moisture diffusion in molding compounds and quality assurance of pems for space applications...
TRANSCRIPT
Moisture Diffusion in Molding Compounds and Quality
Assurance of PEMs for Space Applications
Alexander Teverovsky
QSS Group, Inc./Goddard Operations
MRQW Dec '02 2
No humidity in space. Why moisture concerns?
We need to assure that no moisture related
failures occur during the ground phase
integration and testing period (2 to 5 years).
Quality assurance strategy for PEMs: Moisture prevention Adequate qualification testing
MRQW Dec '02 3
Quality assurance strategy for PEMs for military and space applications
Moisture prevention strategy: Military applications: Wafer Applied Seal for PEM Protection (WASPP);Space applications: Virtual monitoring/simulation of the moisture level variations and baking of components and assemblies.
Adequate testing: Military applications: To assure reliability for 15 to 20 years of storage and operation in harsh humid environments.Space applications: To assure reliability for 5 years maximum in controlled laboratory conditions.
MRQW Dec '02 4
Purpose and Outline
The purpose is to discuss:
Moisture characteristics of MCs and how they can be used for the moisture prevention strategy and for development of adequate testing.The relevance of HAST for PEMs intended for space applications.
Outline:
Part I:• Bake-out conditions for PEMs;• Diffusion characteristics of molding compounds .
Part II:• Acceleration factors of HAST. • What might be an adequate testing?
MRQW Dec '02 5
Part I. Moisture Diffusion Characteristics and Their Measurement
The first step in any MC or PEM degradation process is moisture
diffusion. The characteristic times of
diffusion are important for implementing the moisture prevention strategy.
MRQW Dec '02 6
Characteristic times of moisture diffusion in plastic encapsulated parts
0
0.2
0.4
0.6
0.8
1
0.001 0.01 0.1 1 10
time, t/
C/C
o, M
/Mo
C/Co
M/Mo
Moisture concentration at the die surface and mass losses of a flat package with rated
baking time.
At t = C/Co=0.1,
M/Mo=0.06
Bake-out time:
(T) = h2/D(T)
Bake-out time:
(T) = h2/D(T)
2h – package thickness
The master curve for moisture diffusion
MRQW Dec '02 7
Diffusion characteristics of epoxy encapsulating materials
180 140 120 100 80 60 40 20 T, oC
1.E-14
1.E-13
1.E-12
1.E-11
1.E-10
1.E-09
2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8
reciprocal temperature, 1000/K
D, m
2 /s
{1}{3}{4}{5}{6}{7}{7}{8}HCEMEpoliset
U = 0.43 eV )/exp( kTUDD o )/exp( kTUDD o
Averaged characteristics of MC:
Do = 7.3510-6 m2/sec
U = 0.43 eV
Averaged characteristics of MC:
Do = 7.3510-6 m2/sec
U = 0.43 eV
Data reported in literature.
D85 varies ~ 10 times
MRQW Dec '02 8
Calculated bake times at 125 oC and JEDEC recommendations
Three body thickness groups per IPC/JEDEC J-STD-033A, July 2002:
<1.4 mm; <2 mm;< 4.5 mm
•Ignoring real size of the parts causes significant errors.
•JEDEC is focused on SMT reflow soldering and might be not adequate for moisture control purposes.
•Ignoring real size of the parts causes significant errors.
•JEDEC is focused on SMT reflow soldering and might be not adequate for moisture control purposes.
Note: •2a and 5a are levels
of moisture sensitivity.•part saturated at
30 oC/85% RHIPC-SM-786A, ’95: < >2 mm
0
10
20
30
40
50
60
70
80
0 1 2 3 4 5
package thickness, mm
Ba
ke
tim
e, h
r
JEDEC_2a
JEDEC_5a
calculation
MRQW Dec '02 9
In-situ non-isothermal technique for D(T) measurements
Areas of application Moisture sorption simulation (virtual monitoring of
moisture level). Calculation of bake-out conditions. Lot characterization of molding compound.
