Issue link: https://iconnect007.uberflip.com/i/1313710
DECEMBER 2020 I SMT007 MAGAZINE 81 The samples were mounted in the test rack and kept in the chamber for 24 hours before raising the temperature (85°C) and the humid- ity (75% RH). The first measurement was performed just before the temperature was increased. During the preconditioning phase (85°C and 75% RH without bias), measure- ments were taken every 24 hours. After 96 hours of soaking, the actual CAF phase started, and a bias voltage of 50 V was applied to the samples. Measurements were then taken from all samples every 15 minutes. The test contin- ued for 500 hours, after which the bias was switched off, and the measurement programs stopped. The temperature and humidity were reduced to 25°C and 50% RH. The samples were kept inside the chamber for an additional 24 hours when the final measurement was per- formed. A detailed overview of the HDI CAF test results and associated failure analysis will be published elsewhere. As an example, Figure 5 shows the results for buried vias in a straight configuration with a pitch of 0.8 mm and 1.0 mm for both the warp and weft direction of the glass fibers. Figure 6 shows the results of the microvia test structure. The failure criteria is a drop in resistance of more than one decade compared to the baseline resistance after 96 hours of soak- ing. No failures are observed between micro- vias at 0.5-mm pitch, buried vias at 1.0-mm pitch, or PTHs at 1.27-mm pitch. Some fail- ures occur between buried vias at 0.8-mm pitch. Most failures are detected in the via-to- plane test structures, especially for the PTHs. There is no significant difference between the antipad diameter for basic and complex HDI Figure 5: CAF test results for buried vias in a straight configuration with a pitch of 0.8 mm (left) and 1.0 mm (right) for both the warp (upper row) and weft direction (bottom row) of the glass fibers.