As the products we use become smaller, faster, and have more functionality, 100% test-point access is fast becoming a thing of the past. In the electronics manufacturing environment of today, design for test (DFT) must take into consideration the compatibility of the printed circuit board (PCB), the devices on it, and the targeted test equipment’s capabilities.

The test capabilities are not only electrical, but structural as well as optical. With the improvements in both the software and hardware capabilities of flying probe test systems, manufacturers of all sizes have used this increasingly popular tool for manufacturing and some functional verification in the early life cycles of PCBs.

Flying probe testing has become more popular due to improvements in the technology. The major improvements are in the mechanical accuracy and electrical testing instrumentation that dramatically refine the repeatability, speed, and diagnostic capability of this test method. These enhancements allow the manufacturer to use a “golden board” as a basis for comparison to verify the attributes of items on a bill of materials (BOM).

As part of an integrated solution, the program preparation software imports CAD directly, and generates a test program in a short period of time. This advance, when compared with the earlier approaches of a tedious auto-learning process, is a major advantage of second generation flying probe testers. This typically results in a high fault coverage test in less time, with fewer false calls. The new software that supports flying probe tests also allows the user to automatically place probes on populated test pads that are off-set from the device lead. This will result in fault coverage that in many cases is higher than a bed-of-nails (BON) in a circuit tester, where test pad access is lacking.

The integration of advanced third party instrumentation has allowed flying probe testers to offer some performance testing as well as process testing that would not be available otherwise (i.e. diagnostic testing). The ability of flying probe testers to achieve high fault coverage with pin-level diagnostic resolution is listed as one of the important reasons for the decision to implement this strategy.

Test Access Issues
Since flying probe testers are often used in new product introduction (NPI) and low volume applications, test access is not often available. The lack of large test-point access can be due to a number of reasons such as:

1) Space availability- miniaturized circuit designs do not have space for complete nodal test point access. Miniaturized designs have room for a small number of test points, however, the layout engineers place none since they have no guidelines for optimum placement.

2) Performance considerations- in high frequency circuitry, added test points and associated traces could adversely affect performance.

3) No design for test (DFT) implementation- this can be attributed to time-to-market (TTM) pressures, or a lack of tools to assist with this process.

Test pads should always be used if possible, and those targets should ideally follow the same design rules recommended for BON fixturing. If vias are used, the stacked tolerance of the drilling operation should be taken into consideration. If possible, all vias should be filled with solder, and unmasked to insure that contact is consistent. While leaving a masking layer off of the vias could present a low risk of solder shorts, it will also insure that there is ample nodal contact allowing the flying probe tester greater flexibility in placing probes. This greater flexibility allows for better test speed through probe placement optimization. If the PCB in question has 100% test access using any combination of test pads and vias, then the test engineer’s tasks are simplified, and the electrical test can be implemented with a bed-of-nails ICT system or a flying probe test system such as the SPEA Flying Probe Tester (Figure 6-1).

Considering the above as the exception and not the rule, especially in the early design stages, a flying probe system can be used to increase fault coverage by contacting non-traditional access points. This manner of expanding test access points is only viable if it is carefully and thoughtfully implemented. To use these access points, it is important to have an automatic tool to place the probe in the correct position on the via annulus if the vias are not filled. If the only location available is a populated pad, software can be used (available on certain flying probe testers) that automatically “offsets” the probe placement to avoid hitting the component lead itself. Probing a component lead

should be avoided, because placement inaccuracy could cause damage to the lead or result in a higher quantity of false calls. In addition, the contact pressure applied to a pin which is poorly soldered, or not soldered at all, can effectively mask the very fault that one is trying to detect. If the probes are angled, choosing the correct angle is important to ensure repeatable contact. All of these considerations and rules should ideally be built-in to the test generation software in an intuitive and foolproof manner.

Manufacturing tests should be product and design specific. Any test strategy should take into consideration an estimate of the time needed to stabilize the design, a quantity forecast (both short-term and life), as well as the physical attributes of the product such as test access and technology. Only then can a knowledgeable decision be made on what testing “recipe” to employ. Of course, any decision must include the price-per-test along with the fault coverage achieved by that test.

It is clear that the latest generation of flying probe testers can provide fault coverage that rivals, and, in the case of limited access applications, exceed traditional bed of nails test coverage. That is not to say that flying probe testing is the answer to all test requirements. If utilized correctly, it can provide excellent diagnostics at a low cost, as well as offer process improvement tools that will increase long term quality and reduce the cost of manufacturing.

The EMPF utilizes the SPEA Flying probe tester 4040 in PCB assembly fault testing. It is used on all medium to high density PCB assemblies used in both military and commercial applications. If you would like to see a demonstration of this system, Please contact Jeff Stong at the EMPF at 610-362-1200, extension 224.