QA Technology test probes can be used
over a wide range of temperatures without
affecting their performance. The following
discusses some items to consider when
using QA test probes at the limits of
operating temperature.
The upper temperature limit of a spring
probe is governed by the spring material.
At elevated temperatures the strength
of spring materials is reduced, and yielding
may occur when the probe is deflected.
Although the springs are not likely to
fracture in this situation, they may take
a permanent set and the spring force at
a given deflection will be reduced.
Springs that are made of music wire can
be used without adverse effects at temperatures
up to 250° Fahrenheit [120° C]. Stainless
steel springs can be used up to 400° F
[204° C]. However, 250° F [120° C] is
the recommended upper temperature limit
for spring probes, since the probe user
may not always be certain of the spring
material.
The lower temperature limit of a spring
probe is governed by the lubricant used
within the probe. Lubricants are commonly
used to prevent wear of the precious metal
internal surfaces of the probe, thus extending
probe life and maintaining low electrical
resistance.
The viscosity of lubricants used for
probes will increase as temperature decreases.
The pour point (the temperature at which
the lubricant will barely flow) for the
lubricant used in QA probes is 40° F [5°
C]. If probes are exercised below this
temperature, lubrication may not be adequate,
and galling of the plunger and inside
surface of the probe tube may occur. This
wear could allow the base metal to form
oxides which would greatly reduce electrical
performance.
If, however, it is necessary to perform
tests at temperatures below 40° F [5°
C], the probes can be actuated at room
temperature, and then refrigerated or
moved to the cold environment without
harm to the plating or materials. The
probes should not be exercised when the
temperature of the lubricant is below
the lower temperature limit. In some applications,
movement of the plunger (by deliberate
actuation or even by slight vibration),
at temperatures lower than 40° F [5° C]
is unavoidable. The increased viscosity
of the lubricant at these temperatures
causes sluggish movement of the plungers,
which could result in intermittent contact.
In spite of their reduced life, unlubricated
probes should be used in these cases,
so that plungers will move freely. There
is no known lower temperature limit for
unlubricated probes. The upper temperature
limit for unlubricated probes remains
unchanged. |
