
Outline
•
•
•
•
•
•
•
Plastic Packages
Stress and Test Flows
Thermal Mechanisms
Moisture Mechanisms
Thermo-mechanical Mechanisms
Moisture-mechanical Mechanisms
Technology Update
8/2-4/2011
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© C. Glenn Shirley
130
Package Anatomy
OEM’s heat sink
OEM’s TIM
Intel’s TIM
Made
by
Intel
Silicon chip
Integrated Heat Spreader (IHS)
Underfill
C4 bumps
A “land”
Socket pin
Substrate
Land Grid Array connectors
OEM’s socket
• This is an example of a packaged part as it might be
used in a product
Slide: Scott C. Johnson
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131
Package Anatomy
Silicon chip
C4 bumps
Conductive traces
and vias (yellow)
Polymer insulating
layers (green)
Substrate core
A “land”
OEM socket
pin
• This is a close-up of the package substrate showing
the many layers of conductors and insulators
Slide: Scott C. Johnson
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132
Location: Silicon (vs. Package)
“Back end” = interconnects
“Front end” = transistors
Interlayer
dielectric
(ILD)
Channel
Metal
interconnect
Source
Gate
Gate
Oxide
Drain
Le
Via
Transistor
Slide: Scott C. Johnson
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