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Overcoming the limitations of Flying Probe Test
Pilot line of flying probers.
By Luca Corli, Seica, S.p.A.
When flying probers first entered the market in the late 1980s, their utility was largely limited to testing prototypes and circuits that had no good test points. The bed-of-nails in-circuit test was still king and remained that way for many years. But ICT demands a costly fixture — something that is not practical for short-run circuit boards, so the flying prober has gained in sophistication and popularity.
Today, the gap between flying probe and ICT has narrowed significantly with enormous improvements in the test technology, the software running the tests, and the price differential. In fact, the gap between the two test technologies has decreased to the point that, in many cases, flying probe test is definitely preferred over a bed of nails.
To be sure, the flying prober has an initial cost that is considerably higher than that of a bed of nails system, but flying probers have attracted users because they eliminate test fixtures, the cost of which is hard to justify in many cases.
Historically, the test times achieved by the first flying probe test systems were disappointing — significantly longer than those of conventional in-circuit testers. For this reason, bed of nails systems have continued to find many advocates, even though flying probers offer a much higher degree of flexibility for handling product modifications and considerably lower test setup times. Above all, they are more adaptable to the frenetic pace of the today's global market, which not only consumes large quantities of product, but requires continuous evolution and customization.
Making the Decision
For decision-makers analyzing different ratios, such as
price/performance, setup time/test time, initial price/operating cost, flying probers have gained ground vis-a-vis bed of nails testers. This has happened mainly because of the miniaturization of electronic components, which is an ongoing challenge to board designers who have to find room for those ever-so-elusive test points.
The flying prober has the ability to contact objects much smaller (order of magnitude 0.1mm) than those accessible using a bed of nails fixture — which requires a minimum test point of about 0.8mm). Because of this, flying probers are definitely preferred where the boards are not equipped with specific test points and require contact directly on SMD components to achieve enough test coverage to meet current quality standards.
When a board is not laid out according to "design for testability" criteria and by choice or necessity, the test points are not all distributed on one side of the board, it is immediately understandable how essential it is to have a tester capable of accessing both sides of the PCB simultaneously in order to contact all of the components and to gain access to all of the nets of the circuit under test. In this case, many flying probers are significantly limited, since many of the systems currently on the market offer only single-side probing, which considerably limits test capability and forces the user to create two test programs for the same board — one for each side — which results in wasted time, money and loss of test automation capabilities.
Seica's Pilot V8 test system was designed to overcome this and other limitations of flying probe test. It is the only system of its kind with a vertical architecture, providing eight flying probes (four on each side), plus the ability to test two boards simultaneously. The prober can also be equipped with an automated loader/unloader for operator-free testing.
To probe an electronic board on both sides simultaneously, the force of gravity must be accounted for, since it affects the planarity of a board that is placed horizontally in a test system. The gravity effect is a well-known issue, and for this reason flying probers which have a horizontal architecture, are provided with fixed constraints positioned on the side where the probes are not present, to maintain the planarity of the board and to ensure good probe contact, and therefore test repeatability.
When there are of mobile probes on both sides of the PCB, fixed constraints are obviously not deployable. The vertical architecture design implemented by Seica in the double-sided Pilot V8 prober, eliminates the need for fixed board supports; gravity acts in parallel to the plane of the board under test, therefore it has no effect on its planarity. Contrary to double-sided flying probers with horizontal architecture which need to use one or more mobile probes as fixed supports, all 8 mobile test probes on the Pilot V8 are available to perform testing.
At this point one might think that 8 flying probes are too many to test electronic boards designed for testability, with access to the nets provided all on one side. On the contrary, the Pilot V8 is not oversized in this case, but doubles throughput because of its capability to simultaneously test two boards instead of one, dedicating four flying probes (on one side) to the first board and the other four (on the opposite side) to the second board. Practically speaking, running a single test program, the system is capable of testing two boards in the same length of time that a 4-6 probe system tests just one board. This means a 100 percent reduction of test time for the second board, a significant improvement in test time.
Another common feature of horizontal flying probers is the capability of working autonomously without the presence of an operator, in conjunction with automated loading/unloading magazines. This feature is now also available in the vertically designed Pilot V8, which can be equipped with a fully automated internal board conveyor system, connectable to automated loading/unloading magazines with up to seven board racks for operator-free testing, providing the potential to exploit "lights out" hours to increase production line test throughput.
A True Test Platform
The classic concept of a horizontal flying prober, with 4 flying probes on one side, mainly used to detect process defects on small series of prototypes, constantly manned by an operator, has been outdated. New systems provide a true test platform, considerably more power and versatility and are usable in many different ways, depending on the needs of the moment. The possibilities range from parametric and in-circuit test to boundary scan and functional test, even powering up the board with two additional mobile probes, available as an option on the Pilot V8 (bringing the total number of mobile probes to 10).
The ability to access both sides of the board simultaneously through a single test program guarantees very high fault coverage, while the different "vectorless" techniques, such as FNODE, PWMON or Thermal Scan, enable the testing of repair lots in very short test times by making comparisons to sample boards. This in turn facilitate the creation of a test program in just a few minutes in cases where board CAD data are not available.
Constant growth in terms of performance and testing capabilities has enabled the Pilot V8 to find areas of application beyond prototype and small-to-medium series testing, including the repair of boards returning from the field and reverse engineering, when repair is difficult due to the lack of input data, such as schematics or CAD files.
The VIVA operating system software, common to all of the company's ATEs, has been recently enhanced with the Quick Test module, which enables every user of a Pilot system to write and execute functional tests in minimum time, correctly programming all of the system resources, without any knowledge of the internal architecture or a specific programming language. To complement the standard in-circuit test program with a set of functional tests, the user just needs to have the functional test specification for the board under test and to describe it in the simple, intuitive Quick Test graphical environment.
In addition, Quick Test can also be used to drive the tester in a manual and interactive mode, just like a laboratory instrument, executing functional tests directly, without the need for a complete test program. In this case, an expert user looking to repair boards can utilize the Pilot V8 as a diagnostic tool while maintaining control to carry out the operations deemed most useful, based upon their experience. This capability effectively removes another chronic barrier in flying probers: regardless of the manufacturer and the software used, up until now, functional test on flying probers has represented a real challenge to the less expert user, requiring both a thorough knowledge of a proprietary programming language and of the system hardware architecture. This knowledge is no longer necessary with the Quick Test software. In a matter of seconds, it automatically selects and connects the test instruments required to perform the functional tests requested, moves the probes to the correct positions and manages the power up of the board under test.
Contact: Seica, Inc., 50A Northwestern Drive, Suite 10, Salem, NH 03079
603-890-6002 fax: 603-890-6003 Web:
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