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– 18 – IEC 61709:2017 © IEC 2017
A failure is related to a failure mechanism, which is the basic material behaviour that results
in the failure. For integrated circuits more detailed description is presented in Annex F.
4.2 Thermal modelling
Temperature is a relevant factor affecting failure rate. It is in fact known that temperature and
temperature change has an effect on component reliability. For temperature change see 4.4.
The temperature effect is more significant for some families (active components and
aluminium capacitors with non-solid electrolyte). The models adopted are those which give
the effect of temperature on the predominating failure mechanisms (which are not normally
the “wear-out” mechanisms).
Thermal modelling of components should then be as accurate as possible, in particular
thermal resistance for semiconductors should be measured in order to allow accurate
evaluation of the internal temperature.
When thermal resistance cannot be measured, thermal resistance models provided in
Annex B can be used.
4.3 Mission profile consideration
4.3.1 General
During their life, components experience different conditions of use that represent the major
factors affecting component reliability: the complex of these conditions is defined as mission
profile.
The mission profile defines the required function and the environmental condition as a
function of time. This will vary according to the type of operation that is undertaken. This
operation may be continuous over time at a fixed level, continuous over time at a variable
level or sporadic over time at either a fixed level or a variable level. In some cases switch on
and switch off could be significant and of more importance than the steady state operational
conditions. Careful consideration of the mission profile is needed in order to fully understand
how it affects the component reliability.
The mission profile can be considered to be based on calendar time or on the time of actual
operation or it can be cycle based (e.g. how many times an item is used).
More considerations on mission profile are detailed in Annex D.
4.3.2 Operating and non-operating conditions
A mission profile can be decomposed in several phases, on the basis of the typical use. The
main classification of these phases is between operating and non-operating conditions.
Although this document deals with components, the conditions of use usually refer to the
assembly (equipment or system), and can be defined as follows when they refer to
components.
• Operating: the component is assembled, powered or connected to a load and
functioning at nominal conditions. See 3.1.8.
• Stand-by: the component is assembled, powered or connected to a load, but not
functioning or functioning at conditions lower than nominal. See 3.1.11.
• Dormant: the component is assembled, but not powered or connected to a load. See
4.3.3.
• Storage: the component is not assembled, hence not powered or connected to a load.
During storage components are packed stored in a warehouse. See 4.3.4