In today’s electronics manufacturing market, the race is always on to make smaller and faster electronics equipment with greater functional capabilities. Given this reality, printed circuit boards (PCBs) are increasingly smaller and more densely populated, and 100% test-point access may not be practical, or even possible. Recent improvements in both the software and hardware capabilities of most commercially available Flying Probe test systems have made these them increasingly popular tools for manufacturing verification, as well as some functional verification of PCBs. In this article we will examine some of the characteristics of flying probe test systems and how they may be applied.
Flying probe test systems come in a variety of configurations, but all utilize several mobile armatures equipped with moving probes for contacting test points on PCBs. They are most often used in analog signature analysis, testing of analog components, and continuity (short/open circuit) testing. These moving probes can be programmed to make contact to virtually any point within an x-y grid upon a PCB (Note: There are some systems that provide software options that allow for limited testing on bare boards.).
The operator loads the computer-aided design (CAD) file data of the PCB to be tested into the system software, from which the component value, part reference designation, component orientation, and the position of each land pad or part lead can be extracted. Although the flying probe tester only requires the CAD file, comparisons made against a known good sample (i.e., a “golden” board), will provide for adjusted file information that will help prevent false failure reports.
While heavy volume production lines will often utilize an in-circuit test (ICT) fixture that employs some form of “bed-of-nails” (BON) probe arrangement, this kind of fixture can be very costly for low volume and prototype boards. Although common ICT configurations allow ample control and repeatability of testing within an established production line, these systems must be reconfigured with each modification or change of the board being tested. This can be time consuming and costly. The lack of a required set fixture for flying probe systems gives them a distinct advantage over ICT testers for low volume and prototype applications.
Most recent models can be fitted with armatures that allow the test probes to make contact at more acute angles, allowing greater access to test points, component lands, or vias that were not readily accessible utilizing older models which had more fixed angles. Additionally, most new models boast 24 probes, as opposed to the older generations that were limited to four probes. This increase helps to accelerate test-cycle time and allows for higher fault coverage.
Another way to accelerate test-cycle time and increase throughput is the ability to adjust for parallel components. The latest version of the SPEA 4040 flying probe tester (Figure 3-1) optimizes the speed of its linear motor with magnetic bearings and allows for the probe height to be adjusted for low profile PCBs. These innovations are reducing flying probe test-cycle times to a level comparable to some ICT applications.
A few other considerations include double sided testing (flying probe testers used to be limited to one side, Figure 3-2), and the capability to adjust the amount of pressure the probe uses when making contact with the test point (ultra soft touch). The later, eliminates depressions in the solder joint due to probe contact.
These systems can also be used to test fine and ultra fine pitch boards. While as recently as 2005, the test point limits stood at approximately 4 mils (0.004 inches), current models boast of being able to test down to 1 mil (0.001 inch). This allows for testing of component package sizes that have previously been too small to test, such as 0201, 01005, and RQFPs (power quad flat packs).
While it is clear that flying probe testing is not the answer to all test requirements, the latest generation of flying probe testers does provide fault coverage that rivals and, in some cases, exceeds traditional ICT systems. When properly employed, these systems can provide excellent diagnostics at a low cost and increase the long term quality of manufactured electronic assemblies.
The EMPF has recently added the newest version of the SPEA 4040 flying probe tester to its extensive range of manufacturing equipment. It will be utilized in a variety of applications both military and commercial for both low volume, as well as engineering prototypes. If you would like more information on flying probe test systems or a demonstration of the system we have on site here at the EMPF, please contact Ken Friedman at 610.362.1200, extension 279 or via email at kfriedman@aciusa.org.
