The critical area model is commonly used to estimate the yield loss sensitivity of products to random failure mechanisms, especially shorts. With the shift to dual-damascene copper backends, interconnect opens have become a very important factor in product yield loss. On via chain test structures, we observe a strong dependence of the yield loss as a function of via density. Using the critical area model, we calculate the sensitivity of via chains to metal opens and via opens. By comparison with experimental data, we show that the difference in via chain yields between aluminum line-tungsten plug and dual-damascene copper technologies can be adequately explained by the difference in predominant yield loss mechanism. The presented model allows accurate calibration of yield models for backend yield loss due to opens.