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May 2014 • SMT Magazine 67 automation equipment to fit within a standard 19" wide instrumentation rack and that is dif- ficult to accomplish with automated electrical test solutions that must support large numbers of pins and instruments. Equipment Speeds Assembly equipment speeds have continued to improve since the advent of automated PCB manufacturing. A nearly threefold improve- ment in assembly speeds have placed pressure on downstream test and inspection equipment to improve their operational speeds so as to keep up with the "beat rate" and avoid becom- ing the bottleneck on the production floor. Unlike imaging test systems, the through- put limitations of ICT systems are bound by the physics of performing electrical tests. To per- form electrical tests, relays must be opened and closed, test instruments must be turned on and off, residual voltages must be discharged, com- ponent setup and hold times must be obeyed, and components that are not being tested must be guarded or disabled so that they do not interfere with the components under test. Most of the test time associated with perform- ing electrical tests is related to performing the above activities and the tester instruments and computer are often sitting idle waiting for them to complete. This means that the throughput of ICT systems cannot be dramatically improved by adding enhanced instrumentation or a high- er performance CPU. Because of this, ICT systems can become a bottleneck on the production line forcing man- ufacturers to perform ICT using multiple offline test systems or requiring them to add multiple ICT systems in-line, which increases overall manufacturing floorspace requirements. Electrical measurement Integrity ICT systems must make electrical test con- tact with the product being tested so that it is able to electrically stimulate and measure test points on the circuit board. The product that is being tested usually sits on a bed-of-nails fix- ture that is custom designed to make electrical connections between targeted nets on the PCB and the tester receiver interface. When the test fixture is placed on the ICT system and the PCB is placed in the test fixture, the tester is able to perform electrical tests on all nets that have test access. The fact that ICT systems require a test fix- ture and must physically contact the printed circuit board under test places a greater con- straint on them compared to imaging or other test techniques that do not need to physically touch the circuit board. ICT system designers take great care to keep the signal paths between the tester instruments and the tester receiver as short as possible so as to maintain the integ- rity and reliability of the electrical test measure- ments. To obtain best performance, ICT sys- tems minimize the impedance and inductance in the signal path and try to keep the distance between the instruments and the tester receiver as short as possible. When adding ICT systems to an automat- ed production environment, the manufacturer must accommodate interfacing both the cus- tom test fixture as well as the test instruments inside of the automation handling equipment. figure 2: example automated ICT solution with extra support bays. THE CHaNGING ECoNomICS oF IN-CIRCUIT TEST continues feATuRe