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50 The PCB Magazine • July 2014 compare type of test, which cannot validate the expected value. Most of today's CAD/CAM sys- tems have the ability to output tester data with inclusion of the buried passives. The IPC-356A data format provides the inclusion of the bur- ied passives. What must be included are all the resistor locations or test points, the expected value and the upper and lower tolerance values. There are a few different options for testing the resistors. This can be the use of a universal grid tester (bed-of-nails), a flying probe tester or the use of a manual measurement system. In the end, the accuracy of the readings will depend on the accuracy of the metering system being used, the amount of copper between the test probes and the actual resistor, contact resistance and the resistance of the probes and leads. Most im- portant is that the measurement current should not exceed the current carrying capacity of the resistor. This could lead to permanent damage of the resistor. to test these buried resistors by conventional means only results in a "fail," as usually the resistance is higher than the IPC class require- ment. Further, care has to be maintained to not compromise the resistor itself due to excess cur- rent applied. When we are testing the buried resistor we must take into account the power dissipation of the given resistor. As we know, if we apply too much current to a resistor and overload its power dissipation rating, the resistor will burn. Testing Resistors Electrical testing is required to validate the correct resistor values and also identify faulty or out-of-tolerance resistors. It is recommended that electrical test be performed on both the innerlayer and the final board so that if a re- sistor is faulty at the innerlayer stage it can be scrapped prior to any further value-add to the PCB. Netlist testing must be used as a learn-and- ELECTRICAL TESTING OF PASSIvE COMPONENTS continues Figure 4: thin film resistance calculation.