| 摘要 |
A printed circuit board to be tested on a test fixture mounts sealingly on a movable top plate having through holes located directly beneath test points on the board. A base plate below the top plate is provided with a plurality of upstanding, spring-loaded test probes arranged to pass through the holes in the top plate when the two plates are drawn together by a vacuum established in the space between the plates. A resilient gasket seals the perimeter of the plates defining the vacuum chamber between the plates. The plates are held resiliently apart by compression springs. A vacuum source is connected to the vacuum chamber by a vacuum control. The test probes are of two different heights. When a full vacuum is applied to the vacuum chamber, the plates are pulled close together and all test probes contact test points on the circuit board in a first test mode of operation. The vacuum control includes a sensor of the distance between plates and admits enough vacuum to maintain a preset interplate distance for a second test mode, wherein the plates are spaced apart sufficiently to enable a first population of test probes to contact test points on the circuit board while a second, shorter population of test probes remain spaced apart from the circuit board. An atmospheric venting valve with feedback control facilitates moving from one test mode to another. In another embodiment additional levels or test modes are provided.
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