Socket Set Heights

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

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 leaded components.

Step 3

Calculate the distance (P) from the probe tip to the top of the socket in which it is mounted. Remember to calculate this dimension with the probe compressed to its recommended working travel. See Table for key probe dimensions.

Step 4

Example for 100-25 Series:
H = Spacer + Support Plate + Board Stop
= .062 [1.58] + .188 [4.78] + .125 [3.18] = .375 [9.53]
P = .163 [4.14] (from table)
Set Height = H - P
= .375 [9.53] - .163 [4.14] = .212 [5.39]
Subtract P from H. The result is the proper set height.

To mix-mount probes, 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, .400 [10.16] stroke probes are mounted in the same sockets, at the same set height, as standard .250 [6.35] stroke probes. As a result, they can be interchanged freely from one socket to another as test needs dictate.

Key Probe Dimensions for Calculating Set Heights

 

Probe Tip
Height A

Exposed
Shank B

Probe Tube
Extension C

Recommend
Extension P

025-16*

.010 [0.25]

.053 [1.35]

.020 [0.51]

.083 [2.11]

039-16

.015 [0.38]

.053 [1.35]

.024 [0.61]

.092 [2.11]

039-25

.015 [0.38]

.118 [3.00]

.024 [0.61]

.157 [3.99]

039-40***

.015 [0.38]

.118 [3.00]

.024 [0.61]

.157 [3.99]

050-05*

.040 [1.02]

.000 .000

.040 [1.02]

 

050-16

.020 [0.51]

.051 [1.35]

.000

.073 [1.85]

050-25****

.022 [0.56]

.083 [2.11]

.083 [0.96]

.143 [3.63]

050-40

.022 [0.56]

.098 [2.49]

.043 [1.09]

.163 [4.14]

050-40-D***

.022 [0.56]

.248 [6.30]

.043 [1.09]

.313 [7.95]

075-25

.050 [1.27]

.083 [2.11]

.030 [0.76]

.163 [4.14]

075-40

.050 [1.27]

.083 [2.11]

.030 [0.76]

.163 [4.14]

075-40-D***

.050 [1.27]

.233 [5.92]

.030 [0.76]

.313 [7.95]

100-05*

See Page 30

.000

.005 [0.13]

See Page 30

100-16

.080 [2.03]

.053 [1.35]

.010 [0.25]

.143 [3.63]

100-25

.060 [1.52]

.083 [2.11]

.020 [0.51]

.163 [4.14]

100-40

.060 [1.52]

.083 [2.11]

.020 [0.51]

.163 [4.14]

100-40-D***

.060 [1.52]

.233 [5.92]

.020 [0.51]

.313 [7.95]

125-25

.080 [2.03]

.083 [2.11]

.005 [0.13]

.168 [4.27]

156-25

.100 [2.54]

.083 [2.11]

0.00

.183 [4.65]

187-25

.100 [2.54]

.083 [2.11]

0.00

.183 [4.65]

*100-05, 050-05 and 025-16 Series sockets are all mounted flush (set height is zero).

**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.

***See "Decreased Stroke Probes" for more information.

****When installing the 050-25 socket for the .250 [6.35] stroke probe for dual level testing, the socket must be set .015 [.38] higher to achieve the designed .150 [3.81] tip height difference when the fixture is not actuated.