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.
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)
The table below summarizes the overall pointing accuracy for each series. The average data ranges from a minimum of 0.0000 [0.000] for the 039-25 series to a maximum of 0.0055 [0.140] for the 039-40 series. When comparing pointing accuracy data between a standard probe and an X Probe for a given series, the X Probe will have a better pointing accuracy based on the fact that the X Probe series does not utilize a socket for mounting.
The data represented is from a sample size of fifty (50) parts randomly selected from QA’s inventory. To get a better statistical representation of the data the standard deviation can be added to the average or mean to show how a population of probes from the same series will respond. Plus or minus one standard deviation added to the average is also called +/- one sigma or +/-1σ and represents 64% of all of the readings. Additionally by adding two (2) standard deviations (+/- 2σ) or three (3) standard deviations (+/- 3σ) we can represent 95.44% and 99.74% respectively of all of the readings. These numbers are more useful than the average in that that it gives the designers a higher confidence level that they will be able to meet their design and test objectives.
As space on circuits becomes increasingly limited, reliable contact of smaller test pads becomes a requirement. By improving on manufacturing and assembly methods and designing for testability, false test failures can be greatly reduced.
The probe accuracy specifications listed above can be used together with fixture and circuit board tolerances to accurately size the smallest test pad necessary for reliable contact. For example, the appropriate pointing accuracy specification for the probe from above can be added to the total fixture and circuit board tolerances and multiplied by two to yield the minimum test pad size.
By studying test fixtures and probes in this way, reliable contact can be made while using the minimum possible test pad size.