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32 The PCB Design Magazine • December 2014 feature HDI PWB RELIABILITY continues In order to have a microvia stacked on top of the buried via, what we have to have is a con- ductive cap on the top of the buried via, which can also fail. There may be a separation of the copper cap from the top of the buried via or a crack in the cap of the buried via (Figure 7). Microvias are typically the most robust type of interconnection. Because of their robustness, they are tested at 190°C. When tested at 190°C, robust microvias will survive 500 cycles while weak microvias will fail before 500 cycles. The most common cause of microvia failure is a separation between the base of the microvia and the target pad. The second most com - mon cause of microvia failure is a barrel crack toward the base of the microvia. Other failure modes include corner cracks (seen in copper- filled microvias) and pull out types of failures where the target pad cracks around the base of the microvia. In HDI PW Bs one must also consider con- struction. One may produce multiple microvias structures as either stacked or staggered. Micro- vias that are stacked are about four times more vulnerable to failure than the same structure in which the microvias are staggered. Well fabri- cated one- and two-layer microvias do not usu- ally fail prematurely. Three- and four-stacked microvias tend to fail before 500 cycles when tested at 190°C and are a fabrication challenge. The material damage is monitored in an IST coupon by measuring changes in capacitance Figure 5: Microvia separation. Figure 7: Microvia pull out failure. Figure 6: Microvia barrel crack. Figure 8: Microvia corner crack.