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Electrical Test is an essential part of the manufacturing process. This is the only true way to ensure that the board is working exactly as it was designed electrically. Technology for Electrical Test hasn’t really changed much over the years. Mainly the introduction of flying probe technology and the 4-wire Kelvin testing along with better advancements in capacitive/high frequency modulation testing have been the main changes in the past 2 decades.
The most accurate and expensive way of testing the bare PCB is by 4-wire Kelvin testing. Basically this test uses 4 contacts along one net. 1 set of contacts measures the resistance using voltage, and the other set measures the resistance by using amperage. The result of those two measurements allows the machine to eliminate lead resistance and calculate the resistance of the net being tested in the 0.001 ohm range compared to standard machines having difficulty testing under 5 ohms. The benefit of being able to test with such accuracy is that not only are you able to understand basic connections of the net, you are able to calculate if there is low plating in a via, or other finite issues that a standard electrical test would not be able to pick up. The downside of this test is it very expensive in two ways…one, the machine is rare and fairly expensive, even when added to an existing flying probe tester. The second being, the test itself takes a very long time since it is a straight resistance test of one net at a time.
The next best way to electrically test a board is by doing a “true resistance” test, or some call this 100% net list test. This test can be preformed on a “bed of nails” tester or a flying probe tester. The bed of nails tests all nets at the same time and is great for volume orders. The problem is the bed of nails struggles with very dense areas like HDI. Also, building a fixture for this test is expensive and time consuming. On the flying probe this true resistance test is very time consuming but relative inexpensive. The reason the flying probe tester is very time consuming is that the machine has to place one probe on each of the end points of the net one net at a time. Some machines have up to 16 probes, so theoretically it would be able to test 8 nets at a time but in most cases there is not enough room to simultaneously accomplish this.
The most common test method for low volume work is using the flying probe, but instead of true resistance test, the most common and quickest form of measurement is a capacitive test. Basically a capacitive test generally works by charging the power/ground plane with one probe and measuring the capacitance, (or sometimes the “phase difference”) of each net with the other probes. There is a lot more technology involved in this method than what I’m explaining, but that is the very basic principle of how it works.
Hipot testing is a simple high voltage test to ensure good insulation between the power and ground planes in the 500- 1000 Volt range. This test is a non destructive test that helps ensure there will not be dielectric breakdown in the field. Some flying probe machines are able to do this test automatically, but most often than not this is a manually operated test.
This test method is outlined in IPC-TM-650, section 220.127.116.11, Dielectric Withstanding Voltage (HiPot Method) - Thin Dielectric Layers for Printed Circuit Boards (PCBs)