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80 The PCB Magazine • October 2014 defects shown in Figure 4 are typically what you will detect using 4-Wire Kelvin. Using standard electrical test with continuity thresholds at 10 ohms (IPC Class 3), these defects will go unde- tected as the change in resistance introduced by these defects will not cause enough change to fault at the 10 Ohm range. However, using the Kelvin test these changes in resistance will be detected as the changes although may be only 100–300 milliohms the high resolution mea- surement will fail. 4-Wire Kelvin can also detect voids in circu- lar nets, typically power and ground (Figure 5). A Few Good Questions What is the best way to program for Kelvin Test… …when looking for barrel integrity? It is best to probe the barrel directly from one side to the other side. The barrel would have to be free of unwanted material such as solder mask. The connection to the annular ring/hole is imperative, the slightest variation with throw off the measurement system. When generating your programs it is best to ensure both sides of the barrel are tested in an overlapping fashion. …when looking for specific net resistance? It is important to test all pads open in the mask. This will allow for more accurate read- ings on each individual section of the network under test. Do not use end of node to end of node testing. The networks may be too long, causing unwanted variations in the readings. Keep in mind that depending on the equip- ment used the size of the pad plays a significant WHAT IS 4-WIRE KELVIN? continues figure 4: typical defects detected using 4-wire Kelvin. figure 5: Voids in circular nets.