Issue link: https://iconnect007.uberflip.com/i/1383248
64 PCB007 MAGAZINE I JUNE 2021 ary circuit within itself is not large enough to cause a continuity failure. erefore, the actual inductance of the coil should be measured to ensure a breakdown of the primary, secondary, or both is not pres- ent. Some newer flying probes have developed soware modules that can measure this induc- tance. Just like buried resistance, the induc- tance (in Henry's) is pre-programmed within the netlist and the flying probe measures the inductance of the coil circuit just like any oth- er network. However, this does require an up- graded metering system beyond the standard out-of-box configuration. If you are interested in automated inductance testing, consult your preferred equipment vendor, and discuss what options may be available to you. Another caveat of the electrical test arena is the presence of high-resistance shorts, some- times referred to as high-Z or micro-shorts. e problem these phenomena bring to detec- tion is that they can be present momentarily at the time of test but due to current flow of the test machine being used, they can "blow" like a fuse. Although electrical testers use a mini- mum of current flow during test, these mi- cro-shorts can be extremely small, like slivers or whiskers. It takes a minimum of current to blow them open. A technician may attempt to verify the short reported, only to find that it has disappeared, and the retest of the PCB will pass. Unfortunately, a ticking time-bomb remains. e board now passed and moves on to assembly and finally into service. Howev- er, deep inside the board a process called elec- tro-chemical migration may be taking place. Due to the internal burn of the "blown fuse," there are burn deposits le behind. Over time, this process can migrate metal ions from one electrode to the other—in this case between the once shorted tracks or traces. In the worst case, this can cause an electrical path to return and a high resistance short may reappear. is can cause the final product to have degraded functionality in the field or fail altogether. To combat this, extremely low current should be applied to the circuits while still having the ability to detect the high resistance short. Many times, these shorts will register well above the standard ET parameter for isolation such as 10 or 20 megohm. Using a "micro-short" algo- rithm some test machines can identify these defects without destroying them. Keeping the current to the lowest possible limit while still looking for the high resistance fault is the key. Using standard methodology, the current may exceed the resistance threshold of the short and it heats up and finally burns. is can hap- pen almost immediately so this test should be done prior to standard ET screening. at way the micro-shorts will be identified immediate- ly before higher current is applied. Doing a micro-short test aer standard ET defeats the purpose of the test, as any damage may have already occurred. One must also re- member that these shorts can exhibit very high resistance and a standard "off the shelf " ohm- meter may not have the resolution to verify the high resistance present. So, in micro-short de- tection one must not assume the defect is fake because an ohmmeter does not detect it. If you don't have the correct verification equipment, retest the board. If it fails again, believe the machine, fail the board, and have it examined. Second guessing a fault such as this only results in disaster. Be safe! PCB007 Todd Kolmodin is VP of quality for Gardien Services USA and an expert in electrical test and reliability issues. To read past columns or contact Kolmodin, click here. Table 1: Micro-short detection.