Over the last few decades, major advancements in the semiconductor supply chainhave occurred. These advancements have provided standard foundry processes,physical theories that explain device models, accurate and efficient software,and test equipment. Today, front-end components up to 95 GHz (E-band) and beyondare either commercially available or can be specifically commissioned. In allthese advancements, packaging has been the one area where investments andtechnology have lagged.

In many cases, packaging presents the major bottleneck to overall performance. Astrivial as the connection of components may sound, the unfortunate reality isthat the signal integrity of interconnects quickly limits performance at highfrequencies. Engineers in the digital world are now coming up against some ofthese limitations, and only recently has the field made the investmentsnecessary for these signal integrity issues to be overcome. Even microwaveengineers, whose whole world is high frequency, struggle to find acceptablepackaging solutions. Many will be faced with designs riddled with cripplingmismatch loss, coupling loss, and unacceptable resonances.

For many engineering projects, cost is also an important design criterion anddifferentiating feature. Without careful, directed analysis, engineers may findtheir projects behind schedule and over budget. Further, lower operating costscan be achieved with package designs that encourage simple fabrication,assembly, handling, and test. Packaging research helps to advance new designtechniques and package processes. For these reasons, packaging is rapidlygaining attention as a necessary growth field.

This chapter presents the design and development of thin-film LCP surface mount (SMT) package feed-throughs for DC to Ka-band applications. Three types of feed-through design will be introduced, via feed, bandpass feed, and lumped element feed, that interface signals from the outside world to a component inside a package. The packages are constructed using multilayer LCP films and are surface mounted on a printed circuit board (PCB) for use. The utilization of an all-LCP enclosure provides a hermetic environment for microwave and millimetre-wave monolithic integrated circuits (MMICs). In addition, mounting MMICs inside a package cavity allows enhanced thermal dissipation because the metal submounts can make direct contact with the PCB or motherboard ground through solder or epoxy. Applications that require lightweight, hermetic, and low-loss modules, which can be developed using LCP, include, but are not limited to, vehicular-collision-warning short-range radar, radar for ground-moving vehicles, point-to-point communication, ground–satellite communication, intersatellite links, and airborne radar. A phased-array system for ground-vehicle or airborne applications may require thousands of modules in the RF link. If each module, typically cased in ceramic and metal, were replaced with LCP packages, then the total weight of a system could be reduced by more than 66%, because ceramic [1] is three times denser than LCP [2]; this can lead to improvements in fuel efficiency.

Section 5.1 shows the design, the modeling, and measurement process of a Ka-band package feed-through using vias. The experimental results demonstrate that a package via feed-through including a PCB signal launch structure and bond wires achieves a return loss of better than 20 dB and an insertion loss of less than 0.4 dB at the Ka-band. The package has a measured port-to-port isolation greater than 45 dB up to 40 GHz. An amplifier packaged inside an LCP SMT package is characterized. The measured data are then compared with a circuit-model simulation of the package feed-through, using the on-wafer data of an amplifier to validate the lumped-circuit model.

Package design and fabrication techniques are critical to the high-frequencycommunity. In building improved products, packaging developments are driven byeconomics, performance, and reliability. For example, the telecom industry as awhole is currently pushing to improve electrical performance and lower cost byreplacing the current transceiver designs with new surface mount solutions. Thisbook is intended for electrical engineers involved in designing microwavecircuits. As operating frequencies rise with the emergence of high-speedproducts, engineers will increasingly need a good understanding ofRF/microwave packaging.

This book presents engineering breakthroughs in liquid crystal polymer (LCP)applications to microwave-frequency electronics. It appears that LCP is a highlyattractive platform to achieve low-cost hermetic devices that offer mechanicalflexibility. These benefits are attractive for applications in gigabit wirelesscommunication, radar and imaging systems. Liquid crystal polymer research iscurrently a very hot topic in microwave engineering, with contributions fromseveral research groups and organizations on a global level.

