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52 The PCB Design Magazine • September 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 ro- bust interconnection is a barrel crack that oc- curs 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 in- crease in resistance is considered a robust cou- pon. 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 failing in less than 350 cycles. The PTH may also fail for corner cracks or interconnection separation. Weak buried vias fail typically for barrel cracks in the center zone of the structure simi- lar to PTHs and, less often, for corner crack or interconnect separation. One of the HDI struc- tures includes microvias stacked on buried vias. 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. article