Electronic devices require a greater degree of functionality in smaller spaces. While this trend is rampant in handheld devices (mobile phones, tablets, MP3 players, etc.), the components, materials, and methods used there soon find there way into other applications. For many years underfill materials that have been used to increase the reliability of BGA and CSP packages on SMT boards have focused more on enhancing drop testing reliability versus enhancing thermal cycling performance. With the advent of smaller pitch wafer level chip scale devices (WLCSPs), the thermal cycling enhancement is becoming more critical. These consumer device applications also require that this increase in performance does not affect the other underfill attributes that have become more commonplace, such as: room temperature flow, reworkability, and drop-test improvement. These same performance characteristics can also lead to improved performance for CSP and WLCSP devices used in other applications beyond consumer devices as well. This paper will describe the material properties that affect these various aspects of underfill performance and the challenges around the performance of such materials.
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