Pointing Accuracy

During the testing of Printed Circuit Boards (PCB's), spring loaded test probes contact test sites on the Unit Under Test (UUT) and the specified electrical test is performed. The test sites include but are not limited to pads, vias, leads, posts, components, and connectors. In an ideal situation, the probe tip will make contact with the test site every time. Unfortunately, if not considered during the design stages, the component tolerances between the board, fixture, and probe manufacturers can create a situation where the probes tip miss the test site and a false test failure is encountered. Until recently, detailed pointing accuracy studies concentrated mainly on close-center SMD probes. However, as larger probes are increasingly used for contacting small targets, their accuracy becomes just as important as that of their smaller counterparts.

The information in this section is meant to explain the variables, define the tests, and most importantly, to provide engineers and designers with needed probe accuracy specifications.

Scope

This study presents empirical pointing accuracy data for loaded and bare-board probes made by QA Technology. The information can be used in conjunction with tolerances from the test fixture and PCB boards to properly size test pads for reliable contact. When discussing the ability of a probe to accurately contact its intended target, the effects of standard groups of tolerances must be classified. The tolerances which affect a probe's ability to accurately contact its target can be broadly divided into four groups as follows:

(Refer to Diagram A)

Diagram A

pointing accuracy
  1. "Fixture Offset" tolerances related to the Unit Under Test and the test fixture. This group includes artwork registration; guide pin clearance to the UUT, pin location, pin straightness, location and tolerance of the socket mounting hole, etc.

  2. "Scatter Pattern Offset" tolerances from the probe and receptacle. These tolerances are not affected by actuation of the probe and therefore remain relatively constant. Items such as tilting of the socket in its hole, plunger bend, and eccentricity of the probe tip fall into this category.

  3. "Scatter Pattern Diameter" tolerances from the probe. This group comes from clearances within the probe assembly and varies from one probe actuation to the next, resulting in a roughly circular scatter pattern of probe tip contact points.

  4. "Pointing Accuracy" is the combined effects of the "Scatter Pattern Offset" and 50% of the "Scatter Pattern Diameter". This is measured directly by rotating a probe and socket assembly around the sockets centerline and measuring the Total Indicator Reading (TIR) at the probes tip and dividing by two (2), pointing accuracy = ½ TIR. (Refer to Diagram B) This report focuses on tolerance groups two and three, which are the ones involving test probes. Tolerance group one (Fixture Offset) is controlled by the fixture builder and printed circuit board manufacturer. Group one is typically larger than both the other groups combined. Although the probe manufacturer largely controls groups two and three, the tilt of the socket when installed in its mounting hole is controlled by the fixture builder and is largely dependent on their installation method.

Test Procedure

Fifty probes from each series were inserted into their appropriate sockets and the TIR of the probe and socket assemblies were measured.

The socket was placed into a fixture and a probe was installed into it with a Probe Tool (PT). This was done to replicate how a probe would be installed into a production fixture.

The probe and socket were then mounted in a three-roll concentricity gage at a given set-height and then rotated around its axis.

The set height was determined by the location of the sockets press rings so that the press rings did not interfere with the rolls on the concentricity gage during the test.

The total deviation of the tip was measured with a 50X comparator, recorded as TIR (Total Indicator Reading) and divided by two (2) to get the Pointing Accuracy.

Once the data was collected, the Minimum, Maximum, Average and Standard Deviations were calculated.

pointing accuracy