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SEPTEMBER 2024 I SMT007 MAGAZINE 63 factors influencing coplanarity variation under a component. It was also demonstrated that for the same copper balance strategy, coplanarity increases as the component footprint dimen- sions increase. e profile of retained copper under a com- ponent has a direct relationship with the sur- face topography and coplanarity variation of the PCB footprint for the component. It was shown that as the total removed copper increases, the coplanarity increases. As silicon substrate packages grow in size, it is increas- ingly important to balance the percent of retained copper under the package footprint and match it to the percentage of copper out- side the package on each layer, and balance the total across layers. Large reductions in total copper under a component footprint leads to an hourglass PCB thickness profile and can be a major driver of high localized PCB coplanarity vari- ations under components. Generally, this is the result of having a high concentration of antipads, or incomplete copper floods on power layers under a component resulting in a change in percent retained copper. e impact grows proportionally higher with more and/ or thicker copper layers. is can be optimized by selectively adding non-functional pads for power and ground nets and fill any open areas on both signal and plane layers. Care should be taken when using mixed signal/planes on an individual PCB stackup layer as stacking local- ized areas of high copper under the component footprint can result in complex "W " or "bow- tie" surface profiles. e selection of materials in a PCB stackup can impact the variability and average copla- narity within a given design, especially as com- ponent dimensions increase. e impact of material selection is much less when the design is optimized for copper balancing. e usage of interface analysis obtained by scanning top and bottom surfaces of the PCB, such as the Akrometrix method, can be used to quantify and differentiate the contribution of local PCB thickness variations and the global PCB bow/twist or warpage on the coplanar- ity at a component footprint. e high corre- lation of coplanarity to local PCB thickness variations across materials and copper balance strategies provides an opportunity for design- ers and PCB fabricators to gauge the "good- ness" of a design and material selection by eval- uating the local PCB thickness profile. By com- bining the technique with analysis of the total retained copper profile outside the component footprint and under the component footprint, it is possible to determine where and how cop- per balancing improvements should be imple- mented. is can also be used to prioritize between design changes or material choices vs. process changes when addressing coplanar- ity issues. Figure 15: Delta thickness under component footprint.