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PCB-Nov2014

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58 The PCB Magazine • November 2014 of nails test, the difference from flying probe is that ALL nets are interrogated against one an- other regardless. Although extremely fast, it's rather unnecessary. Flying Probe Here we enter what could be seen as the new world for many OEMs including military. How is this machine going to give me the same results or test the same as my grid tester? New definitions boggle the mind: direct testing, indi- rect testing, adjacency and indirect testing with signature analysis! Well, that's why I'm here. Let's walk through this: Direct Testing The flying probe is going to perform a full resistance test on every net for continuity at the given threshold. (For reference see IPC-9252A w/Amendment 1.) This is no different than the bed of nails. However, when it comes to isola- tion testing the flying probe does this a little differently. The isolation test is performed using adjacency, which I will explain shortly. Indirect Testing (Signature Analysis) This is how the flying probes can compete against the bed of nails. This is known as capac- itance test, discharge test, field test or phase test depending on the type of machine used. Rather than doing a full parametric test on all nodes in the PCB, the flying probe places a reference probe on a plane layer and reads all associat- ed nets to the reference to develop a "master." When the first board is tested the reference is gathered and then a full continuity and adja- cency test is performed. If the PCB tested is ac- ceptable the master values are written. The sub- sequent boards are tested and compared to the master. Deviations from the master will evoke continuity or isolation verification. The World of Adjacency This is where the flying probe makes speed gains. Now with my colleagues in the industry this is where we have taken a leap of faith in reference to manufacturing. In the flying probe world we have made some calculations that we do not need to do every "net to net" test for shorts. This has been accepted by the DLA and also the IPC 9252 Task Force. Adjacency test- ing is defined to testing a single net to all nets adjacent within a given window. The industry standard currently is 1.27 mm or .050". This is where the grid test and flying probe part com- pany. The grid test will test all primary nodes to ALL nodes on the PCB. The flying probe will only check nodes within the adjacency window, aka .050". With that said, we have to drill into the types of adjacency. Line of Sight/Horizontal When checking for discontinuity (shorts), the system scans the PCB on the horizontal plane. In short, Figure 1 applies. As you can see there is a leap of faith implied as if a cata- strophic even were to occur the short from A to E would not be detected. WHAT IS 4-WIRE KELvIN? continues figure 1: line of sight adjacency. (source: iPC) 1. 1.27 mm (0.050 in.). 2. where line of sight adjacency distance is shown as 1.27 mm (0.050 in.), C is tested to b and d, but not to a or e. 3. a short from C to e should also be reported as a C to d and d to e short.

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