SMTA International Conference Proceedings


Authors: David R. Gagnon, Ph.D. et al.
Company: 3M Company
Date Published: 9/22/2002   Conference: SMTA International

Abstract: It has become common practice to use underfill adhesives to increase the thermal cycling reliability of flip chip assemblies. One commonly accepted mechanism for this reliability enhancement is the reduction of solder fatigue by using a highly filled underfill adhesive, with a coefficient of thermal expansion (CTE) matching that of the solder.

These materials are applied in a capillary dispensing process, requiring extra steps for the board assembly process. Although new no-flow type underfills (pre-applied fluxing underfill adhesives) reduce the number of process steps and may potentially reduce processing time and cost, they are currently unfilled and have a relatively high CTE.

For CSP and BGA package reliability enhancement, the default solution has also been to use capillary adhesives. The solder balls of these packages are often larger and solder fatigue is generally less of an issue. Mechanical shock is typically the primary mode of failure. Despite current research efforts, it has not yet been determined whether it is the high Tg/low CTE combination or high adhesion and flexibility that provide the best performance.

Much of this choice is dependent upon the environmental challenges the final device will face. For mechanical reliability, the use of an unfilled capillary or no-flow underfill adhesives may be the preferred choice. The objective of this paper is to review the benefits and trade-offs of a wide array of underfill types in chip and package underfill applications.

Key words: underfill, CSP, BGA, no-flow, adhesive.

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