QFN Package


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QFN Package


Contents


Introduction

Abstract:In this example, a 3mm x 3mm 16-pin Quad Flat No-Lead (QFN) package is optimized to improve the frequency performance using the FEM Simulator.An IC design is not finished until it is packaged. As operating frequencies increase, it becomes more important for IC designers to understand the package’s electrical performance in the early stages of the design process. What is the upper frequency limit that the package can operate? Is it possible to use a lower cost package type? What is the isolation characteristic of the package? As an example, LO to RF leakage performance of an up/down-converter IC depends not only on the IC’s isolation characteristics but also on the package.

Design Challenges

Quad Flat No-Lead packages are low cost and very popular for RFIC, MMIC, and RF SiP applications. The QFN package is a lead-less, near Chip Scale Package (CSP) with a low profile (1.0 mm or less), moderate thermal dissipation, and good electrical performance. Typical QFN packages range from 3.0 x 3.0 mm to 9.0 x 9.0 mm in size.


Figure 1: A 5x5 mm 28-pin QFN package

A 3mm x 3mm 16-pin QFN package performs well up to 15 GHz. Using the full 3D Finite Element Method (FEM) simulator in ADS, the return loss up to 15 GHz is less than -18dB return loss with a typical package mounting scheme. However, new designs require higher operating frequencies, (e.g., up to 20GHz). Using the same package at these frequencies will require modifications to the chip-to-board transition.


Figure 2: 3x3 QFN return loss and insertion loss simulation using FEM

There are a few improvements that will increase the frequency of operation for this package:

  1. Increase the width of input/output transmission lines to make a 50 ohm impedance
  2. Use two lead frames instead of one to minimize the transitional impedance profile and split the double bonding to the two lead frames

These design improvements can be made directly in the ADS layout environment.


Figure 3: Improvements made to the QFN package design

Results

Following the design improvements the QFN package is resimulated using the FEM simulator in ADS. Now the return loss and insertion loss show very good results beyond 20 GHz.


Figure 4: FEM simulation results before and after the design improvements

Conclusion

Design modifications to a 3x3 QFN package improve the performance up to 20 GHz. These modifications can be made directly in the ADS layout environment and then analyzed using the integrated full 3D FEM simulator in ADS, without having to use a standalone EM tool. Note that the ADS layout could also be exported to EMPro and the same FEM simulations could be run, in addition to FDTD simulations.

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