Design analysis of touch chip for enhanced package and board level reliability

Touch chip, a unique bio-sensor to recognize fingerprint of users, is ideally suited for portable consumer applications such as mobile phones, remote controls, tablet PCs, PDAs, and ultra-thin laptop computers as security system. Modeling is a useful and efficient tool for design analysis. In this paper, both package and board level modeling are performed for touch strip, a new generation of touch chip design. The fatigue life, failure location and crack interface of the critical solder ball during thermal cycling test are predicted. It covers 14 design parameters for solder joint reliability analysis, i.e. die size and thickness, substrate thickness, board thickness, mold compound thickness, solder ball geometry, die attach and mold compound material properties, inclusion of polyimide layer, and temperature cycling range. Package level stress analysis is investigated for polyimide thickness and modulus. The findings help to design a more reliable touch chip at both package and board levels.