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High Frequency, Low Loss, Batch Manufactured Package

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by Tim Smith, Bill Rhyne, Robert Reid and Christopher Hatfield, Cubic-Nuvotronics

Semiconductor design and process technologies have advanced considerably at millimeter-wave frequencies and are rapidly increasing toward the terahertz region. However, packaging and module integration technologies have not kept pace and can limit the integration and use of those devices in systems. High-frequency bare die from 40 GHz to 100 GHz are available from manufacturers and distributors. Surface mount packages supporting these devices are unavailable, forcing system architects to choose between size, weight, performance, and cost.

Figure 1: Basic PolyStrata process overview

For systems where RF performance or reliability has been a priority, there was little choice other than the “gold-brick” package, which offers superior environmental protection and reliability and high thermal conductivity for power applications. This approach offers minimal RF losses but is large, heavy, and expensive to build. The “gold-brick” package with connectorized I/Os is not conducive to surface mount assembly lines that are essential for reducing complex electronic system cost points. Conventional plastic packaging with metal lead frames offers low cost packaging solutions but is limited in RF and thermal performance. The plastic material also allows moisture to ingress into the internal cavity, which in turn can cause reliability problems through corrosion or dendritic growth [1].

Ceramic packages have been used for decades to provide high performance, high reliability surface mount packages, with some companies even offering solutions up to 60 GHz. These packages can be hermetically sealed for environmental reliability and use thin or thick film patterning with multilayer lamination or firing with vias to provide an electrical path to the next level interconnects. Approaches using glass as the interposer promise higher frequency operation due to the improved accuracy and technologies for patterning glass and creating through glass vias [2]. In this case, the transition from PCB to glass has an insertion loss of 1.4 dB and the vertical transition through the glass has an insertion loss of 0.4 dB at 40 GHz.

Figure 2: Loss of a PolyStrata coaxial transmission line

Substantial effort is also focused on developing antenna-in-package (AiP) solutions. This has primarily been driven by using millimeter-wave bands in 5G networks [3]. Much of this uses conventional packaging technology and is not hermetic. Automotive radar applications are also pushing this technology forward, and glass-based approaches are also being developed for these applications [4]. An insertion loss of 0.6 dB was reported in the passband of 76-81 GHz [4].

Technology Development

The development discussed in this article is an addition to the PolyStrata® manufacturing process, which is a batch additive metal manufacturing process developed more than a decade ago and has since been discussed in many papers [5,6]. This technology has been demonstrated up to D-band and G-band, showing its high-frequency capabilities [7, 8]. The PolyStrata process is illustrated in Figure 1. The process is compatible with existing interconnect technologies such as solder, conductive epoxy, wire bond, and flip chip. The proven RF performance makes the technology a good fit for replacing plastic lead-frame, LTCC and even organic boards as a package substrate for millimeter-wave RFICs.

Figure 3: Cross-sectional view of micro hermetic feedthrough concept

Cubic-Nuvotronics has developed a patented feedthrough using a small ceramic chip, environmentally sealing the PolyStrata air coax transmission line and enabling a cavity package substrate to be designed to be lidded and sealed using traditional approaches. The package base material is copper, providing 400 W/m-K of isotropic thermal conduction, allowing for low thermal resistance paths through the package base and lid. This permits the package to dissipate heat through the lid by attaching a heat sink directly to the package, with simulated thermal resistances as low as 1° C/W. Packages also support broad bandwidths with insertion loss of less than 0.4 dB and are achievable for frequencies of 90 GHz and above. Figure 2 shows the loss per millimeter of a PolyStrata coaxial transmission line at D-band.

Figure 3 shows how during the transition from the PC board to the PolyStrata rectangular coax lines, the signal line passes through a ceramic hermetic feedthrough. The orange features are the copper of the PolyStrata package and the board metal. The center conductor of the coax passes through a via in the ceramic (shown in white) and down to a landing pad on the board. The metal through the via and around the sides of the ceramic is plated as part of the formation of the copper on that layer. Figure 4 is an SEM image of a cross-section of the tested device that shows how the plated copper forms well around the edges of the feedthrough, which is essential for eliminating any potential leak path.