As we will discuss, using LCP can be challenging at times. The inert chemistry ofLCP, which provides its attractive electrical and mechanical properties, canalso act to hinder actual circuit build. The book gives brief descriptions ofthe theory and provides deep insights into the practical issues of design andrealization with LCP. Numerous real-world examples with expanded explanations ofpreviously published works are included to create a comprehensive and cohesivevolume. We hope to share tips and tricks that we have found for successfullyprocessing LCP for microwave packages and circuit modules. We describesuccessful techniques in using LCP and how to avoid pitfalls.

As defined in IPC/JEDEC J-STD-012, chip-scale packaging (CSP) refers to a packaging method where the final package area dimensions are no larger than 1.2 times the die. Wafer-level packaging refers to a method where a wafer containing multiple chips is processed for packaging before the individual dies have been sawn-cut for separation [1]. While chip-scale packages have been widely available in high-volume production, hermetic packaging at the wafer level is still either at the research stage or in low-volume manufacturing.

The most popular wafer-level packaging technique is wafer-to-wafer bonding for the packaging of microelectromechanical system (MEMS) devices. Several techniques used by industry to package MEMS devices at the wafer level include epoxy seals, glass frit, glass-to-glass anodic bonding, and gold-to-gold bonding [4–9]. These techniques face two major problems. Firstly, organic materials outgas within the MEMS cavity during bonding processes, owing to the wetting compounds in the glass, gold, or epoxy layers. This contamination detrimentally affects the MEMS reliability. Secondly, most bonding processes utilize high temperatures (300 to 400°C), and this can degrade MEMS structures [10]. Furthermore, the available hermetic packages and ceramic or glass feed-throughs have significant losses at microwave frequencies, can be expensive, and add considerable weight to a system. Recent advances in low-temperature hermetic wafer-level packaging have shown promise for packaging multiple integrated circuits at wafer level [34]. One such process involves solder bonding, which combines the benefits of low-temperature conditions and thermodynamically stable eutectic bonding. Another viable alternative for packaging is an organic module, in which compact multilayer substrates house active and passive components; however, this presents even more challenges than those mentioned above. Although multilayer chip-on-flex modules using polyimide films are a proven technology for high-density packaging of microwave circuits [11, 12], polyimide is found to be incompatible with RF MEMS switch packaging owing to its high moisture absorption and high outgassing characteristics and the need to use high outgassing epoxies for lamination. In order to improve performance and provide ease of system integration, a hermetic MEMS package must be made with small, light-weight, planar interfaces constructed with MEMS-compatible materials. Thin-film plastic materials using liquid crystal polymer (LCP) are an attractive possibility for this application, owing to the low-temperature processing requirements for forming an interposer package.

In this chapter we describe and discuss the processes used to fabricate liquid crystal polymer (LCP) for microwave packaging. The normal fabrication techniques have had to be in some cases modified or even reinvented for LCP owing to the unique challenges in working with this inert material. While LCP’s chemical inertness makes it attractive for maintaining electrical performance over wide-ranging environmental conditions, it also means that LCP can be difficult to use in wet chemistries. These issues are complicated further by the fact that LCP is a plastic that can be easily flexed in thin-film form. An inherently anisotropic chemical structure adds an additional complication.

This chapter will progress through the different techniques available to form multilayer circuits and packages using LCP. In section 3.1 we discuss the available LCP formats, which include pellets and laminates. In section 3.2 we explain how copper cladding is added to LCP laminates and made ready for printed circuit board (PCB) processing. Section 3.3 covers the whole process, from laminate material to complete printed circuit board, using standard PCB technology. In section 3.4 we describe advanced process techniques that allow for complex specialized constructions. Lastly, section 3.5 gives a chapter summary.

Liquid crystal polymer (LCP) has interesting characteristics that have garneredthe attention of RF and microwave circuit designers. One major difference fromtypical high-frequency materials is that LCP is a thermoplastic. WithLCP’s advent, engineers have discovered how to integrate high-performanceelectronics directly into the thermoplastic package. The reason why LCP isattractive for RF/microwave electrical design is its unique combinationof excellent electrical and mechanical properties. LCP is extremely stable inthe presence of moisture and exhibits near-hermetic properties. Given itsrelatively low cost, LCP is rapidly becoming the material of choice for newgenerations of electronics requiring increasing integration and performance.This makes LCP a serious candidate for multi-chip module (MCM),system-in-package (SiP), and advanced packaging technology.

