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X Probe Termination Set Heights

An X Probe Termination pin set height is a critical factor in the performance and life of the X Probe. When set too low, the X Probe is under-stroked, reducing the contact force and the probe's ability to penetrate surface contaminants. When set too high, the X Probe is over-stroked, resulting in decreased spring life or possible tip or UUT/PCB damage due to bottoming. To calculate the proper set height, follow these steps:

Set Height Calculations

STEP 1

Make a cross-sectional sketch of the fixture in the actuated (2/3 stroke) position. Diagram A is typical of many vacuum fixtures.

STEP 2

Dimension the thickness of the items that stack up on the top surface of the probe mounting plate. Add these dimensions to get a final distance (H) from the top of the plate to the contact surface of the UUT. Subtract the average lead length from this dimension if contacting component leads.

  • (A) is the distance from the UUT to the top of the Back Plate.
  • (C) is the X Probe tube length (see Table A).
  • (P) is the distance from the X Probe tip to the top of the X Probe tube (see Table A). Remember this dimension is with the X Probe compressed to its recommended working stroke.
  • (A) = (H) + .375 [9.53] (Probe Plate) + .563 [14.29] (optional Spacer Plate). Use actual plate thickness or standoff lengths as per your design.
  • Proper set height = (A) - (C) - (P)
  • Example for X39-25 Series

    H = Spacer + Support Plate + Board Stop
    = .062 [1.58] + .196 [4.98] + .125 [3.18] = .383 [9.73]
    P = .105 [2.67] (from table)
    Set Height = (A) - (C) - (P)
    = 1.321 [33.55] - 1.178 [29.92] - .105 [2.67] = .038 [.97]

    To mix X Probe Series within a fixture, calculate the proper set height for each series used in the fixture. Thicker mounting plates allow the greatest set height range, but hole straightness may suffer. For dual level testing, a .400 [10.16] stroke X Probe is mounted using the same termination pin at the same set height, as a standard .250 [6.35] stroke X Probe. As a result, .250 [6.35] and .400 [10.16] stroke X Probes can be interchanged freely from one termination pin to another as test needs dictate.

    Key Probe Dimensions for Calculating Set Heights
    Series Probe Tube  (Length C) Recommend (Extension P)
    X31-16P 0.650 [16.51] 0.113 [2.87]
    X31-16H 0.650 [16.51] 0.173 [4.39]
    X31-25 1.210 [30.73] 0.133 [3.38]
    X31-40 1.210 [30.73] 0.133 [3.38]
    X31-40-D * 1.210 [30.73] 0.283 [7.19]
    X39-16P 0.650 [16.51] 0.113 [2.87]
    X39-16H 0.650 [16.51] 0.173 [4.39]
    X39-25 1.178 [29.92] 0.105 [2.67]
    X39-40 1.178 [29.92] 0.120 [3.05]
    X39-40-D * 1.178 [29.92] 0.270 [6.86]
    X50-16P 0.650 [16.51] 0.113 [2.87]
    X50-16H 0.650 [16.51] 0.173 [4.39]
    X50-25 1.110 [28.19] 0.133 [3.38]
    X50-40 1.110 [28.19] 0.133 [3.38]
    X50-40-D * 1.110 [28.19] 0.283 [7.19]
    X75-16P 0.650 [16.51] 0.113 [2.87]
    X75-16H 0.650 [16.51] 0.173 [4.39]
    X75-25 1.100 [27.94] 0.143 [3.63]
    X75-40 1.100 [27.94] 0.143 [3.63]
    X75-40-D * 1.100 [27.94] 0.293 [7.44]

    * See Decreased Stroke Probes for more information
    ** To account for irregularities at the hole ends, a margin of at least .020 [0.51] is recommended between the press ring and the closest plate surface.