Concurrent test

Dr. Boctor founded Digitaltest 25 years ago and has pioneered many printed-circuit-board test techniques, including the concurrent-test approach described in this article, which his staff submitted to us in February.

Before his death, preparations for Digitaltest's 25th anniversary celebrations were underway at the company's Stutensee-Blankenloch, Germany, headquarters.

For More Information Read other articles from this issue: Table of contents, May 2005 FEATURES Built for the long haul Test fast clocks on the cheap Concurrent test Spin, look, and measure

The gains in production-line assembly rates have come largely from product designs that use highly integrated components, which combine more functionality into smaller footprints and allow products to shrink in size, as seen in the portable electronic devices we carry. Instead of building individual small boards that go into such products, it is more effective to build large, multi-image boards and later separate them into individual units. This situation arises because of the desire to optimize the balance of load/unload time with machine build time. On consumer and automotive product SMT assembly lines, it is common to see boards with image multiples of two to eight units. On very small units, such as radio key locks, you might find 48 or more units for each board assembled.

Figure 1.  Steering-column sensors, having shapes similar to that of a crescent moon, would present significant handling problems, but multiple images can be assembled in a single rectangular board.

The adoption of smaller component technologies has lead to a reduction in the available space for probe-access test pads at production test. Active components also include more functionality per component, so the number of connections per device has increased, leading to higher connection densities. Because the higher density connections have contact pads much smaller than that required for test probe access, many sections of SMT boards are no longer fully accessible by probe-contact test methods such as in-circuit test (ICT). To make up for the drop in ICT fault coverage, manufacturers have turned to functional test techniques, such as BIST, boundary scan, and in-system-programming (ISP), in addition to conventional performance and compliance test techniques.

Products with multiple functions—such as mobile phones with cameras or pocket PCs that incorporate Bluetooth wireless communication and GPS navigation—further complicate the test process. The same-sized product, produced at the same line beat rate, now has multiple test demands. And tests of such devices must do more than ensure product quality; in many cases, they also must ensure the product complies with standards covering electromagnetic emissions.

How, then, do you develop a test that can handle higher line production rates, multiple image boards, and increased product functionality—without creating a major end-of-line bottleneck? As Eliyahu M. Goldratt has pointed out in several books (Ref. 1, for example), normal test asset costs range from 10% to 25% of the capital cost of the complete assembly line. If traditional thinking is applied to solve the test bottleneck by deploying multiple replicas of the existing test setup, then test costs will rise as a percentage of total line costs—an obviously undesirable situation.

Recent advances in test architecture and test-management software have made concurrent test a feasible alternative. In concurrent test, a flexible, nonmultiplexed test system with integrated functional test resources completes the test on multiple units simultaneously. Allowing the load/unload time to be shared by up to four units, and then completing the basic electrical test on all units simultaneously, the per-unit test time is dramatically reduced. Only when the more expensive functional test resources are needed to complete the test sequence are these used serially, thus ensuring only the lower cost resources need be replicated per unit under test.

In one concurrent-test implementation (Figure 2), a single test platform contains four test heads interfacing to four units under test. Each test head has its own controller managing its test. All stimulus and measurement for each unit is managed with the four test controllers. A master controller manages the complete test cycle and allocates common resources needed to conduct the functional test. These can include test instruments, VXI, PXI, or serial-bus-standard instruments as used in automobile electronics.

Figure 2.  In this concurrent-test implementation, a single test platform contains four test heads interfacing to four units under test. Each test head has its own controller managing its test.

Concurrent test lowers per unit test cost, but the calculation to determine the maximum load for a test process can be complex because, unlike assembly machine capacity, it is more than the addition of load/unload time plus process time. It must include factors for test repeats and test-failure diagnosis, and it must allow some capacity for returns test. Most engineers simplify the calculation by allowing a standard 25% factor for these. The simple calculation of maximum allowable test time is

[(beat rate – (0.25 x beat rate)] – (load + unload time)

Thus, a 15-s unit production beat rate, with a 6-s load plus unload time can tolerate a maximum test time of only 5.25 s.

Once the single-unit test time exceeds this maximum allowance, additional capacity must be found. As previously mentioned, this would traditionally have required replica test systems. But these bring a high overhead of capital, floor-space, and operator time.

Concurrent test can significantly reduce this overhead. By loading/unloading four units and running all tests concurrently, the maximum test time allowed would become

[4 x 15 s – (0.25 x 4 x 15 s)] – (load + unload time) = 39 s.

In any real case, this maximum would be reduced by the serial tests that use common resources. So, the actual achievable maximum will need to be calculated for each application.

The cost-saving calculation is similar. Savings would be gained from requiring one test platform, one floor-space allotment, and one operator instead of four. Some cost for the concurrent test platform must be added, as that platform will have extra resources. But the cost is significantly lower than that required for four replica resources.

Results indicate that significant savings can be achieved using concurrent test systems at board-unit test. The additional benefits of lower demand on floor-space and support resources are equally valuable.