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Advancements in High Frequency Plastic Air Cavity Packaging


by Helen Leung and Erik Sauve, Custom MMIC

RF and microwave design engineers are continuously pushing the development of Monolithic Microwave Integrated Circuits (MMICs) to higher frequencies. New advanced packaging solutions have been introduced to help drive this effort, especially to 40 GHz and beyond. Not all of these packaging solutions are created equal, however, as some will hinder the performance of high frequency MMICs. 

For lower power devices operating at frequencies below 20 GHz, the plastic molded, Quad Flat Non-leaded (QFN) package is a cost effective and well proven approach. Simple and flexible manufacturing processes, along with low material cost, make the QFN an ideal solution for quick turn, high volume products. However, the typical mold compound material used in these packages has a relatively high dielectric constant not suitable for applications above 20 GHz. The dielectric constant also varies widely over temperature, causing performance stability concerns. 

Figure 1: Cross-section of a ceramic air cavity package

Ceramic air cavity QFN packages manufactured with a High Temperature Co-fired Ceramic (HTCC) are also popular for many RF and microwave applications. An example of one such package is shown below in Figure 1. Here, we note the package is comprised of metalized ceramic layers connected by copper filled vias. This construction creates a robust structure that remains stable during temperature extremes. The air cavity provides an excellent RF environment with minimal effect on device performance. 

For power applications that require heat dissipation, an integrated copper or copper alloy heat spreader can be embedded in the die attach paddle, as shown in Figure 2. Despite the improved electrical and thermal conductivity of such construction, this package has a few notable disadvantages. First, due to variations in process tolerances and spacing limitations, this type of package has a much larger footprint than the style shown in Figure 1. Second, the extra layer of ceramic adds inductance along the RF path, making it a less optimal solution for high frequency applications. Finally, the material and manufacturing costs for ceramic air cavity packages are relatively expensive and not optimal for high volume production. 

Figure 2: Cross section of air cavity package with additional copper alloy heat spreader for use in high power applications

 Organic-based packaging is a less expensive alternative to ceramic air cavity packages. The relatively low dielectric constant of the packaging material enables extended frequency operation, and the improved routing flexibility between the metallized layers provides an ideal solution for complex, high density designs. However, maintaining substrate flatness and uniformity throughout the manufacturing process is a constant challenge due to differences in the coefficient of thermal expansion (CTE) between the layers and other structural elements. To address this issue, many substrate manufacturers are developing materials with heat spreaders using copper or copper alloy inserts that help to distribute and dissipate the residual heat more evenly across the substrate, thereby minimizing or eliminating possible distortion. This technology is still relatively new and needs further analysis to determine its viability in mainstream manufacturing. Additionally, environmental conditions such as humidity can have an adverse effect on packaging, including a phenomenon referred to as “popcorning” where absorbed moisture expands during soldering, resulting in a cracked plastic casing leading to device failure.

Here at Custom MMIC, we have utilized the three packaging methods with success in the past but have recently run into limitations as we push the frequency response higher. To address this problem, Custom MMIC has recent­ly deployed a new solution, the plastic air cavity package (or K-package) that can meet the needs of next generation high frequency MMICs. The K-package is a surface mount lead frame-based package with a standard small profile QFN footprint. A cross section is shown in Figure 3. Here, we note a pre-molded manufacturing process creates a defined resin sidewall over the lead frame. The copper lead frame provides a direct and uniform path for RF transitions with minimal performance degradation up to 40 GHz. The air cavity provides the necessary high frequency RF environment for the MMIC, while the flat plastic lid, sealed with B-stage epoxy, protects the MMIC from environmental contaminants.

Figure 3: Cross section of a plastic air cavity package

To illustrate the performance difference between the standard air cavity ceramic package and the K-package, we considered the CMD242, a DC-40 GHz distributed amplifier. The die was assembled into a 4 x 4 mm package of each type and then reflowed onto a probable evaluation circuit board made from a 10 mil Rogers 4350 substrate. The S-parameters of each amplifier were measured using a vector network analyzer with calibration planes brought directly to the ports of the package via wafer probes. In Figure 4, we present a comparison of the input return loss and gain for the CMD242 in these two packages. We note the input return loss is approximately 5 dB lower for the K-package across the entire frequency range, while the gain is nearly identical between both packages, with the K-package having a slight advantage. We note the advantage is likely due to two parameters. First, analysis of the ceramic package has shown that the copper filled vias create a slight degradation in performance at and above 25 GHz, thus limiting the performance as the frequency increases. Second, the copper die paddle on the plastic air cavity package provides a better electrical ground and heat sink than the copper filled vias of the ceramic package.

Figure 4: Comparison of input return loss and gain for the CMD242 distributed amplifier in a ceramic package and an air cavity K-package

Custom MMIC’s deployment of the advanced plastic air cavity K-package offers designers a cost-efficient solution for MMIC devices that operate at frequencies up to 40 GHz and beyond. With the standard small QFN footprint and surface mount interface, this new package makes assembly and implementation of these high performance devices easier and less prone to assembly mishaps, making it a much more reliable and robust solution. As an additional bonus, the simple and flexible manufacturing process allows for quick implementation of future custom designs with minimal tooling required, which translates to lower costs and faster time-to-market.