PCB007 Magazine

PCB-May2017

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94 The PCB Magazine • May 2017 formation as well as the core and foil thick- nesses. If the vertical adjacency window is pro- grammed too shallow, the risk of missing shorts to the adjacent later(s) is possible. If the win- dow is too large you risk picking up too many layers and the test may become much longer than intended. It is necessary to remember that if the adjacency window is changed it can affect the time taken during for the isolation test to be performed. This is directly proportional to the window size. As the adjacency window increas- es the time to perform the test increases, as the Adjacency Window will possibly pick up more net "in range." Figure 1 illustrates how the win- dow can affect the test based on the topography of the PCB using horizontal adjacency. In Figure 1 we see two scenarios, scenario A and scenario B. When performing the isola- tion test this becomes important as the amount of measurements required during the test can be significantly different and affect time to per- form the entire PCB test. In scenario A we see six different nets labeled A through F. We also see an adjacency window of .050". What we see here is when Net A is tested for shorts, it must be tested against nets B through E. This is four measurements. You will notice that Net F is not tested to Net A. Net F does not fall within range of the adjacency window. Now in Figure 1 Scenario B, we have the same adjacency window but in this case, we have nets shown labeled A through C. When the isolation test is performed on net A there will be only one measurement. Net A will be only tested against Net B. Net C is again not within range or inside the adjacency window and therefore is not tested against Net A. So in Figure 1 you can see that PCB density can affect the amount of measurements required to per- form the isolation test and thus affecting time required. Direct Mode Direct mode utilizes direct resistance mea- surements for all nets on the PCB. What this means is that during the continuity (opens) test all test points of the net are tested against the continuity threshold. Any net that violates the required resistance will be reported as a fault. When the isolation test (shorts) is per- formed, each net is probed using the required voltage and isolation parameter. One must re- member that when flying probes perform the isolation test they are performing it via an adja- cency window as defined previously. When direct mode is used, each PCB will take the same amount of time to test. This is because every net will be tested for continuity and isolation the same way each time. PCB 2 will take the same time as PCB 1 as well as PCB 3 and so on. Indirect Mode Indirect mode (also termed indirect testing by signature comparison or discharge testing) is the method where the flying probe develops speed over direct mode testing. In this meth- od, the machine develops a capacitive mas- ter by gathering a capacitive signature of the board and then comparing subsequent boards to it. When the first PCB of an order is tested, the machine places a reference probe or probes down on the PCB reference plane or planes. It will then use the remaining probes to read a sig- nature from all nets and record those finding to the master. Depending on the type of machine, Figure 1: Adjacency windows. FLYING PROBE TESTING VS. IPC-9252B

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