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80 The PCB Magazine • April 2014 Class 2: Dedicated Service Electronic Products This class includes communications equip- ment, sophisticated business machines, instru- ments where high performance and extended life are required and for which uninterrupted service is desired, but not critical. Certain cos- metic imperfections are allowed. Class 3: High-Reliability Electronic Products This class includes the equipment and prod- ucts where continued performance or perfor- mance on demand is critical. Equipment down- time cannot be tolerated and must function when required as in life support items or light control systems. Printed circuit boards in this class are suitable for applications where high levels of assurance are required and service is essential. Requirements for Testing Class 1–3 and 3/A Minimum electrical test requirements are different between IPC classes. This does not mean that opens or shorts are allowed! How- ever, the electrical thresholds to identify these conditions are different. In the electrical test in- dustry we are not only looking for the extreme cases (infinite open or direct short) we are also looking for indicators of what MAY be a poten- tial problem. This is why the minimum require- ments are different between classes. Table 3-1 identifies this. As seen in Table 1, the resistive continu- ity and resistive isolation requirements are much different between Class 1 and Class 3. Notice that indirect continuity and isolation testing by signature comparison is allowed in Classes 1 and 2, but AABUS in Class 3. This does not mean that it cannot be used, but it does require authorization from the OEM or customer for that method to be used on Class 3 product. Class 3/A Exception—Space and Military Avionics IPC-6012, Appendix A This is an exception beyond the standard Class 3 requirement for electrical test. In this case, parameters are specified when testing product under this class. These requirements need to be on the master drawing and com- municated from the OEM to the manufac- turer or this type of test may be overlooked. The differences with Class 3/A are shown in Table 2. Design Concerns vs. Class Requirements From an OEM or designers perspective one must be aware of constraints built into the board design that may cause some difficulty during electrical test. We find this most promi- nent with Class 3 product designs. From Table 1 we see that for continuity resistance there must be no circuit in the board whose resistance is greater than 10 ohms. That is the standard. A couple problems come into play here. If you will look back to note 4, below Table 3-1, you will see a statement regarding referee calcula- tions for circuit length. With that said, a board may be out of tolerance to Class 3 with any net table 1. A SUMMARY OF VARIOUS TEST REQUIREMENTS continues