Board Level Reliability of Large Body Size WlCSP for Automotive Applications
This paper will outline the board-level assembly development and reliability that was undertaken to demonstrate this WLCSP could meet the demands of automotive.
Analysis Lab
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Authored By:
Andrew Mawer, Nishant Lakhera, Mollie Flick and Trung Duong
NXP Semiconductors
TX, USA
Summary
Wafer-level CSP (WLCSP), which is commonly defined as a package that is completely fabricated over the IC layers at the wafer-level, has been around since the mid-1990’s [1]. WLCSP is commonly referred to as “fan-in” since all the package redistribution layers and BGAs are contained within the outline of the IC. Initial applications were mainly flash and DRAM devices with relatively low I/O density. The obvious advantage of WLCSPs is the decrease in form factor and weight versus more conventionally packaged parts. However, another advantage of WLCSP is better electrical performance due to shorter interconnects that also provide lower package, or in this case redistribution layer (RDL), parasitics and may therefore enable higher system speeds and frequencies [2].
For this reason, as well as packaging and test costs that can be lower than some other packaging technology alternatives, WLCSP was chosen for a next generation automotive mmWave transceiver radar application operating at 76-81 GHz [3]. Automotive radar transceivers, which integrate transmit and receive functions on the same silicon, are the primary sensor for automotive driver assistance systems (ADAS). These monolithic microwave integrated circuits (MMICs) are utilized in automotive applications for sensing and identifying the environment and objects around the vehicle. Data on these objects, whether in motion or stationary, such as distance and speed are collected for use in a variety of responses including collision avoidance in these increasingly intelligent vehicles.
Choosing a WLCSP for this demanding automotive application with stringent qualification requirements and a relatively large die presented its challenges [4]. Chief among these challenges is attaining the required reliability at the board-level. WLCSP typically has relatively thin passivation and redistribution layers over the silicon IC which may only minimally mitigate the CTE mismatch between silicon (CTE 2.6 ppm/°C) and the PCB (17 ppm/ºC depending on laminate materials). This paper will outline the board-level assembly development and reliability that was undertaken to demonstrate this WLCSP could meet the demands of automotive. With such a large die size, which is believed to be one of the largest WLCSPs ever qualified for automotive, underfill and edge bond were evaluated to enhance the reliability at the board level.
Conclusions
A large die 76-81 GHz transceiver WLCSP was successfully designed and tested for automotive ADAS applications. Board level reliability testing showed that the WLCSP could be qualified for automotive. Specifically, it was shown that board level underfill and perimeter edge bonding could be used when needed to achieve greatly improved board-level reliability results to meet the most challenging applications.
The specific underfill evaluated was shown to increase board level reliability characteristic lifetime by as much as 10× while the edge bond was shown to increase lifetime as much as 3×. When SMD pads were used on the PCB it reduced the reliability by as much as 2× across all cells. This is due to the higher stress associated with SMD pads that drove bulk solder joint fracturing at that interface. Even though drop testing is not necessarily typical for automotive products, the WLCSP with no underfill or edge bond present passed 1500 g drop testing.
Initially Published in the SMTA Proceedings
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