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Special packaging featur
Pb-fr
e: IC packaging and inter
ee solder creep-fatigue r
connection technologies’ 4th dimension challenge
eliability models updated and extended
Werner Engelmaier
“The data bases for LF-solders have grown, and the favored LF-solders are shifting.
Nevertheless, progress has been made.”
Pb-free solder creep-fatigue
reliability models updated
and extended
It is almost one year ago that a solder creep- still likely that these parameters will see Thus, the modification of the basic fatigue
fatigue reliability model for SAC405/305 slight adjustments once data showing the ductility exponent, m, by the temperature-
Pb-free solders was first published in this independent influences of temperature and time-dependent parameters accounts
column
1
. More information—but far from a and creep time on creep-fatigue life are for the effects of low temperatures and/or
sufficient data base—has become available available. short dwell times resulting in incomplete
in the meantime for SAC405 and SAC305 These five parameters, represent the creep processes.
as well as other Pb-free solders. Further following physical attributes/processes: The model parameters in Table 1
insights into the creep-fatigue behavior of for SAC 205, SAC105 and SnAg Pb-
Pb-free solders have been gained. Thus, • ε’ , the fatigue ductility coefficient, is free solders result from considering the
f
it is now possible to propose models for dependent on the true ductility of the information provided in Figures 1 through
SAC 205, SAC 105 as well as SnAg Pb-free solder; 3.
solders; it also has become evident that the • c
0
is the basic connection between the Please note that much of the data being
proposed model in Reference 1 needs some creep-fatigue process and the resulting shown leaves many test details unreported—
correction. cycles to failure; these are shown as “?”. Attribution of this
All these models are based on the • c
1
is a scaling factor reflecting the data is withheld to protect the guilty.
Engelmaier-Wild creep-fatigue model for temperature dependence of the creep As seen in Figure 4, the difference
eutectic and near-eutectic SnPb solders
2,3
. rates; between SnPb and SAC405/305 is small
In its general format, this model takes the • c
2
is a scaling factor reflecting the time for the higher shear strains more common
form dependence of the completeness of the for accelerated tests; for conditions more
creep process; typical for product operations with proper


'

m
• t
0
represents the time for an essentially ‘Designs-for-Reliability,’ SAC405/305
N
complete creep process at about 50°C; solder joints will fail earlier. In Figures 4
f
(
50%
)
=
1
f
2

ΔD

the shorter t
D
the less complete is the through 6 the difference in the resulting
 
creep process. fatigue lives under accelerated test
conditions and product operating
for which the creep-fatigue ductility
exponent, m, is given by
Solder Model Parameters
1
ε
f
'
c
0
c
1
c
2
t
0




t
= c c
0



1
T
SJ
+ c
2
n 1+

t

m
0
+
D

Engelmaier-Wild Creep-Fatigue Model for SnPb Solders
SnPb 0.325 0.442 6.00e-04 -1.74e-02 360
where ε’ = fatigue ductility coefficient;
f
T
Tentative Creep-Fatigue Model Parameters for Pb-Free Solders
SJ
= mean cyclic solder joint

temperature;
SAC405/305 0.425 0.480 9.30e-04 -1.92e-02 500
t
D
= half-cycle dwell time in
minutes; and
SAC205 0.250 0.480 9.30e-04 -1.92e-02 500
ΔD = cyclic damage term. SAC105 0.225 0.480 9.30e-04 -1.92e-02 500
SnAg 0.275 0.430 6.30e-04 -1.82e-02 400
For the various solders, five parameters
are different as shown in Table 1. It is Table 1. Parameters for solder creep-fatigue model for various solders.
36 – Global SMT & Packaging – September 2009 www.globalsmt.net
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