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AUGUST 2018 I PCB007 MAGAZINE 71 chain breaks first. If the chain's weakest link is thin-plated or brittle copper, then one can expect the failure to occur at this point. Keep in mind that the number of thermal cycles to failures is affected by the strain imposed on the PTH during each cycle. Thus, the concern with additional strain created by the protrud- ing zinc tail. Regardless, it is a given that the zinc tail can have a negative impact on PTH thermal reli- ability. Therefore, it is best to understand how to minimize its formation, or at least provide a means to mitigate the negative effects of the zinc tail. One way to remove zinc is to be somewhat aggressive with the microetch prior to electro- less copper plating. Strong acids, such as hy- drochloric acid, will also remove the zinc with- out etching the copper interconnects. I recom- mend that if zinc tails become an issue, testing of acid soak cleaners, such as hydrochloric ac- id-based and persulphate-based microetchants, be evaluated. PCB007 Michael Carano is VP of technology and business development for RBP Chemical Technology. To reach Carano, or read past columns, click here. Figure 3: Zinc tail evident on C-stage. C-STAGE

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