Wire bonding and reliability challenges of Gel filled TPMS packaging

Tire Pressure Monitoring System (TPMS) device is increasingly in demand as a measure to enhance vehicle and road safety through constant feedback of tyre pressure and condition. In year 2008 and 2012, US and European government has mandated the implementation of TPMS for all passenger cars respectively. This measure has created market size of exceeding 28 million sets in 2012. From 2013 onwards, South Korea government joins the call and demands that all passenger cars with gross weight less than 3.5 tons should be installed with TPMS. The move further expanded the global TPMS demand which has surpassed 30 million sets by 2013. Typical TPMS device consists of a pressure sensor, an accelerometer and a micro-controller chip. It is a multi-die packaging and to protect the pressure sensitive die, encapsulation with low modulus, high CTE gel is needed instead of conventional plastic molding. To meet stringent automotive requirement, TPMS device usually has to pass extensive temperature cycling stress. This has posed more challenges to wire bond where thermal mechanical stress in extensive temperature cycling will aggravate weakness within wire, which increases the risk of wire fatigue crack at wire bending areas such as neck, kink or second bond heel. In this paper, an in-depth analysis of the fatigue failure mode, hypothesis of the failure mechanism, wire material comparison and wire bond process DOE will be presented. Scanning Electron Microscope (SEM) was used extensively to understand the failure mode, location of wire damage and inspect on microscopic structural change, such as the impact of wire bond Heat Affected Zone (HAZ) by material type. From the reliability stress result, it is consistently observed that wire fatigue damage always happened at first kink of corner longest wire of Microcontroller Unit (MCU) to lead, and did not happen on other interchip or shorter side MCU wires. DOE is then carried out to investigate understand this observation, which includes- wire looping, wire material, and gel properties factor. For wire looping profiles, a number of different looping shapes with objective to reduce wire tightness and provide more rounded kink are being evaluated. DOE results showed that smaller kink angle, more loosen and higher looping profile are critical parameters to reduce wire fatigue damage. Besides, in term of wire properties, wire breaking load and HAZ length are found to be important. Base on the DOE result, a solution combining looping optimization and wire selection is being validated. Meanwhile, for gel material change, it is also found that lower modulus gel will provide better protection to wire bond during temperature cycling stress.