In section 2.1, we will first discuss LCP’s chemical properties. The basicchemistry will be introduced, and LCP’s composition will be described anddiscussed. This section is presented from an application stance and is intendedto provide a brief working background to why LCP behaves as it does.

In section 2.2 we describe LCP’s electrical properties. LCP has beencharacterized as having a very low dielectric constant and loss factor over thefrequency range from below 1 GHz up to past 110 GHz [2]. Its low dielectricconstant allows reasonable line impedances to be formed on thin-film materialand, further, minimizes the impact on the nearby electronics as well as thecapacitive detuning effects of packaging. Methods for electricalcharacterization and study results are presented. The test results of LCP inmoisture are also provided, to demonstrate its stable electrical characteristicsover humidity changes.

This chapter presents a number of subsystem-level modules that benefit from anLCP implementation. The first module, in section 7.1, is a long time delay (LTD)circuit with amplitude compensation. This module demonstrates the advantages ofhomogenous dielectric core and ply layers in a multilayer build. Known analyticsolutions for transmission lines may be readily applied for first-pass success.In addition, the homogenous multilayer build achieves amplitude compensationthrough novel LCP transmission line implementations. Lastly, this moduledemonstrates LCP’s surface mount (SMT) component compatibility withcommercially available MEMS switches.

The second module, in section 7.2, is a push–pull amplifier. This moduledemonstrates how LCP’s multilayer construction easily allows minimallyshort bondwires for high-performance chip interconnect. Further, this moduleintegrates high-performance LCP baluns to achieve excellent even-mode distortioncancellation. This module also demonstrates how LCP lends itself naturally tothe higher-level integration of LCP-enhanced passives.

Lastly, a receiver module with a built-in phased-array antenna is described insection 7.3 . In this receiver module , LCP is demonstrated to provide aconvenient platform for mechanically flexible electronics. Passive antennastructures are designed directly into the LCP build. Active semiconductor chipsare packaged into this platform to show how LCP is ideally suited for buildingup large systems.

Each module represents advanced research based on an LCP platform that extendsthe electrical performance and mechanical functionality of today’s subsystemmodules.

In this chapter we evaluate the long-term functionality of LCP packages and the protection they offer against external environments. This emerging flex material has ultra-low moisture absorption and permeation close to that of glass. It is an attractive material for making hermetic packages that can provide reliability in a low-cost and lightweight platform. Prototype LCP packages for RF to millimeter-wave frequencies have been reported recently [1–13]; the emphasis in these publications has been primarily on electrical characteristics, with less focus on the reliability aspects. Among the limited list of publications [7–13], some authors claim that LCP can be used for hermetic packages with long-term reliability. Other groups, including the authors of [13], have performed environmental tests such as measuring the water absorption of LCP-cavity packages by submerging them in water. In this chapter, a variety of reliability tests and results on an LCP package will be reported using standard tests recognized as being required for military and commercial products.

A primary hurdle for LCP packaging, or any hermetic packaging in general, is achieving a high-quality lid-seal process. This hurdle can be exacerbated by LCP’s inert chemical properties, which require a careful approach to processing. In a commercially available LCP-molded lead-frame package, a lid may be attached to a base using epoxy. In an epoxy-sealed package, a typical lid attachment process includes a cure cycle, i.e. 5 psi at 165°C for one hour [6]. Moisture can readily and detrimentally pass through this epoxy layer. In ultrasonic-welded packages, the width of the lid interface to the base must be narrow enough for it to accumulate sufficient ultrasonic energy and melt the LCP. In addition, molding small features in LCP is challenging, as the features may be too thin to form a reliable seal. For these various reasons, and given LCP’s short history, it is not surprising that the leak rate and reliability of LCP packages have not been reported extensively in the literature.