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76 PCB007 MAGAZINE I JUNE 2018 tor and processed through the facility. One set of panels was processed through the previous electroless copper process. Another set was processed through the new electroless. All oth- er process steps were identical. Several coupons each were prepared for IST testing. Initial IST results are shown in Figure 3. Figure 5 shows the results of microsections of the test vehicle coupons after completion of the IST cycles. As the IST results show, the failure was due to cracking in the PTH (barrel). Note the sig- nificant change in resistance detected in the sensing circuit. The power circuit resistance showed little to no change, indicating a robust post connection. It was clear that the failure in the barrel is re- lated to poor ductility of the acid copper elec- trodeposit. There was no evidence of under- cured resin material that could have partially explained the excessive Z-axis expansion. Clearly, the root cause of the defect rested within the plated through-hole. If there was significant change in the post interconnect, one could assume that a less than robust elec- troless copper-to-post interconnection was at least contributing to the failure. But in this in- stance, that was not the case. The failure clear- ly rested within the plated barrel of the via. Thus, possible failure contributors in the PTH are: • Low ductility of the electroplated copper due to additive imbalance or contamina- tion of the plating solutions • Thin electroplated copper in some areas of the via (poor throwing power, rough drilled vias, too low of a current applied) • Columnar grain structure of the plated copper deposit (yielding a deposit of poor ductility) To understand the failure hierarchy, it can best be described as follows: If the PTH barrel is robust (or the post interconnect quality is very poor), the inter-plane connection will receive increased/extended levels of strain and thus fail. If the PTH barrel is not robust, the failure mode of barrel cracking will totally dominate the interconnect performance, thus the stress - es are not given the opportunity to distribute to the innerlayer copper foil interface to cause mi- crocracks. The data collected and the visuals of the microsection clearly indict the plated barrel as the failure mode. For the engineer reading the resistance graph and using the quantitative data of cycles to fail- ure, insight on the failure mode can be gained. It is critical that the actual failure mode is iden- tified. PCB007 References 1. More information on IST can be found by clicking here. 2. IPC-TM-650,2-6-26A. Michael Carano is VP of technology and business development for RBP Chemical Technology. To reach Carano, or read past columns, click here. Figure 5: Close-up of PTH cross-section (left); wider view on right. Clearly, barrel cracking is the failure mode.