Work holders – Holder inserted within work aperture – Holder mounted for movement
Reexamination Certificate
2000-07-31
2001-09-04
Watson, Robert C. (Department: 3723)
Work holders
Holder inserted within work aperture
Holder mounted for movement
C269S903000
Reexamination Certificate
active
06283464
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention generally relates to test fixtures for circuit boards and, more particularly, to an adjustable tooling pin for positioning electronic circuit boards in a vacuum test fixture.
2. Description of the Prior Art
Circuit board designers are constantly faced with the challenge of packaging more and more electronics into a smaller amount of space. Often times, during the design process, circuit test pads must be relocated to other places on the board to make room for other functional components. Test pads are necessary on circuit boards to allow interconnection to test equipment via a test fixture. A test fixture is typically an automated device which comprises a plurality of fixed upwardly projecting spring-loaded test probes. This type of fixture is commonly referred to as a “bed-of-nails” test fixture. Each of the test probes in the bed correspond to a test pad on the circuit board such that when the board is pulled down on top of the probes, each probe is brought into electrical contact with its corresponding test pad.
FIG. 1
shows an example of a bed-of-nails test fixture for testing circuit boards (cards) generally referred to by the reference numeral
10
. The test fixture
10
comprises a probe plate
12
having a plurality of upwardly projecting spring-loaded test probes
14
which resemble a bed-of-nails. Each of the test probes
14
correspond to a test pad
16
on the card
18
under test. The card
18
sits on blocks
20
above a rigid top plate
22
. A tooling pin
24
, fixed to top plate
22
, fits into an alignment or tooling hole in the card
18
to provide alignment for the test probes
14
and the test pad
16
. The guide plate must be made of a non-conductive material so as not to interfere with the electrical signals from the tester. When a vacuum chamber
28
, sealed by a vacuum mat
30
and vacuum seal
32
, is evacuated of air, atmospheric pressure pushes down on the vacuum mat
30
and top plate
22
compressing springs
26
and causing the top plate
22
to move in a downward direction along guide pin
34
. A backer plate
36
, connected to the guide plate
22
, pushes down on backer post
38
which, in turn, pushes down on the circuit board
18
under test. Guide apertures
40
allow probe tips
42
to slide therethrough and make electrical contact with the test pad
16
.
A drawback to the test fixture shown in
FIG. 1
is the non-adjustable design of the tooling pins
24
. This is a problem since card assembly processes constantly change the tooling pin locations. For example, heat from wave solder processes or heat created at card burn-in can change the tooling hole location. The tooling pins
24
must accurately locate the card under test
18
to spring loaded test probes
14
in the test fixture to make sure that each test probe makes good electrical contact to its corresponding test pad
16
. This leads to particular alignment problems for test pads located on 25 mil (0.063 mm), 40 mil (0.101 mm) and 50 mil (0.127) drill centers since the tooling pins
24
are not adjustable.
There are two methods used for aligning the card
18
under test with the bed of test probes
14
. The first method comprises physically changing the location of the tooling pin
24
by plugging the original tooling hole in the top plate
22
and redrilling at a new location. This effort may need to be repeated for each batch of cards where the test pads have changed to again realign the card
18
to the test probes
14
. This particular method of alignment is time consuming and inefficient and can only be preformed a finite number of times before the top plate
22
has too many plugged holes to be useful.
The second method of realigning the card
18
under test with the bed-of-nails test probes
14
involves the use of a sophisticated optical alignment system with micrometer adjustments. However, this method is prohibitively expensive.
As technology drives the sizes of components and test pads down, it becomes increasingly important to have an accurately aligned tooling pin as well as a method for making minute and frequent adjustments at the tester itself while the product is undergoing test.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an adjustable tooling pin for a circuit board test fixture which can easily and repeatedly change the position of the card under test relative to the probes.
It is yet another object of the present invention to provide an adjustable tooling pin which radially moves to any position in an area of the x-y plane over the top plate.
It is yet another object of the present invention to provide an apparatus and method for precisely locating the card under test to the test probes without taking the test fixture off-line, and without toolmaker involvement.
According to the invention, an adjustable tooling pin for a bed-of-nails test fixture which is used for testing circuit boards (cards) is provided. A card to be tested includes a plurality of test pads to be contacted by a corresponding plurality of test probes. The card to be tested is fixed by the adjustable tooling pin to a top plate having apertures therethrough aligned with the test pads. As the top plate is lowered over the test probes the probes pass-through the apertures and contact the test pads. The adjustable tooling pin mechanism comprises a pivot bushing having a channel running from end-to-end into which a slide bar fits. The tooling pin projects from the end of the sliding bar in the z-direction such that a card may be installed over the tooling pin. A locking screw fits through an aperture in the sliding bar and the center of the pivot bushing and is threadably engaged into a threaded insert set in the test fixture top plate.
In operation, a card to be tested is installed over the tooling pin above the top plate. The tooling pin, and thus the card, is radially adjustable in the x-y plane by turning the pivot bushing and by sliding the sliding bar along the pivot bushing channel. Hence, the adjustable tooling pin mechanism of the present invention allows the tooling pin to be precisely realigned while the card is still mounted in the test fixture. This adjustable tooling pin is especially beneficial for modern card technology which requires electronic card assemblies to be densely populated, with the test pads located on 25 mil (0.063 mm), 40 mil (0.101 mm) and 50 mil (0.127 mm) drill centers, and test pad diameters of 0.020″ (0.0508 mm) or less.
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Fraley Lawrence R.
International Business Machines - Corporation
McGuireWoods LLP
Watson Robert C.
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