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PCB-Apr2018

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60 PCB007 MAGAZINE I APRIL 2018 Looking at PTH Voids Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY Introduction Several columns have been published in this space describing the origins of voids in the plated through-hole. However, not all voiding is caused by or has its genesis in the electro- less copper (PTH metalization) process. For this edition of "Trouble in Your Tank," I pres- ent another look at voids and additional root causes that may be less understood. Pre-electroplating Causes of Voids One of the areas often overlooked as a cause of plating voids is a discontinuity in the electro- less copper deposit. However, what if the pan- els were void-free after electroless copper only to v oid later in the manufacturing process? Sev- eral possibilities must be considered when such v oiding is discovered. These include: • Scrubbing prior to resist lamination • Excessive copper removal caused by the microetch prior to electrolytic copper plating • Photoresist residue in the via In general, the type of void one would see is shown Figure 1. The defect depicted is often referred to as a corner void. In this case, cop- per plating is missing on all four corners of the hole. Again, keep in the mind that there was no voiding related to electroless copper. So what could be the root cause of the void shown in Figure 1? It is easy see to the bare dielectric at the corners. Yes, there is sufficient copper plat- ing in the via, however, the slope of the cop- per adjacent to the void has the appearance of an etched copper trace. There is the over- hang of the plated etch resist (tin or tin-lead). The genesis of this etch void is due to the thin knee. That is, the tin or tin-lead on the corner or knee of the hole is less than optimum thick- ness. The thin deposit will not withstand the attack from the etching process, allowing the copper to be etched away. Thin etch-resist plating at the knee of the via can be attributed to improper solution ag- itation in the tin plating process. In addition, when the organic addition agents are out of Figure 1: Example of a corner void. (Source: IPC-9121)

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