Figure 4: SEM cross-section of hermetic feedthrough fabrication demonstration unit

Package Design

A standard package was developed that could support a broad range of RFICs that do not already have packaged options. These are usually lower volume wire bondable millimeter-wave devices above 40 GHz. This package design can support high-power GaN or GaAs MMICs and InP or SiGe devices that require the lowest loss RF interconnection and high thermal conductivity provided by the all-copper construction. Figure 5 shows the package substrate’s basic components that are monolithically fabricated on a wafer.

Figure 5: Microscopic image of a PolyStrata package substrate designed to support a 30 to 40 GHz GaN device with better than 0.32 dB insertion loss and 20 dB of return loss

After package fabrication it will run through die attach, wire bond, and lid attach. These steps can all be run in an automated fashion at the wafer level in high volume. Figure 6 shows the completed 8-in. wafer with the prototype packages. This substrate size contains more than 400 units on a wafer, demonstrating the batch fabrication capability of this technology. The PolyStrata manufacturing technology is currently increasing substrate size to 14-in. panels that will quadruple batch quantity.

Figure 7 shows a package capable of supporting two dies in a compartmentalized substrate and Figure 8 shows a package that can support several I/Os and control lines. A single transition was developed that supports signals from DC to 95 GHz with less than 0.5 dB of insertion loss and 20 dB of return loss (Figure 9).

Figure 6: Fabricated wafer of various prototype package designs
Figure 7: Microscopic image of example package substrate supporting two die with solid metal wall creating separate cavities. The package size is 8 x 7.5 mm.
Figure 8: Microscopic image of PolyStrata package substrate supporting differential RF I/Os and many DC control lines

In summary, the PolyStrata manufacturing process delivers a package product that has proven RF performance from PolyStrata’s recta-coax technology. A wide range of passive structures can be integrated into the package, including couplers and high-power combiners.

These parts are batch manufactured, allowing an impressive economy of scale and requiring minimal or no tooling for minor design changes. The finished packages can be shipped in tape-and-reel format and integrated on a standard pick-and-place line.

Figure 9: ANSYS Electronics Desktop simulation of return loss (a) and insertion loss (b) performance of hermetic feedthrough transition to PCB trace

References

1. G. Zhang, J. Li, Y. Chen, R. Wang, and M. Yang, “Failure analysis for IC plastic and substrate/lead-frame package,” 2014 15th International Conference on Electronic Packaging Technology, 2014, pp. 1225-1228.

2. T. Galler, T. Chaloun, K. Kröhnert, M. Schulz-Ruhtenberg and C. Waldschmidt, “Hermetically Sealed Glass Package for Highly Integrated MMICs,” 2019 49th European Microwave Conference (EuMC), 2019, pp. 292-295.

3 Antenna in Package (AiP) Technology for 5G Growth, Curtis Zwenger and Vic Chaudhry, Chipscale Review, March-April 2020.

4. C. Zhu, Y. Wang, Z. Duan, and Y. Dai, “Design of Patch Antenna in Embedded Glass Fan Out Package for 77-GHz Automotive Radar,” 2020 IEEE Asia-Pacific Microwave Conference (APMC), 2020, pp. 1066-1068.

J. R. Reid, J. M. Oliver, K. Vanhille and D. Sherrer, “Three-dimensional metal micromachining: A disruptive technology for millimeter-wave filters,” 2012 IEEE 12th Topical Meeting on Silicon Monolithic Integrated Circuits in RF Systems, 2012, pp. 17-20.

6. Z. Popovic et al., “Micro-fabricated micro-coaxial millimeter-wave components,” 2008 33rd International Conference on Infrared, Millimeter and Terahertz Waves, 2008, pp. 1-3.

8. J. W. Jordan et al., “Monolithically Fabricated 4096-Element, PolyStrata Broadband D-band Array Demonstrator,” 2019 IEEE MTT-S International Microwave Symposium (IMS), 2019, pp. 1060-1063.

9. J. -M. Rollin, D. Miller, M. Urteaga, Z. M. Griffith and H. Kazemi, “A PolyStrata 820 mW G-Band Solid State Power Amplifier,” 2015 IEEE Compound Semiconductor Integrated Circuit Symposium (CSICS), 2015, pp. 1-4.

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