Solder joint reliability: acceleration transforms and acceleration factors
Werner Engelmaier
“Acceleration factors need to be based on a valid physics-of-failure model—they cannot be reduced
to some ratio of the ΔT’s to some exponent.”
Solder joint reliability:
acceleration transforms and
acceleration factors
While I have written a lot on solder joint f(test) = test cyclic frequency. where
reliability, both leaded and Pb-free, in this F = engineering factor,
~
1.2 to
column [Refs. 1-8] and elsewhere, I have not Both N
f
(field) and N
f
(test) are determined 0.7 for SJs with fillets,
~
1.5 to
directly addressed the issue of acceleration from an accelerated reliability model such 1.0 for SJs without fillets;
factors (AF) previously. This issue does as given in Equation 3 DNP = distance from the neutral
come up frequently, and a common point/plane;
misunderstanding is that somehow ΔCTE = CTE-mismatch;
acceleration factors can be obtained ΔT = cyclic temperature excursion;
without a physics-of-failure model. Any
Equation 3
h = solder joint height for
valid determination of AFs has to come leadless and non-compliant-
from physics-of-failure damage models that where leaded solder joints [see Ref. 9].
describe the material behavior over the ε’
f
= fatigue ductility coefficient
range of parameters of importance—they for the solder alloy; The creep-fatigue exponent, m, which
really cannot be separated. ΔD = the cyclic damage term, and is specific for a given solder alloy and
First, it needs to be understood that m = the creep-fatigue exponent. accounts for the degree of completeness of
there are two different AFs, one based the cyclic creep process is given by
on the number-of-cycles and the other The cyclic damage term, ΔD, is given
on the time-to-failure. The second one is in most cases (for the exception of truly
important because one typically pays per compliant leads, see Ref. 9) by
test time in the temperature chambers.
Equation 5
They are shown in Equations 1 and 2 below
where
Equation 4 T
SJ
= mean cyclic solder joint
temperature;
Equation 1
Solder Model Parameters
ε
f
'
c
0
c
1
c
2
t
0
Equation 2
Engelmaier-Wild Creep-Fatigue Model for SnPb Solders
where
SnPb 0.325 0.442 6.00e-04 -1.74e-02 360
AF(N) = AF in terms of mean cycles-to-
failure;
Engelmaier Creep-Fatigue Model for Pb-Free Solders
AF(t) = AF in terms of mean time-to-
from property data & accelerated test results
failure; SAC405/305 0.240 0.390 9.30e-04 -1.92e-02 100
N
f
(field) = mean cycles-to-failure in the
SAC205 <0.240 TBD TBD TBD 100
field;
N
f
(test) = mean cycles-to-failure in the SAC105 <0.240 TBD TBD TBD 100
test;
SnAg <0.240 TBD TBD TBD ~60
MTTF = mean time-to-failure;
f(field) = field cyclic frequency, and
Table 1. Model parameters for solder creep-fatigue model.
42 – Global SMT & Packaging – August 2009 www.globalsmt.net
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