Selection and Testing of Electronic Components for LM

100
Type of check
Type of component Me- En- El.
chan- viron- param.
ical mental
Resistors, resistor net­
works and potentio­
meters S S
Capacitors
Diodes, transistors
S S
and thyristors S A
Micro circuits S A A
Table 1
Testing of components
A Check of the whole consignment
S Sampling test in accordance with MIL-STD-105
Fig. 10
The solderability tester STE 74 works in accordance
with the solder globule method and is intended
for measuring the solderability of components
and metallized holes in printed boards etc.
The test Item whose solderability is to be measured is lowered
into a molten solder globule that is placed on top of a
heated iron cylinder so that the globule Is divided into two
equal parts. When the solder wets the test item the latter is
completely enclosed by the solder. The wetting time is
measured and Is a measure of the solderability of the test
item.
The lowering speed, solder temperature and quantity of
solder are carefully specified. The solder and test item are
treated with flux and the solder must wet the iron cylinder
hermeticity tests, tension, bending and
torsion tests on the leads and flammabil
ity tests. Electrical tests may com prise
voltage tests, current pulse tests and
power loading tests. Such tests can
continue for periods of 1 000 hours up to
more than 10000 hours depending on
the type of test and the "acceleration
factor", i.e. the size of the load in relation
to specified component data.
Significant component data are measured
before, during and after the
course of the type testing.
Type testing programs for electronic
components also contain instructions
for assessing the test results. However,
the final decision as to whether a certain
type of component should be accepted
is always based on the expert knowledge
of the component specialist.
Quality follow-up
Components delivered by approved
suppliers are inspected on arrival in the
way described in the next section. In
addition a so-called reliability evaluation
is carried out in accordance with a
yearly plan, primarily of recently introduced
components and components
purchased in great quantities.
The reliability evaluation provides a
continuous verification that the com­
ponent quality originally accepted after
type testing is maintained in later component
deliveries.
In this evaluation, which is carried out
on samples taken from the deliveries,
the inner construction of the component
is studied and compared with reference
examples from the type testing.
It can then be ascertained whether the
manufacturer has for example introduced
a new type of silicon chip in a
transistor or changed the connections
to a capacitor foil.
The reliability evaluation also includes
a limited type testing for the purpose
of finding any quality defects in a component
consignment within a few
weeks. It is then possible to prevent the
use of unsatisfactory components in the
production of exchange equipments.
When a component fault is reported in
equipment being manufactured, in the
system testing stage or in operation, a
fault analysis is carried out in order to
determine the cause of the fault and
when necessary to improve the component
quality.
Inspection of components
on arrival
Inspection of purchased components