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PCB-Jan2015

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60 The PCB Magazine • January 2015 or required zero-height (ZHT) via fill. Although a valid process, the misnomer is that this can affect the opposing side of the PCB barrel from valid probing. The "squish" from this can en- capsulate the barrel to the point that probing from the opposing side is invalid. So what is an electrical test engineer to do? Amendment 1 of IPC-9252A states that mid-points should be tested, but, according to Footnote 6, "When conductors are not covered by mask or via plug material." This is all fine when read literally. Mechanical processes in the PCB manufacture are not literal. Capping or filling the via from one side does not guarantee the accessibility of testability from the opposing side! Further, the issue of encroachment exists. If the encroach- ment is absolute, there is no issue, but introduce shift in any direction and the ET probing will be at risk. This is especially prominent when tar- geting larger PTH features where a flying probe cannot probe directly to the center of drill. Fig- ure 1 illustrates an issue that ET must combat to accurately validate subtrates. The systems will offset to the annular ring, but if soldermask is not registered as the CAD reference shows, there will be false "opens" re- ported and will stall the product due to MRB issues. This mis-registration is shown in Fig- ures 2 and 3. If the OEM wishes the full Class III mid-point test it is advisable that they put stipulations on the ICT or mid-points they want tested. Recommendations are only allowing 50–75% squeeze from the opposing side when via filling based on aspect ratio while also not allowing encroachment on the landing pad on the side they wish probed. These examples are somewhat relaxed from what are real challenges but you can see the flavor of what the issue is. PCB Todd Kolmodin is the vice president of quality for gardien services usa, and an expert in electrical test and reliability issues. To read past columns, or to contact Kolmodin, click here. CHALLENGES OF ELECTRICAL TEST continues Figure 1: landing pad encroachment. Figure 3: mis-registration. Figure 2: mis-registration. tESting toDD

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