Part screening is a process designed to remove those parts having inferior
quality and hence reliability.Such screening is accomplished by
subjecting a production lot of parts to various electrical, thermal,
and environmental stress for the purpose of making the weak ones fail.
The part level screening process must be designed to meet the following
- Test and stress levels must be carefully selected to detect inferior
- Tests must be nondestructive and non-degrading to good parts;
- Testing must be adequate to screen out all potential failure mechanisms
of the parts to be screened;
An effective screening program requires a detailed understanding of
materials, fabrication and packaging techniques, electrical and thermal
characteristics, and manufacturing tests performed on the parts to be
screened.In addition, to limit costs, screening should be based upon
the least amount of testing required to provide an effective
Much cost and effort has been expended by DOD agencies and industry
developing reliability screening processes and requirements for the
major types of parts used in military equipment. These requirements
have been detailed in the military specifications for these
parts.Stress screening, particularly at the part level, should not develop any
new or change any of the current, acceptable part level screens.
Screening levels (normally referred to as quality levels) for the three
main categories of military specification parts are as follows: (1)
screened military grade passive electricaI parts (e.g., relays, coils,
resistors and capacitors) are procurable to Established Reliability
(ER) Military Specifications categorized as to ER failure rate level (normally
M through S); (2) military grade discrete semiconductor devices are
procurable to MIL-S-19500 and its detailed slash sheets, and are
categorized as JANTX, JANTXV and JANS screening levels; (3) screened
military grade microcircuits are procurable to MIL-M-38510, are labeled
JAN, and categorized as to quality level (i.e., B or S).
Commercial grade, military grade, and military ER and JAN grade parts are
generally physically and functionally interchangeable with the basic
difference being the failure ratelevels, which can vary by orders of
magnitude. ER and JAN parts have been screened per Military Test
Standards by certified manufacturers as required by the specific part
military specifications. Government inspectors
monitor and periodically survey and recertify these
manufacturers to assure that the high reliability levels of the parts
are maintained from lot to lot.
In addition to the military grade ER and JAN parts, there are some
parts often referred to as “vendor equivalents”.
“Vendor equivalent” parts have been subjected to
screens/tests similar to those required by the ER or JAN military
specifications, but do not meet the fulI requirements of the ER
or JAN military specifications and may not have been
screened/tested on a 100% basis. Such vendor equivalents
exhibit lower failure rates than their commercial counterparts,
but higher than standard military parts when both are subjected
to a military environment.
The purpose of conducting a screen is to “weed out” infant mortality
failures prior to incorporation of that item into its next
higher assembly and/or commitment of the item to its final use.
In keeping with this purpose, the selection of a cost effective screen is
dependent upon many factors which include the end use
environment, the part’s intended life, the ease of conducting
repair, any warranty provisions, and type of application. The decision
process in selecting a cost effective screen involves
understanding the device’s failure mechanisms, then selecting a
screen that will induce the failure mechanisms expected to be inherent in
that device. It is desirable to perform only those
screens which are cost effective to the end user during the
entire life cycle.