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56 The PCB Magazine • December 2014 by Todd kolmodin gardien SerViceS uSa Electrical Test Coverage c o l u m n testiNg todd Recently, I was asked what 100% electrical test coverage is. An interesting query, consider- ing that with electrical test, the answer comes with yet another question: Is 100% really 100%? The first question is are all nets tested? If that answer is yes, then 100% test has been achieved. However, this is where the scenario gets a bit cloudy. It comes down to what is tested and how many actual contacts of the PCB are tested. In standard industry practice for almost all IPC Class II (Level 2) product, the end points of every net are tested for continuity (opens) and all nets are checked for shorts (Figure 1). Keep in mind that with flying probe, shorts are checked by the use of an adjacency window (as discussed in my November column). This standard prac- tice optimizes the intermediate or "mid-points" in a specific net and validates only the end- to-end connectivity based on the continuity threshold parameters. To make it simple, think of this as your light switch at home. The power company supplies the power from the sub-sta- tion to your home and all you are really con- cerned about is that when you flip that switch, the light comes on. This validates your electrical test. You are not necessarily concerned about each pole that the connection comes through, only that your light comes on. Since your light comes on the circuit is valid. Also in conjunction with this, you wish your 60w bulb to light according to voltage supplied. Again if the light illuminates as expected you are not concerned. The circuit is not interfering with any other. The test is 100%. No errors are detected. However, in higher-reliability applications you may wish to add a light to one of the poles between you and the substation, but you see that you cannot because a local avian species has decided to take refuge on that specific pole and has covered the insulators where you need your connection! Does this make your test less than 100%? No. You already validated that your light works at your house. This is where the question arises. In Class III (Level 3) product, the inclusion of mid-points is indicated. Is this "more" of a 100% test? No. The nets have already been 100% tested for functionality but now we are adding redundan- cy testing of the nets already tested. IPC- 9252A states that Class III incorporates mid-point testing, but also has a footnote re- garding accessibility to the mid-points. Process- es such as via fill and via cap can remove the accessibility of test points. In the standard optimization process the nets are validated end-to-end, meaning feed- through holes or vias are only validated to the

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