Flexible test remains upwardly mobile
07 December 2006
Reports of the death of in-circuit test are much exaggerated, says Mike Smith, who argues that ICT plays an important part in the development of a global product.
In the past five to 10 years, UK-based large-scale electronics manufacturing has moved to contractors around the world. Today, facilities here specialise in making low-volume high-mix "niche" products and/or developing production and test processes that they can then ship overseas for ramp-up to high volumes. Many engineers contend that conventional in-circuit and functional test are ill-suited to this new environment. Instead, they have adopted more flexible techniques such as flying-probe testing, AOI, and x-ray inspection.
But these methods come with their own limitations. Even under the best of circumstances, AOI and x-ray can determine only that a circuit "looks" correct, not that it works. Flying probe testing is limited by the number of available probes and by the time it takes to move them into position for each test.
Also, although these methods are appropriate for initial development and early manufacturing here, they often do not work as effectively once routine high-volume production begins, due to limitation in defect detection and diagnostic resolution. Creating another test strategy for the new venue proves both expensive and time-consuming.
Flexible and Scalable
Niche manufacturers in the UK generally produce only a relative handful of boards. Initial testing looks primarily for shorts and opens. A manufacturing-defects analyzer (MDA) serves those needs quite well but it cannot look much beyond those fault types. In adopting an MDA or cursory in-circuit test, a small board manufacturer might depend more heavily on functional test to ensure optimum quality. Unfortunately, that approach proves less successful for a contract manufacturer who often has much less functional knowledge of the product. A more comprehensive in-circuit test can provide the necessary information. In addition, makers of MDAs tend to be local companies that have little presence in the international test market. They offer little opportunity to expand their offerings to meet increasing needs or to handle full-scale production by a contractor.
An ideal test solution would permit a manufacturer to buy a smaller system today that can be expanded as necessary as the customer's needs evolve. One challenge to in-circuit test flexibility is the need to modify fixtures and programs to accommodate the myriad of changes that often accompany early or other low-volume production. Most in-circuit tester vendors minimise costs by reducing the number of drivers and receivers through multiplexing their test pins. In this scheme, one "real" driver/receiver fans out to as many as 16 pins at the board. A single pin cannot serve as both driver and receiver in the same test, complicating the task of assigning nodes to test fixtures, and drastically increasing the pain of implementing engineering changes. The process also requires creating the test program before wiring the fixture.
To address these issues, Teradyne has developed the Ultrapin II 121 and 121a for its TestStation in-circuit tester. Each Ultrapin II card includes 128 real in-circuit pins. Fully loaded TestStations support up to 3,840 real test pins. Users can develop fixtures and programs in parallel, and much more easily make whatever modifications become necessary.
Starting Small
An important aspect of the TestStation architecture is its ability to provide the entire range of capability from MDA-class performance to full in-circuit test with all of the advanced features that you have come to expect. The UltraPin II 121a, an analogue-only variation of the 121, is totally compatible with it. Manufacturers who want only MDA-level equipment can populate the TestStation with 121a cards to perform shorts and opens testing, analogue measurements, and digital-opens detection. A move to a slightly more sophisticated digital-opens test - using Teradyne's FrameScan FX - involves only contacting the local supplier.
But suppose the product and the corresponding testing requirements change. Perhaps new boards coming down the pipe include ISP devices or FLASH memories. Programming those components might require adding one hybrid UltraPin II 121 pin card to an otherwise analogue system. The manufacturer who encounters the need for true digital testing can replace the 121a pin cards with their hybrid equivalents. The common architecture between the two pin card "flavours" means that the migration process protects all fixturing and programming efforts, minimising the cost of the upgrade. Additional hybrid cards (and test pins) increase the fault coverage further, reducing dependence on functional test.
Shipping Costs
One advantage of dealing with a major equipment supplier is that if you move your manufacturing to another contractor or another continent, finding someone with comparable equipment is not difficult. We have customers here who have seamlessly transferred their operations to partners in China, India, Vietnam, Ukraine, and a host of other manufacturing locations. Such companies might initially need only analogue testing, yet require the entire gamut of in-circuit capability when they transfer production.
Transferring operations to a factory elsewhere in the world carries additional challenges. Some companies have experienced considerable disruption when trying to install a supposedly debugged test program at a new location. A test that passes but hovers close to one tolerance limit or the other may fail when installed on another machine. Simple tester-to-tester variation can cause a working program to fail. Conditions on the factory floor - power, temperature, humidity, labor practices - may vary from place to place as well, complicating the transfer.
We provide numerous software tools to help you to develop a test program that runs wherever you need to install it. The software performs compatibility testing, adjusting measurements automatically to achieve the necessary consistency. Again, a vendor with a large installed base world-wide has the experience to help ensure comparable process results regardless of where the test occurs.
Many companies like Dell in the US and Pace in the UK use test as a "quality gate". The test process is very stable. Any failure results from a manufacturing fault, which carries its own recommendation for process correction to prevent future occurrences. Those companies know what to expect from one day to the next. Although it is true that some manufacturers are moving away from in-circuit test in favour of other solutions, the technique nevertheless allows checking the quality of the process from the small-volume startup all the way to the large-volume manufacturer. The available alternatives can complement in-circuit results, but will not soon replace them.
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