Issue link: https://iconnect007.uberflip.com/i/291105
30 The PCB Magazine • April 2014 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 PWBs one must also consider con- struction. One may produce multiple micro- vias structures as either stacked or staggered. Microvias that are stacked are about four times more vulnerable to failure than the same structure in which the microvias are staggered. Well fabricated one- and two-layer microvias do not usually fail prematurely. Three- and four-stacked microvias tend to fail before 500 cycles when tested at 190°C and are a fabrica - tion challenge. lar to PTHs and, less often, for corner crack or interconnect separation. One of the HDI struc- tures includes microvias stacked on buried vias. 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 - HDI PWB RELIABILITY continues Figure 5: microvia separation. Figure 7: microvia pull out failure. Figure 6: microvia barrel crack. Figure 8: microvia corner crack.