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36 SMT Magazine • April 2016 Test and inspection Engineering Parallel to SMT process preparation, the test processes can also be prepared from the central product data. For electrical testing, the essential issue is finding the best ways possible to access all circuit nodes using test points or other ex- posed contacts on the PCB. This heavily draws from the design data to understand the position and nature of every feature of the circuit. Once the best accessibility has been found, the pro- cess preparation system chooses the most ap- propriate probe or pin to be used with respect to the particular tester and fixture parameters. The logic for flying probe is similar to in- circuit test except that accessibility has to be calculated in 3D as the probes go around the product. In each case, the process preparation system creates the necessary output data for- mats to completely describe the test process to the respective machines. Again, many days of work has been replaced with a few minutes. A similar process is provided for optical in- spection machines. Depending on where a ma- chine is placed in the line, the system knows which components are placed on the PCB at that point, or none in the case of solder paste inspection. Again, using the design data, being able to simulate the optical views of the PCB based on each machine's inspection parameters and knowing the physical shape attributes of the parts themselves, process preparation can calculate the required inspection program pa- rameters. In the process of creating all of the various forms of test programs, a good measure of the testability of the PCB can be obtained. It's possi- ble to check whether every element of the PCB can be tested, or whether there are untestable risks. If any of the risks are deemed critical, for example, important areas or features of the PCB cannot be tested, then processes in production can be made or adjusted to compensate. This Figure 10: Test engineering and testability analysis are always based the common product data model. bEsT-PracTicE ProcEss PrEParaTion for Pcb assEmblY