High-frequency modeling of quad flat no-lead packages

In the recent development of semiconductor industries, semiconductor devices and circuits are fabricated by the advanced deep sub-micron and nano technologies, the demand for high-frequency and small-package integrated circuits is rapidly increasing for portable communication systems. Accurate high-frequency modeling of the packages with small sizes is therefore strongly needed. The quad flat no-lead (QFN) package is a chip scale package (CSP) [1]. It is a plastic encapsulated lead-frame package. The lead pads of the package are on the bottom. The QFN package has a much smaller size than conventional packages. The parasitic characteristics of the QFN package are therefore reduced compared to the conventional packages so that the QFN package is suitable for high-frequency portable applications. In this paper, high-frequency modeling of the QFN packages is developed using an analytical method. The modeling methodology is based on the network analyzer measurement of the packages [2] and the ABCD- and S-parameter transformations. The cross-section view of a QFN package is shown in Fig. 1. The package consists of an encapsulated molding compound material, a die pad, gold bond wires, and bottom leads. For the high-frequency modeling of the QFN packages, two package configurations, open path and short path, are studied. The QFN-48 packages with 48 bottom leads are investigated. Fig 2 shows the layouts of QFN-48 packages for the open-path and short-path configurations. In the open-path configuration, the signal leads of the package are opened and the remaining leads are grounded to the die pad through the bond wires. In the short-path configuration, all leads are shorted to the die pad. The high-frequency characteristics of the QFN packages are measured by the network analyzer. The equivalent circuit models of the packages are established according to the physical package structure. The parasitic elements of the QFN packages are extracted using the analytical approach. Fig. 3 shows the modeling results of the package with the open-path configuration. Both the S₁₁ and S₂₁ parameters are displayed. The modeling results for the short-path configuration are shown in Fig. 4. The modeling of the QFN packages exhibits excellent agreement between the simulation and measurement data over a wide frequency range up to 6 GHz. The fast and accurate method using the analytical frequency-domain technique is demonstrated to construct the high-frequency QFN package model.