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Fully Organic Packaging for Applications with Liquid Exposure
Keywords: implantable, liquid crystal polymer, miniaturization
1. Introduction Liquid Crystal Polymer (LCP), a thermoplastic dielectric material with very low water absorption (< 0.04%), high chemical stability and low thermal expansion is best suited both as a substrate material and as the encapsulate for small miniaturized electronic packages and modules. LCP stands out among other polymer materials used for microelectronics. The permeability for water and gases is the lowest among all polymeric materials. With proper design considerations LCP packages can achieve a sufficient low permittivity for exposures in liquid media. 2. Methods Processing techniques for LCP substrates are the same as for other packaging substrate materials. Resolution of lines, spacing and vias are comparable. Multi-layer structure with thin dielectric layers can be made using LCP films. Figure 1. Ultra-thin 6-layer multi- layer stackup with 25 μm thick LCP layers. Minimum via diameter 30 μm. Total thickness: 182 μm. Part of the metal layers can be used to form a coil for communication and power transfer. Simulation and measurement results for small packages with dimensions of less than 10x10 mm2 are discussed. LCP substrates can be connected and sealed without the need for any adhesives by benefiting from its thermoplastic properties. For this purpose localized heat is used to melt a part of the structure locally and produce strong bonding between the involved surfaces. Using this method separated LCP substrates can be sealed and connected in a single process step without the need of any connectors. In another variation this process can be used to seal a completed assembly with an LCP Lid in a single process step. Figure 2: Schematic 3D view into a LCP substrate with an integrated coil and components inside the package. LCP is a homogenous material and can be easily machined with UV lasers with a precision down to the micrometer scale. Cavities, openings for recessed components can be integrated. It is conceivable to even embed a glass window into the lid covering the assembled substrate for optical components. 3. Results Long term soak tests in phosphate buffered saline solution (PBS) at elevated temperatures prove the usability of LCP to encapsulate electronic assemblies in liquid environment. In LCP encapsulated structures passed PBS soak testing at 77C exposed for > 12 months without failure. Silicon test dies embedded in LCP passed tests exposed in concentrated sulfuric acid and saline solution at 50C for > 12 months without failures. 4. Discussion & Conclusion Encapsulation in LCP can provide hermetic sealed and chemically inert miniaturized electronic modules for sensors applications in medical, industrial and automotive markets.
Susan Bagen, Application Development Manager
Micro Systems Technologies, Inc.
Plano, TX

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