Justification for signal management
An accompaniment to "Signal management," from our November 2004 issue.
Bob Stasonis, Pickering Interfaces -- Test & Measurement World, 11/1/2004
The accompanying article, Signal management, describes signal management from a standpoint of sharing resources. There are other reasons to consider signal management in your test strategy. Here are a few examples. These considerations will vary, depending on the application.DiagnosticsIn most cases, functional test is configured as a go/No-go operation. This is because of functional test’s inherent slowness in diagnosing faults. In some industries, however, functional test is moving farther back into the manufacturing process. An example is the cellular phone industry. 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 cell phones. Cell phones are designed to be assembled cheaply, but they do not disassemble very well. So, verifying functionality prior to final test can save on rework costs and reduce potential scrap for phones that can’t be taken apart easily. A signal-management system can automate and speed up calibration of the phone during the in-process test.Probing PCBs
If your design requires the need to probe the PCB for calibration or verification of a particular circuit, then adequate test points will be necessary. But probing the J-lead of a 20-mil device is not very effective. And probing BGAs is impossible. According to the SMTA, test points should be a minimum of 0.040 in. Spacing between pads will vary, depending upon the component height around the test pad, probe size, etc. But 0.200-in. spacing should be a minimum spec, especially for a human probing the area. Given today’s packaging densities, this size pad is almost impossible to provide. In addition, bus speeds would be impaired by such a large test pad. Obviously, test fixturing can be more accurate in hitting a smaller target. It is important in this instance to match the signal switching used along with the instrument such that there is a minimum loading effect on the UUT. Automated or manual test?As volumes and beat rates per line increase, you may look at the possibility of automating the test process. Automated functional test virtually eliminates load and unload time, reducing the need for additional test systems.
The added cost of the handler is usually negated when one considers the improvement in throughput. The down side of automation of test can be the initial hardware cost, integration time, the ability of the test system to keep up with the beat rate of the line, and issues for production should the unit fail. Signal management is a bit of a balancing act in this instance. Sharing resources can greatly lower the cost of the test system. 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 six UUTs, you can perform Test #1 on the first three UUTs and Test #2 on the second three UUTs and then, using signal management, reverse the tests.Operator skill level
In high-density UUTs, probing when calibration or diagnostics are required will likely be almost mandatory. So, the operators will be required to access test points on the UUT. Wherever possible, make sure that the test points are clearly marked. It is also beneficial to consider the probe style and the average operator as well. Questions to ask include:c Is the probe larger than the test point?
c Is there a danger that the probe could short out several points and damage the UUT?
c Is there a shock hazard for the operator?
c Can the average operator quickly identify the test point and probe it? Is the nomenclature large enough to be easily identified?
c How long must the operator hold the probe on a point to get an accurate reading? Small test pads can be an issue if these cycles are long--the operator can slip on a long cycle. Answering the above questions may cause the test engineer to evaluate test probes styles, modify documentation to better identify where the test points are, or even change the qualifications of the operator required. Another way to look at it is to apply signal management to the application and automate the probing.
