Issue link: https://iconnect007.uberflip.com/i/1507822
92 PCB007 MAGAZINE I SEPTEMBER 2023 Notations: N = Number of pieces in a lot n = Number of pieces in a sample from lot p = Average fraction defective q = 1 - p k 1 = e cost to test one unloaded PC board k 2 = e cost of a defective piece that gets into the production line. Includes the cost to discover that this is the defective part, non-recoverable parts, and the cost to replace the PC assembly as well as to retest the replaced assembly. P = Proportion of lot set off for screening (testing) Q = 1 - P How Much to Test Test extent for dedicated fixturing: e most thorough test of a printed circuit board is to check for all possible shorts and to verify there are no open traces on the board. Check- ing for shorts requires a mini- mum of one probe on each cir- cuit network. Checking traces for opens requires a minimum of two probes for each trace. An opens test will usually have more probes to verify every end of a network. Since the cost of fixturing is directly proportional to the number of probes in a test fix- ture, the most thorough test also requires the most expen- sive fixturing. e following table defines general criteria for the types of tests to be per- formed for dedicated fixtures. Test level 1 is a bare mini- mum test but for use only when opens are improbable. Test level 2 is the minimum recom- mended test level. Test level 3 is the most desirable as it eval- uates all shorts and opens at lowest cost. Tests 4 and 5 pinpoint the trou- ble spot to a smaller area. Refer to Figure 4 for examples of test point locations for the differ- ent test extents. No Testing If there is no electrical test in a PC shop, the visual inspection area is burdened with trying to find electrical opens and shorts. As PCBs continue to become denser, with closer spac- ings, more layers, and smaller traces, opens and shorts become more common and finding them through visual inspection gets harder. e most obvious benefit of implementing electri- cal test is that it allows visual inspection people to concentrate on specification tolerances and cosmetic defects, which gives the PCB manu- facturer a marked productivity increase in that area of his shop. Figure 4: Example of test point location and test levels.