Encapsulant materials and processes for wafer level-chip scale packaging (WL-CSP)

Wafer level-chip scale packaging (WL-CSP) has become a popular packaging option in recent years due to its lower profile, faster signal transfer, and smaller size. These features represent the current industry trend toward high performance and miniaturization. Encapsulant materials are usually required to enhance CSP package reliability for thermal cycling and drop resistance. In addition, the encapsulant also provides a certain measure of environmental protection. Among current CSP packaging options such as standard underfill wicking and the so-called no-flow process, the wafer pre-apply process is the most cost-effective packaging method. The wafer level process increases production output and reduces the overall assembly cost significantly, while achieving comparable or better reliability. This paper provides details on the requirements of underfills applied to WL-CSPs, the properties of the new class of materials developed, the optimal wafer process and assembly conditions, and the reliability data obtained to date on WL-CSPs.