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30 The PCB Design Magazine • December 2014 those original readings. A -4% change or great- er indicates significant material damage and a cross section is processed to confirm or refute this finding. The major problem is the implementation of lead-free soldering that requires assembly temperatures of 260°C. The FR-4 material is at its limit to withstand the heat when exposed to 260°C. The Z-axis expansion is at its highest at this temperature, putting extra strain on the copper interconnects. The most common type of failure of a robust interconnection is a barrel crack that occurs in the central zone of the plated through hole (PTH). When tested using thermal cycling to 150°C, this is a wear-out type of failure that happens over hundreds of cycles (500+ cycles). Surviving 500 cycles without any significant increase in resis - tance is considered a robust coupon. In a weak coupon, the failure before 350 cy- cles may relate to a process problem, with the most common problem being thin copper plat- ing. With thin plating, the barrel cracks may still be the cause the failure but it would be fail- ing in less than 350 cycles. The PTH may also fail for corner cracks or interconnection separa- tion. Weak buried vias fail typically with barrel cracks in the center zone of the structure simi- lar to PTHs and, less often, with corner cracks or interconnect separation. One of the HDI struc- tures includes microvias stacked on buried vias. feature HDI PWB RELIABILITY continues Figure 1: Barrel crack due to metal fatigue. Figure 3: Corner or knee crack. Figure 2: Interconnect separation. Figure 4: lifted copper cap on buried via.