Using ALTA
with MultiStress Physics of Failure Models
[Editor's Note: This article has
been updated since its original publication to reflect a more
recent version of the software interface.]
In the "Reliability
Prediction Methods for Electronic Products" article in
Volume 9, Issue 1 of the Reliability Edge newsletter, we
presented several physics of failure models for electronic
components. The article included an example of data analysis
with ReliaSoft's
ALTA software using
an Arrhenius model. However, many physics of failure models are
formulated such that they cannot be used with the one and
twostress models available in ALTA Standard. In this
article, we will analyze accelerated test data using the Peck
model for corrosion in aluminum devices, using the general loglinear model in ALTA PRO.
Comparing the Peck Model and
the GLL Model in ALTA PRO
The Peck model is widely used in the semiconductor industry to
model the effect of three stresses on the life of a device. Each
of these stresses affects the rate of corrosion in devices
containing aluminum or aluminum alloys. The model considers
relative humidity, voltage, and temperature, and it ignores
interactions between the stresses. In addition, the form of the
voltage term has not been experimentally determined. The Peck
model has been applied to failure mechanisms other than
corrosion by changing some of the model parameters.
The Peck model for corrosion is
given by:
where:
 TTF
is the time to failure of the device.
 A_{0}
is a scale factor.
 RH
is the relative humidity in percent.
 f(V)
is an unknown function of voltage in volts.
 E_{a}
is the activation energy in eV.
 k_{B}
is the Boltzmann constant (8.617385x105 eV/K).
 T
is the temperature in Kelvin.
The general loglinear (GLL)
model in ALTA PRO is given by:
where:
 L(X)
is the life of the component subjected to a vector of
stresses,
X,
and
 α_{i}
are model parameters.
The threestress form of the GLL
model becomes:
.
Comparing the Peck model to the
threestress form of the GLL, we obtain the following
relationships:
Making the substitutions
X1
=
log
(R),
X2
= log
S,
and X_{3}
= 1/U
yields:
Note that we have assumed an
inverse power law relationship for the dependence of time to
failure on voltage, i.e.
f(V) = V^{n}.
Therefore, if we enter the stresses relative humidity (R),
voltage (S),
and temperature (U),
in separate columns of an ALTA data sheet, the GLL model
parameters α_{1}
and α_{3}
will be related to the Peck equation through:
Example:
Twentyseven tests were performed at elevated stress
conditions, and the data set is given in Table 1. The data are
copied and pasted directly into an
ALTA folio, and the column headings are customized to
reflect the data, as shown in Figure 1.
Table 1:
Accelerated Corrosion Test Data
Figure 1: Original Test Data in ALTA
The temperature
data were obtained in Celsius; however, absolute temperature
units must be used in the ALTA model. In order to convert the
temperature stress values to Kelvin, the Temperature column is
selected and ALTA > Options > Convert Stress Values is
chosen to automatically convert the temperature
values to Kelvin (see Figure 2).
Figure 2: Automatic Conversion of Temperature Stress Values from
Celsius to Kelvin
Since the Put current
values in Subset ID option is enabled, the column
headings for the temperature stress column and the subset ID are
changed to reflect the unit conversion, as shown in Figure 3.
The GLL model and the associated stress columns are selected, as
is the Weibull distribution.
Figure 3: Accelerated Corrosion Test Data with Temperature in
Absolute Units
Figure 4 shows the
settings used for the transformation of the three stresses that
are selected by clicking Stress Transformation in the
control panel.
Figure 4: Defining the Stress Transformations
The parameters are
computed by clicking Calculate
on the control panel, and Table 2 shows the results
obtained.
Table 2:
Initial Computed ALTA Parameters
Recall that in the Peck model,
the value of the relative humidity exponent is defined as 2.7.
To change this, choose ALTA > Options > Alter Parameters (and
Recalculate) > Alter Alpha > Alter Alpha(1). Then change Alpha(1) to 2.7, as shown in Figure 5.
Figure 5: Altering Alpha(1) in the GLL Model to Match the Peck
Model
Table 3 shows the final
parameters, along with the corresponding Peck model parameters.
Table 3:
Final Computed ALTA Parameters
Conclusion:
The general loglinear model in ALTA PRO was used to compute parameters for the Peck
model for corrosion in devices with aluminum and aluminum
alloys. One of the parameters in ALTA was altered and the
other parameters were recomputed in order to mimic the form of
the Peck model. A similar process can be used to analyze
accelerated test data in ALTA for many physics of failure
models with multiple stresses found in the electronics
literature.