(ROBOCOTS: need rapid assessment methods) Evaluation of moisture leaks along the leads of a
plastic package. Development of adequate moisture stress testing
(HAST alternative).
A. Teverovsky, “A Rapid Technique for Moisture Diffusion Characterization of Molding Compounds in PEMs“, http://nepp.nasa.gov
MRQW Dec '02 10
Part II. Highly Accelerated Stress Test (HAST)
Do we need to use same environmental stress testing as for PEMs intended for
harsh humid conditions?
MRQW Dec '02 11
Accelerated moisture resistance tests
JESD22-A102-C (unbiased autoclave):
121 oC, 100% RH, 24 to 336 hrs (96 hr typical).
JESD22-A118 (unbiased HAST): Cond. A: 130 oC, 85% RH, 96 hrs. Cond. B: 110 oC, 85% RH, 264 hrs.
JESD22-A110-B (biased HAST): Cond. A: 130 oC, 85% RH, 96 hrs. Cond. B: 110 oC, 85% RH, 264 hrs.
Typical HAST conditions: bias at 130 oC, 85% RH96 hrs per JESD22-A110-B.150 hrs per NAVSEA SD18 (Part Requirement & Application
Guide).500 hrs per PRF38535 spec. (for technology characterization).
NASA projects: 250 hrs?
Equivalent to 1000
hours at 85 oC and 85% RH
MRQW Dec '02 12
HAST failure mechanisms
Package levelCorrosion of the leads.
Dendrite formation (on the surface and inside packages).
Swelling/shrinkage:Delaminations;Solder ball failures in PBGA
and flip-chip technology;warpage of large packages;parametric shifts in linear
devices.
-400
-200
0
200
400
0 20 40 60 80 100
relative humidity, %d
V/V
, pp
m
MRQW Dec '02 13
HAST failure mechanisms
Die level:Corrosion of Al metallization.Dendrites between
metallization lines.Leakage currents.Charge instability (lateral, ion
drift, hot electron).
1.E-12
1.E-10
1.E-08
1.E-06
1.E-04
0 2 4 6 8 10 12 14 16 18
gate voltage, V
gat
e cu
rren
t, A
1.E-12
1.E-10
1.E-08
1.E-06
1.E-04
1.E-02
1.E+00
-2.5 -1.5 -0.5 0.5 1.5 2.5
Vg, V
IDS
, A
Curtsy of C.Greenwell
HEXFET failures
Mechanisms of moisture induced
parametric degradation require additional analysis
Mechanisms of moisture induced
parametric degradation require additional analysis
MRQW Dec '02 14
What is the HAST acceleration?
Life time at 25 oC/ 60% ambient conditions
equivalent to HAST: 130oC/85% 100 hrs)
0
100
200
300
400
500
600
700
10 20 30 40 50 60 70
device temperature, oC
life
time,
yea
rs
0
10
20
30
40
50
60
70
80
90
effe
ctiv
e R
H, %
Best-fit model:S. Peck [86];Hallberg and Peck [91];S. Tam [95]; SSB-1 [2000];J. Sweet [02]:
kTERHAt an
f exp)(
Life time increases with device temperature
Life time increases with device temperature
Ea = 0.8 eV; n = 2.7Ea = 0.8 eV; n = 2.7
)(
)(
devs
asaeff TP
TPRHRH
0.7 < Ea < 1.1 eV;
1 < n < 5
MRQW Dec '02 15
Test results on linear devices
Part DC Pack.type
QTYtested
QTYfaile
d
Failuremode
Vref 0112 SOIC8 31 3
1- increased Vout
2 - catastrophic
Opamp
0101 SOIC16 30 2 -
Instr. amp 0033 SOIC8 30 30 Excessive
input currents
FET1 0041 SOT223 29 28 IDS, VGTH
FET2 0040 D2Pak 30 29~ 60%
parametric shift
HAST: 130 oC, 85% RH, 250 hours
11 part types out of 25 failed HAST
11 part types out of 25 failed HAST
MRQW Dec '02 16
Is HAST adequate for normal conditions?
