Reza Ghaffarian, Ph.D.
Jet Propulsion Laboratory, California Institute of Technology
This paper presents assembly challenges and reliability evaluation by thermal cycling for a 2.5D [aka, System in Package (SiP)] in a fine pitch ball grid array (FPBGA). More importantly, it presents the effect of underfilling of the top CSPs assembled onto interposer on the overall SiP reliability behavior. A large number of variables were considered in the design of experiment including evaluation of bare FPBGA without parts on interposer, FPBGA balls either with SAC305 or SnPb solders, FPBGA fully/partially populated with CSPs and flip chip (FC) die on FPBGA interposers, and CSPs with and without underfills on the SiP interposer.
Assembly of mixed SiP packages becomes a challenging task and required to be characterized by X-ray for solder-joint quality and by Shadow Moiré analysis for warpage characterization. Acceptable assemblies with and without underfilling CSPs on interposers were then subjected to thermal cycling between –40C and 125C for reliability evaluation and failure-mechanisms assessment. Details of design, characterization by X-ray and Moiré, as well as reliability behavior of the SiP FPBGA due to thermal cycling exposures are presented. Failure analysis results also presented covering evaluation by optical, scanning electron microscopy (SEM), X-sectioning, and dye-and-pry evaluation. The Weibull plots of cycles to failures for SiP FPBGA with SAC305 and SnPb balls and CSP assemblies with and without underfills were presented. Finally, generic discussions were presented on the positive and negative effects of underfills at assembly level and specific reasons for early failures of SiP FPBGAs with CSPs assembled onto interposer with underfill.