Load board test fixture

Details Load board test fixture Load board test fixture

1999-10-12

2002-04-09

Metjahic, Safet (Department: 2858)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

Of individual circuit component or element

C324S758010, C324S761010

Reexamination Certificate

active

06369593

ABSTRACT:

The present invention concerns a test adapter according to the generic term of claim
1
.
BACKGROUND OF THE INVENTION
Test adapters are used in series production of loaded and unloaded circuit boards in the field of test and repair and serve for simultaneous contacting of a great plurality of contact locations of the circuit board by an automatic test system.
FIG. 3
shows a typical construction of a conventional test adapter. The test adapter comprises a probe plate
1
and receptacles
2
mounted on this plate as well as exchangeable inserted test probes. The test probes
3
have one top called plunger
3
a
for the contact with test points
7
of a circuit board
6
. The circuit board
6
is mounted on a moving plate
4
of the test adapter by the locating pins
5
. Usually the circuit board
6
is pressed against the test probes
3
by vacuum or a hold-down technique not drawn in here, the moving plate
4
performing a vertical movement in the direction to the probe plate
1
.
Usually in accordance with each test method and size of circuit board the test points
7
assigned to between a few hundred and a few thousand test probes are contacted on the circuit board
6
. Here a particularly high economy is offered by the in-circuit-test-method, where, however, all nets of a loaded circuit board have to be contacted each with at least one, in a few cases also several test probes. In order to reach reliable contacting whether special contact surfaces (test pads) are provided on the circuit board in case of nets with only surface-mounted components, or, if possible, existing plated through holes are used. In case of the through hole insertion technique of components decreasing in distribution the connection points of the components can be contacted reliably.
The progressing miniaturization of electronic assemblies requires on the one hand a reduction of the minimum diameter of test pads and plated through holes, which is necessary for the contacting, but on the other hand also the possibility to contact ranges of contact previously not contactable as for instance contact areas of surface-mounted components or thin circuit lines. Just in case of the second requirement very great demands are made on the positioning accuracy.
Here the size of the necessary test contact surfaces depends mainly on the sum of the tolerances of the test probes, on the guidance of the adapter system and on the dimensional stability of circuit board
6
. The factors influencing the size of the necessary test surface on circuit board
6
can be subdivided into three groups:
A first error group, which makes up a considerable portion of the total error, is formed by the mounting-caused misalignment of the receptacles
2
, which receive the test probes
3
, as well as the wobbling clearance of the test probes themselves.
The second important error group concerns the manufacturing or processing-caused deviation of the dimensional stability of circuit board
6
. The most important consequences are the misalignment between hole pattern and circuit line pattern, which is directly reflected as misalignment of the test points with regard to the reference holes of circuit board
6
, as well as a stretch or shrinkage of the entire circuit board
6
as also of the circuit line pattern
10
with regard to the hole pattern. Furthermore an angular displacement between hole pattern and circuit line pattern forms a source of error of this second error group.
FIG. 2
shows the consequences of the misalignment between hole pattern and circuit line pattern. This misalignment is discernible in
FIG. 2
by the non-centric position of the bore hole
24
referred to copper ring
21
surrounding the bore. The same misalignment causes that the drawn-in distance a between mounting hole
20
, through which circuit board
6
is fixed on the moving plate
4
, and test point
23
shows a deviation and with this a test probe
3
lined up relatively to the center of the receiving hole
20
does not meet the contact point
23
whether at center or does not meet it at all, depending on the size of the contact point.
The third error group results from the guidance of circuit board
6
over the tightly positioned locating pins
5
. In order to even out the tolerances of locating pin
5
and receiving bores
20
in circuit board
6
as well as to even out the above named errors of dimensional stability of circuit board
6
the diameter of the locating pins
5
has always be chosen in a manner that a clearance fit is always achieved. Such a clearance fit causes inevitably an positioning error.
It has to be taken into consideration at all sources of error that particularly in case of high production rate circuit boards are got from many different, internationally producing manufacturers, so that close cooperation with the manufacturer in order to reduce tolerances is made more difficult and hardly takes place in practice. Furthermore in an ideal manner automatic or at least easily manageable semi-automatic correction methods are necessary for a running production, because complicated manual correction of occurring tolerances is not compatible with the requirements of the running series production.
Test adapters being aimed in particular at the correction of the above named second error groups, i.e., of the misalignment errors of the board, are known from several patents. In the U.S. Pat. No. 4,820,975 a test adapter with shiftable probe plates is described, where the misalignment is determined by the measuring of reference marks by means of video cameras, and this error will then be adapted automatically over the xy-shiftable probe plate.
An alternative of this solution that requires a lot of technique is the test adapter described later in U.S. Pat. No. 5,321,351, where the locating pins for the circuit board are mounted adjustably and where a justage of each production of circuit boards is also done by the evaluation of reference marks with video cameras.
This test adapter, however, has the disadvantage that the adjusting mechanism for the adjusting pins is only suited to individual adjustments by means of the special adjusting circuit board, so that manual or automatic adaptations within a series of circuit boards cannot be performed. An additional disadvantage results from the locating pins fixed in the probe plate, if, caused by small receiving holes, locating pins with little diameter are needed. In this case the unfavorable relation between diameter and length of the locating pin causes a very low stability. It is a further disadvantage of this version that locating pins fixed in the probe plate abruptly pierce the circuit board at suction of the adapter and thus require additional protection measures to meet the safety regulations.
These three named disadvantages were then removed in the further development according to U.S. Pat. No. 5,408,189. Here the locating pins are tightly fixed in the moving plate, and this can be adjusted continuously over driving elements in relation to the probe plate. In this version, however, the through holes of the spring contact pins through the moving plate must be particularly big, so that for example due to the construction an additional guiding of the plungers in the moving plate is not possible. All three described test adapters are based on the principle that the positions of the circuit board and the one of the receiving bores of the spring contact pins in the probe plate are shifted relatively to each other and that the reference measurement is also carried out between probe plate and circuit board by means of video cameras. Thus the initially described error portions caused by mounting misalignment, wobbling clearance of the probe and clearance of the receiving holes are not taken into consideration and therefore they limit the achievable positioning accuracy. Furthermore for setting up a special adjusting board (golden board) and a complicated setting-up process is necessary, which, for instance, is described in detail in U.S. Pat. No. 5,321,351. Because furthermore the special adjusting plate

Affiliated with

Also associated with

Rating

Load board test fixture does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Load board test fixture, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Load board test fixture will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2832489