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BARE BOARD TEST METHODS
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a trace with seriously errant RF impedance to demonstrate a very solid DC connection (i.e., low DC resistance). The RF impedance of the trace is most strongly affected by the trace s width, thickness, z-axis spacing from the ground plane, the location of adjacent traces, and the relative dielectric constant of the type of insulating core used to build the board. These parameters are usually rather constant within a particular panel, justifying use of coupons as a means of monitoring the delivered product. Standard bare board testing systems (other than flying probes) do not incorporate TDR capability, as the signal paths through the fixture will not pass the fast-risetime TDR signal. TDR is commonly conducted on a test bench manually. TDR test systems inject a very fast-risetime voltage step into one end of the conductor. Discontinuities in the RF impedance level along the conductor result in reflected voltage waves being returned to the driving point, where they are collected by the same probe that injected the signal. The result is usually presented as a graph of RF impedance versus distance from the point of injection. Because of reflections and disturbance occurring at the point of injection, a minimum trace length is needed to get a meaningful measurement. Extremely short networks are not ideal candidates for TDR measurement. A trace 2 in. or more in length is generally required to obtain a useful reading. Longer traces may provide improved accuracy of the result. A branch-off to a second signal path along the measured length will disturb measurement badly; thus an undisturbed signal path provides the best measurement target. A typical TDR result graph is illustrated in Fig. 37.7. This illustrates a region of approximately 50 ohms RF impedance, rising toward infinity as the trace ends in an open circuit at the right, but falling between the upper and lower pass/fail bounds in the region of interest. Sample testing using handheld probes is popular and practical, but the influence of hand and body position on the measurement can be significant, reducing the repeatability of the result. Several vendors offer flying probe solutions dedicated to TDR test. While requiring some additional setup they are faster at higher volumes and greatly improve repeatability.
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FIGURE 37.7
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Typical TDR measurement.
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Test Methods Unique to Flying Probe Systems Because flying probe systems contact pairs (or other limited numbers) of points at any one time, they cannot directly perform precisely the same isolation measurements as can universal grid (and other fixtured) systems. DC isolation is performed between pairs of networks, with the number of measurements reduced by determining which networks are adjacent to the network being tested and therefore likely to cause isolation problems. Otherwise, flying probers are capable of ordinary DC continuity and isolation measurement, as discussed earlier. In addition, most flying probe vendors have developed alternative measurement methods that reduce the number of measurements and therefore reduce time lost to mechanical motion. These methods are discussed in the following text. 37.4.4.1 Indirect Measurement of Isolation or Continuity. Different vendors have developed various implementations of indirect measurement, but in general these methods share a general assumption that a given product network will display a certain amount of capacitive or other electromagnetic coupling to neighboring planes or traces. The amount of coupling is affected by the geometry of the traces involved. If a trace is broken, the remnant will display reduced coupling. Similarly, if a trace is shorted to another trace, the amount of coupling will increase substantially. If the test system measures the amount of coupling from each trace to a ground plane (or other electrical environment), a degree of confidence can be obtained that all traces are intact. No direct measurement of isolation or continuity is necessarily made. Instead, the coupling signature is used to imply the correctness of the configuration. As no direct measurement of isolation or continuity is made, we refer to these methods as indirect measurements. Typically these methods are highly reliable in detecting hard shorts and opens, but may be less effective in detecting distributed contamination or high-resistance connections of several meg ohms or more. Specific methods include constant-current capacitance measurement (charging or discharging variations), voltage-source measurement of Resistance-Capacitance (RC) time constant, alternating current (AC) capacitance measurement, and measurement of the electromagnetic coupling of an AC signal between adjacent networks. The capacitive techniques using constant currents are generally based upon the approximation: C=i dt t or, C = i dv v
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where C is the network capacitance, i is the charge or discharge current, and v is the change in voltage that occurs during the measurement time t. Related, though more complex, behaviors occur for RC time constant and AC coupling measurements. Using the preceding example, note that if two networks are shorted, the resulting capacitance is the sum of their individual capacitances. The larger capacitance causes a slower charge rate than is expected for either net individually. The system may note that these two networks are adjacent and displaying suspiciously similar charge time behavior. The system tags the networks as suspicious, and either fails them or verifies the presence of a short with a DC measurement. Networks containing an open will have reduced capacitance and will charge or discharge too quickly. These conditions are illustrated in Fig. 37.8. To use these indirect methods, most systems require testing of a first article board using standard direct measurement methods for both continuity and isolation. Once the board is known to be good, the signal-coupling behavior is learned for each network on this board.The learned values are saved and compared to measured results from subsequent boards as described previously. Tremendous speed increases are obtained by eliminating repetitive probing of each network at multiple locations, particularly in the case of isolation testing. Some users combine a traditional DC continuity measurement with the indirect method as a substitute for isolation.
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