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48 The PCB Magazine • October 2014 In contrast to the V2V case, for the via-to- plane structure (V2P) at all test vehicles failures were observed. The majority of the failures oc- cur at early stage of the test which might be a sign of residues or impurities due to non-opti- mized manufacturing conditions. Cross-section analyses of the failed samples did not show evi- dence of failure due to dendrite growth. All ob- served dendritic structures can be considered as small compared to the corresponding dielectric gap. However, as it is also shown subsequently for the comb structures, the samples show clear signs of electrochemical migration of copper into the dielectric material. Together with the observed small dendrites this is considered to be a major cause for the observed failures. Concerning the comb structure the results are similar to the results obtained for the V2P structures. Again TV1 and TV2 show better re- sults than TV1. For all samples the majority of the failures are located in the outer layer comb structures, literally in layer 1 and layer 8. Figure 15 shows a detailed plot of the monitored resis- tivity versus test duration for the comb struc- ture on one outer layer (layer 8). As for the V2P structures, the evaluation of cross-sections of the failed coupons did not show evidence of excess filament growth or re- lated electrochemical migration effects which could be identified as root cause for the failure in the HAST test. In fact the comb structures showed significant signs of copper migration at the failed samples. Figure 16 shows an example for a failed structure at TV3 in a darkfield view; Figure 17 shows a darkfield/UV image of a failed comb structure of TV2. Both figures show clearly the migration front at the copper traces with negative po- tential. However, it is also visible that the mi- gration front has not reached the neighboring conductor at the time of failure. So again it is considered that failures at the comb structure are a combined phenomenon where the migra- tion effects together with dendritic growth or THIN PCBS FOR SMARTPHONES continues figure 15: resistivity for comb structure on outer layer (l8) vs. storage time in hast test.