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October 2014 • The PCB Magazine 49 THIN PCBS FOR SMARTPHONES continues impurities such as copper residues finally result in a drop of the resistivity. In general, based on the experiences avail- able at AT&S related to electrochemical migra- tion phenomenon such as dendritic growth, the obtained results are considered as non-critical for all three part numbers, since during manufac- turing of the evaluated TVs in the first step, no specific manufacturing provisions to improve CAF/HAST behavior have been applied. Avail- able experiences show that with special pro- visions—as regularly applied in mass produc- tion—a significant improvement of the electro- chemical migration behavior can be achieved. Conclusion The smartphone market is constantly driving producers of PCBs to reduce board thickness. For an eight-layer board in a modern mobile phone, an any-layer design is considered as almost man- datory, while the thickness target is currently at 600 µm for this kind of stack-up, and it dropped to 400 µm for a rigid eight-layer board in 2013. Current research has shown that with the currently available materials and manufactur- ing processes, it is possible to build rigid any- layer PCBs featuring a build-up of eight layers with a maximum thickness of less than 500 µm. To implement these boards, different manu- facturing approaches were successfully applied: a high-end HDI process combined with via- filling step, and the ALIVH technology. Addi- tionally, it was shown that the combination of pure ALIVH with outer layer HDI, the so called ALIVH-C process, could be successfully ap- plied. Board thicknesses between 443 and 512 µm were manufactured. AT&S has almost 20 years of experience in HDI, and began licensing ALIVH technology in 2011. Thus, ALIVH tech- nology in particular promises still additional potential for improvements in manufacturing of thin boards. figure 16: microscopic darkfield illustration of comb structure on tV3 layer 1, solder mask layer removed by grinding. figure 17: microscopic illustration of comb structure on tV2 layer 8, solder mask layer removed by grinding, left view darkfield, right view with uV lamp.