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44 The PCB Magazine • July 2016 Changes in the milliohm range can flag a po- tential defective barrel early in the process and remove the defective image/panel from further process, thus stopping a potential latent defect or field failure. The standard cross-section is a random sampling of a hole set and just can- not predict the possibility of a plugged hole or debris-entrapped anomaly that will cause a ran- dom defect even though the majority sampling of the panel reports acceptable criteria. 4-wire Kelvin is mainly provided by the flying probe test equipment available today. Electrical Validation Here is where many validations of the prod- uct take place. Here, terms such as TDR, IR, hi- pot, continuity and passive validation are com- mon. Let's look at these one by one. Time domain reflectometry or TDR is a measurement technique used to determine the characteristics of a line by observing reflected waveforms. The analysis is comprised of inject- ing a step or impulse of energy into the sample and observing the reflected waveform. Many shops incorporate a coupon in the panel for this purpose. Terms like stripline and buried stripline are common. These are usually but not always a resistive load. With a resistive load, when the signal or step is applied the resulting waveform is expressed in resistance. Common requirements of characteristic impedance are values in the 50-ohm range as measured on the reflectometer. Insulation resistancetesting or IR testing is a test of dielectric integrity. Somewhat similar to the standard hipot or dielectric breakdown test, the IR test has a specific requirement of the insulation. This is very common when measur- ing insulation between shields and cores. This applies a voltage to the shield and measures the resistance from the adjacent core. This require- ment is usually in the megaohm range. This value can be captured and reported as an accep- tance criteria. Dielectric breakdown or hipot is another test of the dielectric material separating cores. Similar to the IR test, the hipot test applies a ramped test voltage of a period of time and holds (dwells) at that voltage for a specific time. Any unacceptable current leakage during that time results in an immediate shutdown of the test and a fault is reported. Unlike the IR test, this is a "go/no-go" test. The only thing known is that an unacceptable current path exists be- tween the nodes tested. The specific resistance is not reported. This test is mainly used to iden- tify manufacturing defects rather than validate a specific insulation requirement. 4-wire Kelvin or low-resistance testing is a test designed to identify minute changes in re- sistance specifically to plated holes or vias. As with common continuity tests that have thresh- olds from 5−20 ohms, the 4-wire Kelvin mea- sures for fluctuations in the milliohm range. Usually with this test the vias with the highest aspect ratio are selected. Aspect ratio (AR) is de- fined as hole size/board thickness. For example, 0.0098/0.062 = 0.158 or 15.8:1 AR. The sample is tested and a master value set is created. Sub- sequent boards are then tested and compared to the master. Any fluctuations outside of the acceptable control limits are reported as faults. Anomalies such as thin copper or tapered plat- ing are captured with this test. Defects such as these will not be captured during a standard continuity test with thresholds between 5−20 ohms. Buried passive testing validates buried or electrical components built into the board structure. Most common are the buried resis- tors. Today's market mainly uses a resistive ma- terial buried in the PCB where areas are etched away leaving a predetermined resistive charac- teristic or component on the given net. These values are predictable and can be measured and catalogued. The most common method today is by use of the flying probe. When the values are known, this can be programmed into the netlist TEST & MEASUREMENT: THE CASE FOR VALIDATION Figure 2: HiPot/IR tester.