Numerical simulation on the warpage of reconstructed wafer during encapsulation process
Authors: Zhen Hu , Wei Zhao , Xiao Gu , Dong Chen , Haijie Chen , Hong Xu , Tan Kim Hwee 
Published: February 2022

Abstract
At present, the general fan-out package generally uses the molding process and uses epoxy molding compound (EMC) material to complete the reconstruction of wafer, but it is not friendly to small-size chips and high fan-out ratio packaging types. In order to solve the problem of small size and high fan-out ratio and other fan-out packaging, JCET ADVANCED PACKAGING CO., LTD. introduced a new kind of encapsulation material, and the wafer reconstruction process in the fan out package is accomplished through three basic processes, such as the lamination process, the leveling process and the silicon support process. However, it also faces warpage problems after curing. Excessive warpage poses a significant challenge to subsequent RDL fabrication, mid-testing and process automation.
In this paper, the actual process is reasonably assumed and finite element methods are used to study the effects of chip thickness, fan-out area, the encapsulation material properties and support silicon wafer on the warpage after curing. The lamination process is that an airbag slowly squeezes the encapsulation material into the fan-out area, and the pressure of the airbag can be adjusted by the gas in and out. The wafer surface is uneven after the lamination process, so it needs to be leveled by the leveling process. Leveling process is to make the uneven surface smooth by a heavy and proper amount of steel plate. The encapsulation material is soft, so it is necessary to support the silicon wafer in order to enhance the strength of the structure. In this paper, the curing process is divided into two processes: pre curing process and post curing process. The main function of pre curing is to shape the encapsulation material, and then post curing can make the encapsulation material, chip and supporting silicon have a better combination. The lamination, leveling, and silicon support processes do not involve higher temperatures, so it can be assumed that the reconstructed wafer has no residual stress and warpage. In order to save computer resources and shorten the verification period, the geometric model of simulation is simplified under the condition of ensuring accuracy. Through the finite element method to explore the influence of structural characteristics and material characteristics on the wafer warpage, the selection of parameters is determined according to the actual situation, so as to meet the manufacturability of the structure.
The results indicate that with the increase of the thickness of the encapsulated material the encapsulation, wafer warpage is gradually increasing, while the encapsulation material with low modulus and small CTE can improve warpage performance. High modulus, thicker support silicon has a significant effect on reducing warpage, and larger CTE support silicon can also effectively reduce wafer warpage because the larger CTE support silicon reduces the degree of mismatch with the encapsulation CTE. Thinner chips and high fan-out areas tend to increase the warpage of the reconstituted wafer. The study of warpage of package structure and material properties can provide theoretical basis for predicting and improving warpage during the design and evaluation phase of the product.