Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2002-04-02
2003-12-23
Karlsen, Ernest (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S761010
Reexamination Certificate
active
06667628
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to testing systems. Specifically, the present invention relates to electronic testing systems.
2. Description of the Related Art
Modern electronic systems are implemented with a large variety of circuits and devices (e.g. transistors, logic gates, etc). The circuits and devices are often located in a very small area. In addition, the connections between devices are routed within a small area. With such a large density of devices and connections between devices located in a small area, an industry has developed around testing modern electronic systems. The electronic systems are often deployed in circuit boards such as Printed Circuit Boards (PCB). These PCB boards include a large population of devices and a large population of connections between the devices. As a result, a variety of electronic test systems have developed to test Printed Circuit Boards.
A conventional PCB includes devices and a number of connections between the devices. The connections between devices serve as conduits for carrying electrical current between devices. The conduits for carrying electrical current are often referred to as traces. These conduits are also used in testing the devices and ultimately testing the PCB. The conduits running between devices are typically made of a conducting material such as metal. In addition on both the topside and the underside of the PCB, pockets of the metal material called pads, are formed to provide a point of contact for testing.
A conventional electronic test system includes a fixture and tester electronics. The fixture holds a board under test (e.g. PCB). In addition, the fixture aligns the board under test and provides a mechanism for signals generated by the tester electronics to reach the board under test and then return to the tester electronics for analysis. The fixture is positioned on top of the tester electronics.
Electronic test systems perform test on PCB's by sending currents through traces on the board under test. The currents are sent from one point on the PCB to a second point on the PCB. Typically, a contact is made with a pad on the underside of the PCB at a first location and a contact is made with a pad on the underside of the PCB at a second location. Current is then generated from the first location through the traces to the second location. A voltage can then be measured at the second location to determine if the traces and the connection to the traces are working properly. Tester electronics generate and measure the current. The tester electronics typically includes software that controls and automates the process.
Conventional electronic test systems typically include a wired fixture or a wireless fixture. In conventional electronic test systems that include a wired fixture, a board under test (e.g. PCB) is placed on a support located at the top of a fixture. A plurality of probes run through the central area of the fixture. The probes are housed in a probe plate. The probe plate keeps the probes in a substantially vertical position, so that the probes can serve as an electrical pathway for test signals. The probes make contact with the board under test at one end and extend within close proximity to the tester electronics on the other end. Wires are then run from the tester electronics to the probes via pins inserted in the probe plate. As a result, an electrical pathway is established from the tester electronics, across the wires, to the probes and then to the board under test. Test signals are then generated by the tester electronics. The test signals run across the electrical pathway and back to the tester electronics along a similar path. The test signals are then analyzed by the tester electronics.
In a wired fixture, the wires provide an electrical pathway for trace currents. The currents run through wires and then through the probes, to the board under test. However, as the number of devices on PCB's has increased and the sizes of the PCB's have decreased, it has become difficult to place these wires in such a small area. For example, a PCB that is 16 inch by 24 inch may have 3000 to 4000 devices on the board. As a result, 3000 to 4000 wires may need to be connected from the tester electronics to the probes. This results in an incredible amount of congestion in a very small area. In addition if there is a malfunction, it is very difficult to identify a single dysfunctional wire within the 4000 wires. Therefore troubleshooting becomes a major issue.
As a result, a more modern fixture assembly evolved which attempts to eliminate the need for wires in a fixture. This more recent version of the fixture is often referred to as a wireless fixture. In the more recent version, a fixture houses probes, which are used to engage pads on the underside of a board under test. A fixture PCB or wireless PCB is positioned within the fixture and located on an oppositely disposed end of the probes. The wireless PCB includes a plurality of trace patterns for conducting electrical signals within the PCB between pads on both the topside and underside of the wireless PCB. Contact is made between the tester and the underside of the wireless PCB. As a result, an electrical pathway is established between the tester and the wireless PCB. The test signals are routed through the various trace patterns within the wireless PCB. Probes then make contact with the topside of the wireless PCB and an electrical pathway is established between the wireless PCB and the board under test. Ultimately, using the wireless PCB, an electrical pathway is established from the tester, through the wireless PCB, to the board under test.
In order for a conventional electronic test system to function properly, a good electrical pathway must be established between the tester electronics and the board under test. As a result, the contacts and pathway between the tester electronics, the wireless PCB, the probes and the board under test must be established and maintained. In a conventional electronic test system, force is applied to the board under test and the wireless PCB so that the probes can conduct electricity by remaining in contact with both boards. Once the board under test and the wireless PCB are in contact with the probes, the fixture is able to facilitate the transfer of test signals to the board under test. However if there is spacing between either board and the probes, the test signals may not be conducted or may be conducted and produce incorrect readings.
The contact is maintained between the fixture PCB's and the fixture probes by applying downward forces on the board under test and upward force on the wireless PCB. The board under test is often placed in a vacuum-sealed area and downward forces are applied by removing the air out of the vacuum-sealed area. When the air is removed from the vacuum-sealed area, the board under test experiences a downward force (e.g. vacuum force). In addition, spring-loaded tester contact points (e.g. pins), which engage the underside of the wireless PCB, create upward forces on the wireless PCB. The fixture probes maintain their contact as a result of the downward force from board under test and the upward force from the wireless PCB.
As a result of the foregoing configuration, a great deal of stress can build up in a fixture. As mentioned above, the wireless PCB is subject to forces pushing upward so that it maintains contact with the probes. In addition, there are also downward forces applied to the board under test when air is removed from the vacuum sealed chamber or in systems without vacuum sealing, forces appear when the board under test is pulled down into position for testing. The upward forces are transferred through the wireless PCB to the probes. In addition, the downward forces are transferred through the board under test to the probes. Therefore, the probes receive both upward and downward forces in the fixture. Since the probes are mounted in the probe plate, these forces are also transferred to the probe plate.
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Ahrikencheikh Cherif
Stahmer Julie L
Agilent Technologie,s Inc.
Hollington Jermele M.
Karlsen Ernest
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