Designing for IMPEDANCE Testing

摘要

TDR impedance testing of high-speed boards is routine and reliable. For tighter specifications and high-speed performance on standard low-cost base materials, however, automated TDR impedance testing has its advantages. The economics apply whether the need is to test coupons or representative traces on the board. And while testing is one aspect of the equation, it is also vital to understand the production processes most likely to influence yields. Where high-volume test is required there is a choice between fixture and flying probe systems. At first look, it would seem that a fixture system would offer the best throughput. However, in contrast to bare-board testing (BBT) of opens and shorts, a TDR test takes just under one second for single-ended measurement and around 1.5 seconds for differential. Therefore, move time on a flying probe system is not a great factor in test speed. More important, a flying probe impedance tester can self-verify at the probe tip. This is achieved through the use of built-in traceable air-lines on the test table (FIGURE 1). Fixture-based testers using a matrix of RF coaxial switches cannot self-verify in this way, although in applications where a compromise in repeatability and reproducibility (R&R) is acceptable they are a valuable alternative to flying probes.