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.As a result, the bias probes design force the probe to worst case pointing accuracy by default.
The S100 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 IDplunger OD) 

a = extended length of plunger 

b = retained length of plunger 
For Example:
Size S100, 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 ICT100 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 offcenter 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
The 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:
If Y1 > Y2, then
MRL = PRL  ELR
If Y2 >Y1
MRL = MBT + PRL  ELR
For Example:
The Size R100 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 ringdiameter (PRD) and the mounting hole diameter (MHD) by onehalf. Then subtracting that value from the difference between the press ring diameter and the receptacle diameter (RD) multiplied by onehalf.
HO 
= 
1/2 (PRD  RD) 


 1/2 (PRD  MHD) 

= 
Horizontal Offset 
Simplifying 




HO = 1/2 (MHD  RD) 
For the R100 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
The 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 BuildUp
All three characteristics which affect the pointing accuracy of the probe have been calculated for the Size S100.
 Probe Pointing Accuracy = ± .003" (57.69%)
 Probe/Receptacle Concentricity = ± .0000" (0%)
 Receptacle/Mounting Hole ± .0022" (42.31%)
* * 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 S100 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 buildup 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 BuildUp ± 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) 
ICT50J & Size S50J 
S50J 
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) 
ICT50C & Size SC0 
S50C 
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) 
ICT075 & Size S075 
S075 
Standard 
.0020 (0.051) 
.0000 (0.0) 
.0032 (0.081) 
.0052 (0.132) 
.075" Centers
.250" Travel 
ICT075 
Standard 
.0011 (0.028) 
.0000 (0.0) 
.0032 (0.081) 
.0043 (0.109) 

S075 
RX 
.0020 (0.051) 
.0000 (0.0) 
.0000 (0.0) 
.0020 (0.051) 

ICT075 
RX 
.0011 (0.027) 
.0000 (0.0) 
.0000 (0.0) 
.0011 (0.027) 
ICT100 & Size S100 
S100 
Standard 
.0020 (0.051) 
.0000 (0.0) 
.0032 (0.081) 
.0052 (0.132) 
.100" Centers/.250" Travel 
ICT100 
Standard 
.0011 (0.028) 
.0000 (0.0) 
.0032 (0.081) 
.0043 (0.1096) 

S100 
RX 
.0020 (0.051) 
.0000 (0.0) 
.0000 (0.000) 
.0020 (0.051) 

ICT100 
RX 
.0011 (0.028) 
.0000 (0.0) 
.0000 (0.000) 
.0011 (0.028) 
