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October 2014 • The PCB Magazine 47 THIN PCBS FOR SMARTPHONES continues observed over the test time. None of the tested specimen exceeds the limit of -10% of resistivity decrease; the test is considered as passed for all part numbers. At the thermo cycling of the populated sam- ples also no failure was observed as shown in Table 8. Based on the obtained results it can be con- sidered that the thermo cycling behavior will not present the primary challenge in manufac- turing thinner boards. It is commonly consid- ered that major effect influencing the perfor- mance of boards stressed cyclic thermo load is the CTE mismatch between copper at vias and applied dielectric materials. Making the boards thinner does not make the mechanical loads on the copper plating in the plated through holes or the micro vias significantly more severe. This assumption is confirmed by the results of the current work. Additionally no influence of the manufacturing technology was observed. All test vehicles manufactured with the different any- layer technologies passed the limit of 100 cycles in barebord as well as populated configuration. Results HAST Test In contrast to the thermo-cycling the electro- chemical migration behavior seems to be a more critical issue for very thin boards. The subsequent Table 9 shows a summary of the results obtained from HAST testing of the investigated samples. In general the table shows that several fail- ures have been recognized. At the current study the samples with an HDI outer layer—TV1 with the full HDI built up and TV2 with the ALIVH- C built up showed at better HAST performance than TV3. For the HAST test the via-to-via (V2V) struc- ture can be regarded as the least critical case of the three cases evaluated. As expected there was no failure observed at these structures. figure 14: detailed view on delamination area at tV3. table 8: summary of results of thermo cycling test, board level. figure 13: Cross-section after reflow testing, with occurred delamination at tV3. table 9: summary of result of hast testing for thin board test vehicles.