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72 PCB007 MAGAZINE I SEPTEMBER 2020 Introduction For those associated with PCB fabrication, one of the biggest nightmares is the infamous interconnect defect (ICD). Essentially, an ICD is a separation of the plating from the intercon- nect foil. The industry typically classifies this defect into four categories. Type 1 describes the situation where the electroless copper separates from the inner layer copper face. Of course, there are other types of separation that can be encountered. It is well known that the electrolytic copper may separate from the electroless copper, but the electroless copper remains on the inter- connect. This situation is referred to as Type 2 ICD. Whereas Type 1 references the electroless copper separating from the inner connect post or inner plane, there is a third type where the electroless copper deposit separates from itself. This is more like a cohesive failure within the deposit. Type 3 is very difficult to detect and re- quires excellent micro-sectioning and polishing techniques to properly detect and troubleshoot the root cause. Finally, one defect you may en- counter is the fourth type of ICD, D-Sep. Type 1 A type 1 ICD occurs due to a number of fac- tors. First and foremost, an extremely high rate of deposition introduc- es internal stress into the electroless depos- it. When the deposit is highly stressed, most thermal excursions will cause the copper deposit to separate from the inner layer post. The separation can be as little as a hairline fracture to small separations along the face of the post. One solution to mitigating a Type 1 ICD is to slow down the rate of copper deposition. Also, it prevents over-catalyzation (excessive palladium at the interconnect face) and reduces operating temperatures. D-Sep D-sep is also a separation of the electroless copper deposit from the interconnect. How- ever, unlike a Type 1 ICD, which occurs after a thermal excursion (solder shock, assembly, etc.), D-sep occurs without any thermal stress (Figure 1). D-sep is caused by excessive stress in the deposit, allowing the separation that re- sembles the letter "D," hence the name. Other possible root causes relate to the interruption of the plating mechanism that leads to a mis- oriented grain structure of the copper deposit, A Process Engineer's Guide to Interconnect Defects Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY Figure 1: D-sep.

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