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54 SMT007 MAGAZINE I SEPTEMBER 2024 Other works have shown how the choice of PCB materials, fabrication process conditions, and design each impact global PCB bow/twist and warpage 7 . Modeling and observation have shown PCB stackup symmetry is essential for both the thermo-mechanical properties and the thick- nesses selection of materials, with respect to the neutral axis, to manage and minimize PCB bow/twist and global warpage. 7,8,9 Care is usually taken to balance the retained copper between centerline layer pairs across the cen- terline of a PCB, as shown in Figure 2, to pre- vent shiing of the PCB neutral axis that could initiate general warpage. While controlling global PCB bow/twist and warpage is important, it is the local PCB topography or coplanarity of the solder pads directly under an individual package that is critical in determining the SLI solder joint. Prior work highlighted that localized variations of percent copper within an individual PCB layer can result in localized changes in pre- preg thickness leading to core deformation. 10 Regardless of how well percent copper is bal- anced across centerline layer pairs, the percent copper directly under the package is usually much different than the average percent cop- per for an individual layer due to antipad arrays in plane layers or pad/trace density on signal layers, as shown in Figure 3. e impact of these variations increases with higher-layer PCB designs and thicker copper layers for power delivery. As the local total copper under a package is oen different than outside the package foot- print, it is possible to obtain local thickness and surface topography variations across the PCB directly under the package driven by resin shrinkage during the lamination process. 11,12,13 Under packages with large edge dimensions or when using a resin with low flow characteris- Figure 2: Layer pairs and thickness delta vs. delta retained copper. Figure 3: Representative retained copper outside and inside a package footprint area.