Zero static force assembly for wireless test fixtures

Details Zero static force assembly for wireless test fixtures Zero static force assembly for wireless test fixtures

1998-06-03

2001-03-27

Karlsen, Ernest (Department: 2858)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

Of individual circuit component or element

Reexamination Certificate

active

06208158

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a wireless assembly used for in-circuit testing of printed circuit boards. More particularly, the present invention relates to a zero static force assembly for in-circuit testing of printed circuit boards. The zero static force assembly of the present invention provides reliable electrical contact between diagnostic or testing equipment and a unit under test without exerting forces which tend to damage the translator board in conventional in-circuit testing assemblies.
BACKGROUND OF THE INVENTION
Testing of complex electronic devices requires the reliable communication of signals between a unit under test and a testing assembly. Connections to components to be tested are made using external electrical probes applied to exposed test points (pads, vias, or other electrical contact points) on a unit under test.
Automatic testing of electrical circuits requires simultaneous connection to hundreds or thousands of circuit test points. A conventional test fixture used to provide electrical connection to a unit under test includes the so-called “bed-of-nails” assembly having a platform supporting an array of spring loaded probes. In operation, the unit under test (UUT) is placed on the platform and a force pushing the UUT into contact with the exposed probe ends is exerted, compressing the probes slightly, so as to ensure electrical connection therebetween.
The probes are electrically connected to test equipment in one of two common ways. According to one method, electrical connection is accomplished using fixed wiring. According to this method, dedicated wiring is attached between each probe and the test equipment. The dedicated wiring can take the form of wire wrapping in which a thin gauge wire is physically wrapped around an end of the probe. Alternatively, the dedicated wiring can be connected to the probe by force fitting each wire into a slot in each probe. An example of a probe adapted for this sort of wiring is found in U.S. Pat. No. 5,557,213 to Reuter et al. The labor involved in either method of dedicated wiring makes fixed wiring impractical for testing complex circuitry having thousands of test points. Moreover, this method of providing electrical connection between the test equipment and the probe suffers from a disadvantage in that it is difficult to replace a probe in the event of malfunction.
Alternatively, electrical connection may be achieved using a translator board. According to this method, the bottom probe head (in the case of a double-ended probe) is pressed into contact with a pad (electrical contact point) on a translator board, e.g. a printed circuit board, which is electrically connected to the test equipment, and the top probe head is pressed into contact with a corresponding test point on the UUT. This method for providing electrical interconnection provides many advantages over fixed wiring.
However, this method of providing electrical interconnection poses unique problems. Notably, the force required to maintain good electrical contact between the bottom probe head and the translator board tends, over time, to stress and eventually damage the translator board. See, e.g. FIG.
1
.
FIG. 1
shows a conventional wireless assembly
2
used for testing a UUT. The wireless assembly
2
includes a translator board
14
, a guide plate
28
, and a probe plate
10
. The guide plate
28
is supported in a spaced relationship with the translator board
14
by plural spacers
8
(only one spacer shown). In turn, the probe plate
10
is supported in a spaced relationship with the guide plate
28
by plural standoffs
30
(only one standoff shown). Each of the probe plate
10
and the guide plate
28
have corresponding holes configured to receive a double-ended probe socket
20
.
The double-ended probe socket
20
is a generally elongated hollow body formed of an electrically conductive material. A spring loaded contact
12
is provided at a bottom end of the double-ended probe socket
20
, and an opening configured to receive a probe
22
is provided at a top end thereof. The probe socket
20
is inserted through a corresponding hole defined in the probe plate
10
and guide plate
28
. Notably, the guide plate
28
and the probe plate
10
provide lateral support for the probe socket
20
. The open end of the probe socket
20
is configured to receive an end of a probe
22
.
The probe
22
is a conventional spring loaded electrical contact probe such as disclosed in U.S. Pat. No. 4,814,698 to Johnston et al. The probe
22
has a spring loaded head for making electrical contact with a unit under test
18
. In operation, a unit under test
18
is placed on the probe heads such that each head is positioned in contact with a test point
16
on the UUT
18
. Next, a force F
1
along a longitudinal direction of the probe (shown by a corresponding arrow Fl in
FIG. 1
) pressing the UUT
18
into contact with the head of the probe
22
ensures firm electrical connection therebetween.
Force F
2
is exerted by the bottom head (contact)
12
of the probe socket
20
, pushing the contact
12
downward into contact with the translator board
14
. To ensure reliable electrical connection, the force F
2
must be sufficient to properly compress the spring loaded contact
12
of the double-ended probe socket
20
. Force Fl and F
2
are independent of each other.
The total force required to compress the spring loaded contact
12
of each of the hundreds or even thousands of probes required to test the complex circuitry of a UUT is considerable. In a conventional assembly
2
this force is transmitted to and tends to cause permanent damage to the translator board.
By manner of example, a typical probe requires approximately four ounces of pressure to compress the springs and provide reliable electrical connection. Thus, a test fixture having 200 probes would require 200×4=800 ounces, i.e., fifty pounds of pressure! This force is constantly (statically) exerted upon the translator board and tends, over time, to permanently damage the translator board.
A further problem affecting conventional test assemblies relates to transient forces which are transmitted to the translator board each time a force F
1
is applied to the UUT. This problem exists in conventional assemblies despite the use of standoffs. Notably, the transient forces are caused by flexure of the probe plate when subjected to the applied load during testing. Over time the transient force cycles cause wearing of the conductive plating of the pads
17
on the translator board. Ultimately, these transient force cycles result in unreliable electrical connection between the double ended probe socket bottom head
12
and the translator board
14
.
Accordingly, a need exists for a test assembly which maintains good electrical contact between a testing assembly and UUT, does not require fixed wiring, and does not exert static forces on the translator board. A need further exists for a test assembly which provides enhanced flexibility without requiring the use of double-ended probes. Still further, a need exists for a test assembly which eliminates or substantially reduces the transient forces exerted on the translator board.
SUMMARY OF THE INVENTION
Briefly, the present invention includes a wireless assembly for communicating electrical signals between test equipment and a unit under test using a probe. The probe has a head which is electrically connected to the unit under test, and a contact at an opposing end along a longitudinal direction of the probe. The probe contact is operably connected to a selected contact point on the translator board and provides reliable, wireless electrical connection therebetween without exerting a static force on the translator board.
According to a first embodiment the assembly includes a translator board which is electrically connected to the test equipment. The translator board is a printed circuit board having exposed conductive pads which are electrically connected to corresponding test points on the unit under test. Th

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