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Method to measure wafer stiffness in Fan-Out Wafer Level Package
Keywords: Fan-Out Wafer Level Package , Wafer stiffness, Method
Wafer level packaging is an important development trend for IC package design. One of emerging variants is the fan-out wafer level package (FO-WLP), namely Embedded Wafer Level Ball Array (eWLB). In the fan-out process, singulated dies are placed into a "reconstituted wafer" with enough space around each chip, to accommodate second-level connections. Space between dies is filled with molding compound, this construction results in a plurality of microelectronic elements triggering a particular mechanical behavior. However normally all equipment and processes running FO-WLP were designed for processing of Silicon wafers, which have high and homogenous rigidity, in opposition to the "reconstituted wafer". This fact posed several manufacturing and process challenges. Two of the main characteristics associated to the handling and process capabilities are wafer warpage (or bow) and wafer stiffness. These two properties are directly related to success of the wafer handling and wafer processing. Currently different warpage methods and measurement systems are available, but wafer stiffness assessment is not yet explored and to our knowledge there is no commercially available instrument to perform this task. In our research an original experimental method is presented for measuring the wafer stiffness. The method is based on an apparatus specifically designed to quickly record the force-displacement curve of wafer stiffness. Results are demonstrated by varying different characteristics of reconstituted wafers: Si/mold compound ratio; wafer thickness; mold compound; humidity absorption and along process steps. Also correlation between wafer stiffness and wafer warpage is investigated. The method has been successfully used in the field and showed to be a valuable process instrument for both product development and process monitoring.
Jorge Manuel Soares Teixeira, Process integrator
Vila do Conde, Porto
Portugal (PT)

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