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Control of package warpage by package substrate design for low profile package-on-package structure
Keywords: Package warpage, Equivalent CTE of a package substrate, Glass-cloth content ratio in a package substrate
In recent years, with increasing demand for high-density assembly in mobile electronics products, a package-on-package (PoP) structure has been standardized. The demand for lowering the PoP profile is getting stronger as consumers demand thinner mobile devices. To assemble a low profile PoP and mount it on a mother board, it is necessary to have a precise warpage control of PoP components, including its bottom package. It is expected that a flip-chip chip-scale-package (FCCSP) with die-exposed mold will be used as the bottom package of the PoP in the next 2-3 years. In order to control the package warpage, it is necessary to optimize a large number of parameters related to the package substrate, silicon die and mold compound. Among them, equivalent coefficient of thermal expansion (CTE) of the package substrate is one of the most important factors. In this paper, control of the package warpage was studied by focusing on the glass-cloth content ratio in the package substrate. Glass-cloth has the lowest CTE among the constituent materials of the substrate, so it contributes to reduce the equivalent CTE of the substrate. First, equivalent CTE's of package substrates with different glass-cloth contents were experimentally measured. It was found that equivalent CTE has strong correlation with the glass-cloth content ratio. Next, the effect of a large glass-cloth content ratio on package warpage was evaluated. To maximize the glass-cloth content, a substrate with a thick core and thin build-up structure was prepared, and the following package with die-exposed mold was subsequently formed. The substrate was 14mmx14mmx0.33mm and the silicon die was 10mmx10mmx0.1mm. Compared with a similar package with a conventional substrate structure, its warpage, measured experimentally, was about 20% smaller. Consequently, it was concluded that the glass-cloth content ratio is a key factor to control package warpage behavior in the PoP assembly process.
Takeshi Furusawa,
Ogaki, Gifu Pref.

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