PCB Surface Topography and Copper Balancing Under Large Form Factor BGAs

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Editor’s Note: This paper was originally published in the Proceedings of IPC APEX EXPO 2024.

Background

As CPU and GPU packages grow larger and contain higher pin/ball counts, the importance of managing the printed circuit board (PCB) surface coplanarity for package assembly increases. The PCB surface coplanarity under a package is a product of both the global bow/twist of the PCB and the local surface topography under the package. In general, the surface topography is dependent the choice of material and layer stackup and the interaction between the innerlayer copper patterns and prepreg resin flow.

Advances in chiplet design and heterogeneous integration solutions in electronic packaging are enabling complex packages with increasing total die areas, resulting in the need for larger CPU and GPU packages¹. Based on trends and advances in package integration, it is expected that future packages exceeding 100–120 mm on a package edge will become more common. This increases the challenge of the second-level interconnect (SLI) assembly processes when attaching the package to the PCB due to the combined coplanarity and topography variations of the PCB and package. These combined influences between the PCB and package are the key drivers of SLI defects such as solder bridging or solder joint opens during PCB assembly.^2,3 Figure 1 is a graphical depiction of how the global PCB warpage or curvature under the package must be smaller for larger packages to achieve the same PCB coplanarity under the package. 

Figure 1: PCB coplanarity under package.

The characterization of PCB coplanarity under the package footprint has been studied historically, including influences of assembly temperatures on dynamic PCB coplanarity as the PCB and package move together through the assembly reflow temperature profile.^4,5,6 Other works have shown how the choice of PCB materials, fabrication process conditions, and design each impact global PCB bow/twist and warpage⁷. 

To continue reading this article, which originally published in the September 2024 SMT007 Magazine, click here.