Page 28

EETE APR 2014

TEST & MEASUREMENT tional connections which can increase resistance and insertion loss, all of which can change the parameters of the test. Depending on the application, you need to assess the connections within the ITA: Are the connections hard wires? Is there a specially designed PCB to control signal paths and impedances? The latter of course, can add cost, but it may be necessary for an accurate test. So keep some flexibility in your test budget. What can go wrong? The most common mistake seen is when you do not consider the overall specifications when configuring a test system. For example, a measurement channel may have to exhibit particular characteristics – line loss, capacitance, etc. The engineer configuring the system looks at the specifications of the switching subsystem and forgets about the fact that cables & connectors, as well as the characteristics of the instruments, all add to the performance of the measurement channel. Another problem relates to the signals switched. Too often, test engineers miss the specification of a switching system for “Cold Switching” – closing and opening a relay before applying power to the UUT – and “Hot Switching” – closing and opening relays while the UUT is powered up. Hot switching current ratings are typically about half the cold switching – attempting to hot switch currents that are too high can result in damaging the switching system. Diagnostics In most cases, Functional Test is a Go/ No-go operation. However, in some industries, Functional Test is moving farther back into the manufacturing process. Some manufacturers are doing some of the critical measurements at the PCB level, partway through the assembly process. This is because of the disposable nature of electronics today, which are designed to be assembled cheaply, and not disassembled. So verifying functionality prior to final test can save on rework and reduce potential scrap. A Signal Management system can automate Fig. 5: A typical test system block diagram. Fig. 6: Signals passed through test fixture. Fig. 7: Hard wired configuration. and speed up calibration during the in-process test. Probing PCBs If your design requires the need to probe the UUT for calibration or verification, then adequate test points will be necessary. But probing the J-lead of a 20-mil device is not very effective. And BGAs are impossible. Given today’s packaging densities, test pads are almost impossible to provide. In addition, bus speeds Fig. 8: Hard wire or PCB? (Photos Courtesy MacPanel). would be impaired by such a large test pad. Obviously, test fixturing can be more accurate in hitting a smaller target, and more expensive. It is important in this instance to match the signal switching used such that there is a minimum loading effect on the UUT. Automated or manual test? As volumes and bit rates per line increase, you may look at the possibility of automating the test process. Automated Functional test virtually eliminates load/unload time, reducing the need for additional test systems. Signal management is a balancing act in this instance. Sharing resources can greatly lower the cost of the test. But when the functional test system has to keep up with the beat rate of the line, this can be an issue. In this case, look at what test processes can be accomplished in parallel. Also, if the UUTs are tested in a pallet, then splitting up tests can be beneficial. For example, in a pallet of 6 UUTs, you can perform Test #1 on the first three UUTs and Test #2 on the second three UUTs and use signal management to reverse the tests. Operator skill level Probing when calibration or diagnostics are required will likely be almost mandatory. So the operators may be required to access test points. Wherever possible, make sure that the test points are clearly marked. Questions to ask include: Is the probe larger than the test point? Is there a danger that the probe could short out several points and damage the UUT? Is there a shock hazard? Can the average operator quickly identify the test point and probe it? How long must the operator hold the probe on a point to get an accurate reading? Answering the above questions may cause the test engineer to evaluate test probes styles, modify documentation to better identify test points, or even change the qualifications of the operator. Another way is to apply signal management to the application and automate probing. 28 Electronic Engineering Times Europe April 2014 www.electronics-eetimes.com


EETE APR 2014
To see the actual publication please follow the link above