Issue link: https://iconnect007.uberflip.com/i/1476200
66 PCB007 MAGAZINE I AUGUST 2022 Not many 3- to 5-mil traces are going to sur- vive that let alone the electronics in the test machine. Now, what many may not understand about ET and how machines check for opens and shorts is that the "test voltage" requirement is not applied to the opens, or continuity test. e simple fact is that, as shown above, the excessive current would be devastating. Two different processes are in place during the opens test. First, the voltage is limited to a low value, typically 12–20V regardless of the test voltage requirement. is is simply because an open will be present at 10 volts, or the low voltage, thus the high voltage is not required. e second is that during the conti- nuity test the machines use bypass circuitry to limit the current that is applied to the circuit under test. Parallel resistive networks in the machine will shunt the current away from the circuit, allowing only enough to safely take the continuity measurements. Pretty sneaky, huh? Indeed. e higher voltages are applied during the shorts or discontinuity test. Once circuits are validated for opens, the higher voltage is applied to circuits to identify shorts. Again, Ohm's Law is at play. However, with test- ing for shorts, we don't just apply the voltage and hope for the best. In this case, we know the voltage applied but the other variables are unknown. As we are testing network-to-net- work for possible shorts, the resistance value is expected to be very high to infinite. Using V = I x R, V is known, so in my example, I will use 100V. We don't know I or current, but we do know the theoretical resistance R. However, we cannot use "0" or infinite as the law will not allow us. 100 = I x 0? It cannot work when we know we have a known voltage value of 100. So, the shorts threshold must have a value in resistance. Most times it is 10 meg ohms or higher. Now the law works again. 100 = I x 10M or 10,000,000 ohms. Solving, 100V/10M ohms = 0.00001 amps or 10 micro-amps. Now that is still way too much current to allow to pass between circuits if a short may exist. Cur- rent limit triggers are in place during the shorts test that effectively stop the test on the net- work when any current is detected between the nets. erefore, it is relatively rare that a standard automatic electrical test can result in damage to the circuitry. In other words, don't rely on ET to blow up shorts as, in general, we do not apply enough current to cause shorts to burn. e test can heat up the short momen- tarily before the fault trigger but usually will not cause the short to go away. I digressed to theories, so let me come back to voids. Based on my long-winded explana- tion regarding how the opens and shorts tests work, we cannot lean on ET to detect all types of barrel voids. We limit current flow so that we are not blowing fuses (thin traces). Fur- thermore, the difference in resistance of a net- work with an acceptable signature compared to a network with a taper plate or thin copper wall will not be that different. ese changes in resistance are usually in the milli-ohms. Most electrical test machines use a threshold with a tolerance of plus or minus a percentage. For example, 20 ohms ±5%. at is like 1 ohm. A taper plate void scenario may only change the overall resistance of the network by 5–10 milli- ohm. is will not trigger a fault in the conti- nuity test and thus go undetected. Figure 2: Thin copper (magnified) within the barrel that can pass ET.