Issue link: https://iconnect007.uberflip.com/i/1313710
DECEMBER 2020 I SMT007 MAGAZINE 73 ied via sees higher stress than when the pads are tangent. The same configuration cannot be applied for the 0.8-mm pitch fanout as there is not enough space next to the buried via. The component pad, with microvia 1-2 partially inside the pad, is located above the buried via. Due to the smaller pad size, the microvia 1-2 is located above the buried via, which is consid- ered a reliability risk. The target pad of micro- via 2-3 is placed tangent to the pad of the bur- ied via on the opposite side of the buried via. Two IST coupon designs are included on the panel to cover the most critical design features of the BGA coupon. Next to a TVX coupon for through-vias, an SLX coupon with buried vias and microvias at 0.8-mm pitch is included (microvia configuration as described previ- ously). This SLX coupon contains three sense circuits: buried via (S1), buried plus microvias (S2), and staggered microvias (S3). The cou- pons are placed on the panel in close proxim- ity to the BGA coupons. To assess the CAF performance of a given HDI PCB technology, an HDI CAF test vehi- cle is required that resembles the final prod- uct as much as possible (build-up, via con- figuration, routing, etc.). The HDI component fanout is represented by including buried vias with a pitch of 0.8-mm and 1.0-mm and micro- vias with a pitch of 0.5 mm. The highest CAF risk for through-vias on HDI PCBs is the high- density connectors with a pitch of 1.27 mm. The minimum distance between a via and a ground plane for buried vias and through-vias is defined by the design rules for minimum conductor spacing (75 µm for basic HDI and 50 µm for complex HDI). The drill diameter for buried vias (0.3 mm) and through-vias (0.5 mm) is copied from the qualification test vehi- cle design for the basic HDI technology. Microvias are considered less prone to CAF due to the use of laser drilling instead of mechanical drilling, and their smaller diameter results in a smaller contact area with the glass fibers. Nevertheless, the wall-to-wall spacing of a microvia with 125 µm drill diameter at 0.5-mm pitch is only 375 µm. This is far below the wall-to-wall spacing seen for buried vias or PTHs. To assess this risk, a dedicated micro- via test structure is added to the HDI CAF test vehicle. Due to the independent drilling pro- cesses, stacked microvias are not considered a higher risk for CAF compared to staggered microvias. Six types of via-to-via test structures are included in the HDI CAF test vehicle: 0.3-mm drill buried vias with 0.8-mm pitch in a straight and a staggered alignment, 0.3-mm drill buried Figure 2: BGA coupon with real and daisy-chain component fanout for 1.0-mm and 0.8-mm pitch component (left) and build-up of the qualification test vehicle (right).