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, athorough 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. As 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 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
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