Page 10 - ChipScale_Jan-Feb_2020-digital

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]

A utomotive radar technology

at 77GHz for advanced
driver assistance systems
(ADAS) and autonomous driving requires
a package solution, which provides
both superior RF performance 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 first company to introduce
the eWLB package technology to the
automotive market in 2012 [1], only three
years after introducing this technology to
the consumer market [2].
Characteristic to eWLB package Figure 1: Typical fatigue modes evolving during TCoB cycling to release thermomechanical stress, and its radial
technology is the signal routing directly orientation with respect to the package center.
on top of the silicon device and package what drives thermo-mechanical lifetime stress. In addition to the 2D analysis of the
body by using thin dielectric layers for behavior and what fatigue modes do fatigue modes, new analysis techniques
electrical insulation and thin copper we have?); 2) RF performance (what have been developed to investigate the
film layers for electrical redistribution. will impact the RF-performance?); and 3D orientation across the entire package
These characteristics offer low parasitic 3) Thermal performance (how to set (see lower portion of Figure 1). Scanning
inductances, shorter signal pathways, up thermal management and what will acoustic microscopy (SAM) and new
and together with more freedom in change thermal behavior?). delayering methods are applied to reveal
designing the layout of the redistribution the solder ball fatigue and the UBM-RDL
layers (RDL), they provide an excellent Thermomechanical behavior interfaces, respectively. For all fatigue
RF transition. Frontend Si-technologies The eWLB package with its short modes, the orientation of the fatigue
and processes became standard for interconnection means there is no material modes is radial from the edge of the
this backend package technology. layer at the package side that is able to package to the center.
Today, this low-cost wafer-level eWLB buffer any coefficient of thermal expansion With trends like increased die or
packaging solution with its attractive RF (CTE) mismatch. The solder balls are package size and the demand to withstand
performance is now used in our second- exposed to the full CTE mismatch a higher number of cycles on thicker or
generation automotive radar technologies between the printed circuit board (PCB) stiffer board stacks, several measures
and is also widely used by others for many and the eWLB’s main components (silicon of the eWLB technology are needed in
automotive radar systems at 77GHz. and molding compound). order to improve the TCoB robustness.
In the following sections, we describe Cross sections of typical fatigue modes In Figure 2, a few examples of those
how the eWLB technology fulfills the are shown in the upper part of Figure 1. measures are listed and rated.
demanding performance and reliability Under bump metallization (UBM) fatigue, Besides introducing underfill or
requirements of automotive radar sensors. RDL fatigue and solder ball fatigue evolve corner bond, reducing the size or adding
Three important topics will be addressed: during temperature cycling on board redundant balls have the most positive
1) Thermomechanical behavior (i.e., (TCoB) to release thermomechanical effect on TCoB. The min/max temperature

8 8 Chip Scale Review January • February • 2020 [ChipScaleReview.com]