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.