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Surface Mount Technology (SMT) has changed and will continue to change electronics manufacturing.  As pad size has decreased the demand for improved pointing accuracy has increased. In this section, a thorough explanation of probe pointing accuracy and the interrelationship between pointing accuracy and pad size (target size) is discussed.

The pointing accuracy of a probe can be divided into three categories.

  • Probe pointing accuracy
  • Probe/receptacle concentricity
  • Receptacle/mounting hole concentricity

Probe Pointing Accuracy


Probe pointing accuracy is defined as the variation in the actual location of the probe tip from test to test and is internal to the probe. Probe pointing accuracy is influenced by the following factors:

  • Straightness of the Plunger
  • Maximum Working Clearance
  • Retained Length of Plunger
  • Extended Length of Plunger
  • Probe Design

Until recently, all probes were designed with an inherent bias to ensure positive electrical contact between the plunger and barrel.PointingAccuracyAs a result, the bias probes design force the probe to worst case pointing accuracy by default.

The S-100 design, with a reduced clearance at the top of the barrel, improves pointing accuracy of a bias design by reducing the allowable angle at which the plunger sits in the barrel. The ICT Series eliminates biasing completely. The bifurcated beams at the top of the barrel force the plunger to perfect center, without sacrificing positive electrical contact.


The formula below is a simplified version of calculating pointing accuracy for standard spring contact probes. 

e = ±c (.625a/b + .125)

where e = pointing accuracy
  c = max. working clearance (barrel ID-plunger OD)
  a = extended length of plunger
  b = retained length of plunger

For Example:

Size S-100, use the following formula: 
e = ±c(.625a/b + .125)
  = ±0.002((.625 x .330/.232)+ .125)
  = ±0.002(.625 x 1.422 + .125)
  = ±0.002(.8888 + .125)
  = ±0.002(1.0138)
  e = ±0.002

For the ICT-100 Probe, pointing accuracy is calculated using the following formula: 
e = 1/2c(a/b)
  = 1/2(.0016)(.330/.232)
  = .0008(1.422)
  = .0011

Probe/Receptacle Concentricity

The probe/receptacle concentricity is defined as the offset or angle, which occurs when the probe rests inside the receptacle. The factors influencing this dimension are as follows:
  • Barrel Outside Diameter
  • Receptacle Inside Diameter
  • Straightness of the Receptacle
  • Detent Location and Design

Typically, the clearance between the outside diameter of the barrel and the inside diameter of the probe is 0.001".

In a single detent design, the probe is pushed off-center to one side of the receptacle. This results in a .0005" offset from the centerline of the receptacle. In the four detent design, the detents which are 90° offset from each other, center the probe in the receptacle.

Receptacle/Mounting Hole Concentricity

ConcentricityThe receptacle/mounting hole concentricity is defined as the offset that occurs when the receptacle is press fit into the mounting hole. Factors which influence the receptacle/mounting hole concentricity
  • Mounting Plate Thickness
  • Mounting Hole Size
  • Receptacle Diameter
  • Receptacle Straightness

Calculating the offset of the receptacle/mounting hole is a complicated procedure. The worst case offset can be easily calculated.

Step 1

The first step is to determine the maximum retained length of the plunger below or above the centerline of the press ring. Using the figure below:RetainedLength

If Y1 > Y2, then
 MRL = PRL - ELR


If Y2 >Y1
 MRL = MBT + PRL - ELR
   







For Example:

RetainedLengthChartThe Size R-100 receptacle has a press ring location of .300" from the top of the receptacle to the bottom of the press ring. Typically, press rings are .030" in length. Therefore, the centerline location of the press ring is .285". If the Mounting Board Thickness (MBT) is .375", then Y1 > Y2 as long as the extension length does notexceed .0975" [PRL - (MBT/2)].

Step 2

The second step is to determine the horizontal offset (HO) of the receptacle in the mounting hole. This is calculated by multiplying the difference between the press ringOffsetdiameter (PRD) and the mounting hole diameter (MHD) by one-half. Then subtracting that value from the difference between the press ring diameter and the receptacle diameter (RD) multiplied by one-half.

HO =    1/2 (PRD - RD)
    - 1/2 (PRD - MHD)
  =   Horizontal Offset
Simplifying    
    HO = 1/2 (MHD - RD)

For the R-100 receptacle,
Press Ring Diameter - PRD = .071" 
Mounting Hole Diameter - MHD = .068" to .070"
Receptacle Diameter -  RD = .066" 

Therefore: 
For .068" diameter mounting hole 
   HO - 1/2 (.068 - .066) = .001" 

For .070" diameter mounting hole 
   HO = 1/2 (.070 - .066) = .002
 

Step 3

OffsetChartThe third step is to determine the maximum angle at which the receptacle can be offset in the mounting hole. This can be calculated using right triangles.

The table following lists the angle (Q) for the minimum and maximum horizontal offsets (HO) at the maximum retained lengths (MRL) listed in the previous table. The formula used to calculate the angle is:

   TAN θ =HO/MRL 

 

Step 4

Once the angle of the receptacle in the mounting hole has been determined, the forth step is to determine the offset of the probe tip from the center line of the mounting hole.  The total offsset (TO) of the probe tip fromthe centerline of the receptacle canbe calculated usinf the following formula.
 
