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46 The PCB Magazine • October 2014 cess. Rather, it has to be considered that the FR-4 HDI prepreg material used is a most recent generation of FR-4 material that has been op- timized for reflow behavior and has a known outstanding reflow performance. Thus, it is as expected that TV3 shows the best performance in MSL testing. If the cross-sections of the failures at TV2 and TV3 are investigated in more detail, it is visible that the delamination failures occur in both cases which feature design characteristics known to be critical for reflow testing, such as structures with full copper areas. The analyses did not show evidence for an interface failure that could be related to the applied copper sur- face adhesion promotion system. Instead, the cross-sections show a mixed failure mode with cohesive failure in the prepreg layer as well as adhesive failure at the interface (Figure 14). In order to identify the root cause for delamina- tion at the current samples more work has to be done; however, although the performance of TV2 and TV3 in the reflow process did not reach the level required for mass production boards, it is considered as acceptable, taking into account that the samples are built within a development project without any process optimization steps. Thermo cycling results As described earlier, the temperature cy- cling was done on bareboard coupons as well as on populated ones. Neither at the bareboard samples nor at the populated ones were failures THIN PCBS FOR SMARTPHONES continues figure 10: mean thickness of the manufactured thin boards (tV1 to tV3). figure 11: Cross-section of tV1 (3+2+3 hdi/fV buildup). figure 12: Cross-section of tV2 (1+a6+1 aliVh-C build-up). table 7: results of reflow testing of thin board test vehicles.