Recent industry trends to continue enabling increased server system performance and packaging density has driven the need to implement larger form factor hybrid land grid array (LGA) attached organic modules. In addition, given the need to package multiple modules on a single printed circuit board (PCB) assembly, PCB cross-sections and their corresponding physical properties (e.g., flatness, etc.) as well as module bottom surface metallurgy (BSM) co-planarity require a more detailed understanding of impacts to the compliant as well as the soldered connector interfaces. Lastly, the migration to lead (Pb)-free solders has further complicated the issue given both the change in material properties as well as processing temperatures.
In this paper we will discuss the mechanical stress analysis and evaluation tests assessment of a recently developed 50 mm square organic processor module, hybrid LGA attached to a multiple site PCB. The analysis presented will highlight the methodology to identify both connector soldered stress and predicted contact load variation across the module's mated interface. Key parameters discuss will include the PCB flatness, Organic substrate BSM co-planarity (both predicted and measured) and the Hybrid LGA as-soldered contact co-planarity. Corroborating predicted analytical results, we will discuss various evaluation tests performed to validate the design's integrity. Key tests include, pressure sensitive film (PSF) studies and environment stress exposures, including thermal shock, mechanical shock and vibration and seismic exposure. Post test electrical integrity and test sample construction analysis, including 3D x-ray and mechanical cross-section, will also be described.
The analysis process and testing described will provide a method to evaluate more challenging hybrid LGA applications as both module sizes and/or number applied per PCB assembly increase and Pb-free assembly is introduced in future applications.
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