SMTA International Conference Proceedings


EFFECT OF UNFILLED UNDERFILL ON BOARD LEVEL RELIABILITY

Author: Edward S. Ibe et al.
Company: Zymet, Inc.
Date Published: 9/25/2005   Conference: SMTA International


Abstract: The work reported here explores the effect of underfill encapsulants on drop test and thermal cycle reliability of area array packages, such as BGA’s, CSP’s, and WL-CSP’s. An unfilled underfill was found to provide superior drop test performance. A silica filled underfill was found to provide superior thermal cycle performance. However, if the unfilled underfill provides adequate thermal cycle performance, its use yields substantial process benefits.

Underfill encapsulants were originally developed to encapsulate flip chip IC’s, to enhance their resistance to thermo-mechanical stresses; i.e., thermal cycle performance. A flip chip, a piece of silicon, can have a much lower coefficient of thermal expansion (CTE) than the substrate onto which it is assembled. During thermal cycling, the CTE mismatch results in relative movement of the chip and the board resulting in early mechanical fatigue of solder joints.

Underfill encapsulants bond the chip to the board, reducing the relative movement during thermal cycling, increasing the fatigue life of the solder joints. These encapsulants are typically epoxies filled with silica, to reduce the composition’s CTE. The use of underfill encapsulants has proliferated to encapsulation of area array packages such as CSP's and BGA's. These packages are normally capable of surviving thermal cycling requirements without being encapsulated.

However, they were not designed to withstand repeated mechanical shock, which is typical for many of the applications in which they are used. For example, CSP's and BGA's are used in mobile phones, which users often drop, and are used in automotive and military electronics, which have to survive years of vibration and/or severe shock. To enhance the packages’ resistance to mechanical stresses, a filled underfill, with lower CTE, may not be necessary.

Keywords: underfill, reliability, BGA, CSP, WL-CSP, drop, thermal cycle



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