Issue link: https://iconnect007.uberflip.com/i/1526114
SEPTEMBER 2024 I SMT007 MAGAZINE 55 tics there can be insufficient resin flow during the lamination cycle that can impact local PCB thicknesses and surface topography. Figure 4a shows the classical assumption where the PCB surfaces aer the lamination process are planar with the press platen plates. Figure 4b shows the potential PCB cross-sectional deformation resulting from resin shrinkage. When the resin volume is uniform across the PCB, the impact of resin shrinkage is not noticeable, but when the resin volume is not uniform across the PCB and not supported by PCB copper shapes, the impact of resin shrinkage will be proportional to the change in resin volume. In some cases, under large package foot- prints, designers may stack power or shielding shapes which result in islands of higher total copper within a local area under the package. In these cases, the mechanism of resin shrink- age could generate non-standard profiles such as "W " shapes. Experimental Methodology A test vehicle was designed to evaluate a range of copper balancing distributions under various package footprint sizes ranging from 28 x 28 mm to 89 x 89 mm. Figure 5 shows the test vehicle stackup and basic design floorplan. To drive a high range of total copper and emulate high-end server and network systems, a sym- metrical 22-layer construction with target 2.50 mm final thickness was selected which con- sisted of six layers of 2-ounce copper located at the center and six layers of 1-ounce copper at the top and bottom of the stackup. e basic floorplan form-factor was 205 x 285 mm with a set of components and connector placements. Component footprints included a large 89 x 89 mm footprint at 1 mm pitch located at the cen- ter of the floorplan, two 37 x 37 mm footprints at 0.8 mm pitch, and two 28 x 218 mm foot- prints at 0.65 mm. A power channel from the large package footprint was also included to Figure 4: PCB thickness under package with and without resin shrinkage. Figure 5: Test vehicle stackup and floorplan.