Signetics Programmable Logic Devices - Al Kossow's Bitsavers

Signetics Programmable Logic Devices
Quality and Reliability
METHODS OF FAILURE RATE
CALCULATIONS
Actual Device Hours = Number of Devices x
Number of Hours. In order to determine the
Equivalent Hours derated to a given operation
temperature, the junction temperatures of the
devices should be calculated using the known
thermal resistance of the package (9JA) and the
power dissipation of the devices:
(1)
Using the Arrhenius relation, the test temperatura
and the derated operation temperature will
yield the thermal acceleration factor from
T, to T2:
R, A.exp[~J [EAJ [1 1 ]
R; = A.exp[ EAJ = exp k r;-;:;
kT2
k = 8.617 X 10-6 eViKelvin (BoI12mann's
constant)
A = Proportionality constant for a given
failure mechanism
R, = mean time to failure@T,
R. = mean time to failure @ T.
EA = activation energy for the failure
mechanism
T, = operating temperature
T. = life test temperature
(2)
An additional 55 X acceleration factor should
be added !or !he 150"C/6.5V dynamic life test
due to the time-dependent oxide failure acceleration
(20% higher than specified power supply
voltage).
Multiplying the actual device hours by the acceleration
factor for each failure mechanism
will result in the equivalent hours.
Poisson statistics are applied to estimate the
performance of the population from the life test
results of a sample test. This is useful when the
probability of failures is small and the failures
occur randomly in time. A commonly used formula
for estimating the failure rate is the 'chisquared'
equation:
x·
Fe= - x 100%
2nt
(3)
Fc = calculated failure rate estimate
(in %11000 hrs) at upper confidence limit
X· = 'chi-squared" value for 2FA + 2 degrees
n
t
offreedom for DC where FA is the number
of actual failures (X2 comes from
available tables for a known DC)
= 1-8, where 8 is the confidence limit
(8 is stated in %).
= number of units in test
= test time in thousands of hours
(equivalent)
Equation 3 will calculate the estimated failure
rates/l000 hrs for 60% confidence level (industry
standard) for each failure mechanism.
THE SURE PROGRAM
The SURE (Systematic Uniform Reliability
Evaluation) program audits/monitors products
from all Signetics' divisions under a variety of
accelerated environmental stress conditions.
this program, first introduced in 1964, has
evolved to suit changing product complexities
and performance requirements.
The SURE program has two major functions:
long-term accelerated stress performance audit
and a short-term accelerated stress monitor.
In the case of Memory products, samples are
selected that represent product groups from ali
wafer fabrication and assembly locations.
SURE REPORTS
The data from these test matrices provides a
basic understanding of product capability, an
indication of major failure mechanisms and an
estimated failure rate resulting from each
stress. This data is compiled periodically and is
available to customers upon request
January 1990
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