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EETE MAY 2014

INTERCONECTS & CIRCUIT PACKAGING Silicon interposers for efficient 3D integration By Stéphane Bellenger The comon point with articles relating progress in the electronic components world is the search for form factor reduction and high performance. Since the 90s, designers have been working on 3D integration (Multi-Chip Package, stacked die, System in Package) which brings highly efficient solutions to achieve these goals. Several products have been developed, in particular the interposers. The interposer can be assimilated to a packaging platform serving as a high density substrate with a redistribution layer and offering, unlike traditional packages, the reduced pitch capabilities required by advanced IC technology nodes. Fig. 1: (left) Schematic of IPDiA 2D interposer with PICS IPD and external active dies in flip-chip or chip-on-silicon technologies. (right) IPD RF module for W-CDMA & GSM RF transceiver. Table 1: Comparison with several substrates of dimensional features. Table 2: Comparison with several substrates of thermal and thermo-mechanical features. In other words, the interposer plays the role of a space transformer from the IC to the applicative module. It also allows usually incompatible technologies to be mixed on the same platform, therefore leading to heterogeneous integration (System in Package on interposer). Combined with Through Silicon Vias, it opens the doors to an optimized form factor world (system volume, weight and footprint) with improved performance (higher transmission speed, lower power consumption and RF parasitic reduction). From an applicative point of view, interposers were first imagined to be used as a pure packaging platform dedicated to dies with large I/O number (high density BGA). They have evolved towards 3D structures to meet the demands of CCD imager, mobile phone and consumer applications. Now, an additional range of applications can be reached with the so-called 2.5D interposers. This new approach offers an economic model perfectly adapted to related portable products, implantable medical devices, avionics and defense. IPDiA, leading manufacturer of passive components, has developed a range of silicon interposers which, when combined with Integrated Passive Devices (IPD) and Through Silicon Vias (TSV), offer an evolutionary solution to these market segments. Several types of material can be considered as interposer substrate, each offering intrinsic properties that need to be seriously considered prior to any other considerations. Silicon is one, and is chosen for the following reasons: first, silicon is a stable base substrate that presents a very small CTE (coefficient of thermal expansion) mismatch with attached external ICs. Since the active parts are in fact often made of silicon themselves, the thermomechanical stresses encountered during processing and lifetime application are minimized, thereby increasing the reliability. Silicon therefore offers a very good trade-off between thermal conductivity and thickness. It is also perfectly adapted to via or micro-via technology (including via last technology) and provides wider possibilities in terms of pitch, via diameter and via density. Lastly, it enables passive devices to be integrated (IPD technology) and is compatible with ICs and MEMS. Main dimensional as well as thermal and thermo-mechanical characteristics for different types of substrate are summarized in the tables 1 and 2. The different structures The interposers can be divided into three families. The three structures show the common key advantages of enabling external integration of active dies without their packaging, as well as integration (externally or internally) of passive devices - see figure 1. 2D silicon interposer: This two vertical stage structure is an intermediate solution in terms of footprint. Due to the connections from the sides of the interposer to the final module, space saving is not totally Stéphane Bellenger is Assembly & Interposer Market Segment Director and Customer Support & Packaging expert at IPDiA – www.ipdia.com 32 Electronic Engineering Times Europe May 2014 www.electronics-eetimes.com


EETE MAY 2014
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