Issue link: https://iconnect007.uberflip.com/i/340751
July 2014 • The PCB Magazine 51 For most automated test systems the follow- ing guidelines should be applied: 1. Input the power rating (watts) of the resistor, if this feature is supported by the test system 2. Limit the test current used to 20mA A flying probe (or grid test) machine basical- ly contains a high-speed ohmmeter that allows rapid gathering of resistance values for nets on the PCB. Once the resistor value is known the ma- chine will make the necessary tests to the resis- tors and catalogue the results. It will compare the expected value with the read value (com- pensating for the allowed tolerances) and inter- nally log the resistor net as pass or fail. Once the entire test of the PCB is complete, a report ticket will be printed, stating that the PCB has passed or failed and identifying all resistors that failed in addition to the standard continuity test re- sults. Many machines of today can also supply a report of all the resistor values read for the given PCB for statistical process monitoring. Protecting the Resistors When testing the resistors via a flying probe, care must be taken to limit the voltage/current drive to the measurement. Flying probes have ELECTRICAL TESTING OF PASSIvE COMPONENTS continues two methodologies when taking measure- ments: constant current and constant voltage. Depending on the design of the flying probe you have could cause some concerns. If the BR record was provided in the raw netlist the machine should make the appropriate determi- nations of how to best test the resistor. If the system is solely constant current source (grid/ prober) it should be able to automatically de- tect if the test parameters are in range. If the system has constant voltage capability, then it can auto-range itself to the appropriate power level. A constant current system is only limited by its internal supply voltage. With a constant current scenario the actual power or "wattage" is not monitored and can be detrimental to the UUT. The constant current method is far too re- strictive for the range of resistors that are in use today. These measurements should be done in a constant voltage mode. In this scenario the cur- rent is based on the actual resistance read and not the calculated or anticipated value. In this scenario it is a simple calculation of E*I, where E is the voltage applied and I is the current. In a constant voltage scenario the resistance is only needed to calculate the required voltage of the source. The measurement parameters are con- structed using the E=P/I formula where E =Volt- age, P=Power and I = Current. The measurement is taken at the calculated E and the resultant I is measured and calculated in ohms. Capacitors Just as with the buried resistors, capacitance can also be embedded into the PCB. This is done by the use of a capacitive core material lami- nated into the PCB. This removes the need for surface decoupling capacitors and as with the removal of some of the surface resistors allows more space on the surface of the PCB and allows the overall size to be reduced. However this introduces a challenge with electrical test. With the buried capacitance in the PCB false shorts can be reported by the tes- ter. The standard electrical test for shorts applies a voltage (primary point) and reads for current leakage on adjacent nets in the case of the fly- ing probe or any nets in the case of the bed-of- nails grid test machine. A specific threshold is set on the machine so that nets to one another Figure 5: resistor measurement with current limiting.