Chip Scale Review - January February 2020

8 Chip Scale Review January • February • 2020 [ChipScaleReview.com] stress. In addition to the 2D analysis of the fatigue modes, new analysis techniques have been developed to investigate the 3D orientation across the entire package (see lower portion of Figure 1 ). Scanning acoustic microscopy (SAM) and new delayering methods are applied to reveal the solder ball fatigue and the UBM-RDL interfaces, respectively. For all fatigue modes, the orientation of the fatigue modes is radial from the edge of the package to the center. With t rends like increased die or package size and the demand to withstand a higher number of cycles on thicker or stiffer board stacks, several measures of the eWLB technology are needed in order to improve the TCoB robustness. In Figure 2 , a few examples of those measures are listed and rated. Besides i nt roduci ng under f ill or corner bond, reducing the size or adding redundant balls have the most positive effect on TCoB. The min/max temperature utomotive radar technology at 77GHz for advanced driver assistance systems (ADAS) and autonomous driving requires a package solution, which provides both super ior RF per formance and fulfills the strict automotive reliability requirements. In the past, the automotive industry used predominantly very mature semiconductors and packages. Today, however, a car will need to use the latest packaging technology to provide the best solution for ADAS sensors. One example of this trend is the embedded wafer-level ball grid array (eWLB) package. Infineon was the f irst company to int roduce the eWLB package technology to the automotive market in 2012 [1], only three years after introducing this technology to the consumer market [2]. Cha r acter ist ic to eWLB package technology is the signal routing directly on top of the silicon device and package body by using thin dielectric layers for electrical insulation and thin copper film layers for electrical redistribution. These characteristics offer low parasitic inductances, shorter signal pathways, and together with more f reedom in designing the layout of the redistribution layers (RDL), they provide an excellent RF transition. Frontend Si-technologies and processes became st andard for t h i s ba ckend pa ck age t e ch nolog y. Today, this low-cost wafer-level eWLB packaging solution with its attractive RF performance is now used in our second- generation automotive radar technologies and is also widely used by others for many automotive radar systems at 77GHz. In the following sections, we describe how the eWLB technology fulfills the demanding performance and reliability requirements of automotive radar sensors. Three important topics will be addressed: 1) Thermomechanical behavior (i.e., what drives thermo-mechanical lifetime behavior and what fatigue modes do we have?); 2) RF performance (what will impact the RF-performance?); and 3) Thermal performance (how to set up thermal management and what will change thermal behavior?). Thermomechanical behavior The eWLB package with its shor t interconnection means there is no material layer at the package side that is able to buffer any coefficient of thermal expansion (CTE) mismatch. The solder balls are exposed to t he f ull CTE mismatch between the printed circuit board (PCB) and the eWLB’s main components (silicon and molding compound). Cross sections of typical fatigue modes are shown in the upper part of Figure 1 . Under bump metallization (UBM) fatigue, RDL fatigue and solder ball fatigue evolve during temperature cycling on board (TCoB) to release thermomechanical A Reliability and performance of wafer-level fan-out packaging for automotive radar By Walter Hartner, Martin Niessner, Francesca Arcioni, Markus Fink, Christian Geissler, Gerhard Haubner, Maciej Wojnowski [Infineon Technologies AG] Figure 1: Typical fatigue modes evolving during TCoB cycling to release thermomechanical stress, and its radial orientation with respect to the package center.

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