Test fixtures for contacting assembled printed circuit boards

Details Test fixtures for contacting assembled printed circuit boards Test fixtures for contacting assembled printed circuit boards

2000-02-23

2002-07-23

Le, N. (Department: 2858)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

Of individual circuit component or element

Reexamination Certificate

active

06424163

ABSTRACT:

FIELD OF THE INVENTION
The present invention concerns test fixtures for assembled printed circuit boards with an assembled printed circuit board mounted on a probe plate and to be tested and with a probe plate supporting test probes.
BACKGROUND OF THE INVENTION
Test fixtures for assembled printed circuit boards are predominantly used in conjunction with automatic test systems and make possible an individual quickly changeable electrical connection between the circuit board and the connecting interface of the test systems. Depending on the size of the circuit boards and test methods 200 up to about 5000 test points are simultaneously contacted with one test fixture.
The construction of a test fixture is illustrated in principle in
FIG. 2
a
. Into a probe plate
1
tightly mounted receptacles
4
are seated, into which test probes
5
are inserted. The test probes
5
are mostly held in the receptacles
4
by a clamping fixation (for example at reduced cross-section of the receptacles
4
) and have preferably an internal, spring-loaded movable plunger
8
forming the test prod of the test probe
5
. Correspondingly to the distance of the test points
11
on the circuit board
3
to be tested and to the resulting distance of the contact points
11
to each other test probes
5
and receptacles
4
are of varying diameter. Today versions are usual, which are designed for a distance of up to 100 mils (2.54 mm), 75 mils (1.79 mm), 50 mils (1.27 mm) respectively each.
For contacting, the specimen
3
fixed on a probe plate
2
is preferably pressed against the movable plungers
8
forming the test prods of the test probes
5
either by vacuum or by mechanical push down fingers in such way that the test probes
5
contact the contact points
11
.
The underside of the receptacles
4
is mostly provided with a contact element
6
having a wire wrap post. The receptacles
4
are then wired up from the wire wrap post to the interface.
In case of alternative embodiments, so-called adapters without wiring (
FIG. 2
b
), the receptacles
4
have also a spring contact element
7
at the underside and contact either (not shown in the drawing) an individually manufactured connection board or a universal grid board from a so-called interface cassette.
The test probes
5
must be replaceable easily and very quickly in case of defect or wear, because a defective test fixture causes immediately a standstill of production in an automated production line and consequently high costs.
The costs of the test probes
5
increase overproportionately with decreasing diameter of the test probes
5
. Stability and life decrease plainly at test probes
5
getting smaller. In case of probe
5
suitable for the 50 mil grid measure the internal moved plunger
8
and the spring
5
reach already diameters of about 0.45 mm.
The progressing miniaturisation and with it the increasing closeness of the contact points
11
on the specimen
3
therefore necessitates the search for alternative solutions to achieve in case of small distance of the test probes to each other possibly high stability and reasonable costs of the test probes
5
.
Since many years test probes
12
are known on the market, where, as shown in
FIG. 3
, in order to reduce the total diameter connection pins
13
are fixed in the probe receptacle
14
clamping from below.
Because here the external diameter of the receptacle
4
tightly mounted enlarges no more the total diameter (in case of the test probe
12
illustrated in
FIG. 3
, a tightly mounted receptacle
4
for test probe
12
is not necessary for the purpose of contacting), so test probes
12
, which are applied in case of assembling with receptacles
4
only for a minimal grid of 75 mils (1.79 mm), can be used up to a distance of 50 mils (1.27 mm) in case of assembling without tightly mounted receptacles, and previous 50 mil test probes
12
can be used up to a distance of 40 mils (1 mm).
It is a disadvantage of this method, however, that the replacement of probes is made considerably more difficult. While in case of the conventional fixing method, where the test probes
5
are held in the tightly mounted receptacles
4
by frictional resistance and contact them, the test probes
5
can be easily taken from above from the tightly mounted receptacles
4
without opening the internal part of the adapter (for example the wiring area of the contact elements
6
,
7
), and can be inserted again into the tightly mounted receptacles
4
, in case of the test probes
12
described in
FIG. 3
an opening of the wiring area made inaccessible for the non-specialist is necessary to remove or replace the test probes
12
in order to draw first the connecting pin
13
from the receptacle
14
. After that the test probes
12
can be pulled out of the probe plate
1
from above and be replaced. After inserting a new test probe
12
into the probe plate
1
from above the wiring area has to be opened again in order to insert the connecting pin
13
back into the receptacle
14
.
The replacement of all test probes
12
, which is necessary in regular time intervals in case of a high number of boards to be tested, will be considerably more complicated, because each replacement has to be executed individually and with access to the wiring area and to the top side of the probe plate
1
of the test fixture.
A further disadvantage is the only incomplete reduction of the diameter, because in order to prevent a slipping of the test probe
12
downwards (i.e., towards the wiring area) through the probe plate
1
an additional retaining ring
15
on top of the probe plate
1
at the test probe
12
is necessary.
An alternative known solution, which avoids the disadvantages named above, is illustrated in FIG.
4
. Also here, test probes
5
are used, which are not carried in tightly mounted receptacles
4
, but directly in bores, which are placed in a preferably particularly thick probe plate
1
. The test probes
5
free to move in the bores in axial direction are supported with their lower end (averted from the test prod) by the contact parts
17
mounted on a probe plate
16
, and consequently in testing the test probes
5
are pressed against the contact parts
17
by the pressure force of the specimen
3
to the test probes
5
.
A disadvantage of this solution is at first the additional contact interface between the test probes
5
and the contact parts
17
necessary for the accessibility of the adapter, because now the test probe is no more connected tightly in frictional resistance with the contact parts
17
replacing of the receptacles
4
. This transition contact is loaded at any change of specimen
3
, i.e., up to several hundred thousand times during the life of an adapter, and therefore it involves the risk of additional contact failures and increased contact transition resistance respectively.
A further disadvantage arises by the fact that in order to achieve the necessary contact spring force a clearance fit of the test probes
5
in the bores in the probe plate
1
must be ensured in any case to transfer the contact pressure on plunger
8
of the test probe to the contact location on the contact part
17
. Resulting from manufacturing tolerances hardly avoidable in the manufacturing of the test probes
5
and in making the bores the required clearance fit necessitates a considerably larger wobbling clearance of the probes
5
.
It is a third disadvantage that either in the first manufacturing of the test fixture and also in later alterations expensive provisions have to be made (for example deposition of an elastic holding layer
18
consisting of an elastic material on the probe plate
1
), so that the test probes
5
will not be pulled out after testing, when the circuit board is taken off. This danger exists, because for the test of circuit boards
3
soldered with soldering wave and therefore contaminated with remains of flux particularly very sharp probes are necessary, which often hook up in the softer solder.
Because furthermore from several reasons the concept described above cannot be combine

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