Issue link: https://iconnect007.uberflip.com/i/1313710
74 SMT007 MAGAZINE I DECEMBER 2020 vias with 1.0-mm pitch straight and staggered, 0.5-mm drill through-via with 1.27-mm pitch straight, and the microvias with 0.5-mm pitch straight. (Straight alignment refers to vias which are directly adjacent, while the stag- gered alignment offsets the second via with half of the pitch. The term staggered can cre- ate confusion with the microvia configuration. IPC-9691B uses the terms in-line and diagonal, respectively.) A number of these test structures are duplicated to cover both the warp and weft fiber direction. Although via-to-via filament growth is regarded as the dominant form of CAF, via-to- plane test structures remain relevant for HDI. Four via-to-plane test structures are designed with buried vias or PTHs. The diameter of the antipad is the sum of the pad diameter and two times the minimum as-designed spacing for basic HDI (75 µm) or complex HDI (50 µm). In the case of the buried vias (0.6-mm pad diameter), the antipad diameters will be 0.75 mm and 0.7 mm. For the PTHs (0.8-mm pad diameter), this becomes 0.95 mm and 0.9 mm. The test structures consist of 17x17 vias with all internal non-functional pads removed to ensure the failure mechanism is from the hole wall to the conductor. A single design is used to cover both the basic HDI and the complex HDI technology. To mimic the presence of an additional microvia layer, a dummy lamination is performed before the PTH drilling. This covers the risk of material embrit- tlement as a result of multiple laminations, affecting the quality of the drilled holes. The layout of the HDI CAF TV is shown in Figure 3. The distance between connected vias of a given test structure is varied to avoid pos- sible alignment with the glass fiber bundles, resulting in worst-case (or best-case) perfor- mance. For the same reason, additional spac- ing between groups of vias in staggered test structures is foreseen. Test Methods The qualification test vehicle is subjected to the qualification test flow for HDI technol- ogy as defined in ECSS-Q-ST-70-60C. Group 1 (visual inspection and non-destructive test) consists of visual inspection, dimensional Figure 3: Design and build-up of the HDI CAF test vehicle.