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PCB007-Aug2022

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64 PCB007 MAGAZINE I AUGUST 2022 Electrical test is best known for identifying routine opens and shorts. But what has plagued ET, and manufacturers in general, is the bar- rel void. ese voids are breaks in the plat- ing of the drilled barrel that result in circuitry "opens" that pass through the stackup from one layer to another. In some cases, the break in plating is severe and results in an immediate open condition that is detected during electri- cal test. However, certain plating anomalies can be present that will go undetected during stan- dard ET. What we must remember is that elec- tricity must follow Ohm's Law no matter what conditions exist. at is: V = I x R, where V = Voltage, I = Current, and R = Resistance. e most significant hidden plating anomaly to barrels is thin copper, sometimes known as "taper plate" as shown in Figures 1 and 2. is is a condition where copper may be accept- able close to the outer layers of the PCB but may become thinner as distance increases to the center of the depth of the barrel. Although copper exists, it is a dangerous anomaly that can routinely go undetected. Now I can hear the gallery saying, "Todd, how is this possible? e thin copper should blow like a fuse under the formula of Ohm's Law." Well, if Ohm's Law were to be applied straight away, you would likely be correct, along with most circuitry in the board. e reason? Most small trace widths could not stand up to the abuse. ET vs. Ohm's Law is law cannot be broken. How- ever, we can adapt to the law while still maintaining safety to the PCB and under ly ing c irc uitr y. Many specifications call for test voltages to be applied during test. Indus- try and military requirements are 40V minimum, so let's use that. Cal- culators ready? Okay, we see 40V. Let's say the continuity resistance threshold is 20 ohms. Now plug those into Ohm's Law: 20V = I x 20. Ohm's Law can be simplified for solving any variable. For instance, V/R = I or V/I = R. Anyway, we have 40 volts and 20 ohms. Solving, V/R = I we see 40/20 = I, or 2 amps. Breaking the Void Testing Todd by Todd Kolmodin, GARDIEN SERVICES USA Figure 1: Taper plate voiding indicated (left barrel).

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