TAN θ = TO/TE
 
Where  
 TE = total extension from the press ring to the tip of the probe.
 TE = PRL + Extended length of the plunger
  = .285 + .330
  = .615

Using the information in the table below, the TO has been calculated for the angles determined in Step 3. This information indicates that to minimize angular misalignment of the receptacle in the mounting hole:

  • The smallest possible mounting hole should be used to minimize the horizontal offset of the receptacle in the mounting hole.
  • Increasing the maximum retained length available will always minimize the misalignment.
  • Since the maximum retained length (MRL) is critical to the misalignment it is important to note the effect of the mounting board thickness (MBT).
    • Increase Mounting Board thickness to decrease Total Offset
    • Decrease Mounting Board thickness to increase Total Offset

Worst Case Tolerance Build-Up

All three characteristics which affect the pointing accuracy of the probe have been calculated for the Size S-100.
  • Probe Pointing Accuracy = ± .003" (57.69%)
  • Probe/Receptacle Concentricity = ± .0000" (0%)
  • Receptacle/Mounting Hole ± .0022" (42.31%) TotalOffsetChart
                * * Flush mounted, minimum horizontal offset 

Total Pointing Accuracy 
= ± .003 + .000 + .0022
= ± .0052 (100%) (worst case) 

Probes and Receptacles for Improving Total Pointing Accuracy

Analyzing the distribution of Pointing Accuracy for the Size S-100 Probe, .250" stroke, it is found that 42% of the total misalignment is contributed to the receptacle mounting and 58% to the probe pointing accuracy. The percentages will vary with the probe size and style. 

The table on below details pointing accuracy for standard, SX and ICT probe designs for various sizes. Also included is the standard and RX receptacles effect on pointing accuracy.

Minimum Required Total Pointing Accuracy

Knowing the Total Pointing Accuracy (probe/receptacle/ mounting hole) is useful information in determining the ability of the probe to accurately hit the target. However, tolerance build-up in the fixture and component placement also affect the probe's ability to hit the target.

Typical PCB and Fixture Tolerances 
  • Pad Size ± 0.002"
  • Pad Location ± 0.003"
  • Effect of Angle of Drilled Socket Hole ± 0.003"
  • Tooling Hole Size ± 0.003"
  • Tooling Hole Pin ± 0.0005"
  • Worst Case Tolerance Build-Up ± 0.0115"

Typical Probe Mounting Tolerance

  • Probe Location to Tooling Hole ± 0.003"
The following formula determines the required total pointing accuracy needed to accurately probe with fixture tolerances considered.

Minimum Required Target Size (MRTS) 
Total Pointing Accuracy (TPA) = ±0.0100 
Target Location Tolerance (TLT) = ±0.0115 
Probe Mounting Tolerance (PMT) = ±0.0030 

MRTS = TPA + TLT + PMT
= 0.0100 + 0.0115 + 0.0030 
= ±0.0245
= 0.049 diameter pad 

The Total Pointing Accuracy Figure can be changed to accommodate the style of probe being used. The figure of 0.010" was chosen for demonstration purposes only.

If the target size is known, then the formula to determine the minimum required total pointing accuracy of a probe is: 

TPA = (1/2 x MRTS) - TLT - PMT 
For a pad size of .035", the required total pointing accuracy would be: 

TPA = (0.5 x 0.035) - 0.0115 - 0.003 
TPA = ± 0.0030" 
 
Comparison of Combined Pointing Accuracy 
 
All dimensions are in inches (millimeters).
Probe
Series
Probe
Design
Receptacle
Design
Category 1
Probe
Category 2
Probe in
Receptacle
Category 3
Receptacle
Mounting Hole
Combined
Worst
Case
Size 0 Standard Standard .0014 (0.036) .0005 (0.013) .0012 (0.031) .0031 (0.079)
.050" Centers
.100" Travel
Standard RX .0014 (0.036) .0005 (0.013) .0000 (0.0) .0019 (0.048)
  SX Standard .0007 (0.018) .0005 (0.013 .0012 (0.031) .0024 (0.061)
  SX RX .0007 (0.018) .0005 (0.013) .0000 (0.0) .0012 (0.031)
ICT-50J & Size S-50J S-50J Standard .0024 (0.061) .0000 (0.0) .0022 (0.056) .0046 (0.117)
.050" Centers
.250" Travel
ICT 50J Standard .0008 (0.020) .0000 (0.0) .0022 (0.056) .0030 (0.076)
ICT-50C & Size SC0 S-50C Standard .0024 (0.061) .0000 (0.0) .0022 (0.056) .0045 (0.114)
.050" Centers
.250" Travel
ICT 50C Standard .0008 (0.020) .0000 (0.0) .0022 (0.056) .0028 (0.071)
Size 1 Standard Standard .0034 (0.086) .0000 (0.0) .0011 (0.028) .0045 (0.114)
.075" Centers
.100" Travel
SX Standard .0017 (0.043) .0000 (0.0) .0011 (0.028) .0028 (0.071)
ICT-075 & Size S-075 S-075 Standard .0020 (0.051) .0000 (0.0) .0032 (0.081) .0052 (0.132)
.075" Centers
.250" Travel
ICT-075 Standard .0011 (0.028) .0000 (0.0) .0032 (0.081) .0043 (0.109)
  S-075 RX .0020 (0.051) .0000 (0.0) .0000 (0.0) .0020 (0.051)
  ICT-075 RX .0011 (0.027) .0000 (0.0) .0000 (0.0) .0011 (0.027)
ICT-100 & Size S-100 S-100 Standard .0020 (0.051) .0000 (0.0) .0032 (0.081) .0052 (0.132)
.100" Centers/.250" Travel ICT-100 Standard .0011 (0.028) .0000 (0.0) .0032 (0.081) .0043 (0.1096)
  S-100 RX .0020 (0.051) .0000 (0.0) .0000 (0.000) .0020 (0.051)
  ICT-100 RX .0011 (0.028) .0000 (0.0) .0000 (0.000) .0011 (0.028)