Testing temperature (130 oC) is above the operational range for many parts.
Degradation of molding compound: Decrease of Tg (up to 100 oC in resins and up to
30 oC in MCs). Enhanced creep. Decrease of the tensile stress and adhesion.
The model accelerates mostly corrosion failures, but corrosion is no longer a prime concern.
Moisture assisted hot-carrier degradation mighthave U < 0.
MRQW Dec '02 17
Irreversible changes in MCs: An example
00.20.40.60.8
11.21.41.61.8
bake 85 deg.C 110 deg.C 130 deg.C 150 deg.C bake
mas
s/vo
lum
e de
viat
ion,
%
MC1_m
MC1_v
MC2_m
MC2_v
bake: 125 deg.C for 96 hr
dM
dV
Test: moisture uptake and swelling after HAST were measured on two epoxy molding compounds.
Conditions: saturation with moisture at 85% RH and different temperatures.
Additional mechanical stresses due to swelling: AE[(MC - LF)T + m], At MC LF mechanical stresses are due only to moisture sorption.
An increase in volume might cause delaminations and mechanical failures.
MRQW Dec '02 18
Do we need moisture testing?
Ionic dissociation in polymer: Charge carriers are impurity ions generated by dissociation of a salt MAThe equilibrium constant: no is the concentration of salt molecules; f is the fractional degree of dissociation
AMAM
f
nf
AM
AMK
1
][][ 02
kT
UKK
0
0 expIn a medium with low : Uo is the energy of ions separation in vacuum
kT
UAn
2exp 0 2. Another reason:
Moisture activates ionic impurities similar to temperature. Moisture
testing might accelerate degradation mechanisms related to charge
instability.
2. Another reason:Moisture activates ionic impurities
similar to temperature. Moisture testing might accelerate degradation
mechanisms related to charge instability.
w
mcwmcmcmc m
~
~0
1. Moisture concentration
at the die surface at RT is approximately the same as at
high T/RH conditions.
1. Moisture concentration
at the die surface at RT is approximately the same as at
high T/RH conditions.
MRQW Dec '02 19
A possible alternative to HAST: MS + HTB
Soaking in humidity
chamber for 48 hrs at
110 oC and 85% RH
Note: C/Co = 1 corresponds to the equilibrium moisture saturation at 100% RH.
Testing at 85 oC.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.01 0.1 1 10 100
time, hr
C/C
o,
M/M
o C/Co soaking
M/Mo soaking
C/Co testing
M/Mo testing
Moisture evolution in a 2 mm thick package
MRQW Dec '02 20
Calculated test time and equivalent time of operation
0
5
10
15
20
25
30
60 70 80 90 100 110
test temperature, oC
test
tim
e, h
r
0
1
2
3
4
5
6
7
equ
ival
ent
time,
yea
r
Soaking conditions:RH = 95%,T = 110 oC,
t = 30 hr
Test conditions:RHeff 90%
70 oC < T < 105 oCOperating conditions
(environment):RH =50%T = 20 oCPackage thickness 2 mm
MRQW Dec '02 21
HAST alternative: MS + HTB testing
Input: Package size; D(T) data; ground phase conditions; maximum storage and operation temperatures.
Set RH and T for MS conditions
MS + HTB Test conditionsMS + HTB Test conditions
C(t) simulation
Estimate MS time
Set T for HTB conditions
Calculate A and
C(t) simulation
Estimate HTB time
Is A & suitable?
Algorithm for calculation of MS+HTB testing conditions
+-
MRQW Dec '02 22
ConclusionSuggested quality assurance strategy: moisture content control (virtual monitoring and baking) + adequate testing.The moisture prevention strategy can be implemented by assessment of moisture content and calculations of the bake-out conditions based on D(T) data. The existing HAST conditions are too harsh for PEMs intended for space applications. A possible alternative to HAST might be MS + HTB testing. Additional analysis of moisture induced parametric degradation and acceleration factors is necessary.