Miniaturized Stacked Die QFN for Tire Pressure Monitoring System Applications

Tire Pressure Monitoring Systems (TPMS) are electronic wireless systems that monitor and report air pressure inside pneumatic tires in real time. An example of a TPMS module integrated with the valve stem and showing the typical tire mounting location is shown in Figure 1. For their safety and fuel economy benefits, starting with the mid-2000's, active TPMS were mandated on many vehicles worldwide. The NHTS estimates that there are approximately 23,000 accidents and 535 fatalities annually involving tire underinflation and blowouts [1]. The use of TPMS has been shown to result in improved fuel economy and therefore reduced carbon emissions [2]. TPMS in passenger vehicles was mandated in the US as of Sept. 1, 2007 under the TREAD Act, in the European Union as of Nov. 1, 2012 and in South Korea as of Jan.1, 2013. Countries like Russia, Indonesia, the Philippines, Israel, Malaysia, Turkey and many others soon followed [3]. The first TPMS systems were large and bulky with a significant electronics content [4]. Since that time TPMS electronics have gotten more energy efficient and form factors have come down dramatically. This paper will outline an effort to miniaturize an existing 1.0 mm pitch, 7x7x2.2 mm body size 24 lead QFN (Quad Flat No Leads) TPMS down to a 4x4x1.98 mm body size QFN with 0.5 mm pitch that would still meet automotive AEC Grade 1 reliability requirements. The original 7x7 mm three die QFN package consisted of an ASIC, a pressure sensor and an accelerometer. This miniaturization led to many technical challenges at both the package and board level. This paper will primarily address the board level reliability (BLR) challenges encountered due to the large silicon to package ratio along with the 50% reduction in pitch. Through a series of test vehicles with variables such as QFN leadframe surface finish, lead shape and size, wettable flank (WF) technology and anchors pads, the BLR was successfully improved to the point where it met application requirements.