| Test probes used
in production testing will eventually
get dirty enough to cause contact
problems. The following steps
will eliminate contact problems
caused by dirty probes:
- Practice preventive maintenance
on fixtures, test boards and
the test environment to prolong
probe life.
- Use techniques approved by
the probe and fixture manufacturers
to clean the probe tips.
- Develop a maintenance program,
which defines intervals at which
probes are cleaned or replaced.
Some recommendations to help
keep test probes clean:
Test Environment - The
test environment is one of the
largest contributors to probe
contamination. Minimize airborne
contamination such as dust, clothing
fibers or particles from a nearby
wave-solder machine to improve
contact reliability.
Circuit Boards - Printed
circuit boards, which are being
tested, should be as clean as
possible. If testing boards coated
with no-clean flux, choose low-solids
fluxes and fine-tune process controls
to minimize the amount of flux
applied to the board. Testing
contaminated boards will not only
cause poor contact on new probes,
but will leave residues behind
on the probe tips, which impede
the next test as well.
Dust Covers - Use dust
covers over idle fixtures to prevent
airborne contaminants from settling
on the probe tips. In the case
of vacuum fixtures, dust that
settles on the board test area
is drawn directly onto the test
probes when the fixture is first
put into use.
Air Filters - When a vacuum fixture
is released, room air rushes into
the fixture around the test probes.
Protect the probes from airborne
contamination by installing an
air filter in the release port.
Receiver Bays - Like the
probes in test fixtures, probes
which are exposed on a test system's
receiver bay should also be protected.
Keep bays covered with either
a dust cover or a test fixture,
and maintain clean electrical
contact surfaces on all fixtures. |
 |
In some cases, especially in
high volume production (where
the probes see many cycles over
a short time) it may be practical
to clean the tips of the probes.
Virtually all manufacturers of
low-resistance, long-life probes
use some sort of lubricant to
prolong the life of the probe's
internal sliding contact surfaces.
Cleaning a probe by bathing it
in Freon or other solvent will
remove this important lubricant.
Even spot-cleaning the probe tips
with solvent can wash particles
down into the critical internal
surfaces where they can drastically
affect performance.
To clean probe tips, remove lint,
fibers, flux, and other contaminants
by gently brushing the probe tips
with a small brush and vacuuming
away the dislodged particles.
A brush with nylon or natural
fiber bristles works well; metallic
bristles may damage the probe
plating and are not recommended.
A practical maintenance program
for fixtures can save considerable
time and money at the production
level. Testing becomes more reliable,
thus reducing the chance of false
failures and lost rework expense.
Diagnosing contact problems as
they arise and replacing test
probes one at a time is more expensive
than replacing probes on regular
intervals. Use cycle counters
on test fixtures to help establish
a maintenance program, which calls
for cleaning or replacing probes
after a predetermined number of
cycles.
Developing such a program requires
some tracking to determine the
average life of the probes in
a particular application. Since
test conditions vary widely, it
is difficult to generalize probe
life. Some applications call for
replacement as often as every
few thousand cycles, while probes
in clean environments or applications
with wide electrical tolerances
can last far longer.
Better test yields and reduced
downtime are the rewards for keeping
fixtures and probes in top condition. |
