
100 Improving reliability and quality for product success
This concept of the acceleration factor is expressed as follows:
*
AF
S
S
E
k T T
n
a
= ⋅ −
0 0
1 1
exp
(5.5)
where S
0
, T
0
are the stress and temperature under normal use conditions,
S, T are the stress and temperature under severe test conditions, n is the
exponent of stress, E
a
is the activation energy, and k is the Boltzmann con-
stant. If the exponent (n) and activation energy (E
a
) are found, the AF can
be calculated. The lifetime, or anticipated time to failure, divided by the
AF becomes the requisite minimum test period to identify failures. The
actual test hours can obviously not be less tha ...