Board Level Reliability of Automotive Grade WLCSP for Radar Applications

Wafer-Level Chip Scale Packages (WLCSPs) are becoming commonplace in the industry due to their small form factor. Applications include industrial and automotive which demand high reliability performance. Additionally, WLCSPs may be superior in some implementations to other package options for RF performance in the mmWave spectrum, which is desired for automotive radar application. But board level reliability can be a challenge for some WLCSP package due to CTE mismatch between Si and PCB. Variety of factors including PCB materials, sphere alloys, and board level underfills can influence the board level reliability of WLCSP packages. In this study the industry's first auto grade 1 capable large WLCSP package. (~ 72 mm2 body size, 18×15 BGA array, 0.5 mm pitch) is presented. Board level underfill application was utilized to achieve automotive grade board level reliability. Underfills are typically selected based on thermomechanical properties of unaged materials. An understanding of the evolution of underfill material properties under thermal aging is important for selecting a stable material capable of meeting the reliability requirements. This study evaluates board level underfills and edge bond materials in the form of stand-alone samples and applied to a large daisy-chain WLCSP. The underfilled daisy-chain WLCSPs and the stand-alone samples are placed in a -40/125C air cycling chamber (1 cycle/hour). Glass transition temperature (Tg), elastic modulus (E), and coefficient of thermal expansion (CTE) are measured using Dynamic Mechanical Analysis (DMA) and Thermomechanical Analysis (TMA) on the stand-alone samples at various intervals to monitor the evolution of material properties. Simultaneously, the underfilled daisy chain WLCSPs are monitored electrically using an event detector. The combination of material property measurements and cycles to electrical failure can be used to correlate underfill material properties and WLCSP board-level reliability. The results of this study can provide material property guidance for underfill selection.