Test fixture alignment system for printed circuit boards

A test fixture for testing printed circuit boards and the like, the test fixture being of the type having a board mounting plate, an array of test probes mounted on a probe plate for contact with a pattern of test points in a printed circuit array on the board, the circuit array being positioned on the board with reference to a fiducial mark on the board, the board also having an alignment device positioned thereon in alignment with the printed circuit array, the test fixture including:

• a tooling pin rigidly affixed to the board mounting plate for engaging the alignment device on the board to hold the board and its circuit array in a fixed position relative to an array of test probes on the probe plate;
• optical fiber sensing means mounted in a fixed position on the probe plate for sensing the fiducial mark on the board and producing an image that represents the position of the fiducial mark;
• means for producing a fixed reference point that represents a fixed reference position with which the image of the fiducial mark position can be aligned to thereby align the array of test probes with the pattern of test points on the board;
• and
• drive means for moving the board mounting plate relative to the probe plate and the test probes for thereby moving the board relative to the fixed sensing means to correct any misalignment of the circuit array relative to the array of test probes, the sensing means producing said output to indicate the precise movement of the board mounting plate to an adjusted position relative to the probe plate necessary to align the sensing means image of the fiducial mark with the fixed reference point to thereby indicate precise alignment of the test probes with corresponding test points in the circuit array on the board;
• the drive means comprising an X-axis drive motor and a Y-axis drive motor each having an output shaft coupled to the board mounting plate to produce X and Y-axis travel of the board mounting plate, and further drive means to produce Z-axis rotational motion of the board mounting plate to move the circuit board relative to and generally parallel to the fixed probe plate and sensing means to thereby correct said misalignment.

A method for the diagnostic testing of printed circuit boards on a test fixture of the type having a board mounting plate, an array of test probes mounted on a probe plate for contact with a pattern of test points in a printed circuit array on a board under test, wherein the circuit board under test is held in pressure contact with the test probes to provide electrical contact between diagnostic testing equipment external to the fixture and the circuits on the board under test, the circuit array being positioned on the board with reference to a fiducial mark on the board, the board also having an alignment device positioned thereon for alignment with the circuit array, the method comprising:

• aligning a tooling pin on the board mounting plate with the alignment device on the board to hold the board and its circuit array in a fixed position relative to the probe plate and the pattern of test probes on the probe plate;
• mounting an optical sensing means to a fixed position relative to the probe plate for producing an image of the fiducial mark on the board which is held by the tooling pin;
• mounting X-axis and Y-axis drive motors to the board mounting plate, the drive motors being operative to move the board mounting plate and the circuit board in X and Y-axis movements and Z-axis rotational movements relative to the probe plate and its array of test probes;
• producing a reference image of a fixed reference point that represents a known correct position with which the fiducial mark image can be aligned to correctly align the array of test probes with the pattern of test points on the board;
• and
• operating the X-axis and Y-axis drive motors for adjusting the position of the board mounting plate relative to and generally parallel to the fixed probe plate to thereby move the board relative to the test probes and the fixed position of the optical sensing means to align the image of the fiducial mark with the reference image and thereby correctly align the array of test probes with the pattern of test points on the board, including electronically producing an optical image of the fixed reference point on a monitor and operating the drive motors to align an image of the fiducial mark on the monitor with the fixed reference point to produce said correct alignment.

A test fixture comprising:

• a board mounting plate for holding a board under test, the board having an array of test points in a printed circuit aligned on the board with reference to a fiducial mark on the board;
• a probe plate having an array of test probes for alignment with the test points in the circuits on the board;
• the board mounting plate and probe plate being movable toward each other to provide contact between the test probes and corresponding test points in the circuits on the board;
• optical sensing means in a fixed position on the probe plate for sensing the position of the fiducial mark on the board and producing a first image on a monitor to represent the position of the fiducial mark;
• means for producing a second image on the monitor representing a fixed reference point with which the first image is aligned to indicate a correct alignment of the test probes and the test points on the board under test;
• and
• an X-axis drive motor and a Y-axis drive motor in which each motor has an output shaft coupled to the board mounting plate to produce X and Y-axis travel and Z-axis rotational motion of the board mounting plate to move the board relative to and generally parallel to the fixed probe plate and thereby align the first and second images to correctly align the test probes with the test points in the circuits on the board under test.