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72 The PCB Magazine • June 2014 Indirect Testing Indirect testing however uses a reference plane or planes to develop a "signature" or ca- pacitive master. This is why you will see the term "indirect testing by signature compari- son." When this methodology is used the first board tested will go through the capacitive gather and then a full continuity and isolation (using adjacency) test against the set threshold parameters. Once this is complete the signa- ture or master is written. On the second and subsequent boards the capacitive gather is done and then compared to the "signature" or mas- ter. Only nets that are identified as "outside" of the signature or master comparison will be retested under resistive parameters for either opens or shorts. The entire board does not re- ceive the full resistive test for opens and shorts as provided by the direct test method. Capaci- tive scanners also fall into the arena of indirect testing. The UUT is placed on the scanner and two passes are made. This is the same as the capacitive gather on the flying probe but thou- sands of probes are used rather than the limi- tation of arms on the flying probe. This elimi- nates up to 90% of the test that a general flying probe would use. Then, using a flying probe, the resistive retest is performed. The end result is the same, but time in testing using this com- bination is reduced significantly. IPC and Military Considerations Now the question arises as to when one can safely test using indirect testing by signature comparison? In general, IPC-9252A amend- ment 1 (Table 2) does allow the use of indirect testing across all performance classes A–C. How- ever, the caveat here is that with Class 3 (C) this must be agreed upon between the manufacturer and end user, known as AABUS (as agreed upon between user and supplier). Also what must be agreed upon is the use of adjacency (isolation) testing on Class 3 (C) product. One should al- ways review the procurement document or fab- rication drawing as many Class 3 (C), Class 3/A (aerospace and military avionics) and high-reli- ability medical products will not allow the use of flying probe at all. Further it must be understood that not all remaining military product can be tested using indirect testing by signature comparison. Al- though the defense logistics agency (DLA) has released updated revisions to MIL-PRF-31032, MIL-PRF-50884 and MIL-PRF-55110, one can- not assume that any test method can be used on military product. Although the DLA uses IPC- table 2: iPc-9252a amendment 1. FLyINg PRoBE: INDIRECT TESTINg VS. MILITARy continues