Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2001-07-18
2003-09-30
Karlsen, Ernest (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S761010
Reexamination Certificate
active
06628130
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates generally to test equipment used to test printed circuit boards and more specifically to the use of a wireless fixture in a printed circuit board test system.
2. Discussion of the Prior Art
After printed circuit boards (PCB's) or other electrical interconnect planes or devices have been manufactured and loaded with components, and before they can be used or placed into assembled products, they should be tested to verify that all required electrical connections have been properly completed and that all necessary electrical components have been attached or mounted to the board in proper position and with proper orientation. Other reasons for testing printed circuit boards are to determine and verify whether the proper components have been used and whether they are of the proper value. It is also necessary to determine whether each component performs properly (i.e., in accordance with the specification). Some electrical components and electro-mechanical components also may require adjustment after installation.
Loaded-board testing has complex multiplexed tester resources and is capable of probing soldered leads, vias and testpads on loaded boards with topside and bottom side components. Loaded-board testing includes analog and digital tests, such as tests for electrical connectivity, voltage, resistance, capacitance, inductance, circuit function, device function, polarity, vector testing, vectorless testing, and circuit functional testing. Loaded-board testing requires very low contact resistance between the test targets and the fixture components.
Advances in circuit board and electronic component packaging technology have escalated the probe spacing demands placed on loaded-board test equipment. Existing state-of-the-art technology requires loaded-board test equipment capable of accessing test targets which are spaced apart by 50 mils (center to center) or less, where test targets are physical features on a PCB or electronic component which may be probed during testing. One of the greatest challenges faced by loaded-board test equipment manufacturers now and in the future is a high false failure and test malfunction rate caused by physical and electrical contact problems. These problems are exacerbated by existing fixture limitations in probing accuracy, probing pitch (center to center spacing), and surface contamination.
As component and board geometries shrink and become denser, loaded-board testing becomes more difficult using standard fixtures. Existing shortwire, loaded-board fixtures can consistently hit test targets equal to or greater than 35 mils in diameter with equal to or greater than 75-mil pitch. Targets that are smaller or more closely spaced cannot be probed with consistency due to prohibitive component and system tolerance stack-ups.
A variety of test fixtures have heretofore been available for testing loaded boards on test equipment. A device under test (DUT) often embodies a PCB loaded with electronic components and electronic hardware. One approach that addresses the limited-access problems during testing is the use of wireless fixtures. The term “limited-access” refers to something that cannot easily be reached, or accessed, due to physical restrictions or constraints. For example, a limited-access PCB may contain many targets that are too closely spaced to accurately probe using existing fixture technology.
The term “standard-access” refers to that which can be reached, or accessed, using existing fixture technology. The use of wireless fixtures allows a greater coupling of test targets to test resources.
FIG. 1
shows one wireless fixture that attempts to address limited-access problems during testing. The wireless fixture of
FIG. 1
shows a fragmentary cross-sectional view of a representative wireless fixture that is currently used in the Agilent 3070 test system. The PCB
120
provides electrical connections between pins
100
and double-ended sockets
20
. The PCB
120
also has holes for mounting screws
80
. This PCB
120
has permanently attached pins
100
. The test system interface probe
90
provides the interface between the PCB
120
and the test system. The test system interface probe
90
allows the pin
100
to provide an electrical interface between the PCB
120
and the test system. The alignment plate
110
is used to orient the pin
100
properly with respect to the test system interface. The double-ended socket
20
is a receptacle for a standard spring probe
30
used to make electrical contact to the DUT
130
. The double-ended socket
20
is press mounted into the probe mounting plate
10
and has a spring loaded probe tip that makes electrical contact to PCB
120
. Probe mounting plate
10
is attached to the fixture frame
40
, which is coupled to alignment plate
110
. A guide plate
70
, comprising a thin insulating sheet, is used to protect the double-ended socket
20
during assembly. The guide plate
70
is oriented relative to the probe mounting plate
10
and the PCB
120
by the use of a standoff
50
between the guide plate
70
and the probe mounting plate
10
and a standoff
60
between the PCB
120
and the guide plate
70
. A number of screws
80
are used to resist the spring forces generated by the double end sockets
20
and the test system interface probes
90
. These screws
80
pass through the PCB
120
, then the first standoff
60
, the guide plate
70
, the second standoff
50
, and finally enter the probe mounting plate
10
.
Current wireless fixtures need to assemble the printed circuit board by attaching pins and require the printed circuit board to carry loads imposed by springs used in electrical contacts. Installation and soldering of pins onto a circuit board is not a common process for typical fixture builders. The materials typically used for constructing printed circuit boards are not very stiff. The travel distance for the probe tip in a typical double-ended socket is 0.100 inch so PCB supports must be close together to adequately control deflection of the PCB. In current implementations this leads to the use of a large number of screws and standoffs for mounting the PCB to the probe mounting plate. In order to keep the PCB relatively flat during fixture assembly these mounting screws must be carefully tightened in small increments. This is very time consuming and is a decrement to disassembling the fixture if changes are needed. The standoffs and screws must be located where double ended sockets are not required. Determining an adequate number of mounting screws and locating them must be done before the PCB can be routed which can lead to longer fixture build times. The space needed for the mounting holes in the PCB takes away space for routing traces and can increase the number of layers required which increases the cost of the PCB.
There is a need for an improved wireless test fixture that places less reliance on the printed circuit board or other electrical interconnect device as a structural element, that causes less deflection of the PCB due to contact forces, and that does not require screw holes in the area of double ended sockets. There is a further need for such an improved wireless test fixture that has an assembly process that does not require pins to be attached to the PCB, that allows the PCB to be routed independent of spacer location, that is easier to assemble and disassemble when changes must be made, and that allows faster assembly due to the reduction in the number of screws required. There is a further need in the art for a wireless fixture that allows the reuse of some components when the wireless fixture is retired.
SUMMARY OF THE INVENTION
It is an object of the invention to allow routing of the printed circuit board or other electrical interconnect plane in the wireless test fixture independent of spacer location.
It is further an object of the invention that the wireless test fixture use pins separate from the printed circuit board.
It is further an object of the invention that the assemb
Slutz Robert A.
Williams Michael C.
Agilent Technologie,s Inc.
Hollington Jermele
Karlsen Ernest
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